Balanced, gated-mode photon detector for quantum-bit discrimination at 1550 nm

A photon detector that combines two avalanche photodiodes (APDs) has been demonstrated for quantum-bit discrimination at 1550 nm. Spikes accompanied by signals in a gated mode were canceled by balanced output from the two APDs. The spike cancellation enabled one to reduce the threshold in the discriminators and thus the gate pulse voltage. The dark count probability and afterpulse probability were reduced to $7\times {10}^{-7}$ and $7\times {10}^{-4}$, respectively, without affecting the detection efficiency (11%) at 178 K.