Absence of heating in a uniform Fermi gas created by periodic driving

Abstract

Ultracold atoms are a powerful resource for quantum technologies. As such, they are usually confined in an external potential that often depends on the atomic spin, which may lead to inhomogeneous broadening, phase separation and decoherence. Dynamical decoupling provides an approach to mitigate these effects by applying an external field that induces rapid spin rotations. However, a continuous periodic driving of a generic interacting many-body system eventually heats it up. The question is whether dynamical decoupling can be applied at intermediate times without altering the underlying physics. Here we answer this question affirmatively for a strongly interacting degenerate Fermi gas held in a flat box-like potential. We counteract most of the gravitational force by applying an external magnetic field with an appropriate gradient. Since the magnetic force, and consequently, the whole potential, is spin-dependent, we employ rf to induce a rapid spin rotation. The driving causes atoms in both spin states to experience the same time-average flat potential, leading to a uniform cloud. Most importantly, we find that when the driving frequency is high enough, there is no heating on experimentally relevant timescales, and physical observables are similar to those of a stationary gas. In particular, we measure the pair-condensation fraction of a fermionic superfluid at unitarity and the contact parameter in the BEC-BCS crossover. The condensate fraction exhibits a non-monotonic dependence on the drive frequency and reaches a value higher than its value without driving. The contact agrees with recent theories and calculations for a uniform stationary gas. Our results establish that a strongly-interacting quantum gas can be dynamically decoupled from a spin-dependent potential for long periods of time without modifying its intrinsic many-body behavior.

Quantum Gases in Optical Boxes

“…The second issue is that many exciting possibilities rely on specific features of different atomic species and their mixtures, but the methods for the levitation of gases in 3D box traps are generally species specific. In current experiments one can study mixtures of species that have very similar ratios of mass and magnetic moment [16,139], but creating arbitrary homogeneous mixtures of different chemical elements, different isotopes, or even just different spin states of the same isotope, is an open challenge.…”

Experimental realization of fragmented models in tilted Fermi-Hubbard chains

“…In order to tune the naturally-given tilt difference between the spin components which is caused by the different m F quantum numbers, we employ the technique of RF dressing [35,36]. During the time evolution we apply an RF field to drive the transition from |↓ to |↑ .…”

“…The spin-dependent tilt ∆ σ in Hamiltonian (1) introduces additional constraints. In order to tune this spindependence in the experiment we employ the technique of radio-frequency (RF) dressing [31,35], where we use an additional RF field to couple the two spins. Thereby we realize dressed states that see a weighted average of ∆ ↑ and ∆ ↓ .…”

Strong Fulde-Ferrell Larkin-Ovchinnikov pairing fluctuations in polarized Fermi systems

“…To this end, we suggest that the projection technique introduced some time ago to detect pair condensation in a strongly interacting Fermi gas [16,17] could as well be used here to measure a "projected" pair-momentum distribution, whereby the occurrence of a pronounced peak at the same Q 0 of the pair susceptibility would provide unambiguous evidence for strong FFLO pair fluctuations. We explicitly calculate this projected pair-momentum distribution, and conclude that its peak at a finite Q 0 should most readily be observed with a box-like trapping potential [18][19][20][21].…”

“…A somewhat related quantity of more direct access to experiments with ultracold gases should be the "projected" pair-momentum distribution n proj pair (Q), which is the momentum distribution of the molecules formed after a rapid sweep of the magnetic field to the BEC side of the crossover. Measurements of the projected pairmomentum distribution have already been successfully applied to detect condensation (or quasi-condensation) of fermionic pairs across the BCS-BEC crossover, both in three [16,17,21,45] and two [46] dimensions, and were also proposed some time ago to detect FFLO superfluidity in trapped Fermi gases [47]. On physical grounds, we expect that, even in the normal phase, strong FFLO pairing fluctuations should result into a peak of n proj pair (Q) at the same finite Q 0 found for χ pair (Q).…”