What is the role of the quantum efficiency of a microwave photon detector in quantum network applications?

The quantum efficiency of a microwave photon detector in quantum network applications measures the probability that an incoming microwave photon is successfully detected, and it directly determines the performance of quantum communication protocols that rely on detecting individual microwave photons. The quantum efficiency eta = (number of detected photons) / (number of incident photons). For quantum network applications: the detector must distinguish between 0 and 1 photons in a microwave pulse (single-photon detection). The quantum efficiency determines: the communication success probability (in a quantum network: two nodes exchange entangled microwave photons through a cable or waveguide; the probability of successfully establishing entanglement is proportional to eta × eta (both detectors must fire)), the protocol overhead (lower eta requires more attempts to establish entanglement; the number of attempts scales as 1/eta^2 for two-detector protocols), and the achievable quantum state fidelity (dark counts (false detections) dilute the entangled state fidelity; the signal-to-dark-count ratio depends on eta). Current state of the art: microwave single-photon detectors are much less mature than optical single-photon detectors. Types: qubit-based detector (a transmon qubit absorbs the photon and is measured; eta approximately 50-90% demonstrated in labs), JPA-based detector (a JPA amplifies the photon to a measurable level; detection efficiency limited by the amplifier's added noise), and bolometric detector (measures the energy deposited by the photon; very slow response but potentially high efficiency).