Session E05: Degenerate Fermi Gas I

Pauli blocking enhanced life-times and cooperative radiation in a 2D Fermi gas

The observation of  Pauli blocking of atomic spontaneous decay via direct measurements of the atomic population requires the use of long-lived atomic gases where quantum statistics,  atom recoil  and cooperative  radiative processes are all relevant.  We develop a theoretical framework capable of simultaneously accounting for these effects and apply it to atoms featuring an effective  $\Lambda$ level  structure confined in a single 2D pancake or arrays of pancakes.  We identify a parameter window in which a factor of two extension in the atomic lifetime clearly attributable to Pauli blocking  should be experimentally observable in deeply degenerate gases with  $\sim  10^{3} $ atoms. Consistent with these theory predictions, we experimentally observe  a $\sim$20\% suppressed excited state decay rate in the moderately degenerate regime on the  ${}^{1}\rm{S_0}-{}^{3}\rm{P_1}$ transition in $^{87}$Sr atoms.

Ref: arXiv:2108.02819

    

Excitation Spectrum and Superfluid Gap of an Ultracold Fermi Gas

Ultracold Fermi gases with tunable interactions have enabled the realization of the famous BEC-BCS crossover from a Bose-Einstein condensate (BEC) of molecules to a Bardeen-Cooper-Schrieffer (BCS) superfluid of weakly bound Cooper pairs. Using Bragg spectroscopy, we measure the full momentum-resolved low-energy excitation spectrum of strongly interacting Fermi gases and directly observe the smooth transformation from a bosonic to a fermionic superfluid that takes place in the BEC-BCS crossover. Moreover, we determine the evolution of the superfluid gap from these spectra and find excellent agreement with previous experiments and self-consistent T-matrix calculations both in the BEC and crossover regime. However, towards the BCS regime, a calculation that includes the effects of particle-hole correlations shows better agreement with our data.   

Observing the effect of dimensionality on the stability of strongly interacting fermionic superfluids

We use ultracold Fermi gases as a model system to directly probe the influence of dimensionality on the stability of fermionic superfluids. Using momentum-resolved Bragg spectroscopy, we measure the superfluid gap of of two- and three-dimensional homogeneous Fermi gases over a wide range of interaction strengths. We find that the superfluid gap follows a universal function of the chemical potential, irrespective of the dimensionality. Comparing our results with other fermionic superfluids suggests that strong correlations and short coherence lengths are more important for the stability of fermionic superfluids than the dimensionality of the system.   

Realization of resonantly interacting Fermi mixtures of 6Li and 53Cr atoms

We report on the realization of resonantly interacting Fermi mixtures of ⁶Li and ⁵³Cr atoms. 

Employing a set of 50 Feshbach resonances, observed in several spin combinations between 0G and 1500G, we develop an accurate quantum collisional model for our system. From this we derive scattering lengths and unobserved resonance locations for ⁶Li-⁵³Cr, and provide accurate predictions for the collisional properties of other Li-Cr isotopic pairs.

A set of strong s-wave resonances, essentially immune to two-body inelastic losses, are identified at about 1.4kG.

We measure the magnetic field width of one among these features to be about 0.5G, in agreement with our model prediction.

Exploiting the same resonance, we show resonant tuning of inter-species elastic interactions, characterized through radio-frequency spectroscopy protocols.

Finally, we discuss our all-optical route to reach simultaneous quantum degeneracy in our mixture, comprising about 2.2x10⁵ Li atoms at T/T_F=0.3 and 6x10⁴ Cr atoms at T/T_F=0.9, respectively.

Our work paves the way to the observation of novel few- and many-body phenomena in mass-imbalanced Fermi gases, and to the realization of ultracold samples of LiCr ground-state molecules featuring both electric and magnetic dipole moments.   

Controlling persistent currents in fermionic rings via phase imprinting

Here I report on our recent results on exciting persistent currents in ring-shaped atomic Fermi superfluids. By employing a phase imprinting technique, we populate finite and controllable circulation states throughout the BEC-BCS crossover. We make use of a second disk condensate as a local oscillator to extract the circulation state of the ring by means of an interferometric probe, which allows us to monitor the current in time. By increasing the number of phase imprinting pulses, we are able to deterministically populate high circulation states of the fermionic ring superfluids, which are observed to be as long lived as the atomic sample. Finally, we induce the current decay by inserting a defect inside the ring trap to trigger dissipation. In the BEC and BCS regime, the high circulation currents are observed to decay via the emission of vortices, in well agreement with Gross-Pitaevskii numerical simulation for the first case. On the other hand, in the UFG all the accessible circulation states are found to be robust against dissipation. Our work demonstrates phase imprinting as a powerful technique to control the circulation state of a superfluid, opening for further studies on superfluidity and dissipation of strongly-interacting fermions.   

Observation of Anomalous Decay in a Uniform Three-Component Fermi Gas

We investigate the stability of a three-component Fermi gas with contact interactions. Starting with a quasi-homogeneous ultracold gas in an optical box trap, we prepare a spin-1-like Fermi gas using the three lowest Zeeman levels of ⁶Li and observe the resulting decay of the spin populations over time. We observe competition between three-body recombination of free atoms and two-body-like inelastic collision of atoms and dimers. We report the first observation of losses in a spin-imbalanced, three-component mixture; the spin-imbalanced system shows surprising dynamics that cannot be described by a simple loss rate expression. We propose an empirically determined loss rate expression which describes both balanced and imbalanced mixtures.   

Optical Control of Interactions in an Energy-Space Spin Lattice

A spin lattice in energy-space is created by a weakly-interacting Fermi gas, which conserves the single-atom energy states over long time scales. Forward scattering produces effective long-range site-to-site couplings, which we control using optical fields. We demonstrate optical tuning of the site-to-site couplings by inducing spin waves over timescales of hundreds of milliseconds. This optically-controlled many-body system serves as a new platform for simulations of spin lattices and studies of information scrambling.   

A Diagrammatic Monte Carlo study of the resonant Fermi Polaron

The Fermi polaron is an impurity interacting with a sea of fermions. It is an exemplary system to study impurity problems, strongly imbalanced Fermi gases and quasiparticles. For strong interactions diagrammatic Monte Carlo provides a reliable tool to calculate various properties of the system quantitatively and from first principles. In my talk I will show how diagrammatic Monte Carlo, resummation techniques and numerical analytic continuation can be applied to obtain the spectral function of the resonant Fermi polaron, which can be measured in cold atoms experiments.   

Probing molecular spectral functions in ultracold Fermi gases via Raman spectroscopy

The detection of many strongly correlated phases remains a challenge in experimental physics. Here we present a theoretical proposal to directly detect the momentum resolved spectral function of molecular states in ultracold two-component Fermi mixtures, using conventional spectroscopic techniques in the quantum impurity limit. The detection of this quantity has remained so far elusive. We will then show how this new spectroscopic technique can be used to observe precursors of strongly correlated phases in Fermi mixtures, including possibly the FFLO phase, in excited states in the impurity limit. Our proposal will enable the detection of the dispersion relation and the excitation spectrum of composite particles dressed by a many-body environment. We further discuss possible experimental realizations.   

Tunneling dynamics of small two-component Fermi systems

Tunneling is an inherently quantum mechanical process. Interactions between particles can speed the single-particle tunneling dynamics up or slow it down; they can also trigger correlated tunneling of pairs or clusters. We will report on our theoretical work in progress on describing the tunneling dynamics of small two-component Fermi systems in an effectively one-dimensional trap. We use the imaginary time evolution to prepare the initial state and real time evolution to simulate the system dynamics. The particle flux out of the trap is monitored and tunneling rates are extracted. We will discuss the numerical techniques such as the alternating direction implicit two-dimensional Crank-Nicholson scheme, its efficiency, and initial results. Extensions to three dimensions (three one-dimensional particles) will be discussed.