Rewinding time with photons

We present and experimentally implement a universal rewinding protocol for two-level quantum systems. The protocol takes an unknown quantum state, evolving under an unknown Hamiltonian, and brings it back to its initial state. Unlike previous known time-translation protocols, ours can achieve an arbitrarily high probability of success, and is furthermore asymptotically optimal in the sense that it only requires O(T) amount of time to rewind the system by T units of time. Our experimental demonstration applies the protocol to qubits encoded in the polarization degree of photons, where the time evolution under a Hamiltonian for time ΔT is mimicked by applying the corresponding unitary operator. We compare our results to a classical strategy and find that the quantum protocol achieves a dramatically higher fidelity that is independent of choice of initial state, Hamiltonian and length of time to be rewound.