Dr. Andrea Alberti  

You can download here the CV of Andrea Alberti.
Listen to the interview with Forschung aktuell of Deutschlandfunk on the atom sorting machine (“Sortiergerät für Atome: Forscher präparieren Quantenregister im Rekordtempo” broadcast on March 9th, 2017). These results have been recently published in Phys. Rev. Lett. Phys. Rev. Lett. 118, 065302 (2017).
We report on the observation of a topologically protected edge state at the interface between two topologically distinct domains of the SuSchriefferHeeger model, which we implement in arrays of evanescently coupled dielectricloaded surface plasmon polariton waveguides. Direct evidence of the topological character of the edge state is obtained through several independent experiments: Its spatial localization at the interface as well as the restriction to one sublattice is confirmed by realspace leakage radiation microscopy. The corresponding momentumresolved spectrum obtained by Fourier imaging reveals the midgap position of the edge state as predicted by theory.
Quantum statistics have a profound impact on the properties of systems composed of identical particles. In this Letter, we demonstrate that the quantum statistics of a pair of identical massive particles can be probed by a direct measurement of the exchange symmetry of their wave function even in conditions where the particles always remain spatially well separated and thus the exchange contribution to their interaction energy is negligible. We present two protocols revealing the bosonic or fermionic nature of a pair of particles and discuss possible implementations with a pair of trapped atoms or ions.
We show that the bulk winding number characterizing onedimensional topological insulators with chiral symmetry can be detected from the displacement of a single particle, observed via losses. Losses represent the effect of repeated weak measurements on one sublattice only, which interrupt the dynamics periodically. When these do not detect the particle, they realize negative measurements. Our repeated measurement scheme covers both timeindependent and periodically driven (Floquet) topological insulators, with or without spatial disorder. In the limit of rapidly repeated, vanishingly weak measurements, our scheme describes nonHermitian Hamiltonians, as the lossy SuSchriefferHeeger model of Rudner and Levitov, [Phys. Rev. Lett. 102, 065703 (2009)]. We find, contrary to intuition, that the time needed to detect the winding number can be made shorter by decreasing the efficiency of the measurement. We illustrate our results on a discretetime quantum walk, and propose ways of testing them experimentally.
We study the relation between the global topology of the Hofstadter butterfly of a multiband insulator and the topological invariants of the underlying Hamiltonian. The global topology of the butterfly, i.e., the displacement of the energy gaps as the magnetic field is varied by one flux quantum, is determined by the spectral flow of energy eigenstates crossing gaps as the field is tuned. We find that for each gap this spectral flow is equal to the topological invariant of the gap, i.e., the net number of edge modes traversing the gap. For periodically driven systems, our results apply to the spectrum of quasienergies. In this case, the spectral flow of the sum of all the quasienergies gives directly the RudnerLindnerBergLevin invariant that characterizes the topological phases of a periodically driven system.
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.
Even scientific grade optical glasses show birefringence when small external forces are applied to the sample. Stressinduced birefringence can be particularly detrimental to the state of polarization of light when a laser beam is transmitted through the glass. This is especially the case for glass windows of a vacuum chamber. Since compensation of spatially inhomogeneous birefringence is extremely challenging, it should be prevented by proper design of the vacuum chamber. Birefringence below 0.2 nm/cm can be achieved by thoroughly choosing glass material with low stress optical coefficient and mounting geometry. Applications strongly depend on light polarization are quantum technologies such as precision metrology, quantum computation and quantum simulations based on ions or atoms.
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 report on the state of the art of quantum walk experiments with neutral atoms in statedependent optical lattices. We demonstrate a novel statedependent transport technique enabling the control of two spinselective sublattices in a fully independent fashion. This transport technique allowed us to carry out a test of singleparticle quantum interference based on the violation of the LeggettGarg inequality and, more recently, to probe twoparticle quantum interference effects with neutral atoms cooled into the motional ground state. These experiments lay the groundwork for the study of discretetime quantum walks of strongly interacting, indistinguishable particles to demonstrate quantum cellular automata of neutral atoms.
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 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.
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 wavefunction 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 quasiparticle effects in quantum many body systems.
We report on the realization of dynamical control of transport for ultracold 88Sr atoms loaded in an accelerated amplitudemodulated onedimensional (1D) optical lattice. We show that the behavior of the dynamical system can be viewed as if traveling wave packets were moving in a static lattice whose energy dispersion can be tailored at will in width, amplitude and phase. One basic control operation is a reversible switch between Wannier–Stark localization and driven transport based on coherent tunneling. Performing modulation sequences of this operation within a Loschmidtecho scheme, we are able to reverse the atomic group velocities at once. We then apply the technique to demonstrate a novel mirror for matter waves working independently of the momentum state. We finally discuss advantages of amplitude over previously reported phase modulation techniques for applications in force measurements at micrometric scales.
The manipulation of matter waves had an important role in the history of quantum mechanics. The first experimental validation of matterwave behaviour was the observation of diffraction of matter by crystals, followed by interference experiments with electrons, neutrons, atoms and molecules using gratings and Young's double slit. More recently, matterwave manipulation has become a building block for quantum devices such as quantum sensors and it has an essential role in a number of proposals for implementing quantum computers. Here, we demonstrate the coherent control of the spatial extent of an atomic wavefunction by reversibly stretching and shrinking the wavefunction over a distance of more than one millimetre. The quantumcoherent process is fully deterministic, reversible and in quantitative agreement with an analytical model. The simplicity of its experimental implementation could ease applications in the field of quantum transport and quantum processing.
We report the realization of a quantum device for force sensing at the micrometric scale. We trap an ultracold Sr88 atomic cloud with a onedimensional (1D) optical lattice; then we place the atomic sample close to a test surface using the same optical lattice as an elevator. We demonstrate precise positioning of the sample at the micrometer scale. By observing the Bloch oscillations of atoms into the 1D optical standing wave, we are able to measure the total force on the atoms along the lattice axis, with a spatial resolution of few micrometers. We also demonstrate a technique for transverse displacement of the atoms, allowing us to perform measurements near either transparent or reflective test surfaces. In order to reduce the minimum distance from the surface, we compress the longitudinal size of the atomic sample by means of an optical tweezer. This system is suited for studies of atomsurface interaction at short distance, such as measurement of the Casimir force and the search for possible nonNewtonian gravity effects.
Atomic wave packets loaded into a phasemodulated vertical opticallattice potential exhibit a coherent delocalization dynamics arising from intraband transitions among WannierStark levels. WannierStark intraband transitions are here observed by monitoring the in situ wavepacket extent. By varying the modulation frequency, we find resonances at integer multiples of the Bloch frequency. The resonances show a Fourierlimited width for interrogation times up to 2 s. This can also be used to determine the gravity acceleration with ppm resolution.