Quantum Metrology via Repeated Quantum Nondemolition Measurements in "Photon Box"

In quantum metrology schemes, one generally needs to prepare m copies of N entangled particles, such as entangled photon states, and then they are detected in a destructive process to estimate an unknown parameter. Here, we present a novel experimental scheme for estimating this parameter by using repeated indirect quantum nondemolition measurements in the setup called "photon box". This interaction-based scheme is able to achieve the phase sensitivity scaling as 1/N with a Fock state of N photons. Moreover, we only need to prepare one initial N-photon state and it can be used repetitively for m trials of measurements. This new scheme is shown to sustain the quantum advantage for a much longer time than the damping time of Fock state and be more robust than the common strategy with exotic entangled states. To overcome the 2π/N periodic error in the estimation of the true parameter, we can employ a cascaded strategy by adding a real-time feedback interferometric layout.