Quantum phase sensing with states out of thermal equilibrium

Abstract

Interferometry can be viewed generally as the measurement of a relative phase between two subsystems. I consider the problem of interfering a quantum resource state with a thermal bath, drawing a precise connection between the athermality of the resource and the resulting phase sensitivity. This is done by finding the fundamental sensing precision limit under the minimal conditions of global unitarity and energy conservation. The results here apply both to general finite-dimensional systems and to linear quantum optics. The same techniques further upper-bound the speed at which a system and bath can jointly evolve under an energy-conserving interaction.