Publication
Ultracold gases provide a controlled environment that is ideal for studying many intriguing phenomena associated with quantum correlated systems. Current efforts are directed towards the identification of magnetic properties, as well as the creation and detection of exotic quantum phases. In this context, a mapping of the spin polarization of the atoms to the state of a single-mode light beam has been proposed. Here we introduce a quantum-limited interferometer that realizes such an atom–light interface with high spatial resolution. We measure the probability distribution of the local spin polarization in a trapped Fermi gas, showing a reduction of spin fluctuations by up to 4.6(3) dB below shot noise in weakly interacting Fermi gases, and by 9.4(8) dB for strong interactions. We deduce the magnetic susceptibility as a function of temperature and discuss our measurements in terms of an entanglement witness.