Bulletin of the American Physical Society
52nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 66, Number 6
Monday–Friday, May 31–June 4 2021; Virtual; Time Zone: Central Daylight Time, USA
Session U08: Quantum systems with long range interactionsLive
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Chair: Tim Langen, Stuttgard |
Thursday, June 3, 2021 2:00PM - 2:12PM Live |
U08.00001: Time crystals in a shaken atom-cavity system Jim P Skulte, Phatthamon Kongkhambut, Hans Keßler, Andreas Hemmerich, Jayson Cosme, Ludwig Mathey Periodically driven atoms in a high finesse optical cavity enjoy a very rich phase diagram. By off resonant driving the equilibrium properties of the system can be renormalised in a controlled fashion, while resonant driving allows for new non-equilibrium phases such as time crystalline phases and dynamical density wave orders as recently reported. In this talk, I will discuss the emergence of an incommensurate time crystal by a phase-modulated transverse pump field resulting in a shaken pump lattice. This periodically driven system exhibits macroscopic oscillations in the number of cavity photons and order parameters at noninteger multiples of the driving period, which signals the appearance of an incommensurate time crystal. The subharmonic oscillatory motion corresponds to dynamical switching between symmetry-broken states, which are nonequilibrium bond ordered density wave states. Employing a semiclassical phase-space representation for the driven-dissipative quantum dynamics, we confirm the rigidity and persistence of the time crystalline phase. We identify experimentally relevant parameter regimes for which the time crystal phase is long lived, and map out the dynamical phase diagram. I will further present preliminary experimental results that confirm our theoretical predictions. |
Thursday, June 3, 2021 2:12PM - 2:24PM Live |
U08.00002: Active Suppression of Dephasing and Observation of Rabi Oscillations in Near Resonant Dipole-Dipole Interactions Chengxing He, Robert R Jones We have shown that pulsed electric fields can be employed to actively suppress macroscopic dephasing in dipole-dipole mediated, resonant energy transfer between nearly degenerate atom pairs states in cold random Rydberg ensembles. In particular, we have studied the Stark-tuned (nearly) resonant interaction, 32p3/2+32p3/2 → 32s+33s in a Rb magneto-optical trap (MOT). In an electric field of 11.6V/cm, the 32p3/2+32p3/2 and 32s+33s states in a pair of non-interacting 85Rb atoms are degenerate, and following laser excitation of an isolated 32p3/2 atom pair, the state of the system will coherently evolve into a coherent superposition of 32p3/2+32p3/2 and 32s+33s, exhibiting Rabi oscillations in the probability for finding the atoms in the 32s+33s state. However, in a random ensemble with a large number of nearest neighbor atom pairs, the Rabi frequencies vary significantly over the sample, resulting in macroscopic dephasing of the coherent evolution in each atom pair, such that no Rabi oscillations are observed in the total 32s+33s population. We have implemented a scheme that is a time-domain analog to spatial quasi-phase matching of non-linear optical processes that effectively reduces the variation in Rabi frequencies over the cold atom ensemble. Specifically, we use pulsed electric fields to coherently transport the system across the resonance, from the high to low field side and back, enhancing the off-resonant population transfer and reducing the effective Rabi rate (and its spread). Simulations predict an increase in dephasing times by a factor of 2N where N is the number of back and forth cycles. To date, we have experimentally observed the suppression of dephasing, the reduction of the effective Rabi frequency, and the population transfer enhancement for N=1 and 2. We are exploring the potential of the method for suppressing dephasing associated with atom motion as well. |
Thursday, June 3, 2021 2:24PM - 2:36PM Live |
U08.00003: Roton Excitations in an Oblate Dipolar Quantum Gas Jan-Niklas Schmidt, Jens Hertkorn, Mingyang Guo, Fabian Boettcher, Matthias Schmidt, Kevin Ng, Sean Graham, Tim Langen, Martin W Zwierlein, Tilman Pfau Elementary excitations leave a characteristic footprint in the fluctuations of a quantum mechanical system. We report on first signatures of radial and angular roton excitations around a droplet crystallization transition in oblate dipolar Bose-Einstein condensates by direct in situ measurements of the density fluctuations near this transition. This approach allows for a direct extraction of the static structure factor simultaneously at all momenta, which we use to identify the radial and angular excitations by the characteristic symmetries of their spatial patterns. The fluctuations peak as a function of interaction strength indicating the crystallization transition of the system. We connect the crystallization mechanism with the softening of the angular roton modes by comparing our observations to a theoretically calculated excitation spectrum. This understanding is an important step towards the realization of a dipolar supersolid in two-dimensional oblate trapping geometries. |
Thursday, June 3, 2021 2:36PM - 2:48PM Live |
U08.00004: Collective decay in inverted three-dimensional and two-dimensional atomic arrays Oriol Rubies-Bigorda, Susanne F Yelin Since the first studies of super radiance by Dicke in the 1950s, collective effects have caught the interest of the physics community. Although in this first study all particles were assumed to be in the same position, super and sub radiance persist even for extended samples larger than a resonant wavelength. We hereby study the collective decay of inverted three- and two-dimensional arrays with sub wavelength spacing using the Keldysh contour formalism and show that the time evolution of the system presents a super radiant outburst and a subradiant regime for small enough spacings. These systems can be currently realized experimentally. |
Thursday, June 3, 2021 2:48PM - 3:00PM Live |
U08.00005: Many-body Rabi oscillations for atoms confined in a state-insensitive optical lattice Yin Li, Yefeng Mei, Huy N Nguyen, Paul R Berman, Alexander M Kuzmich Coherent many body Rabi oscillations between the ground state and a collective Rydberg state can be realized in the regime of the Rydberg excitation blockade for a microscopic atomic ensemble. Here we utilize a magic-wavelength optical lattice with the ground-Rydberg coherence time extended up to 30 \mu s to observe such oscillations as a function of driving duration in an ensemble of ~300 atoms. The oscillations are described by a theoretical model which includes a range of decoherence mechanisms. |
Thursday, June 3, 2021 3:00PM - 3:12PM Live |
U08.00006: Dipole collision and energy dissipation in 2D Unitary Fermi Gases and BCS Andrea Barresi, Antoine Boulet We study propagation and collisions of vortex dipoles and quadrupoles in fermionic superfluids, both weakly and strongly interacting. The aim of the research is to explore the impact of the fermionic nature of the system on the dynamics of the vortices. The effects generated by Andreev states, localized within the cores, on dissipative processes will be discussed. Finally, results of exploratory large-scale simulations of many vortex dynamics in 2D their link to turbulence will be presented. |
Thursday, June 3, 2021 3:12PM - 3:24PM Live |
U08.00007: Nonclassical Light from Exciton Interactions in a Two-Dimensional Quantum Mirror Valentin Walther, Lida Zhang, Susanne F Yelin, Thomas Pohl Excitons in a semiconductor monolayer form a collective resonance that can reflect resonant light with extraordinarily high efficiency. Here, we investigate the nonlinear optical properties of such atomistically thin mirrors and show that finite-range interactions between excitons can lead to the generation of highly non-classical light. We describe two scenarios, in which optical nonlinearities arise either from direct photon coupling to excitons in excited Rydberg states or from resonant two-photon excitation of Rydberg excitons with finite-range interactions. The latter case yields conditions of electromagnetically induced transparency and thereby provides an efficient mechanism for single-photon switching between high transmission and reflectance of the monolayer, with a tunable dynamical timescale of the emerging photon-photon interactions. Remarkably, it turns out that the resulting high degree of photon correlations remains virtually unaffected by Rydberg-state decoherence, in excess of non-radiative decoherence observed for ground-state excitons in two-dimensional semiconductors. This robustness to imperfections suggests a promising new approach to quantum photonics at the level of individual photons. |
Thursday, June 3, 2021 3:24PM - 3:36PM Live |
U08.00008: Pattern Formation in Oblate Quantum Ferrofluids: from Supersolids to Superglasses Jens Hertkorn, Jan-Niklas Schmidt, Mingyang Guo, Fabian Boettcher, Kevin Ng, Sean Graham, Paul Uerlings, Hans Peter Büchler, Tim Langen, Martin W Zwierlein, Tilman Pfau Pattern formation is a ubiquitous phenomenon observed in nonlinear and out-of-equilibrium systems. In equilibrium, pattern formation can occur in classical ferrofluids, which exhibit strong dipolar interactions. Here we study quantum ferrofluids, for which density patterns that form in elongated geometries were recently shown to be manifestations of supersolids. We theoretically investigate the phase diagram of quantum ferrofluids in oblate trap geometries and find a wide range of exotic states of matter. In analogy to the patterns in classical ferrofluids, we find honeycomb and labyrinthine states featuring strong density connections. They can be considered two-dimensional manifestations of quantum liquids that spontaneously develop crystalline or amorphous density patterns. Supersolid droplets with two-dimensional crystal structures represent the beginning of the phase diagram at low densities. We show that the quantum fluctuations, responsible for the stabilization of these exotic phases, lead to modified scaling properties generally applicable to quantum ferrofluids in which beyond-mean field effects are important. This allows to find the supersolid droplet, honeycomb and superglass labyrinthine states for a wide variety of trap geometries, interaction strengths, and atom numbers. Our study illuminates the origin of the various possible morphologies of quantum ferrofluids, highlights their emergent supersolid and superglass properties and shows that their occurrence is generic of strongly dipolar interacting systems stabilized by beyond mean-field effects. |
Thursday, June 3, 2021 3:36PM - 3:48PM Live |
U08.00009: Two-dimensional supersolidity in a dipolar quantum gas Claudia Politi Supersolids are quantum states combining the phase coherence of a superfluid with the spatial periodicity of a solid. Predicted 60 years ago and long searched in solid helium, supersolidity was recently observed in dipolar gases [1-3]. In these systems, in a narrow interaction-parameter range, it is possible to reach spontaneous density modulation and global phase coherence. So far, supersolidity in dipolar systems has only been demonstrated along a single direction, as a linear chain of phase-coherent “droplets”. Differently from other experiments, we perform direct evaporative cooling into the supersolid state from a thermal cloud, which allows us to reach high atom numbers and states with large number of droplets. By loosening the transversal trap confinement, we observe a phase transition from a linear to a zig-zag structure, establishing supersolid properties in two dimensions [4]. This step brings our system closer to the supersolid states predicted in solid helium and opens up the possibility of studying rich excitation spectra, as well as vortex formation. |
Thursday, June 3, 2021 3:48PM - 4:00PM Live |
U08.00010: Casimir force between two objects with temperature distribution Chen Xiao-Fan In this paper, the Casimir force between two objects with temperature distribution is studied. |
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