OSCAR Reports

an SFB/TR 185 magazine

Second Funding Period, Issue 6

Author: Priv.-Doz. Dr. Axel Pelster

**Observing the loss and revival of long-range phase coherence through disorder quenches**

Benjamin Nagler, Sian Barbosa, Jennifer Koch, Giuliano Orso, and Artur Widera

PNAS** 119, **e2111078118 (2022)

Understanding the combined effects of disorder and interactions in quantum many-body systems is an intriguing topic of condensed matter physics. While most studies have considered a static disorder, the work of Nagler et al. investigates systematically the relaxation dynamics of a three-dimensional Bose-Einstein condensate of ^{6}Li_{2} molecules after switching on or off a laser speckle potential. In particular, the experiment-theory collaboration studies the transient and steady-state atomic density distribution as well as the ability of the gas to hydrodynamically expand, directly reflecting long-range phase coherence. Comparing experimental measurements with large-scale numerical simulations it is found that, when the gas is exposed to disorder, long-range phase coherence breaks down one order of magnitude faster than the density distribution responds. In the opposite case, surprisingly, long-range coherence takes two orders of magnitude longer times to revive than the density relaxation. In the future, it will be interesting to study with this set-up the dynamical response of quantum gases along the whole crossover from a Bose-Einstein condensate (BEC) to a Bardeen-Cooper-Schrieffer (BCS) type superfluid to explore the impact of quenched disorder on resonantly interacting superfluids.

Fig. 1 Schematic illustration of experimental setup: trapped sample (yellow ellipsoid) and speckle beam (green volume) producing randomly distributed anisotropic grains. Insets show section of speckle intensity distribution in xy-plane and in situ absorption image of BEC in disorder.

**A Kapitza Pendulum for Ultracold Atoms**

Jian Jiang, Erik Bernhart, Marvin Röhrle, Jens Benary, Marvin Beck, Christian Baals, and Herwig Ott

arXiv:2112.10954 (2021)

Manipulating the states and dynamics of a system by a periodic drive is known as Floquet engineering. Kapitza first physically explained and experimentally investigated such kind of Floquet engineering for an inverted pendulum with a vibrating suspension point. The most prominent example of this kind is the Paul trap, where a saddle point potential is modulated periodically to create a confining two-dimensional harmonic potential for ions.

The publication of Jiang et al. experimentally demonstrates the realization of a theoretical proposal of a Kapitza trap from 2003. This confinement is created by a time-periodic attractive and repulsive potential, which are localized in space and have a vanishing time average. Employing a rapid time-periodic Gaussian potential, realized with red- and blue-detuned focused laser beams, allows to confine ultracold ^{87}Rb atoms with an effective ring barrier potential. In the future this approach should allow to enter the regime of a quantum Kapitza pendulum, so that further insight into the physics of tunneling processes becomes possible.

Fig.2: Kapitza trap: experimental setup (left) and absorption image of the atomic cloud (right).

**Quantum Rabi dynamics of trapped atoms far in the deep strong coupling regime**

Johannes Koch, Geram Hunanyan, Till Ockenfels, Enrique Rico, Enrique Solano. and Martin Weitz

arXiv:2112.12488v1 (2021)

The coupling of a two-level system with an electromagnetic field, whose fully quantized field version is known as the quantum Rabi model, is among the central topics of quantum physics and recent quantum information technologies. When the coupling strength becomes stronger than the decoherence rate the so-called strong coupling regime is reached, with mixed states of the two-level system and the field mode becoming relevant. And when the coupling strength reaches even the field mode frequency the deep strong coupling regime is approached, where excitations can be created out of the vacuum.

OSCAR Project C5 demonstrates the realization of a periodic variant of the quantum Rabi model using two coupled quantized mechanical modes of cold ^{87}Rb atoms in optical potentials, which allows to reach the Rabi coupling strength of 6.5 times the field mode frequency. Thus, for the first time, the coupling term dominates over all other energy scales. Field mode creation and annihilation upon e.g., de-excitation of the two-level system approach equal magnitudes, so the atomic dynamics becomes observable in this novel experimental regime, revealing a subcycle timescale raise in bosonic field mode excitations in good agreement with theoretical predictions. In a measurement recorded in the basis of the coupling term of the quantum Rabi Hamiltonian, the observed dynamics freezes for small frequency splittings of the two-level system, but revives for larger splittings. This new concept nourishes the prospect to realize quantum-engineering applications in yet unexplored parameter regimes.

Fig. 3: Atoms are exposed to the combined potential obtained by superimposing a harmonic trapping potential (left) generated by a focused CO_{2}-laser beam and a lattice potential (right). For atoms moving in the combined potential, the oscillation in the harmonic trapping potential of frequency ω and the oscillation at the first band gap of the lattice with frequency ω_{q} are extremely strongly coupled.