Session K09: Degenerate Fermi Gas II

A multi-purpose quantum simulation setup with Lithium 6 atoms.

Quantum science has become an extremely active area of research, relying on physical realizations of large and well-controlled quantum systems. With quantum gases of ultracold atoms, experiments have had tremendous success in tackling quantum many-body problems, which are notoriously challenging, thanks to their capabilities to create quantum systems with incremental complexity. However, a limitation of the current approaches is that, typically, each quantum gas experiment is optimized to address a certain type of problems. One often has to choose between lattice or continuous systems, short or long-range interactions, single-particle detection or bulk measurements, etc. In this talk, we will present a design for a 6Li-based quantum gas platform that aims to provide wide-ranging capabilities, where one could address a variety of quantum problems that have historically been treated on different setups. The use of 6Li makes the setup suitable for the study of BEC-BCS physics, Fermi Hubbard Physics and Rydberg systems, while the architecture is compatible with a quantum gas microscope and tailored optical potentials. This two-chamber setup relies on robust and easy to implement techniques, allowing to produce low entropy ensembles of fermions with temperatures and atom numbers that are compatible with a wide range of many-body problems.   

Thermalization dynamics of a dissipative two-level system in a Fermi bath with contact interactions

We study the dynamics of a quasi-homogeneous Fermi gas under a strong internal-state drive. A highly spin-imbalanced weakly-interacting mixture is created in two internal states in a box trap. The minority is then driven to a third internal state with a radio-frequency field, where this third state interacts via unitary limited interactions with the majority. We investigate the dynamics of the minority under a strong drive with a Rabi frequency much larger than the Fermi energy of the host atoms. We extract the Rabi frequency renormalized by interactions and the relaxation of the effective magnetization (the relative population difference). We observe that after a few cycles, the system reaches a non-trivial steady state characterized by a detuning-dependent magnetization. This system is understood as dissipative two-level systems, which evolve to their thermal equilibrium state via interactions with the fermionic bath. We extract the effective T1 and T2 times characterizing the decay to the steady-state as well as the mean-field shift of the resonance frequency of the two-level systems. Our study paves the way to studying equilibrium states of unitary Fermi gases in the presence of strong driving fields.   

Fermi Polaron lifetimes at finite momentum from a functional-renormalization-group approach

We study theoretically the lifetimes of the attractive and the repulsive Fermi Polaron and the molecule at finite momentum in both two and three dimensions. To this end, we developed a new technique that allows for the computation of Green's functions in the whole complex frequency plane using exact analytical continuation within the functional renormalization group. While conventional approaches like the NSCT method cannot determine these lifetimes, we are able to find the momentum dependent lifetime at different interaction strengths of both the attractive and repulsive polaron as well as the molecule. In our talk we discuss our findings and talk about possible experiments which could be conducted.   

Hydrodynamic Relaxation in a Strongly Interacting Fermi Gas

We report new time-domain, free evolution methods for measuring hydrodynamic transport coefficients in a strongly interacting Fermi gas. We measure the free decay of a spatially periodic density profile in a two-state mixture of ⁶Li in the normal fluid phase, confined in a box potential. This spatial profile is initially created in thermal equilibrium by a perturbing potential. After the perturbation is abruptly extinguished, the dominant spatial Fourier component exhibits an exponentially decaying (thermally diffusive) mode and a decaying oscillatory (first sound) mode, enabling independent measurement of the thermal conductivity and the shear viscosity directly.   

Persistent current decay in an atomic ring trap

The motion of quantized vortices is of fundamental importance in different phenomena in condensed matter physics such as the resistive behaviour of superconductors or many other dissipative collective phenomena in superfluids. In particular, here we investigate theoretically their role on the decay of persistent current in ring-shaped atomic superfluids and in the presence of a small defect. In our studies, we model a recent experiment of ⁶Li at LENS in the limit of Bose-Einstein condensate of molecules. The numerical simulations are performed at T=0 by solving the Gross-Pitaevskii equation. A finite circulation state is excited by imprinting a phase in the equilibrium condensate wavefunction. In the absence of the defect, the current is persistent and the superflow is dissipatonless up to a maximum circulation. When a small defect instead is introduced, there is a critical velocity, i.e. a critical circulation, at which the vortices are emitted into the superfluid causing phase slips and thus a transition of the quantized initial circulation states to lower values (i.e. a decay of the current in time). We then give a microscopic description of such mechanism and its dependence on defect parameters and circulation state.   

On the Stability of the Repulsive Fermi Gas with Short-Range Interactions

We study the stability of a homogeneous spin-1/2 Fermi gas with repulsive short-range interactions. This many-body repulsive 'branch' is metastable towards the formation of Feshbach bound states, via three-body recombination. We measure the universal recombination coefficient $K_3$ and observe universal scalings with the average kinetic energy per particle and two-body scattering length. The scaling exponents are in excellent agreement with the linear energy dependence arising from a three-body threshold law involving two indistinguishable fermions and the $a^{6}$ scaling for three-body collisions in two-component Fermi gas under zero-range approximation. The interplay of the momentum dependence of the recombination coefficient and the Fermi statistics leads to non-trivial temperature dynamics, alternatively heating or cooling depending on the initial quantum degeneracy. The universal scaling with interactions extends over four orders of magnitude and beyond the expected range of validity of the three-body recombination law.   

Stability of quantum degenerate fermionic polar molecules without and with microwave shielding

A stabilization of a fermionic molecular gas towards collapse in attractive head-to-tail collisions and its evaporative cooling below the Fermi temperature has so far been achieved in two ways. Either a strong dc electric field is applied to confine the molecular motion to 2D [1] or inelastic collisions in 3D are strongly suppressed by applying a circularly polarized microwave field [2]. Here we use a Hartree-Fock mean-field theory [3,4] in order to determine the 3D properties of quantum degenerate fermionic molecules. In particular, we compare the stability diagrams occurring without (with) microwave shielding, where a (negative) dipole-dipole interaction is present. In case that the orientation of the electric dipoles with respect to the trap axes is unknown, we outline how to reconstruct it from time-of-flight absorption measurements.

[1] G. Valtolina, et al., Nature 588, 239 (2020).

[2] A. Schindewolf, et al., arXiv:2201.05143 (2022).

[3] V. Veljic, et al., New J. Phys. 20, 093016 (2018).

[4] V. Veljic, A. Pelster, and A. Balaz, Phys. Rev. Res. 1, 012009 (2019).   

Density fluctuations in three & two-dimensional SU(N) Fermi gases

For a degenerate SU(N) Fermi gas, spin multiplicity N affects its thermodynamics and quantum fluctuations, especially in the deep degenerate regime. Although a lot of efforts have been put in investigating intriguing properties of SU(N) Fermi gases, the thermodynamics of imbalanced high spin system remains unexplored.

Here, we systematically investigate density fluctuations in two-(2D) and three-dimensional (3D) SU(N) gases in variable temperatures T/T_F and spin configurations. Especially, we carefully examine the imbalanced SU(N) system in the context of density fluctuation (i.e. thermodynamic compressibility), which will expand our understanding of SU(N)-symmetric Fermi liquids both in 2D and 3D.   

Observation of superfluid current through a dissipative quantum point contact

We experimentally and theoretically confirm the robustness of fermionic superfluidity to spin-dependent particle dissipation in a unitary Fermi gas. By locally illuminating a quantum point contact connecting two superfluid reservoirs with a beam resonant with one of the two spin states, we engineer particle loss and measure its effect on the signature of superfluidity in our system: a non-Ohmic supercurrent carried by multiple Andreev reflections (MAR). We develop a mean-field model in the Keldysh formalism that quantitatively reproduces our observations. We find that there is no critical dissipation strength where the supercurrent vanishes. Instead, it smoothly decays towards zero with increasing dissipation, indicating a surprising robustness of MAR. Our model also predicts that Onsager's reciprocal relations are violated due to broken detailed balance, which we hope to soon experimentally confirm as it can have significant implications for dissipative engineering of transport properties.   

A new experimental apparatus for exploring the SU(N) Fermi-Hubbard model

The Fermi-Hubbard model is a cornerstone model of condensed matter physics. Extending this model to the SU(N) symmetric case promises new exotic magnetic phases beyond the limits of natural materials. The behavior of such systems is hard to predict theoretically. Experimentally however, the SU(N) Fermi-Hubbard model can be realized and studied with the fermionic isotopes of alkaline-earth atoms, where the interactions are SU(N) symmetric, i.e., independent of the internal state of the atom.