A balanced gated-mode photon detector for qubit discrimination in 1550 nm

A photon detector combining the two avalanche photon diodes (APD) has been demonstrated for qubit discrimination in 1550 nm. Spikes accompanied with the signals in 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 7×10 and 10, respectively, without affecting the detection efficiency (11 %) at 178 K. One of the key devices for the optical implementation of quantum information technology is a photon detector to determine the quantum states, or to discriminate qubits. In discriminating qubits, we often split the photons according to the state (|0〉 or |1〉) and detect them. For example, qubits encoded on polarization states can be discriminated by a polarization beam splitter followed by two photon detectors. Phase information will be also obtained by detecting photons at the two output ports of a Mach-Zehnder interferometer. We can find application of the qubit discrimination in quantum key distribution (QKD) experiments[1, 2, 3, 4]. The use of two identical photon detectors is necessary for such demonstrations. Though a single photon detector can act as two detectors by the use of time division multiplexing technique[1], this will increase the loss in the detectors. Once we admit the requirement of two detectors, we should make the most use of them. We show, in the following, the improved detector performance by taking the differential signals of the two photon detectors. The photon detectors should show high detection efficiency, low dark counts, and short response time. The ratio of the detection efficiency η to the dark count probability Pd determines the error rate, and so the range of QKD transmission[2, 5, 6, 7]. The ratio Pd/ηshould be less than 10 −3 for 100 km fiber transmission in 1550 nm even with an ideal single photon source. Clock