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 , 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 .
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.
 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)
 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)