  We report on a gated single-photon detector based on InGaAs/InP avalanche
photodiodes (APDs) with a single-photon detection efficiency exceeding 55% at
1550 nm. Our detector is gated at 1 GHz and employs the self-differencing
technique for gate transient suppression. It can operate nearly dead time free,
except for the one clock cycle dead time intrinsic to self-differencing, and we
demonstrate a count rate of 500 Mcps. We present a careful analysis of the
optimal driving conditions of the APD measured with a dead time free detector
characterization setup. It is found that a shortened gate width of 360 ps
together with an increased driving signal amplitude and operation at higher
temperatures leads to improved performance of the detector. We achieve an
afterpulse probability of 7% at 50% detection efficiency with dead time free
measurement and a record efficiency for InGaAs/InP APDs of 55% at an afterpulse
probability of only 10.2% with a moderate dead time of 10 ns.
