Single-photon detectors have achieved impressive performance, and have led to a number of new scientific discoveries and technological applications. Existing models of photodetectors are semiclassical in that the field-matter interaction is treated perturbatively and time-separated from physical processes in the absorbing matter. An open question is whether a fully quantum detector, whereby the optical field, the optical absorption, and the amplification are considered as one quantum system, could have improved performance. Here we develop a theoretical model of such photodetectors and employ simulations to reveal the critical role played by quantum coherence and amplification backaction in dictating the performance. We show that coherence and backaction lead to tradeoffs between detector metrics, and also determine optimal system designs through control of the quantum-classical interface. Importantly, we establish the design parameters that result in a perfect photodetector with 100% efficiency, no dark counts, and minimal jitter, thus paving the route for next generation detectors.
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