Temporally non-local effects in optical detection

In the quantum mechanical world every measurement performed on a system causes a curios non-classical effect, the so-called backaction. The backaction reflects our updated knowledge of the system’s state post-measurement, and in the case of a two-level system being monitored by a single photodetector, the detection of a photon causes the system to collapse on its ground state. The monitoring of quantum systems via photodetection is vital in current and emerging quantum technologies, allowing for, e.g., high precision estimation of system parameters, quantum feedback control, and heralding of certain system events of interest.

Conventional measurement theory in quantum optics neglects any ‘trivial’ distance between the observed system and the monitoring detector setup. In this work we reveal the impact of such signal delays on our understanding of the measured system, and we demonstrate how the mere addition of a Michelson-like interferometer in the measurement scheme destroys any direct relation between the measured detector signal and the emitter system state, causing photons detected at the detectors to be in a superposition of several distinct emission times. We analyze these non-trivial signal delays through both quantum trajectory simulations and correlation functions.