Session H02: Bose and Fermi gases in optical boxes

Fermions in an Optical Box - Tales of Stability

For the past two decades harmonically trapped ultracold atomic gases have been used with great success to study fundamental many-body physics in flexible experimental settings. However, the resulting gas density inhomogeneity in those traps makes it challenging to study paradigmatic uniform-system physics (such as critical behavior near phase transitions) or complex quantum dynamics. The realization of homogeneous quantum gases trapped in optical boxes has marked a milestone in the quantum simulation program with ultracold atoms [1]. These textbook systems have proved to be a powerful playground by simplifying the interpretation of experimental measurements, by making more direct connections to theories of the many-body problem that generally rely on the translational symmetry of the system, and by altogether enabling previously inaccessible experiments. I will present a set of studies with ultracold fermions trapped in a box of light. This system is particularly suitable to study problems of fermion stability, of which I will discuss two cases: the spin-1/2 Fermi gas with repulsive contact interactions [2], and the three-component Fermi gas with spin-population imbalance [3]. Both studies lead to surprises, highlighting how spatial homogeneity not only simplifies the connection between experiments and theory, but can also unveil unexpected outcomes.   

Quench dynamics of tunable Bose gases in an optically painted 2D box

A quantum gas loaded in an optical box presents a model homogeneous system for exploring quantum many-body dynamics. While trap uniformity has been the main attraction for realizing intricate many-body states or for studying non-equilibrium dynamics without suffering from inhomogeneous effects, the existence of sharp edges in a box could also lead to unexpected, but fascinating consequences in out-of-equilibrium dynamics that were not seen in conventional harmonic traps. In this talk, I will discuss two examples of quench dynamics from atomic superfluids trapped in two-dimensional (2D) boxes. I will first present our recent studies of interaction quenches to attractive Bose gases, where we observe instability-induced quasiparticle pair-creations and can characterize their quantum entanglement. This dynamics in a uniform trap eventually leads to fragmentation and formation of solitons among other intricate dynamics. In the second example, I will discuss quench dynamics of a repulsive Bose gas and show how the interaction with the box boundary could lead to spontaneously patterned defect formation in a superfluid - from ring dark solitons to vortex dipole necklaces - which would open a doorway towards forming complex vortex quantum matters in an optical box. I will discuss our on-going studies on out-of-equilibrium dynamics using tunable Bose gases in optically painted 2D boxes.   

Wave turbulence in box-trapped Bose gases

Studies of turbulent cascades of excitations in box-trapped Bose gases [1] have in recent years offered a new view on this topic, thanks to the possibilities to observe the system on all relevant lengthscales and timescales, tune the dissipation lengthscale and the strength of interactions driving the cascade, and measure the scale-invariant particle and energy fluxes carried by the cascade. I will give an overview of our recent and ongoing studies on this topic, including the time-resolved observation of the 'birth of turbulence' in a 2D gas [2] and the experimental construction of an equilibrium-like universal equation of state for wave turbulence in a 3D gas [3].

[1] Emergence of a turbulent cascade in a quantum gas, N. Navon, A. L. Gaunt, R. P. Smith, and Z. Hadzibabic, Nature 539, 72 (2016). 

[2] Emergence of Isotropy and Dynamic Scaling in 2D Wave Turbulence in a Homogeneous Bose Gas, Phys. Rev. Lett. 129, 190402 (2022).

[3] Universal equation of state for wave turbulence in a quantum gas, L. H. Dogra et al., arXiv:2212.08652   

Fermionic superfluids in two and three dimensions

Understanding the origins of unconventional superconductivity has been a major focus of condensed matter physics for many decades. While many questions remain unanswered, experiments have found the highest critical temperatures in layered two-dimensional materials. However, to what extent the remarkable stability of these strongly correlated 2D superfluids is affected by their reduced dimensionality is still an open question. Here, I will review our work studying superfluidity in homogeneous 2D and 3D Fermi gases, in particular measurements of the dynamic structure factor of 2D [1,2] and 3D [3] superfluids. Using Bragg spectroscopy, we determine the critical velocity and the superfluid gap in the BEC-BCS crossover, allowing for detailed comparisons with and benchmarks for theory. Our measurements enable us to directly study the role of reduced dimensionality on strongly correlated superfluids [2].

In the outlook I will report on our ongoing investigation of spin-imbalanced 2D Fermi gases.

[1] L. Sobirey et al., Science 372, 844 (2021)

[2] L. Sobirey et al., PRL 129, 083601 (2022)

[3] H. Biss et al., PRL 128, 100401 (2022)