  This paper presents a detailed investigation of superconducting nanowire
single-photon detectors (SNSPDs) biased with microwave and direct currents. We
developed a hybrid detector, which allows the operation in the rf and dc
operation mode. With this hybrid detector, we are able to compare the count
rates of the same nanowire biased with dc and rf currents. Furthermore, we
demonstrate the use of the oscillating current in the rf operation mode as a
reference signal in a synchronous single-photon detection mode.
