  High-frequency gating InGaAs/InP single-photon detectors (SPDs) are widely
used for applications requiring single-photon detection in the near-infrared
region such as quantum key distribution. Reducing SPD size is highly desired
for practical use, which is favorable to the implementation of further system
integration. Here we present, to the best of our knowledge, the most compact
high-frequency sine wave gating (SWG) InGaAs/InP SPD. We design and fabricate
an InGaAs/InP single-photon avalanche diode (SPAD) with optimized semiconductor
structure, and then encapsulate the SPAD chip and a mini-thermoelectric cooler
inside a butterfly package with a size of 12.5 mm $\times$ 22 mm $\times$ 10
mm. Moreover, we implement a monolithic readout circuit for the SWG SPD in
order to replace the quenching electronics that is previously designed with
board-level integration. Finally, the components of SPAD, monolithic readout
circuit and the affiliated circuits are integrated into a single module with a
size of 13 cm $\times$ 8 cm $\times$ 4 cm. Compared with the 1.25 GHz SWG
InGaAs/InP SPD module (25 cm $\times$ 10 cm $\times$ 33 cm) designed in 2012,
the volume of our miniaturized SPD is reduced by 95\%. After the
characterization, the SPD exhibits excellent performance with a photon
detection efficiency of 30\%, a dark count rate of 2.0 kcps and an afterpulse
probability of 8.8\% under the conditions of 1.25 GHz gating rate, 100 ns
hold-off time and 243 K. Also, we perform the stability test over one week, and
the results show the high reliability of the miniaturized SPD module.
