Reversibility of Nonequilibrium Dynamics in a Unitary Fermi Gas
We theoretically describe and experimentally observe the reversibility of a nonequilibrium state in a unitary Fermi gas confined in a time-dependent trap. Unlike the equilibrium case, reversibility out of equilibrium imposes a stringent condition: the accumulated dynamical phase must be an integer multiple of $2π$, i.e., $Δϕ= 2ω_{0} s(T) = 2nπ$, where $ω_{0}$ is the initial trapping frequency and $s(T)$ is the rescaled time after a total evolution time $T$. We demonstrate the reversibility of an initial nonequilibrium state in an isotropic unitary Fermi gas by driving the optical trap through a round trip. We find two types of reversible pathways, the space-time scaling-invariant and polynomial shortcut-to-adiabaticity (STA) protocols. The phase condition restricts the round-trip time $T$ to discrete values, a distinguishing feature of nonequilibrium dynamics. Our work establishes a clear criterion for the quantum control of nonequilibrium many-body systems beyond linear response and provides practical pathways to reversibility.