Figure 1: (Top) Experimental scheme of the measurement of the energy of the Fermi polaron. For a free minority atom (left), the resonance condition for a transition between states $|g\rangle$ and $|e\rangle$ is obtained at a frequency $\nu$. When the atom is immersed in a Fermi sea of majority atoms (right), the energy of $|g\rangle$ is modified and the resonance frequency is shifted to a new value $\nu \prime$. The energy $E$ of the polaron is then simply obtained from $E=h(\nu -\nu \prime )$, where $h$ is Planck’s constant. (Bottom) Structure of an imbalanced Fermi cloud. The cloud is organized in concentric shells. The core (pink) is a superfluid with equal spin populations, while the rim (orange) is an ideal gas of majority atoms. The MIT group observes in addition an intermediate normal phase composed of a normal mixture of both spin species (blue).