Floquet Time Crystals As Quantum Sensors of Ac Fields

We discuss the performance of discrete time crystals (DTCs) as quantum sensors. The long-range spatial and time ordering displayed by DTCs, leads to an exponentially slow heating, turning DTC into advantageous sensors. Specifically, their performance (determined by the quantum Fisher information) to estimate AC fields can overcome the shot-noise limit while allowing for extremely long time sensing protocols. Since the collective response of the many-body interactions stabilizes the DTC dynamics against noise, these sensors become moreover robust to imperfections in the protocol. The performance of such a sensor can also be used in a dual role to probe the presence or absence of a many-body localized phase.