Physicists love to study interacting fermions. Not only are such studies practical (for instance to understand materials), but they also address open questions about our world from the highest observed energy scales (at CERN) to the lowest achieved energy scales (with ultracold atoms). Astrophysical objects (such as neutron stars) and primordial states of matter (quark-gluon plasmas) also consist of strongly correlated fermions.
In this colloquium, I will discuss how ultracold neutral Fermi gases are used to probe many-body dynamics and transport. We have a remarkable degree of control: by changing the ambient magnetic field, we can tune the interaction strength, near a so-called Feshbach resonance. When s-wave collisions are tuned to resonance (called the "unitary" limit), the s-wave scattering length diverges, leaving a scale-invariant system. It is well understood that thermodynamics are "universal" in this regime, i.e., that all unitary Fermi gases have the same equation of state, no matter what the origin of the inter-particle interaction.
Still under debate are transport properties, although they too should be universal. We have recently measured spin transport in a unitary Fermi gas. The starting point of our measurements is a transversely spin-polarised gas, where each atom is in a superposition of the lowest two Zeeman eigenstates. In the presence of an external field gradient, a spin texture develops across the cloud, which drives diffusive spin currents. The slow diffusive relaxation reveals strong scattering in the unitary gas, such that the inferred value of the transport lifetime is near a conjectured quantum limit.