IAP logo UniBonn logo
EnglishDeutsch
  • Schrift vergrößern
  • Standard-Schriftgröße
  • Schriftgröße verkleinern

Quantentechnologie

Dieter Meschedes Forschungsgruppe
Home Gruppenmitglieder Wolfgang Alt
Drucken
Gruppenmitglieder
Dr. Wolfgang Alt
Kontakt
Position: Leitender Wissenschaftler
Field of research: Few-atom quantum systems
Fibre cavity QED
Digital quantum simulators
Address:
Institut für Angewandte Physik
Wegelerstr. 8
D-53115 Bonn
Germany
Office room: 318
E-mail: Diese E-Mail-Adresse ist gegen Spambots geschützt! JavaScript muss aktiviert werden, damit sie angezeigt werden kann.
Office: +49 228 73-3471
Fax: +49 228 73-3474
 

Publications

2017

  • 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)arXivBibTeXPDF
    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, S. Brakhane, W. Alt, D. Meschede, J. Zopes and A. Alberti
    Fast, high-precision optical polarization synthesizer for ultracold-atom experiments, arXiv:1608.02410, (2017)arXivBibTeXPDF
    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.

  • 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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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.

2016

  • 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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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.

  • 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 (editor's suggestion) 94, 013620 (2016)arXivBibTeXPDF
    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.

2015

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

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)arXivBibTeXPDF
    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. Mølmer
    Hidden Markov Model of atomic quantum jump dynamics in an optically probed cavity, Phys. Rev. A 89, 043839 (2014)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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
    A digital atom interferometer with single particle control on a discretized spacetime geometry, PNAS 109, 9770 (2012)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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.

  • U. Wiedemann, W. Alt and D. Meschede
    Switching photochromic molecules adsorbed on optical microfibres, Opt. Express 20, 12710-12720 (2012)BibTeXPDF
    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)arXivBibTeXPDF
    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)BibTeXPDF
    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)arXivBibTeXPDF
    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.

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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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)BibTeXPDF
    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.

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)arXivBibTeXPDF
    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.
  • 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)arXivBibTeXPDF
    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.
  • 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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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.

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)arXivBibTeXPDF
    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.

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.
  • 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)arXivBibTeXPDF
    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
    Number-triggered loading and collisional redistribution of neutral atoms in a standing wave dipole trap, New J. Phys. 8, 259 (2006)BibTeXPDF
    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.
  • 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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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.
  • 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)arXivBibTeXPDF
    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.

2004

  • W. Alt
    Optical control of single neutral atoms, (2004), DoktorarbeitBibTeXPDF
  • 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)BibTeXPDF
    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

  • 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)BibTeXPDF
    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.
  • 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)arXivBibTeXPDF
    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)arXivBibTeXPDF
    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.

2002

  • W. Alt
    An objective lens for efficient fluoresence detection of single atoms, Optik 113, 142 (2002)arXivBibTeXPDF
    ABSTRACT »
    We present the design of a diffraction limited, long working distance monochromatic objective lens for efficient light collection. Consisting of four spherical lenses, it has a numerical aperture of 0.29, an effective focal length of 36 mm and a working distance of 36.5 mm. This inexpensive system allows us to detect 8·10^4 fluorescence photons per second from a single cesium atom stored in a magneto-optical trap.

2001

  • S. Kuhr, W. Alt, D. Schrader, M. Müller, V. Gomer and D. Meschede
    Deterministic Delivery of a Single Atom, Science 293, 278 (2001)BibTeXPDF
    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)arXivBibTeXPDF
    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.

2000

  • 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)arXivBibTeXPDF
    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.