Watching fermions step in sync

Superfluids are remarkable substances that can flow endlessly without dissipating energy. As such, their response to certain disturbances can be very different to the response of a normal fluid. In systems made up of fermionic constituents (particles with half-integer spin), superfluidity is generally accompanied by the formation of a Bose-Einstein condensate of fermion pairs, yet observing this pair condensation, particularly in strongly interacting superfluids, has long proven challenging. Here, we present a new technique of spatially resolved Bragg spectroscopy, a special kind of disturbance, which has allowed us to reveal a strong local signature of pair condensation. We have used this technique on an ultracold Fermi gas with resonant interactions to measure the homogeneous density-density response function and seen the dramatic change in the basic excitations when the gas is cooled across the superfluid transition. A special feature of our experiments is the way we determine the local Bragg response. As is often the case, our atom clouds are confined in a harmonic potential which means the atomic density varies with position. Standard imaging techniques necessarily integrate over regions with widely varying densities making the identification of local features, such as the precise conditions at a phase boundary, very difficult. Our protocol for obtaining homogeneous parameters, based on measuring and differentiating 1D data, overcomes this limitation. It is easily adapted to finding other parameters and can work in situations where other techniques fail, opening the way to new types of measurement in trapped quantum gases.

The work has been published in Physical Review Letters, and has been selected as an Editor's Suggestion.

Link to paper: M. Lingham et al., Phys. Rev. Lett. 112, 100404 (2014)