Dr. Wolfgang Alt  

We have designed, built, and characterized a high resolution objective lens that is compatible with an ultrahigh vacuum environment. The lens system ex ploits the principle of the Weierstrasssphere 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 twolens 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 nearfield 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.
We create lowentropy states of neutral atoms by utilizing a conceptually new opticallattice technique that relies on a highprecision, highbandwidth synthesis of light polarization. Polarizationsynthesized optical lattices provide two fully controllable optical lattice potentials, each of them confining only atoms in either one of the two longlived 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 atomsorting algorithm with logarithmic complexity, which is enabled by polarizationsynthesized optical lattices. Vibrational entropy is subsequently removed by sideband cooling methods. Our results pave the way for a bottomup approach to creating ultralowentropy states of a manybody system.
We propose a novel approach to precisely synthesize arbitrary polarization states of light with a high modulation bandwidth. Our approach consists in superposing 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. To assess the precision of the synthesized polarization states, we characterize static spatial variations of the polarization over the wavefront, as well as the noise spectral density of temporal fluctuations. We find that static polarization distortions limit the extinction ratio to 2x10^{5}, corresponding to a 0.01% reduction of the degree of polarization (DOP). We also obtain that temporal fluctuations give rise to a 0.2º uncertainty in the state of polarization (SOP). We recently demonstrated an application of the polarization synthesizer (Robens et al., arXiv:1608.02410) to create two fully independent, controllable optical lattices, which trap atoms depending on their internal spin state. Probing ultracold atoms in polarizationsynthesized optical lattices, we obtain an independent, complementary characterization of the optical performance of the polarization synthesizer.
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 interactionfree measurement scheme can be put in direct relation with falsification tests of the macrorealistic worldview. Here we have implemented the "bomb test" with a single atom trapped in a spindependent optical lattice to show explicitly a violation of the LeggettGarg inequality—a quantitative criterion fulfilled by macrorealistic physical theories. To perform interactionfree 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 pushout technique. Allowing decoherence to dominate the evolution of our system causes a transition from quantum to classical behavior in fulfillment of the LeggettGarg inequality.
Discretetime 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 twodimensional discretetime 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 spindependent optical lattices to realize spatial boundaries between distinct topological phases. Our proposal relies on a new scheme to implement spindependent discrete shift operations in a twodimensional 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.
We report on image processing techniques and experimental procedures to determine the latticesite 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 superresolution 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 superresolution 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 subpixel 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.
Fiber FabryPerot cavities, formed by micromachined mirrors on the endfacets 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 FabryPerot resonators and stabilize their length for experiments in cavity quantum electrodynamics with neutral atoms. A piezomechanically actuated cavity with feedback based on the PoundDreverHall 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 selflocking and external temperature tuning. Furthermore, we present a general analysis of the mode matching problem in fiber FabryPerot 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 fibergenerated background photons. We expect that our results contribute towards the integration of highfinesse fiber FabryPerot cavities into compact and robust quantumenabled devices in the future.
We report on an ultralow birefringence dodecagonal glass cell for ultrahigh vacuum applications. The epoxybonded trapezoidal windows of the cell are made of SF57 glass, which exhibits a very low stressinduced 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.
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 LeggettGarg inequality. Our results rigorously excludes (i.e., falsifies) any explanation of quantum transport based on classical, welldefined trajectories. We use socalled ideal negative measurements—an essential requisite for any genuine LeggettGarg test—to acquire information about the atom’s position, yet avoiding any direct interaction with it. The interactionfree 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 LeggettGarg 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.
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.
We discuss decoherence in discretetime quantum walks in terms of a phenomenological model that distinguishes spin and spatial decoherence. We identify the dominating mechanisms that affect quantumwalk 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 longtime spin dephasing mechanisms. We introduce the concept of coherence length in the discretetime quantum walk, which quantifies the range of spatial coherences. Unexpectedly, we find that quasistationary 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.
We demonstrate cooling of the motion of a single neutral atom confined by a dipole trap inside a highfinesse optical resonator. Cooling of the vibrational motion results from electromagnetically induced transparency (EIT)–like interference in an atomic lambdatype 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.
We present an insitu method to measure the birefringence of a single vacuum window by means of microwave spectroscopy on an ensemble of cold atoms. Stressinduced birefringence can cause an ellipticity in the polarization of an initially linearlypolarized 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.