  Afterpulsing noise in InGaAs/InP single photon avalanche photodiodes (APDs)
is caused by carrier trapping and can be suppressed successfully through
limiting the avalanche charge via sub-nanosecond gating. Detection of faint
avalanches requires an electronic circuit that is able to effectively remove
the gate-induced capacitive response while keeping photon signals intact. Here
we demonstrate a novel ultra-narrowband interference circuit (UNIC) that can
reject the capacitive response by up to 80 dB per stage with little distortion
to avalanche signals. Cascading two UNIC's in a readout circuit, we were able
to enable high count rate of up to 700 MC/s and low afterpulsing of 0.5 % at a
detection efficiency of 25.3 % for 1.25 GHz sinusoidally gated InGaAs/InP APDs.
At -30 degree C, we measured 1 % afterpulsing at a detection efficiency of 21.2
%.
