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Density-wave ordering in a unitary Fermi gas with photon-mediated interactions

By Jean-Philippe, EPFL

The distinctive feature of cold atomic gases is the ability to control the interactions between particles at will. This allows to program a quantum gas to induce a wide variety of quantum phases, from superfluids or insulators to more exotic such as supersolids. In our work [1], we demonstrate the realization of doubly tunable Fermi gas, featuring simultaneous and independent control over both short range and long-range, all-to-all interaction induced by photons in a cavity. We show that the photon-induced interactions drive the system towards a density-wave ordered state, offering a fully controlled example of the interplay of charge-density wave and superfluid orders.

When two atoms are placed in a high-finesse cavity, a photon emitted by one atom is likely to be channeled in the cavity mode, such that the other atom has the possibility to scatter it as well. This leads to a mechanism photon-exchange between atoms, effectively creating an interaction. The strength of this interaction is controlled by the rate at which atoms can scatter photons in cavity: by illuminating the atoms from the side with more or less power, this rate can be tuned up or down, providing complete control of the interaction strength [2].

Artist view illustrating a gas of lithium atoms inside a high-finesse cavity, and subject to a transverse illumination by a laser beam. For strong enough illumination, the gas forms a macroscopic density wave pattern originating from photon exchanges between atoms via the cavity © Harald Ritsch

We have mapped out the phase diagram of the Fermi gas as a function of the strength of both the short- and long-range interactions and found a regime of strong long-range interactions where a density wave ordered phase emerges, regardless of the short-range interaction strength. We also observed the precursors of the ordering in the form of a strongly enhanced density-wave susceptibility even without ordering.

There are a lot of questions emerging from the observation of this new system: is the superfluid order suppressed when the system undergoes density-wave ordering, or does it form a new type of layered superfluid ? How does the order forms dynamically, what are the time-scales and lengths scales associated to it ? How far do the concepts developed in the solid-state context for complex superconductors apply to this situation ? A new generation of theoretical and experimental studies will try to answer these questions in the near future.

[1] V. Helson, T. Zwettler, F. Mivehvar, E.Collela, K.Roux, H. Konishi, H. Ritsch, J.P. Brantut
Density-wave ordering in a unitary Fermi gas with photon-mediated interactions
Nature (in press – available here)

[2] F. Mivehvar, F. Piazza, T. Donner and H. Ritsch
Cavity QED with quantum gases: new paradigms in many-body physics
Advances in Physics 70 1-153 (2021)

Based on article published in Nature

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