Many-body spin echo

We show that quantum coherence produces an observable many-body signature in the dynamics of few-fermion Hubbard systems (describing cold atoms in optical lattices, coupled quantum dots, or small molecules) in the form of a revival in the transition probabilities echoing a flip of the system's itinerant spins. Contrary to its single-particle (Hahn) version, this many-body spin echo is not dephased by strong interactions or spin-orbit coupling, and constitutes a benchmark of genuine many-body coherence. A physical picture that allows for the analytical study of this nonperturbative effect is provided by a semiclassical approach in Fock space, where coherence arises from interfering amplitudes associated with multiple chaotic mean-field solutions with action degeneracies due to antiunitary symmetries. The analytical predictions resulting from our semiclassical approach are in excellent quantitative agreement with corresponding numerical simulations. The latter, moreover, confirm that the shape of the echo profile is independent of the interaction, while its amplitude and sign universally depend only on the number of flipped spins and the spin-orbit coupling phase.