Our Experiments

Emergent s-wave interactions in quasi-low-dimensional Fermi gases

We investigate the scattering properties of a quasi-one-dimensional (q1D) and quasi-two-dimensional (q2D) systems of spin-polarized Fermi gases near a p-wave Feshbach resonance. Strong confinement creates these quasi-low-dimensional spaces by creating a large energy gap between any 1D or 2D excitations and any excitations into higher dimensions. This also means that when activated, excitations in the strongly confined direction are discrete, and can be considered a new "orbital" degree of freedom.

In this scenario, we discover a new phenomenon: a low-energy scattering channel that has symmetric particle-exchange parity in the low-dimensional subspace, as if the underlying interactions were s-wave! This is normally forbidden to identical fermions, because their exchange symmetry must be odd, whereas s-waves, d-waves, etc have even parity. 

Employing radiofrequency (rf) spectroscopy, we observe both power-law scalings and dimensional-crossover features that are indicative of emergent s-wave interaction channels. In a q1D system [1], we measure both the odd-wave and even-wave “contact” parameters for the first time. In a q2D gas [2], we demonstrate the formation of two types of low-energy dimers, possessing either s-wave or p-wave symmetry, through rf spin-flip association from an orbital mixture. Our observations are compared to predictions from new scattering models that include the full range of possible pair wave functions in q1D and q2D [3]. 

These findings underscore how gapped orbital degrees of freedom can control scattering symmetry in strongly confined ultracold gases.

[1] K. G. Jackson, C. J. Dale, J. Maki, K. G. S. Xie, B. A. Olsen, D. J. M. Ahmed-Braun, Shizhong Zhang, J. H. Thywissen

Emergent s-wave interactions between identical fermions in quasi-one-dimensional geometries

Phys. Rev. X 13, 021013 (2023) [doi: 10.1103/PhysRevX.13.021013]

[2] C. J. Dale, K. G. S. Xie, K. Pond Grehan, Shizhong Zhang, J. Maki, J. H. Thywissen

Emergent s-wave interactions in orbitally active quasi-two-dimensional Fermi gases

Phys. Rev. A 110, L051302 (2024) [doi: 10.1103/PhysRevA.110.L051302]

[3] J. Maki, C. J. Dale, J. H. Thywissen, Shizhong Zhang

Radio-frequency spectroscopy and the dimensional crossover in interacting spin-polarized Fermi gases

Phys. Rev. A 110, 053314 (2024) [doi: 10.1103/PhysRevA.110.053314]

Observation of unitary p-wave interactions between fermions in an optical lattice

The realisation of exchange-antisymmetric pair wavefunctions in controllable quantum systems, such as ultracold gases, could enable new types of quantum simulations, topological quantum gates, and exotic few-body states. However, p-wave and other antisymmetric interactions are weak in naturally occurring systems, and their enhancement via Feshbach resonances in ultracold systems has been limited by three-body loss. Here we create isolated pairs of spin-polarised fermionic atoms in a multi-orbital three-dimensional optical lattice. We spectroscopically measure elastic p-wave interaction energies of strongly interacting pairs of atoms near a magnetic Feshbach resonance and find pair lifetimes to be up to fifty times larger than in free space. We demonstrate that on-site interaction strengths can be widely tuned by the magnetic field and confinement strength but collapse onto a universal single-parameter curve when rescaled by the harmonic energy and length scales of a single lattice site. Since three-body processes are absent within our approach, we are able to observe elastic unitary p-wave interactions for the first time. We take the first steps towards coherent temporal control via Rabi oscillations between free-atom and interacting-pair states. All experimental observations are compared both to an exact solution for two harmonically confined atoms interacting via a p-wave pseudopotential, and to numerical solutions using an ab-initio interaction potential. The understanding and control of on-site p-wave interactions provides a necessary component for the assembly of multi-orbital lattice models, and a starting point for investigations of how to protect such a system from three-body recombination even in the presence of tunnelling.

V. Venu, P. Xu, M. Mamaev, F. Corapi, T. Bilitewski, J. P. D'Incao, C. J. Fujiwara, A. M. Rey,  J. H. Thywissen

Unitary p-wave interactions between fermions in an optical lattice

Nature 613, 262–267 (2023)

[doi: 10.1038/s41586-022-05405-6][Full-text link][UofT press release][JILA press release]

Probing open and closed channel p-wave resonances

We study the interplay between a p-wave Feshbach resonance and an open channel shape resonance using a variety of theoretical techniques and experimental measurements. Good agreement between our rf spectra (above in blue) and theoretical lineshape theory (red lines) allow us to precisely characterize this resonance. The cartoons (b,d,f,h) show the levels involved in resonant association (RA) and spin flip association (SFA)  spectroscopy for both bound and quasi-bound states. We find excellent agreement with coupled-channels calculations and develop a simplified two-channel model that takes into account the open channel shape resonance, the Feshbach resonance, and weak dipole-dipole interactions.

Ahmed-Braun et al., Physical Review Research 3, 033269 (2021)
[doi: 10.1103/PhysRevResearch.3.033269]

Transition between dynamical phases

We have observed a phase transition between two dynamical phases in a harmonically trapped ultracold Fermi gas. This transition is between a paramagnet like state (A), in which the total magnetization of the trapped atoms decreases quickly, and a dynamical ferromagnet state (C-E) where the magnetization is protected by the opening of an energy gap. The agreement between our data and theory (red lines) validates the mapping between our harmonically trapped atoms and the collective Heisenberg spin model on a lattice composed of the harmonic oscillator modes.

Smale et al., Science Advances, 5, eaax1568 (2019)
[doi:10.1126/sciadv.aax1568] [journal link]

Measuring a.c. conductivity in an optical lattice

Using a.c. conductivity spectra for ultracold potassium 40 in a cubic lattice, we find the spectral weight at low frequency. For variable number (N), deposited heat (Q), scattering length (as), and lattice depth (V), the data collapses onto the expected response of a single-band Hubbard model. At high T/t (temperature divided by tunnelling energy), the data approaches a 1/T regime (dashed black line). Inset: Scattering does not "destroy" conductivity, but only moves it from one part of the spectrum to another, leaving spectral weight and band mass relatively unchanged. 

Anderson et al., Physical Review Letters, 122, 153602 (2019)
[doi: 10.1103/PhysRevLett.122.153602] [journal link]

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