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Single-photon detectors are semiconductor devices sensitive enough to detect individual photons - the smallest possible units of light. They're essential for quantum key distribution where information is encoded in single photons. The detector is typically an avalanche photodiode (APD) operated at high voltage. When a single photon hits it, it triggers an "avalanche" of current you can measure. The challenge: these detectors have "dark counts" (false triggers from random noise) and "dead time" (a recovery period after each detection where they can't detect). To reduce dark counts, detectors are cooled - sometimes to -90�C using thermoelectric coolers, or to 4 Kelvin using cryogenic systems. Different semiconductor materials detect different wavelengths: silicon for visible light, indium gallium arsenide (InGaAs) for the 1550nm telecom wavelength used in fiber networks. Detection efficiency is typically 10-50%, meaning many photons go undetected - this is accounted for in QKD security proofs.
Single-photon detectors are the technological bottleneck in QKD - they limit speed, distance, and cost. Current research focuses on superconducting nanowire single-photon detectors (SNSPDs) with 90%+ efficiency, microsecond recovery time, and extremely low dark counts. These enable longer distance, higher key rates, and better security margins. As detector technology improves, QKD becomes more practical and affordable.
For QNu Labs' Armos, detector choice determines system performance. Better detectors mean longer secure distance, higher key generation rates, and fewer false alarms. They also affect cost - cryogenic detectors are expensive but perform better, while thermoelectric cooling is cheaper but limits performance. The engineering tradeoff balances security, performance, and cost for commercial deployment.
Quantum key distribution receiver systems, quantum communication receivers, quantum random number generators using photon detection, quantum computing readout systems, time-of-flight measurements, photon timing applications, quantum sensing systems
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