  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 7x10^-7 and 10^-4, respectively, without affecting
the detection efficiency (11 %) at 178 K.
