QUROPE Quantum Information Processing and Communication in Europe

L. A. Sidorenkov, M. K. Tey, R. Grimm, Y-H Hou, L. Pitaevskii and S. Stringari
Nature 498, 78-81 (2013)

Superfluidity is a macroscopic quantum phenomenon occurring in systems as diverse as liquid helium and
neutron stars. It occurs below a critical temperature and leads to peculiar behaviour such as frictionless flow
and the formation of quantized vortices. Many finite-temperature superfluid phenomena can be explained in
terms of a two-fluid mixture comprising a normal component that behaves like a usual fluid, and a superfluid
component with zero viscosity and zero entropy. The two-component nature of a superfluid is manifest in
'second sound', an entropy wave in which the superfluid and the non-superfluid components oscillate with
opposite phases.

In their paper Sidorenkov and colleagues report the observation of second sound in a strongly interacting,
ultracold Fermi gas. This enables them to extract for the first time the temperature dependence of the
superfluid fraction, a previously inaccessible quantity. One the one hand their experimental results provide a
benchmark against which theories of strongly interacting quantum gases will have to measure up, while on
the other hand the experimental techniques developed may find applications for the study of dynamics and
transport in Fermi gases.