# Quantum technologies

Dieter Meschede's research group
Group members
Professor Dr. Dieter Meschede
 Field of research: Few-atom quantum systemsFibre cavity QEDDigital quantum simulators
 Address: Institut für Angewandte Physik Wegelerstr. 8 D-53115 Bonn Germany
 Office room: 307/308
 E-mail: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it. Office: +49 228 73-3477/3478 Fax: +49 228 73-3474

## Publications

### 2020

• M. R. Lam, N. Peter, T. Groh, W. Alt, C. Robens, D. Meschede, A. Negretti, S. Montangero, T. Calarco and A. Alberti
Demonstration of quantum brachistochrones between distant states of an atom, arXiv, (2020)arXivBibTeX
ABSTRACT »

Transforming an initial quantum state into a target state through the fastest possible route—a quantum brachistochrone—is a fundamental challenge for many technologies based on quantum mechanics. Here, we demonstrate fast coherent transport of an atomic wave packet over a distance of 15 times its size—a paradigmatic case of quantum processes where the target state cannot be reached through a local transformation. Our measurements of the transport fidelity reveal the existence of a minimum duration—a quantum speed limit—for the coherent splitting and recombination of matter waves. We obtain physical insight into this limit by relying on a geometric interpretation of quantum state dynamics. These results shed light upon a fundamental limit of quantum state dynamics and are expected to find relevant applications in quantum sensing and quantum computing.

• E. Uruñuela, W. Alt, E. Keiler, D. Meschede, D. Pandey, H. Pfeifer and T. Macha
Ground-State Cooling of a Single Atom in a High-Bandwidth Cavity, Phys. Rev. A 101, 023415 (2020)arXivBibTeX
ABSTRACT »

We report on vibrational ground-state cooling of a single neutral atom coupled to a high-bandwidth Fabry-Pérot cavity. The cooling process relies on degenerate Raman sideband transitions driven by dipole trap beams, which confine the atoms in three dimensions. We infer a one-dimensional motional ground-state population close to 90% by means of Raman spectroscopy. Moreover, lifetime measurements of a cavity-coupled atom exceeding 40 s imply three-dimensional cooling of the atomic motion, which makes this resource-efficient technique particularly interesting for cavity experiments with limited optical access.

• T. Macha, E. Uruñuela, W. Alt, M. Ammenwerth, D. Pandey, H. Pfeifer and D. Meschede
Non-adiabatic Storage of Short Light Pulses in an Atom-Cavity System, Phys. Rev. A 101, 053406 (2020)arXivBibTeX
ABSTRACT »

We demonstrate the storage of 5 ns light pulses in a single rubidium atom coupled to a fiber-based optical resonator. Our storage protocol addresses a regime beyond the conventional adiabatic limit and approaches the theoretical bandwidth limit. We extract the optimal control laser pulse properties from a numerical simulation of our system and measure storage efficiencies of (8.1±1.1)%, in close agreement with the maximum expected efficiency. Such well-controlled and high-bandwidth atom-photon interfaces are key components for future hybrid quantum networks.

### 2019

• M. Sajid, J. K. Asbóth, D. Meschede, R. F. Werner and A. Alberti
Creating anomalous Floquet Chern insulators with magnetic quantum walks, Phys. Rev. B (Editors' Suggestion) 99, 214303 (2019)arXivBibTeX
ABSTRACT »

We propose a realistic scheme to construct anomalous Floquet Chern topological insulators using spin-1/2 particles carrying out a discrete-time quantum walk in a two-dimensional lattice. By Floquet engineering the quantum-walk protocol, an Aharonov-Bohm geometric phase is imprinted onto closed-loop paths in the lattice, thus realizing an abelian gauge field—the analog of a magnetic flux threading a two-dimensional electron gas. We show that in the strong field regime, when the flux per plaquette is a sizable fraction of the flux quantum, magnetic quantum walks give rise to nearly flat energy bands featuring nonvanishing Chern numbers. Furthermore, we find that because of the nonperturbative nature of the periodic driving, a second topological number—the so-called RLBL invariant—is necessary to fully characterize the anomalous Floquet topological phases of magnetic quantum walks and to compute the number of topologically protected edge modes expected at the boundaries between different phases. In the second part of this article, we discuss an implementation of this scheme using neutral atoms in two-dimensional spin-dependent optical lattices, which enables the generation of arbitrary magnetic-field landscapes, including those with sharp boundaries. The robust atom transport, which is observed along boundaries separating regions of different field strength, reveals the topological character of the Floquet Chern bands.

### 2018

• J. Gallego, W. Alt, T. Macha, M. Martinez-Dorantes, D. Pandey and D. Meschede
Strong Purcell effect on a neutral atom trapped in an open fiber cavity, Phys. Rev. Lett. 121, 17360 (2018)arXivBibTeX
ABSTRACT »

We observe a sixfold Purcell broadening of the D2 line of an optically trapped 87Rb atom strongly coupled to a fiber cavity. Under external illumination by a near-resonant laser, up to 90% of the atom's fluorescence is emitted into the resonant cavity mode. The sub-Poissonian statistics of the cavity output and the Purcell enhancement of the atomic decay rate are confirmed by the observation of a strongly narrowed antibunching dip in the photon autocorrelation function. The photon leakage through the higher-transmission mirror of the single-sided resonator is the dominant contribution to the field decay (κ≈2π×50 MHz), thus offering a high-bandwidth, fiber-coupled channel for photonic interfaces such as quantum memories and single-photon sources.

• M. Zopf, T. Macha, R. Keil, E. Uruñuela, Y. Chen, W. Alt, L. Ratschbacher, F. Ding, D. Meschede and O. G. Schmidt
Frequency feedback for two-photon interference from separate quantum dots, Phys. Rev. B 98, 161302(R) (2018)arXivBibTeX
ABSTRACT »

We employ active feedback to stabilize the frequency of single photons emitted by two separate quantum dots to an atomic standard. The transmission of a rubidium-based Faraday filter serves as the error signal for frequency stabilization. We achieve a residual frequency deviation of <30 MHz, which is less than 1.5% of the quantum dot linewidth. Long-term stability is demonstrated by Hong-Ou-Mandel interference between photons from the two quantum dots. Their internal dephasing limits the expected visibility to V = 40%. We observe Vlock = (41±5)% for frequency-stabilized dots as opposed to Vfree = (31±7)% for free-running emission. Our technique reaches the maximally expected visibility for the given system and therefore facilitates quantum networks with indistinguishable photons from distributed sources.

• C. Robens, S. Brakhane, W. Alt, D. Meschede, J. Zopes and A. Alberti
Fast, high-precision optical polarization synthesizer for ultracold-atom experiments, Phys. Rev. Applied 9, 034016 (2018)arXivBibTeX
ABSTRACT »

We present a novel approach to precisely synthesize arbitrary polarization states of light with a high modulation bandwidth. Our approach consists in superimposing two laser light fields with the same wavelength, but with opposite circular polarizations, where the phase and amplitude of each light field are individually controlled. We find that the polarization-synthesized beam reaches a degree of polarization of 99.99%, which is mainly limited by static spatial variations of the polarization state over the beam profile. We also find that the depolarization caused by temporal fluctuations of the polarization state is about two orders of magnitude smaller. In a recent work, Robens et al. [Phys. Rev. Lett. 118, 065302 (2017)] demonstrated an application of the polarization synthesizer to create two independently controllable optical lattices, which trap atoms depending on their internal spin state. We here use ultracold atoms in polarization-synthesized optical lattices to give an independent, in situ demonstration of the performance of the polarization synthesizer.

• D. Meschede
Superradiators created atom by atom, Science 359, 641 (2018)BibTeX
ABSTRACT »

High radiation rates are usually associated with macroscopic lasers. Laser radiation is “coherent”—its amplitude and phase are well-defined—but its generation requires energy inputs to overcome loss. Excited atoms spontaneously emit in a random and incoherent fashion, and for N such atoms, the emission rate simply increases as N. However, if these atoms are in close proximity and coherently coupled by a radiation field, this microscopic ensemble acts as a single emitter whose emission rate increases as N2 and becomes “superradiant,” to use Dicke's terminology (1). On page 662 of this issue, Kim et al. (2) show the buildup of coherent light fields through collective emission from atomic radiators injected one by one into a resonator field. There is only one atom ever in the cavity, but the emission is still collective and superradiant. These results suggest another route toward thresholdless lasing.

• M. Martinez-Dorantes, W. Alt, J. Gallego, S. Ghosh, L. Ratschbacher and D. Meschede
State-dependent fluorescence of neutral atoms in optical potentials, Phys. Rev. A 97, 023410 (2018)arXivBibTeX
ABSTRACT »

Recently we have demonstrated scalable, nondestructive, and high-fidelity detection of the internal state of 87Rb neutral atoms in optical dipole traps using state-dependent fluorescence imaging [M. Martinez-Dorantes, W. Alt, J. Gallego, S. Ghosh, L. Ratschbacher, Y. Völzke, and D. Meschede, Phys. Rev. Lett. 119, 180503 (2017)]. In this paper we provide experimental procedures and interpretations to overcome the detrimental effects of heating-induced trap losses and state leakage. We present models for the dynamics of optically trapped atoms during state-dependent fluorescence imaging and verify our results by comparing Monte Carlo simulations with experimental data. Our systematic study of dipole force fluctuations heating in optical traps during near-resonant illumination shows that off-resonant light is preferable for state detection in tightly confining optical potentials.

### 2017

• C. F. Roos, A. Alberti, D. Meschede, P. Hauke and H. Häffner
Revealing quantum statistics with a pair of distant atoms, Phys. Rev. Lett. (Featured in Physics & Editors' Suggestion) 119, 160401 (2017)arXivBibTeX
ABSTRACT »

Quantum statistics have a profound impact on the properties of systems composed of identical particles. At the most elementary level, Bose and Fermi quantum statistics di er in the exchange phase, either 0 or π, which the wavefunction acquires when two identical particles are exchanged. In this Letter, we demonstrate that the exchange phase can be directly probed with a pair of massive particles by physically exchanging their positions. We present two protocols where the particles always remain spatially well separated, thus ensuring that the exchange contribution to their interaction energy is negligible and that the detected signal can only be attributed to the exchange symmetry of the wavefunction. We discuss possible implementations with a pair of trapped atoms or ions.

• M. Martinez-Dorantes, W. Alt, J. Gallego, S. Ghosh, L. Ratschbacher, Y. Völzke and D. Meschede
Fast Nondestructive Parallel Readout of Neutral Atom Registers in Optical Potentials, Phys. Rev. Lett. 119, 180503 (2017)arXivBibTeX
ABSTRACT »

We demonstrate the parallel and nondestructive readout of the hyperfine state for optically trapped 87Rb atoms. The scheme is based on state-selective fluorescence imaging and achieves detection fidelities > 98% within 10 ms, while keeping 99% of the atoms trapped. For the readout of dense arrays of neutral atoms in optical lattices, where the fluorescence images of neighboring atoms overlap, we apply a novel image analysis technique using Bayesian inference to determine the internal state of multiple atoms. Our method is scalable to large neutral atom registers relevant for future quantum information processing tasks requiring fast and nondestructive readout and can also be used for the simultaneous readout of quantum information stored in internal qubit states and in the atoms’ positions.

• C. Robens, J. Zopes, W. Alt, S. Brakhane, D. Meschede and A. Alberti
Low-entropy states of neutral atoms in polarization-synthesized optical lattices, Phys. Rev. Lett. 118, 065302 (2017)arXivBibTeX
ABSTRACT »

We create low-entropy states of neutral atoms by utilizing a conceptually new optical-lattice technique that relies on a high-precision, high-bandwidth synthesis of light polarization. Polarization-synthesized optical lattices provide two fully controllable optical lattice potentials, each of them confining only atoms in either one of the two long-lived hyperfine states. By employing one lattice as the storage register and the other one as the shift register, we provide a proof of concept using four atoms that selected regions of the periodic potential can be filled with one particle per site. We expect that our results can be scaled up to thousands of atoms by employing an atom-sorting algorithm with logarithmic complexity, which is enabled by polarization-synthesized optical lattices. Vibrational entropy is subsequently removed by sideband cooling methods. Our results pave the way for a bottom-up approach to creating ultralow-entropy states of a many-body system.

• C. Robens, S. Brakhane, W. Alt, F. Kleißler, D. Meschede, G. Moon, G. Ramola and A. Alberti
High numerical aperture (NA = 0.92) objective lens for imaging and addressing of cold atoms, Opt. Lett. 42, 1043 (2017)arXivBibTeX
ABSTRACT »

We have designed, built, and characterized a high- resolution objective lens that is compatible with an ultra-high vacuum environment. The lens system ex- ploits the principle of the Weierstrass-sphere solid immersion lens to reach a numerical aperture (NA) of 0.92. Tailored to the requirements of optical lattice experiments, the objective lens features a relatively long working distance of 150 μm. Our two-lens design is remarkably insensitive to mechanical tolerances in spite of the large NA. Additionally, we demonstrate the application of a tapered optical fiber tip, as used in scanning near-field optical microscopy, to measure the point spread function of a high NA optical system. From the point spread function, we infer the wavefront aberration for the entire field of view of about 75 μm. Pushing the NA of an optical system to its ultimate limit enables novel applications in quantum technolo- gies such as quantum control of atoms in optical mi- crotraps with an unprecedented spatial resolution and photon collection efficiency.

• R. Wengenmayr - interview mit Dieter Meschede
Quantentechnologie wird immer noch als Kuriosität wahrgenommen, Phys. Unserer Zeit 4, 169 (2017)BibTeX
ABSTRACT »

Quanteninformationstechnologien gehören zu den boomenden Forschungsgebieten. Längst geht es nicht mehr allein um Grundlagenforschung, sondern um handfeste technische Anwendungen. Im Fokus ist die abhörsichere Kommunikation. Ein Interview mit Dieter Meschede, Leiter der Arbeitsgruppe Quantentechnologie an der Universität Bonn und künftiger Präsident der Deutschen Physikalischen Gesellschaft.

### 2016

• T. Groh, S. Brakhane, W. Alt, D. Meschede, J. Asbóth and A. Alberti
Robustness of topologically protected edge states in quantum walk experiments with neutral atoms, Phys. Rev. A (Editors' Suggestion) 94, 013620 (2016)arXivBibTeX
ABSTRACT »

Discrete-time quantum walks allow Floquet topological insulator materials to be explored using controllable systems such as ultracold atoms in optical lattices. By numerical simulations, we study the robustness of topologically protected edge states in the presence of decoherence in one- and two-dimensional discrete-time quantum walks. We also develop a simple analytical model quantifying the robustness of these edge states against either spin or spatial dephasing, predicting an exponential decay of the population of topologically protected edge states. Moreover, we present an experimental proposal based on neutral atoms in spin-dependent optical lattices to realize spatial boundaries between distinct topological phases. Our proposal relies on a new scheme to implement spin-dependent discrete shift operations in a two-dimensional optical lattice. We analyze under realistic decoherence conditions the experimental feasibility of observing unidirectional, dissipationless transport of matter waves along boundaries separating distinct topological domains.

• J. Gallego, S. Ghosh, S. K. Alavi, W. Alt, M. Martinez-Dorantes, D. Meschede and L. Ratschbacher
High Finesse Fiber Fabry-Perot Cavities: Stabilization and Mode Matching Analysis, Appl. Phys. B 122, 47 (2016)arXivBibTeX
ABSTRACT »

Fiber Fabry-Perot cavities, formed by micro-machined mirrors on the end-facets of optical fibers, are used in an increasing number of technical and scientific applications, where they typically require precise stabilization of their optical resonances. Here, we study two different approaches to construct fiber Fabry-Perot resonators and stabilize their length for experiments in cavity quantum electrodynamics with neutral atoms. A piezo-mechanically actuated cavity with feedback based on the Pound-Drever-Hall locking technique is compared to a novel rigid cavity design that makes use of the high passive stability of a monolithic cavity spacer and employs thermal self-locking and external temperature tuning. Furthermore, we present a general analysis of the mode matching problem in fiber Fabry-Perot cavities, which explains the asymmetry in their reflective line shapes and has important implications for the optimal alignment of the fiber resonators. Finally, we discuss the issue of fiber-generated background photons. We expect that our results contribute towards the integration of high-finesse fiber Fabry-Perot cavities into compact and robust quantum-enabled devices in the future.

• A. Alberti, C. Robens, W. Alt, S. Brakhane, M. Karski, R. Reimann, A. Widera and D. Meschede
Super-resolution microscopy of single atoms in optical lattices, New J. Phys. 18, 053010 (2016)arXivBibTeX
ABSTRACT »

We report on image processing techniques and experimental procedures to determine the lattice-site positions of single atoms in an optical lattice with high reliability, even for limited acquisition time or optical resolution. Determining the positions of atoms beyond the diffraction limit relies on parametric deconvolution in close analogy to methods employed in super-resolution microscopy. We develop a deconvolution method that makes effective use of the prior knowledge of the optical transfer function, noise properties, and discreteness of the optical lattice. We show that accurate knowledge of the image formation process enables a dramatic improvement on the localization reliability. This allows us to demonstrate super-resolution of the atoms' position in closely packed ensembles where the separation between particles cannot be directly optically resolved. Furthermore, we demonstrate experimental methods to precisely reconstruct the point spread function with sub-pixel resolution from fluorescence images of single atoms, and we give a mathematical foundation thereof. We also discuss discretized image sampling in pixel detectors and provide a quantitative model of noise sources in electron multiplying CCD cameras. The techniques developed here are not only beneficial to neutral atom experiments, but could also be employed to improve the localization precision of trapped ions for ultra precise force sensing.

• C. Robens, W. Alt, C. Emary, D. Meschede and A. Alberti
Atomic "bomb testing": the Elitzur-Vaidman experiment violates the Leggett-Garg inequality, Appl. Phys. B 123, 12 (2016)arXivBibTeX
ABSTRACT »

Elitzur and Vaidman have proposed a measurement scheme that, based on the quantum superposition principle, allows one to detect the presence of an object—in a dramatic scenario, a bomb—without interacting with it. It was pointed out by Ghirardi that this interaction-free measurement scheme can be put in direct relation with falsification tests of the macro-realistic worldview. Here we have implemented the "bomb test" with a single atom trapped in a spin-dependent optical lattice to show explicitly a violation of the Leggett-Garg inequality—a quantitative criterion fulfilled by macro-realistic physical theories. To perform interaction-free measurements, we have implemented a novel measurement method that correlates spin and position of the atom. This method, which quantum mechanically entangles spin and position, finds general application for spin measurements, thereby avoiding the shortcomings inherent in the widely used push-out technique. Allowing decoherence to dominate the evolution of our system causes a transition from quantum to classical behavior in fulfillment of the Leggett-Garg inequality.

### 2015

• C. Robens, W. Alt, D. Meschede, C. Emary and A. Alberti
Ideal Negative Measurements in Quantum Walks Disprove Theories Based on Classical Trajectories, Phys. Rev. X (Featured in Physics) 5, 011003 (2015)arXivBibTeX
ABSTRACT »

We report on a stringent test of the nonclassicality of the motion of a massive quantum particle, which propagates on a discrete lattice. Measuring temporal correlations of the position of single atoms performing a quantum walk, we observe a 6σ violation of the Leggett-Garg inequality. Our results rigorously excludes (i.e., falsifies) any explanation of quantum transport based on classical, well-defined trajectories. We use so-called ideal negative measurements—an essential requisite for any genuine Leggett-Garg test—to acquire information about the atom’s position, yet avoiding any direct interaction with it. The interaction-free measurement is based on a novel atom transport system, which allows us to directly probe the absence rather than the presence of atoms at a chosen lattice site. Beyond the fundamental aspect of this test, we demonstrate the application of the Leggett-Garg correlation function as a witness of quantum superposition. Here, we employ the witness to discriminate different types of walks spanning from merely classical to wholly quantum dynamics.

• R. Reimann, W. Alt, T. Kampschulte, T. Macha, L. Ratschbacher, N. Thau, S. Yoon and D. Meschede
Cavity-Modified Collective Rayleigh Scattering of Two Atoms, Phys. Rev. Lett. 114, 023601 (2015)arXivBibTeX
ABSTRACT »

We report on the observation of cooperative radiation of exactly two neutral atoms strongly coupled to the single mode field of an optical cavity, which is close to the lossless-cavity limit. Monitoring the cavity output power, we observe constructive and destructive interference of collective Rayleigh scattering for certain relative distances between the two atoms. Because of cavity backaction onto the atoms, the cavity output power for the constructive two-atom case (N=2) is almost equal to the single-emitter case (N=1), which is in contrast to free-space where one would expect an N^2 scaling of the power. These effects are quantitatively explained by a classical model as well as by a quantum mechanical model based on Dicke states. We extract information on the relative phases of the light fields at the atom positions and employ advanced cooling to reduce the jump rate between the constructive and destructive atom configurations. Thereby we improve the control over the system to a level where the implementation of two-atom entanglement schemes involving optical cavities becomes realistic.

• S. Brakhane, W. Alt, D. Meschede, C. Robens, G. Moon and A. Alberti
Ultra-low birefringence dodecagonal vacuum glass cell, Rev. Sci. Instrum. 86, 126108 (2015)arXivBibTeX
ABSTRACT »

We report on an ultra-low birefringence dodecagonal glass cell for ultra-high vacuum applications. The epoxy-bonded trapezoidal windows of the cell are made of SF57 glass, which exhibits a very low stress-induced birefringence. We characterize the birefringence Δn of each window with the cell under vacuum conditions, obtaining values around 10-8. After baking the cell at 150 ºC, we reach a pressure below 10-10 mbar. In addition, each window is antireflection coated on both sides, which is highly desirable for quantum optics experiments and precision measurements.

• S. Brakhane, W. Alt, D. Meschede, C. Robens and A. Alberti
Polarisationserhaltende Vakuum-Zelle zur Anwendung oder Messung elektromagnetischer Wellen im Vakuum, Patent pending, (2015)BibTeX
ABSTRACT »

Die Erfindung betrifft ein Verfahren, eine Vorrichtung und die Verwendung einer Vorrichtung zur Anwendung oder Messung polarisierter elektromagnetischer Strahlung im Vakuum, wobei die Doppelbrechung Δn < 10-6 beträgt.

• C. Robens, S. Brakhane, D. Meschede and A. Alberti
Quantum Walks With Neutral Atoms: Quantum Interference Effects of One and Two Particles, Proceedings of the XXII International Conference ICOLS, (2015)arXivBibTeX
ABSTRACT »

We report on the state of the art of quantum walk experiments with neutral atoms in state-dependent optical lattices. We demonstrate a novel state-dependent transport technique enabling the control of two spin-selective sublattices in a fully independent fashion. This transport technique allowed us to carry out a test of single-particle quantum interference based on the violation of the Leggett-Garg inequality and, more recently, to probe two-particle quantum interference effects with neutral atoms cooled into the motional ground state. These experiments lay the groundwork for the study of discrete-time quantum walks of strongly interacting, indistinguishable particles to demonstrate quantum cellular automata of neutral atoms.

### 2014

• T. Kampschulte, W. Alt, S. Manz, M. Martinez-Dorantes, R. Reimann, S. Yoon, D. Meschede, M. Bienert and G. Morigi
Electromagnetically-induced-transparency control of single-atom motion in an optical cavity, Phys. Rev. A 89, 033404 (2014)arXivBibTeX
ABSTRACT »

We demonstrate cooling of the motion of a single neutral atom confined by a dipole trap inside a high-finesse optical resonator. Cooling of the vibrational motion results from electromagnetically induced transparency (EIT)–like interference in an atomic lambda-type configuration, where one transition is strongly coupled to the cavity mode and the other is driven by an external control laser. Good qualitative agreement with the theoretical predictions is found for the explored parameter ranges. Further, we demonstrate EIT cooling of atoms in the dipole trap in free space, reaching the ground state of axial motion. By means of a direct comparison with the cooling inside the resonator, the role of the cavity becomes evident by an additional cooling resonance. These results pave the way towards a controlled interaction among atomic, photonic, and mechanical degrees of freedom.

• S. Gammelmark, W. Alt, T. Kampschulte, D. Meschede and K. Mølmer
Hidden Markov Model of atomic quantum jump dynamics in an optically probed cavity, Phys. Rev. A 89, 043839 (2014)arXivBibTeX
ABSTRACT »

We analyze the quantum jumps of an atom interacting with a cavity field, where strong coupling makes the cavity transmission depend on the time-dependent atomic state. In our analysis we employ a Bayesian approach that conditions the population of the atomic states at time t on the cavity transmission observed both before and after t, and we show that the state assignment by this approach is more decisive than the usual conditional quantum states based on only earlier measurement data. We also provide an iterative protocol which, together with the atomic state populations, simultaneously estimates the atomic jump rates and the transmission signal distributions from the measurement data. Finally, we take into account technical fluctuations in the observed signal, e.g., due to spatial motion of the atom within the cavity, by representing atomic states by several hidden states, thereby significantly improving the state's recovery.

• A. Alberti, W. Alt, R. Werner and D. Meschede
Decoherence Models for Discrete-Time Quantum Walks and their Application to Neutral Atom Experiments, New J. Phys. 16, 123052 (2014)arXivBibTeX
ABSTRACT »

We discuss decoherence in discrete-time quantum walks in terms of a phenomenological model that distinguishes spin and spatial decoherence. We identify the dominating mechanisms that affect quantum-walk experiments realized with neutral atoms walking in an optical lattice.

From the measured spatial distributions, we determine with good precision the amount of decoherence per step, which provides a quantitative indication of the quality of our quantum walks. In particular, we find that spin decoherence is the main mechanism responsible for the loss of coherence in our experiment. We also find that the sole observation of ballistic—instead of diffusive—expansion in position space is not a good indicator of the range of coherent delocalization.

We provide further physical insight by distinguishing the effects of short- and long-time spin dephasing mechanisms. We introduce the concept of coherence length in the discrete-time quantum walk, which quantifies the range of spatial coherences. Unexpectedly, we find that quasi-stationary dephasing does not modify the local properties of the quantum walk, but instead affects spatial coherences.

For a visual representation of decoherence phenomena in phase space, we have developed a formalism based on a discrete analogue of the Wigner function. We show that the effects of spin and spatial decoherence differ dramatically in momentum space.

• R. Reimann, W. Alt, T. Macha, D. Meschede, N. Thau, S. Yoon and L. Ratschbacher
Carrier-free Raman manipulation of trapped neutral atoms, New J. Phys. 16, 113042 (2014)arXivBibTeX
ABSTRACT »

We experimentally realize an enhanced Raman control scheme for neutral atoms that features an intrinsic suppression of the two-photon carrier transition, but retains the sidebands which couple to the external degrees of freedom of the trapped atoms. This is achieved by trapping the atom at the node of a blue detuned standing wave dipole trap, that acts as one field for the two-photon Raman coupling. The improved ratio between cooling and heating processes in this configuration enables a five times lower fundamental temperature limit for resolved sideband cooling. We apply this method to perform Raman cooling to the two-dimensional vibrational ground state and to coherently manipulate the atomic motion. The presented scheme requires minimal additional resources and can be applied to experiments with challenging optical access, as we demonstrate by our implementation for atoms strongly coupled to an optical cavity.

### 2013

• A. Steffen, W. Alt, M. Genske, D. Meschede, C. Robens and A. Alberti
In-situ measurement of vacuum window birefringence by atomic spectroscopy, Rev. Sci. Instrum. 84, 126103 (2013)arXivBibTeX
ABSTRACT »

We present an in-situ method to measure the birefringence of a single vacuum window by means of microwave spectroscopy on an ensemble of cold atoms. Stress-induced birefringence can cause an ellipticity in the polarization of an initially linearly-polarized laser beam. The amount of ellipticity can be reconstructed by measuring the differential vector light shift of an atomic hyperfine transition. Measuring the ellipticity as a function of the linear polarization angle allows us to infer the amount of birefringence Δn at the level of 10-8 and identify the orientation of the optical axes. The key benefit of this method is the ability to separately characterize each vacuum window, allowing the birefringence to be precisely compensated in existing vacuum apparatuses.

• M. Genske, W. Alt, A. Steffen, A. H. Werner, R. F. Werner, D. Meschede and A. Alberti
Electric quantum walks with individual atoms, Phys. Rev. Lett. 110, 190601 (2013)arXivBibTeX
ABSTRACT »
We report on the experimental realization of electric quantum walks, which mimic the effect of an electric field on a charged particle in a lattice. Starting from a textbook implementation of discrete-time quantum walks, we introduce an extra operation in each step to implement the effect of the field. The recorded dynamics of such a quantum particle exhibits features closely related to Bloch oscillations and interband tunneling. In particular, we explore the regime of strong fields, demonstrating contrasting quantum behaviors: quantum resonances vs. dynamical localization depending on whether the accumulated Bloch phase is a rational or irrational fraction of 2π.
• N. Belmechri, L. Förster, W. Alt, A. Widera, D. Meschede and A. Alberti
Microwave control of atomic motional states in a spin-dependent optical lattice, J. Phys. B: At. Mol. Opt. Phys. 46, 104006 (2013)arXivBibTeX
ABSTRACT »
Spin-dependent optical potentials allow us to use microwave radiation to manipulate the motional state of trapped neutral atoms (Förster et al. 2009 Phys. Rev. Lett. 103, 233001). Here, we discuss this method in greater detail, comparing it to the widely-employed Raman sideband coupling method. We provide a simplified model for sideband cooling in a spin-dependent potential, and we discuss it in terms of the generalized Lamb-Dicke parameter. Using a master equation formalism, we present a quantitative analysis of the cooling performance for our experiment, which can be generalized to other experimental settings. We additionally use microwave sideband transitions to engineer motional Fock states and coherent states, and we devise a technique for measuring the population distribution of the prepared states.

### 2012

• A. Steffen, A. Alberti, W. Alt, N. Belmechri, S. Hild, M. Karski, A. Widera and D. Meschede
arXivBibTeX
ABSTRACT »

Engineering quantum particle systems, such as quantum simulators and quantum cellular automata, relies on full coherent control of quantum paths at the single particle level. Here we present an atom interferometer operating with single trapped atoms, where single particle wave packets are controlled through spin-dependent potentials. The interferometer is constructed from a sequence of discrete operations based on a set of elementary building blocks, which permit composing arbitrary interferometer geometries in a digital manner. We use this modularity to devise a space-time analogue of the well-known spin echo technique, yielding insight into decoherence mechanisms. We also demonstrate mesoscopic delocalization of single atoms with a separation-to-localization ratio exceeding 500; this result suggests their utilization beyond quantum logic applications as nano-resolution quantum probes in precision measurements, being able to measure potential gradients with precision 5×10-4 in units of gravitational acceleration g.

• S. Brakhane, W. Alt, T. Kampschulte, M. Martinez-Dorantes, R. Reimann, S. Yoon, A. Widera and D. Meschede
Bayesian Feedback Control of a Two-Atom Spin-State in an Atom-Cavity System, Phys. Rev. Lett. 109, 173601 (2012)arXivBibTeX
ABSTRACT »

We experimentally demonstrate real-time feedback control of the joint spin-state of two neutral Caesium atoms inside a high finesse optical cavity. The quantum states are discriminated by their different cavity transmission levels. A Bayesian update formalism is used to estimate state occupation probabilities as well as transition rates. We stabilize the balanced two-atom mixed state, which is deterministically inaccessible, via feedback control and find very good agreement with Monte-Carlo simulations. On average, the feedback loops achieves near optimal conditions by steering the system to the target state marginally exceeding the time to retrieve information about its state.

• N. Spethmann, F. Kindermann, S. John, C. Weber, D. Meschede and A. Widera
Dynamics of single neutral impurity atoms immersed in an ultracold gas, Phys. Rev. Lett. 109, 235301 (2012)arXivBibTeX
ABSTRACT »
We report on controlled doping of an ultracold Rb gas with single neutral Cs impurity atoms. Elastic two-body collisions lead to a rapid thermalization of the impurity inside the Rb gas, representing the first realization of an ultracold gas doped with a precisely known number of impurity atoms interacting via s-wave collisions. Inelastic interactions are restricted to a single three-body recombination channel in a highly controlled and pure setting, which allows to determine the Rb-Rb-Cs three-body loss rate with unprecedented precision. Our results pave the way for a coherently interacting hybrid system of individually controllable impurities in a quantum many-body system.
• A. Ahlbrecht, A. Alberti, D. Meschede, V. B. Scholz, A. H. Werner and R. F. Werner
Molecular binding in interacting quantum walks, New J. Phys. 14, 073050 (2012)arXivBibTeX
ABSTRACT »

We show that the presence of an interaction in the quantum walk of two atoms leads to the formation of a stable compound, a molecular state. The wave-function of the molecule decays exponentially in the relative position of the two atoms, hence it constitutes a true bound state. Furthermore, for a certain class of interactions we develop an effective theory and find that the dynamics of the molecule is described by a quantum walk in its own right. We propose a setup for the experimental realization as well as sketch the possibility to observe quasi-particle effects in quantum many body systems.

• U. Wiedemann, W. Alt and D. Meschede
Switching photochromic molecules adsorbed on optical microfibres, Opt. Express 20, 12710-12720 (2012)BibTeX
ABSTRACT »
The internal state of organic photochromic spiropyran molecules adsorbed on optical microfibres is optically controlled and measured by state-dependent light absorption. Repeated switching between the states is achieved by exposure to the evanescent field of a few nanowatts of light guided in the microfibre. By adjusting the microfibre evanescent field strength the dynamic equilibrium state of the molecules is controlled. Time-resolved photoswitching dynamics are measured and modelled with a rate equation model. We also study how many times the photochromic system can be switched before undergoing significant photochemical degradation.

### 2011

• M. Karski, L. Förster, J. Choi, W. Alt, A. Alberti, A. Widera and D. Meschede
Direct Observation and Analysis of Spin-Dependent Transport of Single Atoms in a 1D Optical Lattice, J. Korean Phys. Soc. 59, 2947 (2011)arXivBibTeX
ABSTRACT »

We have directly observed spin-dependent transport of single cesium atoms in a 1D optical lattice. A superposition of two circularly polarized standing waves is generated from two counter propagating, linearly polarized laser beams. Rotation of one of the polarizations by π causes displacement of the σ+- and σ-lattices by one lattice site. Unidirectional transport over several lattice sites is achieved by rotating the polarization back and forth and flipping the spin after each transport step. We have analyzed the transport efficiency over 10 and more lattice sites, and discussed and quantified relevant error sources.

• A. Widera, W. Alt and D. Meschede
Coherently Walking, Rocking and Blinding Single Neutral Atoms, J. Phys.: Conf. Ser. 264, 012021 (2011)BibTeX
ABSTRACT »
Advances in the preparation and detection, but most importantly in the coherent manipulation of single neutral atoms have allowed the observation of intriguing phenomena of quantum physics in recent years. We discuss developments to prepare and detect single neutral atoms in a one-dimensional optical lattice potential with single site resolution. Moreover, using two different experimental techniques, a state-dependent optical lattice potential on the one hand and a high-finesse optical cavity on the other hand, we have obtained coherent control over single neutral atoms. The former has enabled us to observe the quantum walk of atoms in position space, and to coherently control the motion of trapped atoms via microwave radiation. The latter offers a means to non-destructively detect the atomic spin state, thereby revealing quantum jumps of single atoms, or the altered optical properties of single atoms when subject to electromagnetically-induced transparency.
• R. Garcia-Fernandez, W. Alt, F. Bruse, C. Dan, K. Karapetyan, O. Rehband, A. Stiebeiner, U. Wiedemann, D. Meschede and A. Rauschenbeutel
Optical nanofibers and spectroscopy, Applied Physics B 105, 3–15 (2011)arXivBibTeX
ABSTRACT »
We review our recent progress in the production and characterization of tapered optical fibers with a sub-wavelength diameter waist. Such fibers exhibit a pronounced evanescent field and are therefore a useful tool for highly sensitive evanescent wave spectroscopy of adsorbates on the fiber waist or of the medium surrounding. We use a carefully designed flame pulling process that allows us to realize preset fiber diameter profiles. In order to determine the waist diameter and to verify the fiber profile, we employ scanning electron microscope measurements and a novel accurate in situ optical method based on harmonic generation. We use our fibers for linear and non-linear absorption and fluorescence spectroscopy of surface-adsorbed organic molecules and investigate their agglomeration dynamics. Furthermore, we apply our spectroscopic method to quantum dots on the surface of the fiber waist and to caesium vapor surrounding the fiber. Finally, towards dispersive measurements, we present our first results on building and testing a single-fiber bi-modal interferometer.
• A. Mawardi, S. Hild, A. Widera and D. Meschede
ABCD-treatment of a propagating doughnut beam generated by a spiral phase plate, Optics Express 19, 21205-21210 (2011)BibTeX
ABSTRACT »
We apply the Collins-Huygens integral to analytically describe propagation of a doughnut beam generated by a spiral phase plate. Measured beam profiles in free space and through an ABCD-lens system illustrate excellent agreement with theory. Applications range from the creation of optical beams with angular momentum to microscopy to trapping neutral atoms. The method extends to other beam shaping components, too.

### 2010

• T. Kampschulte, W. Alt, S. Brakhane, M. Eckstein, R. Reimann, A. Widera and D. Meschede
Optical control of the refractive index of a single atom, Phys. Rev. Lett. 105, 153603 (2010)arXivBibTeX
ABSTRACT »

We experimentally demonstrate the elementary case of electromagnetically induced transparency with a single atom inside an optical cavity probed by a weak field. We observe the modification of the dispersive and absorptive properties of the atom by changing the frequency of a control light field. Moreover, a strong cooling effect has been observed at two-photon resonance, increasing the storage time of our atoms twenty-fold to about 16 seconds. Our result points towards all-optical switching with single photons.

• M. Karski, L. Förster, J. Choi, A. Steffen, N. Belmechri, W. Alt, D. Meschede and A. Widera
Imprinting Patterns of Neutral Atoms in an Optical Lattice using Magnetic Resonance Techniques, New J. Phys. 12, 065027 (2010)arXivBibTeX
ABSTRACT »
We prepare arbitrary patterns of neutral atoms in a one-dimensional (1D) optical lattice with single-site precision using microwave radiation in a magnetic field gradient. We give a detailed account of the current limitations and propose methods to overcome them. Our results have direct relevance for addressing planes, strings or single atoms in higher-dimensional optical lattices for quantum information processing or quantum simulations with standard methods in current experiments. Furthermore, our findings pave the way for arbitrary single-qubit control with single-site resolution.
• S. Reick, K. Mølmer, W. Alt, M. Eckstein, T. Kampschulte, L. Kong, R. Reimann, A. Thobe, A. Widera and D. Meschede
Analyzing quantum jumps of one and two atoms strongly coupled to an optical cavity, J. Opt. Soc. Am. B 27, A152 (2010)arXivBibTeX
ABSTRACT »
We induce quantum jumps between the hyperfine ground states of one and two Cesium atoms, strongly coupled to the mode of a high-finesse optical resonator, and analyze the resulting random telegraph signals. We identify experimental parameters to deduce the atomic spin state nondestructively from the stream of photons transmitted through the cavity, achieving a compromise between a good signal-to-noise ratio and minimal measurement-induced perturbations. In order to extract optimum information about the spin dynamics from the photon count signal, a Bayesian update formalism is employed, which yields time-dependent probabilities for the atoms to be in either hyperfine state. We discuss the effect of super-Poissonian photon number distributions caused by atomic motion.
• U. Wiedemann, K. Karapetyan, C. Dan, D. Pritzkau, W. Alt, S. Irsen and D. Meschede
Measurement of submicrometre diameters of tapered optical fibres using harmonic generation, Opt. Express 18, 7693–7704 (2010)BibTeX
ABSTRACT »
Applications of subwavelength-diameter optical fibres in nonlinear optics require precise knowledge of the submicrometre fibre waist diameter. We demonstrate a new technique for optical measurement of the diameter based on second- and third-harmonic generation with an accuracy of better than 2%. To generate the harmonic light, inter-modal phase matching must be achieved. We find that the phase-matching condition allows us to unambiguously deduce the fibre diameter from the wavelength of the harmonic light. High-resolution scanning electron microscope imaging is used to verify the results.
• C. Weber, S. John, N. Spethmann, D. Meschede and A. Widera
Single Cs Atoms as Collisional Probes in a large Rb Magneto-Optical Trap, Phys. Rev. A 82, 042722 (2010)arXivBibTeX
ABSTRACT »
We study cold interspecies collisions of cesium and rubidium in a strongly imbalanced system with single and few Cs atoms. Observation of the single-atom fluorescence dynamics yields insight into light-induced loss mechanisms, while both subsystems can remain in steady state. This significantly simplifies the analysis of the dynamics, as Cs-Cs collisions are effectively absent and the majority component remains unaffected, allowing us to extract a precise value of the Rb-Cs collision parameter. Extending our results to ground-state collisions would allow to use single neutral atoms as coherent probes for larger quantum systems.

### 2009

• M. Karski, L. Förster, J. Choi, W. Alt, A. Widera and D. Meschede
Nearest-Neighbor Detection of Atoms in a 1D Optical Lattice by Fluorescence Imaging, Phys. Rev. Lett. 102, 053001 (2009)arXivBibTeX
ABSTRACT »
We overcome the diffraction limit in fluorescence imaging of neutral atoms in a sparsely filled one-dimensional optical lattice. At a periodicity of 433 nm, we reliably infer the separation of two atoms down to nearest neighbors. We observe light induced losses of atoms occupying the same lattice site, while for atoms in adjacent lattice sites, no losses due to light induced interactions occur. Our method points towards characterization of correlated quantum states in optical lattice systems with filling factors of up to one atom per lattice site.
• J. Kim, D. Haubrich and D. Meschede
Efficient sub-Doppler laser cooling of an Indium atomic beam, Opt. Express 17, 21216-21221 (2009)BibTeX
ABSTRACT »
We have realized efficient transverse cooling of an Indium atomic beam by combining optical pumping with a closed cycle UV laser cooling transition at 325.6 nm. The transverse velocity of the atomic beam is reduced to 13.5 ±3.8 cm/s, well below the Doppler cooling limit. The fraction of laser-cooled In atoms is enhanced to 12±3 % by optical pumping in the present experiment. It can be scaled up to approach 100% efficiency in cooling, providing high brightness atomic beams for further applications. Our results establish In on the map of elements suitable for applications involving laser cooling.
• L. Förster, M. Karski, J. Choi, A. Steffen, W. Alt, D. Meschede, A. Widera, E. Montano, J. H. Lee, W. Rakreungdet and P. S. Jessen
Microwave Control of Atomic Motion in Optical Lattices, Phys. Rev. Lett. 103, 233001 (2009)arXivBibTeX
ABSTRACT »
We control the quantum mechanical motion of neutral atoms in an optical lattice by driving microwave transitions between spin states whose trapping potentials are spatially offset. Control of this offset with nanometer precision allows for adjustment of the coupling strength between different motional states, analogous to an adjustable effective Lamb-Dicke factor. This is used both for efficient one-dimensional sideband cooling of individual atoms to a vibrational ground state population of 97% and to drive coherent Rabi oscillation between arbitrary pairs of vibrational states. We further show that microwaves can drive well resolved transitions between motional states in maximally offset, shallow lattices, and thus in principle allow for coherent control of long-range quantum transport.
• M. Karski, L. Förster, J. Choi, A. Steffen, W. Alt, D. Meschede and A. Widera
Quantum Walk in Position Space with Single Optically Trapped Atoms, Science 325, 174 (2009)arXivBibTeX
ABSTRACT »
The quantum walk is the quantum analog of the well-known random walk, which forms the basis for models and applications in many realms of science. Its properties are markedly different from the classical counterpart and might lead to extensive applications in quantum information science. In our experiment, we implemented a quantum walk on the line with single neutral atoms by deterministically delocalizing them over the sites of a one-dimensional spin-dependent optical lattice. With the use of site-resolved fluorescence imaging, the final wave function is characterized by local quantum state tomography, and its spatial coherence is demonstrated. Our system allows the observation of the quantum-to-classical transition and paves the way for applications, such as quantum cellular automata.
• J. Kim, D. Haubrich, B. Klöter and D. Meschede
Strong effective saturation by optical pumping in three-level systems, Phys. Rev. A 80, 063801 (2009)BibTeX
ABSTRACT »
We have studied nonlinear absorption from the In P1/2,3/2 ground-state doublet in a resistively heated high-temperature cell and a hollow cathode lamp. Using probe and pump lasers at 410 and 451 nm, respectively, absorption spectra with nonlinear properties caused by saturated absorption, coherent dark resonances, and optical pumping are observed. A theoretical description in terms of a density-matrix theory agrees very well with the observed spectra and identifies optical pumping as a dominating process of broadening in the stepwise contribution rather than velocity-changing collisions. Our experiments suggest that the theory used here is widely applicable in saturation spectroscopy on three-level Λ systems.
• M. Khudaverdyan, W. Alt, T. Kampschulte, S. Reick, A. Thobe, A. Widera and D. Meschede
Quantum jumps and spin dynamics of interacting atoms in a strongly coupled atom-cavity system , Phys. Rev. Lett. 103, 123006 (2009)arXivBibTeX
ABSTRACT »
We experimentally investigate the spin dynamics of one and two neutral atoms strongly coupled to a high finesse optical cavity. We observe quantum jumps between hyperfine ground states of a single atom. The interaction-induced normal-mode splitting of the atom-cavity system is measured via the atomic excitation. Moreover, we observe the mutual influence of two atoms simultaneously coupled to the cavity mode.

### 2008

• M. Khudaverdyan, W. Alt, I. Dotsenko, T. Kampschulte, K. Lenhard, A. Rauschenbeutel, S. Reick, K. Schörner, A. Widera and D. Meschede
Controlled insertion and retrieval of atoms coupled to a high-finesse optical resonator, New J. Phys. 10, 073023 (2008)arXivBibTeX
ABSTRACT »
We experimentally investigate the interaction between one and two atoms and the field of a high-finesse optical resonator. Laser-cooled caesium atoms are transported into the cavity using an optical dipole trap. We monitor the interaction dynamics of a single atom strongly coupled to the resonator mode for several hundred milliseconds by observing the cavity transmission. Moreover, we investigate the position-dependent coupling of one and two atoms by shuttling them through the cavity mode. We demonstrate an alternative method, which suppresses heating effects, to analyze the atom-field interaction by retrieving the atom from the cavity and by measuring its final state.
• J. Kim and D. Meschede
Continuous-wave coherent ultraviolet source at 326 nm based on frequency trippling of fiber amplifiers, Opt. Express 16, 16803-16808 (2008)BibTeX
ABSTRACT »
We have demonstrated a tunable single frequency source of continuous-wave (CW) coherent ultraviolet (UV) radiation at λ_3ω = 326 nm. Laser light of a tunable diode laser at λ_ω = 977 nm was split and injected into two independent fiber amplifiers yielding 1 W and 0.4 W, respectively. The 1 W branch was resonantly frequency doubled, resulting in 120 mW of useful power at λ_2ω = 488 nm. The third harmonic was generated by summation of the second branch of λ_ω and λ_2ω which were enhanced by a doubly resonant cavity. This light source has TEM_00 character and can be continuously tuned over more than 18 GHz. It is of interest for efficient laser cooling of In and potentially other applications.
• B. Klöter, C. Weber, D. Haubrich, D. Meschede and H. Metcalf
Laser cooling of an indium atomic beam enabled by magnetic fields, Phys. Rev. A 77, 033402 (2008)BibTeX
ABSTRACT »
We demonstrate magnetic field enabled optical forces on a neutral indium atomic beam in a light field consisting of five frequencies. The role of dark magnetic ground state sublevels is studied and enables us to cool the atomic beam transversely to near the Doppler limit with laser frequencies tuned above the atomic resonance. The effect of laser cooling can be explained with transient effects in the light potential created by the standing wave light field where the atoms are optically pumped into the dark states and recycled by Larmor precession.

### 2007

• G. Sagué, E. Vetsch, W. Alt, D. Meschede and A. Rauschenbeutel
Cold-Atom Physics Using Ultrathin Optical Fibers: Light-Induced Dipole Forces and Surface Interactions, Phys. Rev. Lett. 99, 163602 (2007)arXivBibTeX
ABSTRACT »
The strong evanescent field around ultrathin unclad optical fibers bears a high potential for detecting, trapping, and manipulating cold atoms. Introducing such a fiber into a cold-atom cloud, we investigate the interaction of a small number of cold cesium atoms with the guided fiber mode and with the fiber surface. Using high resolution spectroscopy, we observe and analyze light-induced dipole forces, van der Waals interaction, and a significant enhancement of the spontaneous emission rate of the atoms. The latter can be assigned to the modification of the vacuum modes by the fiber.
• M. Haas, V. Leung, D. Frese, D. Haubrich, S. John, C. Weber, A. Rauschenbeutel and D. Meschede
Species-selective microwave cooling of a mixture of rubidium and caesium atoms, New J. Phys. 9, 147 (2007)BibTeX
ABSTRACT »
We have sympathetically cooled a small sample of 133Cs atoms with 87Rb to below 1 μK. Evaporative cooling was realized with microwave radiation driving the Rb ground-state hyperfine transition. By analysing the sympathetic cooling dynamics, we derive a lower limit of the modulus of the Rb–Cs interspecies triplet s-wave scattering length of 200 a_0. For temperatures below 5 μK we observe strong non-exponential losses of the Cs sample in the presence of the Rb sample.
• F. Warken, E. Vetsch, D. Meschede, M. Sokolowski and A. Rauschenbeutel
Ultra-sensitive surface absorption spectroscopy using sub-wavelength diameter optical fibers, Opt. Express 15, 11952-11958 (2007)arXivBibTeX
ABSTRACT »
The guided modes of sub-wavelength diameter air-clad optical fibers exhibit a pronounced evanescent field. The absorption of particles on the fiber surface is therefore readily detected via the fiber transmission. We show that the resulting absorption for a given surface coverage can be orders of magnitude higher than for conventional surface spectroscopy. As a demonstration, we present measurements on sub-monolayers of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) molecules at ambient conditions, revealing the agglomeration dynamics on a second to minutes timescale.
• D. M. Segal, P. L. Knight and D. Meschede
Quantum information processing using selectively addressed atoms, J. Mod. Opt. 54, 1537-1540 (2007)BibTeX
ABSTRACT »
In summary, a much clearer picture is now emerging regarding the various atomic and optical approaches. In this article, the worldwide state of the art in this important area of quantum information processing is discussed. The QGATES project encompassed both theoretical and experimental work in the general areas of trapped neutral atoms, cavity QED and trapped ions.

### 2006

• Y. Miroshnychenko, W. Alt, I. Dotsenko, L. Förster, M. Khudaverdyan, D. Meschede, D. Schrader and A. Rauschenbeutel
An atom-sorting machine, Nature 442, 151 (2006)BibTeX
ABSTRACT »
Laser cooling and trapping techniques allow us to control and manipulate neutral atoms. Here we rearrange, with submicrometre precision, the positions and ordering of laser-trapped atoms within strings by manipulating individual atoms with optical tweezers. Strings of equidistant atoms created in this way could serve as a scalable memory for quantum information.
• D. Meschede and A. Rauschenbeutel
Manipulating single atoms, Adv. At. Mol. Opt. Phys. 53, 75 (2006)BibTeX
ABSTRACT »
Neutral atoms are interesting candidates for experimentally investigating the transition from well-understood quantum objects to many particle and macroscopic physics. Furthermore, the ability to control neutral atoms at the single atom level opens new routes to applications such as quantum information processing and metrology. We summarize experimental methods and findings in the preparation, detection, and manipulation of trapped individual neutral atoms. The high efficiency and the observed long coherence times of the presented methods are favorable for future applications in quantum information processing.
• Y. Miroshnychenko, W. Alt, I. Dotsenko, L. Förster, M. Khudaverdyan, A. Rauschenbeutel and D. Meschede
Precision preparation of strings of trapped neutral atoms, New J. Phys. 8, 191 (2006)arXivBibTeX
ABSTRACT »
We have recently demonstrated the creation of regular strings of neutral caesium atoms in a standing wave optical dipole trap using optical tweezers [Y. Miroshnychenko et al., Nature, in press (2006)]. The rearrangement is realized atom-by-atom, extracting an atom and re-inserting it at the desired position with sub-micrometer resolution. We describe our experimental setup and present detailed measurements as well as simple analytical models for the resolution of the extraction process, for the precision of the insertion, and for heating processes. We compare two different methods of insertion, one of which permits the placement of two atoms into one optical micropotential. The theoretical models largely explain our experimental results and allow us to identify the main limiting factors for the precision and efficiency of the manipulations. Strategies for future improvements are discussed.
• L. Förster, W. Alt, I. Dotsenko, M. Khudaverdyan, D. Meschede, Y. Miroshnychenko, S. Reick and A. Rauschenbeutel
BibTeX
ABSTRACT »
We implement a technique for loading a preset number of up to 19 atoms from a magneto-optical trap into a standing wave optical dipole trap. The efficiency of our technique is characterized by measuring the atom number before and after the loading process. Our analysis reveals details of the trap dynamics that are usually masked when working with larger atomic ensembles. In particular, we identify a low-loss collisional blockade mechanism. It forces the atoms to redistribute in the periodic potential until they are all stored in individual trapping sites, thereby strongly reducing site occupation number fluctuations.
• A. Camposeo, O. Maragò, B. Fazio, B. Klöter, D. Meschede, U. Rasbach, C. Weber and E. Arimondo
Resist-assisted atom lithography with group III elements, Appl. Phys. B 85, 487-491 (2006)BibTeX
ABSTRACT »
Resist-assisted atom lithography with group III elements, specifically with gallium and indium, is demonstrated. Self-assembled monolayers (SAM) of nonanethiols prepared on thin sputtered gold films were exposed to a beam of neutral gallium and indium atoms through a physical mask. The interaction of the Ga and In atoms with the nonanethiol layer, followed by a wet etching process, creates well defined structures on the gold film, with features below 100 nm. The threshold of the lithographic process was estimated by optical methods and found to be around 3 gallium atoms and 12 indium atoms per thiol molecule. Our experiments suggest that resist-assisted atom lithography can be realized with group III elements and possibly extended to new neutral atomic species.
• Y. Miroshnychenko, W. Alt, I. Dotsenko, L. Förster, M. Khudaverdyan, D. Meschede, S. Reick and A. Rauschenbeutel
Inserting two atoms into a single optical micropotential, Phys. Rev. Lett. 97, 243003 (2006)arXivBibTeX
ABSTRACT »
We recently demonstrated that strings of trapped atoms inside a standing wave optical dipole trap can be rearranged using optical tweezers [Y. Miroshnychenko et al., Nature, in press (2006)]. This technique allows us to actively set the interatomic separations on the scale of the individual trapping potential wells. Here, we use such a distance-control operation to insert two atoms into the same potential well. The detected success rate of this manipulation is 16(+4/-3) %, in agreement with the predictions of a theoretical model based on our independently determined experimental parameters.

### 2005

• I. Dotsenko, W. Alt, M. Khudaverdyan, S. Kuhr, D. Meschede, Y. Miroshnychenko, D. Schrader and A. Rauschenbeutel
Submicrometer position control of single trapped neutral atoms, Phys. Rev. Lett. 95, 033002 (2005)arXivBibTeX
ABSTRACT »

We optically detect the positions of single neutral cesium atoms stored in a standing wave dipole trap with a sub-wavelength resolution of 143 nm rms. The distance between two simultaneously trapped atoms is measured with an even higher precision of 36 nm rms. We resolve the discreteness of the interatomic distances due to the 532 nm spatial period of the standing wave potential and infer the exact number of trapping potential wells separating the atoms. Finally, combining an initial position detection with a controlled transport, we place single atoms at a predetermined position along the trap axis to within 300 nm rms.

• M. Khudaverdyan, W. Alt, I. Dotsenko, L. Förster, S. Kuhr, D. Meschede, Y. Miroshnychenko, D. Schrader and A. Rauschenbeutel
Adiabatic Quantum State Manipulation of Single Trapped Atoms, Phys. Rev. A 71, 031404 (2005)arXivBibTeX
ABSTRACT »
We use microwave induced adiabatic passages for selective spin flips within a string of optically trapped individual neutral Cs atoms. We position-dependently shift the atomic transition frequency with a magnetic field gradient. To flip the spin of a selected atom, we optically measure its position and sweep the microwave frequency across its respective resonance frequency. We analyze the addressing resolution and the experimental robustness of this scheme. Furthermore, we show that adiabatic spin flips can also be induced with a fixed microwave frequency by deterministically transporting the atoms across the position of resonance.
• M. Mützel, M. Müller, D. Haubrich, U. Rasbach, D. Meschede, C. O’Dwyer, G. Gay, B. V. d. Lesegno, J. Weiner, K. Ludolph, G. Georgiev and E. Oesterschulze
The atom pencil: serial writing in the sub-micrometre domain, Appl. Phys. B 80, 941 (2005)BibTeX
ABSTRACT »
The atom pencil we describe here is a versatile tool that writes arbitrary structures by atomic deposition in a serial lithographic process. This device consists of a transversely laser-cooled and collimated cesium atomic beam that passes through a 4-pole atom-flux concentrator and impinges on to micron- and sub-micron-sized apertures. The aperture translates above a fixed substrate and enables the writing of sharp features with sizes down to 280 nm. We have investigated the writing and clogging properties of an atom pencil tip fabricated from silicon oxide pyramids perforated at the tip apex with a sub-micron aperture.
• Y. Louyer, D. Meschede and A. Rauschenbeutel
Tunable Whispering Gallery Mode Resonators for Cavity Quantum Electrodynamics, Phys. Rev. A 72, 031801(R) (2005)arXivBibTeX
ABSTRACT »
We theoretically study the properties of highly prolate shaped dielectric microresonators. Such resonators sustain whispering gallery modes that exhibit two spatially well separated regions with enhanced field strength. The field per photon on the resonator surface is significantly higher than e.g. for equatorial whispering gallery modes in microsphere resonators with a comparable mode volume. At the same time, the frequency spacing of these modes is much more favorable, so that a tuning range of several free spectral ranges should be attainable. We discuss the possible application of such resonators for cavity quantum electrodynamics experiments with neutral atoms and reveal distinct advantages with respect to existing concepts.
• S. Kuhr, W. Alt, D. Schrader, I. Dotsenko, Y. Miroshnychenko, A. Rauschenbeutel and D. Meschede
Analysis of dephasing mechanisms in a standing-wave dipole trap, Phys. Rev. A 72, 023406 (2005)arXivBibTeX
ABSTRACT »
We study in detail the mechanisms causing dephasing of hyperfine coherences of cesium atoms confined by a far off-resonant standing wave optical dipole trap [S. Kuhr et al., Phys. Rev. Lett. 91, 213002 (2003)]. Using Ramsey spectroscopy and spin echo techniques, we measure the reversible and irreversible dephasing times of the ground state coherences. We present an analytical model to interpret the experimental data and identify the homogeneous and inhomogeneous dephasing mechanisms. Our scheme to prepare and detect the atomic hyperfine state is applied at the level of a single atom as well as for ensembles of up to 50 atoms.
• C. O'Dwyer, G. Gay, B. d. Lesegno, J. Weiner, M. Mützel, D. Haubrich, D. Meschede, K. Ludolph, G. Georgiev and E. Oesterschulze
Advancing atomic nanolithography: cold atomic Cs beam exposure of alkanethiol self-assembled monolayers, J. Phys.: Conference Series 119, 109 (2005)BibTeX
ABSTRACT »
We report the results of a study into the quality of functionalized surfaces for nanolithographic imaging. Self-assembled monolayer (SAM) coverage, subsequent post-etch pattern definition and minimum feature size all depend on the quality of the Au substrate used in atomic nanolithographic experiments. We find sputtered Au substrates yield much smoother surfaces and a higher density of {111} oriented grains than evaporated Au surfaces. A detailed study of the self-assembly mechanism using molecular resolution AFM and STM has shown that the monolayer is composed of domains with sizes typically of 5-25 nm, and multiple molecular domains can exist within one Au grain. Exposure of the SAM to an optically-cooled atomic Cs beam traversing a two-dimensional array of submicron material masks ans also standing wave optical masks allowed determination of the minimum average Cs dose (2 Cs atoms per SAM molecule) and the realization of
• D. Yanyshev, B. Grishanin, V. Zadkov and D. Meschede
Dynamics of atoms interacting via the radiation field in an optical dipole trap, Laser Physics 15, 1189-1203 (2005)BibTeX
ABSTRACT »
Theoretical study and computer simulation results for the stochastic dynamics of atoms localized in an optical dipole trap are presented. This dynamics is governed by the optical trap potential, cooling due to the Doppler effect, and heating due to the emission and absorption of virtual photons, i.e., due to the resonant dipole-dipole interactions (RDDI). It is shown that the RDDI becomes essential for closely spaced atoms, but the effect can be significantly improved by irradiating the atoms in the trap with an additional resonance probe laser beam. By varying both the optical dipole trap parameters and intensity of the probe laser field, the role of RDDI in the atomic dynamics in the trap is clarified in detail.

### 2004

• D. Schrader, I. Dotsenko, M. Khudaverdyan, Y. Miroshnychenko, A. Rauschenbeutel and D. Meschede
Neutral Atom Quantum Register, Phys. Rev. Lett. 93, 150501 (2004)arXivBibTeX
ABSTRACT »
We demonstrate the realization of a quantum register using a string of single neutral atoms which are trapped in an optical dipole trap. The atoms are selectively and coherently manipulated in a magnetic field gradient using microwave radiation. Our addressing scheme operates with a high spatial resolution and qubit rotations on individual atoms are performed with 99% contrast. In a final read-out operation we analyze each individual atomic state. Finally, we have measured the coherence time and identified the predominant dephasing mechanism for our register.
• U. Rasbach, J. Wang, R. dela Torre, V. Leung, B. Klöter, D. Meschede, T. Varzhapetyan and D. Sarkisyan
One- and two-color laser spectroscopy of indium vapor in an all-sapphire cell, Phys. Rev. A 70, 033810 (2004)BibTeX
ABSTRACT »
We present saturation and polarization laser spectroscopy experiments of indium vapor with a single color on the 410 nm transition and with two colors at 410 and 451 nm. The spectra observed by polarization spectroscopy are discussed in terms of a quantitative model. The line shapes observed with two-color spectroscopy can phenomenologically be described taking into account hyperfine changing collisions, velocity changing collisions, and dark resonances. As an application, we actively stabilized a 410 nm diode laser on the resonances of saturation and polarization spectroscopy, and obtained long term frequency stabilities in the 100 kHz–1 MHz range.
• I. Dotsenko, W. Alt, S. Kuhr, D. Schrader, M. Müller, Y. Miroshnychenko, V. Gomer, A. Rauschenbeutel and D. Meschede
Application of electro-optically generated light fields for Raman spectroscopy of trapped Cesium atoms, Appl. Phys. B 78, 711-717 (2004)BibTeX
ABSTRACT »
We present an apparatus for generating a multi-frequency laser field to coherently couple the F=3 and F=4 ground state of trapped cesium atoms through Raman transitions. We use a single frequency diode laser and generate sidebands by means of a 9.2 GHz electro-optic modulator. With an interferometer, we separated the sidebands and carrier, sending them to the trapped atoms in opposite directions. The Rabi oscillation of the populations of F=3 and F=4 is monitored. We find that due to destructive quantum interference of two simultaneous Raman transitions the expected Rabi frequency is reduced by a factor that is in quantitative agreement with theoretical expectations. It is demonstrated how this interference can be suppressed experimentally. Besides, we demonstrate the application of the setup for Raman spectroscopy of Zeeman sublevels and of the vibrational states of a small number of trapped atoms.

### 2003

• D. Meschede
Atom lithography, Yearbook of Science and Technology, McGraw-Hill, (2003)BibTeX
ABSTRACT »
Atom lithography designates a physical method where the forces exerted by interfering laser beams on the atoms of an atomic beam are used to steer the atoms into nanostructures fabricated on a plane surface. While atom lithography is the most frequently used term, the method is also known as light-force lithography and atomic nanofabrication (ANF).
• Y. Miroshnychenko, D. Schrader, S. Kuhr, W. Alt, I. Dotsenko, M. Khudaverdyan, A. Rauschenbeutel and D. Meschede
Continued imaging of the transport of a single neutral atom, Opt. Express 11, 3498-3502 (2003)BibTeX
ABSTRACT »
We have continuously imaged the controlled motion of a single atom as well as of a small number of distinguishable atoms with observation times exceeding one minute. The Cesium atoms are confined to potential wells of a standing wave optical dipole trap which allows to transport them over macroscopic distances. The atoms are imaged by an intensified CCD camera, and spatial resolution near the diffraction limit is obtained.
• M. Mützel, U. Rasbach, D. Meschede, C. Burstedde, J. Braun, A. Kunoth, K. Peithmann and K. Buse
Atomic nanofabrication with complex light fields, Appl. Phys. B 77, 1-9 (2003)BibTeX
ABSTRACT »
The method of neutral atom lithography allows one to transfer to a substrate a 2D intensity modulation of an atomic beam imposed by an inhomogeneous light field. The complexity of the pattern depends on the properties of the light field constructed from the superposition of multiple laser beams. For the design of suitable light fields we present a mathematical model with a corresponding numerical simulation of the so-called inverse problem. Furthermore, details of an experiment carried out with a holographically reconstructed light field are discussed.
• D. Schrader, S. Kuhr, W. Alt, Y. Miroshnychenko, I. Dotsenko, W. Rosenfeld, M. Khudaverdyan, V. Gomer, A. Rauschenbeutel and D. Meschede
Controlled transport of single neutral atom qubits, Proceedings of the 16th ICOLS, (2003)BibTeX
ABSTRACT »
We have prepared and detected quantum coherences of trapped cesium atoms with long dephasing times. Controlled transport by an “optical conveyor belt” over macroscopic distances preserves the atomic coherence with slight reduction of coherence time. The dominating dephasing effects are experimentally identified and found to be of technical rather than fundamental nature.
• S. Kuhr, W. Alt, D. Schrader, I. Dotsenko, Y. Miroshnychenko, W. Rosenfeld, M. Khudaverdyan, V. Gomer, A. Rauschenbeutel and D. Meschede
Coherence properties and quantum state transportation in an optical conveyor belt, Phys. Rev. Lett. 91, 213002 (2003)arXivBibTeX
ABSTRACT »
We have prepared and detected quantum coherences with long dephasing times at the level of single trapped cesium atoms. Controlled transport by an "optical conveyor belt" over macroscopic distances preserves the atomic coherence with slight reduction of coherence time. The limiting dephasing effects are experimentally identified and are of technical rather than fundamental nature. We present an analytical model of the reversible and irreversible dephasing mechanisms. Coherent quantum bit operations along with quantum state transport open the route towards a "quantum shift register" of individual neutral atoms.
• W. Alt, D. Schrader, S. Kuhr, M. Müller, V. Gomer and D. Meschede
Single atoms in a standing-wave dipole trap, Phys. Rev. A 67, 033403 (2003)arXivBibTeX
ABSTRACT »
We trap a single cesium atom in a standing-wave optical dipole trap. Special experimental procedures, designed to work with single atoms, are used to measure the oscillation frequency and the atomic energy distribution in the dipole trap. These methods rely on unambiguously detecting presence or loss of the atom using its resonance fluorescence in the magneto-optical trap.
• D. Meschede and H. Metcalf
Atomic nanofabrication: atomic deposition and lithography by laser and magnetic forces, J. Phys. D: Appl. Phys. 36, R17-R38 (2003)BibTeX
ABSTRACT »
Atomic deposition on a surface can be controlled at the nanometre scale by means of optical and magnetic forces. Impingement of atoms on the surface can lead to growth of a structured array (direct deposition) or to chemical modifications of the surface (neutral atom lithography). In this report we survey requirements, present the current results, and explore the potential applications of this method of nanofabrication.

### 2002

• B. Ueberholz, S. Kuhr, D. Frese, V. Gomer and D. Meschede
Cold collisions in a high-gradient magneto-optical trap, J. Phys. B: At. Mol. Opt. Phys. 35, 4899 (2002)BibTeX
ABSTRACT »
We present a detailed analysis of the cold collision measurements performed in a high-gradient magneto-optical trapwith a few trapped Cs atomsfirst presented in Ueberholz et al (J. Phys. B: At.Mol. Opt. Phys. 33 (2000) L135). The ability to observe individual loss events allows us to identify two-body collisions that lead to the escape of only one of the colliding atoms (up to 10% of all collisional losses). Possible origins of these events are discussed here. We also observed strong modifications of the total loss rate with variations in the repumping laser intensity. This is explained by a simple semiclassical model based on optical suppression of hyperfine-changing collisions between ground-state atoms.
• M. Mützel, S. Tandler, D. Haubrich, D. Meschede, K. Peithmann, M. Flaspöhler and K. Buse
Atom Lithography with a Holographic Light Mask, Phys. Rev. Lett. 88, 083601 (2002)BibTeX
ABSTRACT »
In atom lithography with optical masks, deposition of an atomic beam on a given substrate is controlled by a standing light-wave field. The lateral intensity distribution of the light field is transferred to the substrate with nanometer scale. We have tailored a complex pattern of this intensity distribution through diffraction of a laser beam from a hologram that is stored in a photorefractive crystal. This method can be extended to superpose 1000 or more laser beams. The method is furthermore applicable during growth processes and thus allows full 3D structuring of suitable materials with periodic and nonperiodic patterns at nanometer scales.

### 2001

• S. Kuhr, W. Alt, D. Schrader, M. Müller, V. Gomer and D. Meschede
Deterministic Delivery of a Single Atom, Science 293, 278 (2001)BibTeX
ABSTRACT »
We report the realization of a deterministic source of single atoms. A standing-wave dipole trap is loaded with one or any desired number of cold cesium atoms from a magneto-optical trap. By controlling the motion of the standing wave, we adiabatically transport the atom with submicrometer precision over macroscopic distances on the order of a centimeter. The displaced atom is observed directly in the dipole trap by fluorescence detection. The trapping field can also be accelerated to eject a single atom into free flight with well-defined velocities.
• D. Schrader, S. Kuhr, W. Alt, M. Müller, V. Gomer and D. Meschede
An optical conveyor belt for single neutral atoms, Appl. Phys. B 73, 819 (2001)arXivBibTeX
ABSTRACT »
Using optical dipole forces we have realized controlled transport of a single or any desired small number of neutral atoms over a distance of a centimeter with sub-micrometer precision. A standing wave dipole trap is loaded with a prescribed number of cesium atoms from a magneto-optical trap. Mutual detuning of the counter-propagating laser beams moves the interference pattern, allowing us to accelerate and stop the atoms at preselected points along the standing wave. The transportation efficiency is close to 100%. This optical "single-atom conveyor belt" represents a versatile tool for future experiments requiring deterministic delivery of a prescribed number of atoms on demand.
• R. Bertram, H. Merimeche, M. Mützel, H. Metcalf, D. Haubrich and D. Meschede
Magnetic whispering-gallery mirror for atoms, Phys. Rev. A 63, 053405 (2001)BibTeX
ABSTRACT »
Videotape with a sinusoidal magnetization of 31 μm wavelength is used to reflect Cs atoms with unit reflectivity in a 75 m/s atomic beam. The atoms serve as a probe, allowing us to measure the magnetic field at the surface. A technique is presented for mounting the videotape so that its surface can be curved to a specific shape or made flexible. We show that such a reflector provides high-quality grazing-incidence atom optics and we demonstrate deflections as large as 23° in a whispering-gallery geometry.
• V. Gomer and D. Meschede
A single trapped atom: Light-matter interaction at the microscopic level, Ann. Phys. (Leipzig) 10, 9-18 (2001)BibTeX
ABSTRACT »
For a single trapped atom the fluctuations of resonance fluorescence reveal its dynamic evolution at all relevant time scales. We review experimental results, extend interpretations and express expectations for future systems with fully controlled quantum properties.

### 2000

• M. Mützel, D. Haubrich and D. Meschede
Nanoscale focusing of atoms with a pulsed standing wave, Appl. Phys. B 70, 689 (2000)BibTeX
ABSTRACT »
We have theoretically and experimentally investigated the focusing properties of a detuned pulsed standing wave onto a beam of neutral atoms. In close analogy to the continuous-wave situation the dipole force leads to a periodic focusing of atoms with a period of λ/2, provided an adiabatic condition is fulfilled. Pulsed laser light is conveniently converted to short wavelengths and hence offers advantages in the application of atom lithography with elements of technological interest having blue or UV resonance lines.
• F. Lison, P. Schuh, D. Haubrich and D. Meschede
High-brilliance Zeeman-slowed cesium atomic beam, Phys. Rev. A 61, 013405 (2000)BibTeX
ABSTRACT »
We have built a Zeeman-slower apparatus which produces a slow and cold cesium atomic beam. The atomic beam has a mean velocity in the range 35–120 m/s and a high atomic current of more than 2×10^10 cold atoms/s. A small longitudinal velocity spread was achieved by optimizing the termination of the slowing process. The measured value of less than 1 m/s is consistent with a numerical simulation of the slowing process. With a magnetic lens and a tilted two-dimensional optical molasses stage, the slow atomic beam is transversely compressed, collimated, and deflected. We achieve a transverse temperature below the Doppler limit. The brilliance of this beam has been determined to be 7×10^23 atoms s^-1 m^-2 sr^-1. By optical pumping the slow atomic beam can be polarized in the outermost magnetic substates F=4,mF=±4, of the cesium ground state. This brilliant beam is an ideal source for experiments in atom optics and atom lithography.
• H. Leinen, D. Gläßner, H. Metcalf, R. Wynands, D. Haubrich and D. Meschede
GaN blue diode lasers: a spectroscopist's view, Appl. Phys. B 70, 567 (2000)BibTeX
ABSTRACT »

We have characterized the spectroscopic properties of one of the first samples of blue-emitting diode lasers based on GaN. With such a laser diode operated inside a standard extended cavity arrangement we find a mode-hop free tuning range of more than 20 GHz and a linewidth of 10 MHz. Doppler-free spectroscopy on an indium atomic beam reveals the isotope shift between the two major indium isotopes as well as efficient optical pumping.

• D. Frese, B. Ueberholz, S. Kuhr, W. Alt, D. Schrader, V. Gomer and D. Meschede
Single Atoms in an Optical Dipole Trap: Towards a Deterministic Source of Cold Atoms, Phys. Rev. Lett. 85, 3777 (2000)arXivBibTeX
ABSTRACT »
We describe a simple experimental technique which allows us to store a small and deterministic number of neutral atoms in an optical dipole trap. The desired atom number is prepared in a magneto-optical trap overlapped with a single focused Nd:YAG laser beam. Dipole trap loading efficiency of 100% and storage times of about one minute have been achieved. We have also prepared atoms in a certain hyperfine state and demonstrated the feasibility of a state-selective detection via resonance fluorescence at the level of a few neutral atoms. A spin relaxation time of the polarized sample of $4.2\pm 0.7$ s has been measured. Possible applications are briefly discussed.
• H. Schadwinkel, V. Gomer, U. Reiter, B. Ueberholz and D. Meschede
Quantum Fluctuations of a Single Trapped Atom: Transient Rabi Oscillations and Magnetic Bistability, IEEE J. of Quantum Electronics 36, 1358 (2000)arXivBibTeX
ABSTRACT »
Isolation of a single atomic particle and monitoring its resonance fluorescence is a powerful tool for studies of quantum effects in radiation-matter interactions. We present observations of quantum dynamics of an isolated neutral atom stored in a magneto-optical trap. By means of photon orrelations in the atom's resonance fluorescence we demonstrate the well-known phenomenon of photon antibunching which corresponds to transient Rabi oscillations in the atom. Through polarization-sensitive photon correlations, we show a novel example of resolved quantum fluctuations: pontaneous magnetic orientation of an atom. These effects can only be observed with a single atom.
• P. Rosenbusch, J. Retter, B. Hall, E. Hinds, F. Lison, D. Haubrich and D. Meschede
Reflection of cold atoms by a cobalt single crystal, Appl. Phys. B 70, 661 (2000)BibTeX
ABSTRACT »
We have demonstrated that a cobalt single crystal can be used to make a remarkably smooth retro-reflector for cold paramagnetic atoms. The crystal is cut so that its surface lies in the (0001) plane and the atoms are reflected by the magnetic field above the surface due to the self-organized pattern of magnetic domains in the material. We find that the reflectivity for suitably polarized atoms exceeds 90% and may well be unity. We use the angular spread of a reflected atom cloud to measure the roughness of the mirror. We find that the angular variation of the equivalent hard reflecting surface is (3.1±0.3°)rms for atoms dropped onto the mirror from a height of 2 cm.
• B. Ueberholz, S. Kuhr, D. Frese, D. Meschede and V. Gomer
Counting Cold Collisions, J. Phys. B: At. Mol. Opt. Phys. 33, L135 (2000)arXivBibTeX
ABSTRACT »
We have experimentally explored a novel possibility to study exoergic cold atomic collisions. Trapping of small countable atom numbers in a shallow magneto-optical trap and monitoring of their temporal dynamics allows us to directly observe isolated two-body atomic collisions and provides detailed information on loss statistics. A substantial fraction of such cold collisional events has been found to result in the loss of one atom only. We have also observed for the first time a strong optical suppression of ground-state hyperfine-changing collisions in the trap by its repump laser field.

### 1999

• F. Lison, D. Haubrich, P. Schuh and D. Meschede
Reflection of a slow cesium atomic beam from a naturally magnetized Nd-Fe-B surface, Appl. Phys. B 69, 501 (1999)BibTeX
ABSTRACT »
We have demonstrated the partly directed reflection of a slow cesium atomic beam by using the natural magnetic stray field above a Nd-Fe-B surface. From these experiments we determine the reflectivity and a minimum value for the magnetic stray field directly at the surface.
• A. Goepfert, F. Lison, R. Schütze, R. Wynands, D. Haubrich and D. Meschede
Efficient magnetic guiding and deflection of atomic beams with moderate velocities, Appl. Phys. B 69, 217 (1999)BibTeX
ABSTRACT »
We have studied guidance and deflection of a beam of cesium atoms by a strong toroidal magnetic quadrupole field. The beam guide is made from permanent magnets sustaining a radial field gradient of 2.8 T/cm. Atoms with moderate longitudinal velocities ranging from 30 m/s to 70 m/s were inserted across the 10-mm-diameter aperture of a 24.5° arc with radius 300 mm. We have measured transmission and beam divergence and find good agreement with ray-tracing calculations and analytical estimates. The magnetic beam guide allows for 100% transmission of heavy atoms over large angles.
• H. Schadwinkel, U. Reiter, V. Gomer and D. Meschede
Magneto-optical trap as an optical lattice, Phys. Rev. A 61, 013409 (1999)BibTeX
ABSTRACT »
We study the magneto-optical trap (MOT) as an optical lattice with a setup providing full phase control for all light fields. Although completely different light fields are possible for various phases, we have found experimental evidence that stored atoms are generally localized in micropotentials of the six-beam lattice. The influence of the phase variation is surprisingly small, suggesting that the robust behavior of the MOT is a consequence of this fact. We find furthermore good agreement of our experimental data with a simple theoretical model which reduces the complicated MOT to a description of steady-state atoms localized at points of the deepest adiabatic light-shift potential.
• A. Nagel, S. Brandt, D. Meschede and R. Wynands
Light shift of coherent population trapping resonances, Europhys. Lett. 48, 385-389 (1999)BibTeX
ABSTRACT »
We have measured the spectral position of the absorption minimum in a coherent population trapping resonance in thermal cesium vapor as a function of light intensity. The dependence of position on intensity is found to be almost linear. We have furthermore studied the dependence of this light shift on neon buffer gas pressure and find a strong reduction for higher pressures. So the addition of a buffer gas not only reduces the linewidth of the resonance but also a very important systematic effect for precision measurements.

### 1998

• V. Gomer, F. Strauch, B. Ueberholz, S. Knappe and D. Meschede
Single-atom dynamics revealed by photon correlations, Phys. Rev. A 58, R1657 (1998)BibTeX
ABSTRACT »
We have studied a single neutral atom stored in a magneto-optical trap by recording arrival times of fluorescence photons emitted by the atom. Photon correlations at nanosecond scales (Rabi oscillations), at microseconds (intensity and polarization correlations), and also at milliseconds (position correlations) reveal the dynamical behavior of the atomic excitation, of the atomic orientation, and of its transport in the trap at both the optical wavelength scale and the trap size.
• D. Meschede, V. Gomer and H. Monien
Atomic Bose-Einstein Condensates: A Model for Macroscopic Quantum Systems, Naturwissenschaften 85, 203 85, 203-218 (1998)BibTeX
ABSTRACT »
A novel type of macroscopic quantum system has recently become available through the experimental realization of Bose condensates from neutral atoms. We review experimental results and the elementary quantum mechanical approach and outline advanced theoretical concepts regarding finite size, potentials, dimensionality, and interactions.
• V. Gomer, B. Ueberholz, S. Knappe, F. Strauch, D. Frese and D. Meschede
Decoding the dynamics of a single trapped atom from photon correlations, Appl. Phys. B 67, 689 (1998)BibTeX
ABSTRACT »
Information on the dynamics of a single neutral atom can be decoded from fluctuations in the resonance fluorescence. We have measured two-time photon correlations of individual cesium atoms stored in a magneto-optical trap. We observe strong correlations at nanosecond scales (Rabi oscillations), at microseconds (intensity and polarization correlations), and also at milliscconds (position correlations) revealing the dynamical behavior of the atomic excitation, of the atomic orientation, and of its transport in the trap at both the optical wavelength scale and the trap size. In this article we compare our experimental results with a simplified model of an atom moving through an optical lattice. We investigate the influence of light-field topogaphy and of the multilevel character of the atom on the shape and the visibility of the correlations.
• B. A. Grishanin, V. N. Zadkov and D. Meschede
Modification of resonance fluorescence and absorption in a lambda system by four-wave mixing, Phys. Rev. A 58, 4235 (1998)BibTeX
ABSTRACT »
A universal mechanism destroying coherence in a Λ system driven by two resonant laser fields due to four-photon interactions is analyzed theoretically. It is shown that this mechanism gives rise to novel spectral structures in resonance fluorescence. The “dark resonance” in absorption (dispersion) spectra is affected as well.
• A. Rauschenbeutel, H. Schadwinkel, V. Gomer and D. Meschede
Standing light fields for cold atoms with intrinsically stable and variable time phases, Opt. Comm. 148, 45 (1998)BibTeX
ABSTRACT »
We present a novel method to realise a standing light field with a stable configuration in two or three dimensions. A single standing wave formed by two counterpropagating beams is folded and brought into intersection with itself. The values of the relative timephases are stable, a priori known, and can be altered arbitrarily by means of retardation plates. The polarisation configurations of three orthogonal standing waves include the standard magnetooptical trap and a novel three-dimensional pure polarisation lattice which we have investigated in a first spectroscopic measurement, providing strong evidence for atomic localisation in both cases.
• R. Wynands, A. Nagel, S. Brandt, D. Meschede and A. Weis
Selection rules and line strengths of Zeeman-split dark resonances, Phys. Rev. A 58, 196 (1998)BibTeX
ABSTRACT »
In a weak magnetic field coherent dark resonances in cesium vapor are split into up to 15 resolved components, depending on field direction and laser polarizations. We find that the selection rules are different for vapor cells with and without buffer gas due to a change in multipolarity of the two-photon coupling. At low laser intensities or sufficiently high buffer-gas pressure optical pumping between different dark resonances can be neglected so that a simple model allows one to calculate the relative line strengths, giving complete agreement with the experimental spectra.
• A. Nagel, L. Graf, A. Naumov, E. Mariotti, V. Biancalana, D. Meschede and R. Wynands
Experimental realization of coherent dark-state magnetometers , Europhys. Lett. 44, 31 (1998)BibTeX
ABSTRACT »
Coherent population trapping resonances in cesium vapor can be used to determine DC flux densities in the range from 1 μT to 1 mT with typically 3·10^−5 relative uncertainty. For fields modulated at a few kHz, we find sensitivities of below 10 pT within 0.5 s integration time. From the signal-to-noise ratio the sensitivity can be extrapolated to 500 fT/√Hz. A quantitative understanding of the lineshape allows to detect DC fields of several nT even when the Zeeman components of the resonance are not resolved.
• B. A. Grishanin, V. N. Zadkov and D. Meschede
Effect of four-photon interactions on coherent population trapping in Lambda-systems, J. Exp. Theor. Phys. 86, 79-92 (1998)BibTeX
ABSTRACT »
The resonance fluorescence spectrum of a Λ-system excited by two resonant light fields is calculated using a Markov analysis. Analytical formulas are derived in the strong-field limit within and beyond the rotating wave approximation. It is shown that the resonance fluorescence of the system does not vanish during coherent population trapping. Its spectrum consists of two multiplets which are similar to a triplet in the resonance fluorescence spectrum of a two-level atom and lie at the electronic transition frequencies, together with two triplets located at the frequencies of four-photon processes involving the optical excitation fields. The latter are fundamental in character and impose limits on the lower bound of the dephasing rate for the Raman resonance owing to the effect of radiative decay of the dipole transitions on the dynamics of the ground state. The effect of four-photon dephasing on the absorption spectrum of a Λ-system is analyzed and found to lead to a substantial reduction in the depth of a dip in the absorption spectrum which vanishes as the laser field strength is increased. Zh. Éksp. Teor. Fiz. 113, 144–167 (January 1998)

### 1997

• A. Goepfert, I. Bloch, D. Haubrich, F. Lison, R. Schütze, R. Wynands and D. Meschede
Stimulated focusing and deflection of an atomic beam using picosecond laser pulses, Phys. Rev. A 56, R3354 (1997)BibTeX
ABSTRACT »

Using the stimulated force exerted by counterpropagating π pulses from a mode-locked Ti:sapphire laser we have focused a beam of laser-cooled cesium atoms along one dimension to about 57% of its original width in the detection zone. We determined the force profile outside and inside the overlap region of the pulses and found agreement with an earlier theoretical prediction. The scheme does not require an effective two-level system and is therefore suitable for a large variety of elements.

• D. Haubrich, D. Meschede, T. Pfau and J. Mlynek
Atomlithographie, Phys. J. 53, 523 (1997)BibTeX
ABSTRACT »

Grofitechnisch angewandte optische Lithographieverfahrenstofien bei Auflosungen unter 100 nm auf eine prinzipielle Grenze. Die Lithographie mit neutralen Atomstrahlen bietet dazu insbesondere beim parallelen Schreiben periodischer Strukturen eine interessante, noch junge Alternative. Indem man die Wechselwirkung der Atome mit Lichtmasken ausnutzt, gelingt es, grofiflachig periodische Linienmuster und verschiedene zweidimensional periodische Strukturen mit einer Auflosung unter 100 nm zu erzeugen. Dabei werden Atome entweder direkt auf einem Substrat deponiert oder zur Modifikation organischer Resists genutzt. Der folgende Beitrag gibt einen herblick iiber den Stand der Lithographie mit Atomstrahlen.

• O. Harms, M. Zehnpfennig, V. Gomer and D. Meschede
Photodetachment spectroscopy of stored H- ions, J. Phys. B:. At. Mol. Phys. 30, 3781 (1997)BibTeX
ABSTRACT »

Threshold photodetachment of negative hydrogen ions stored in a Penning trap has been studied. The electron afﬁnity of hydrogen is determined to 6082.8(7) cm−1 in good agreement with previous experiments. The Wigner law has been found to be valid in a region of 400 cm−1 above the threshold.

• D. Meschede, I. Bloch, A. Goepfert, D. Haubrich, M. Kreis, F. Lison, R. Schuetze and R. Wynands
Atom optics with permanent magnetic components, Proc. SPIE 2995, Atom Optics 2995, 191 (1997)BibTeX
ABSTRACT »

We have fabricated and investigated efficient magnetic lenses, waveguides, and mirrors from rare earth permanent materials. They are affordable and maintenance free. In contrast to corresponding light force components they do not need any supplies, they have large apertures, high reflectivity, and there is no spontaneous emission. The cylindrical shape of magnetic components is furthermore well suited to steer atomic beams.

• F. Strauch, V. Gomer, H. Schadwinkel, B. Ueberholz, D. Haubrich and D. Meschede
Diffraction by cold atoms, Opt. Comm. 145, 57 (1997)BibTeX
ABSTRACT »
We have observed diffraction of a laser probe beam by a trapped sample of cold atoms. The effect is only visible in the vicinity of a resonance line. The observed diffraction pattern arises from interference of the incident and scattered light wave, allowing reconstruction of geometric properties of the trapped sample from the holographic record.
• V. Gomer, O. Harms, D. Haubrich, H. Schadwinkel, F. Strauch, B. Ueberholz, S. aus der Wiesche and D. Meschede
Magnetostatic traps for charged and neutral particles, Hyperfine Interactions 109, 281-292 (1997)BibTeX
ABSTRACT »
We have constructed magnetostatic traps from permanent magnets for trapping charged and neutral atoms. Two storage experiments are presented: a compact Penning trap for light ions and magnetic trapping of single neutral atoms. The dynamics of cold neutral atoms and their loss mechanisms in a quadrupole magnetostatic trap are discussed.
• F. Lison, H. Adams, D. Haubrich, M. Kreis, S.Nowak and D. Meschede
Nanoscale atomic lithography with a cesium atomic beam, Appl. Phys. B 65, 419 (1997)BibTeX
ABSTRACT »
We have demonstrated the lithographic production of a periodic nanostructure by focusing a transversely laser cooled cesium atomic beam with a standing-wave light field. With a self-assembled monolayer used as the resist on a gold surface, exposure to cesium atoms locally changes the wetability. Subsequently a wet-etching process transfers the pattern to the underlying gold film. We have generated lines with a separation of half the wavelength of the cesium D2 line (852 nm) and a width of about 120 nm and covering a large area of approximately 1 mm^2.

### 1996

• D. Haubrich, H. Schadwinkel, F. Strauch, B. Ueberholz, R. Wynands and D. Meschede
Observation of individual neutral atoms in magnetic and magneto-optical traps, Europhys. Lett. 34, 663 (1996)BibTeX
ABSTRACT »
We have identified and photographed individual cesium atoms in a magneto-optical trap with steep magnetic gradients. By switching off the trapping light fields, single atoms were released to a bound state of the magnetic potential. A storage time of 38 s was measured for purely magnetic trapping, whereas a storage time of 147 s was observed in the corresponding magneto-optical trap.
• W. G. Kaenders, F. Lison, I. Müller, A. Richter, R. Wynands and D. Meschede
Refractive components for magnetic atom optics, Phys. Rev. A 54, 5067 (1996)BibTeX
ABSTRACT »

With strong rare-earth permanent magnets we have built highly refractive atom-optical components for laser-cooled atoms. We have studied the influence of axially symmetric multipole components on a cesium atomic beam. In analogy to traditional optics the action of a quadrupole ring parallels a conical prism, or axicon. Hexapole lenses were applied for focusing with a more than 1000-fold increase in atomic flux density at the focal spot and for imaging with the atomic beam. Two hexapole lenses were combined to form a telescope, which was operated off axis in order to separate fast thermal and slowed atoms. The experiments can approximately be described in terms of geometrical optics.

• M. Kreis, F. Lison, D. Haubrich, D. Meschede, S. Nowak, T. Pfau and J. Mlynek
Pattern generation with cesium atomic beams at nanometer scales, Appl. Phys. B 63, 649 (1996)BibTeX
ABSTRACT »
We have demonstrated that a cesium atomic beam can be used to pattern a gold surface using a self assembling monolayer (SAM) as a resist. A 12.5 µm period mesh was used as a proximity mask for the atomic beam. The cesium atoms locally change the wetability of the SAM, which allows a wet etching reagent to remove the underlying gold in the exposed regions. An edge resolution of better than 100 nm was obtained. The experiment suggests that this method can either be used as a sensitive position detector with nanometer resolution in atom optics, or for nanostructuring in a resist technique.

### 1994

• A. Höpe, D. Haubrich, H. Schadwinkel, F. Strauch and D. Meschede
Optical Trapping in a Cesium Cell with Linearly Polarized Light and at Zero Magnetic Field, Europhys. Lett. 28, 7 (1994)BibTeX
ABSTRACT »

We have found that linearly polarized light can be used efficiently for optical trapping of cesium atoms in a magnetic-quadrupole field. The number and density of atoms of the trapped samples are comparable to a standard magneto-optical trap with σ+ - σ polarized light, but the influence of the magnetic-quadrupole strength is strikingly different. When the polarization of counterpropagating light beams is orthogonal, trapping is observed also for zero magnetic field.

• O. Schmidt, K. Knaak, R. Wynands and D. Meschede
Cesium saturation spectroscopy revisited: How to reverse peaks and observe narrow resonance, Appl. Phys. B 59, 167 (1994)BibTeX
ABSTRACT »

The complex magnetic structure of the cesium atom is responsible for the interesting behaviour of its saturated absorption spectra, e.g., a two-fold sign reversal of a crossover resonance, under various polarization configurations with and without applied magnetic fields. We show that this morphology is a result of optical pumping processes including coherent population trapping which, under normal laboratory conditions, prevent the atoms from reaching an equilibrium situation. Our interpretation is useful for an intuitive and rapid understanding of this important tool in high-resolution spectroscopy.

### 1993

• W. G. Kaenders, V. Frerichs, F. Schröder and D. Meschede
Pure multipoles from strong permanent magnets: analytic and experimental results, Hyperfine. Interact. 76, 221 (1993)BibTeX
ABSTRACT »

We present the construction of arbitrary multipole field configurations from strong permanent magnets for trapping charged or neutral particles. A general analytic method for the design of three-dimensional magnetic multipoles is discussed for an idealized continuously varying magnetisation taking advantage of the superposition principle. Simple recipes for constructing magnetic dipole and quadrupole fields are given with two types of elements, axially and radially magnetised rings. Cylindrical magnet components not only give free access to the experimental region of interest, but also allow for some tunability to reduce undesirable higher multipole orders. Measurements confirm theoretical predictions achieving useful magnetic fields of 1 T and steep gradients of 3 T/cm with high purity over several ccm.

### 1992

• V. Frerichs, W. G. Kaenders and D. Meschede
Analytic construction of magnetic multipoles from cylindric permanent magnets, Applied Physics A Solids and Surfaces 55, 242--249 (1992)BibTeX
ABSTRACT »

We discuss an analytic method for the design of three-dimensional magnetic multipoles from permanent magnet materials. The concept is explicited with an idealized, continuously varying magnetization. The effect of segmentation for realistic implementations is discussed. As an example we present an open, experimentally accessible cylindric structure for a dipole and a quadrupole field with high purity. The fields are useful over several cm3.