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The Japan Society of Applied Physics

Entangled photon pair detection by SSPDs with a SFQ coincidence circuit

Rintaro Fujimoto, Shoichi Murakami, Rikizo Ikuta, Toshiki Kobayashi, Shigehito Miki, Shigeyuki Miyajima, Masahiro Yabuno, Hirotaka Terai, Nobuyuki Imoto, Takashi Yamamoto

Published Jan 26, 2021• 0 citations
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We present a single-flux-quantum (SFQ) based digital correlator to trace independent signals from two superconducting single-photon detectors (SSPDs) triggering its inputs. In our design two SSPDs are magnetically coupled to inputs of a readout system where direct current (DC)-to-SFQ converters are used to convert transient SSPD output pulses, triggered by detection of single-photon events, to SFQ pulses. The coincidence verification of SFQ pulses, generated by the two DC-to-SFQ converters, is performed with a modified SFQ coincidence buffer. The coincidence buffer is designed to generate an SFQ output pulse only when its both inputs are triggered simultaneously, or within a preset margin time. The output of the coincidence buffer is connected via, this time, an SFQ-to-DC converter, to a pulse counter operated at room temperature. We performed extensive simulations of both the SSPD equivalent circuit and correlator redout elements for the proposed coincidence scheme, using a WRSpice and PSCAN2 simulation platforms that are specifically designated to model Josephson junctions and widely used to simulate operation of the SFQ circuitry. In particular, we investigated our coincidence correlator scheme for measurements of the second-order correlation function, used to demonstrate the antibunching effect in the single-photon detection of non-classical light.
We report on the timing jitter characterization of the superconducting single flux quantum (SFQ) coincidence circuit, which is an essential component of the superconducting coincidence photon counter. Two superconducting nanowire single photon detectors (SSPDs), each of which is irradiated with optically time-controlled photons, are connected to the SFQ coincidence circuit, and the timing jitter of the SFQ circuit is evaluated by changing the relative time delay between two input ports for the SFQ comparator unit. We successfully observe the transition curve of the probability of obtaining the signal from the SFQ coincidence circuit by sweeping photon arrival time to each SSPD and confirm that this curve shifts temporally upon changing the bias current to the Josephson transmission line (JTL) in the SFQ circuit. A systematic investigation reveals that the relation between time delay and the bias current to JTL can be estimated. The full width half maximum timing jitter of the SFQ circuit is 1.1 ps, which is sufficiently low so that it does not influence the entire timing jitter of the coincidence photon counter.
We demonstrate the operation of a coincidence photon detector with a short timing jitter consisting of two superconducting nanowire single photon detectors (SSPDs) and a single flux quantum (SFQ) circuit. By utilizing the timing discrimination capability of the SFQ coincidence circuit, the full width at half maximum timing jitter of the entire coincidence photon detector was evaluated as 32.3 ps, which is 36 ps less than that of the standard commercial time correlated single photon counting module, and the timing jitter of the SSPD was estimated as ∼15 ps. Owing to the short timing jitter characteristics, our coincidence photon detector could correctly capture the effect of pulse-width broadening by insertion of an optical bandpass filter. We have also demonstrated that our coincidence photon detection clearly shows Hong-Ou-Mandel interference with a weak coherent pulse. These results are a crucial step to realizing high timing resolution coincidence measurements, ushering in a technology for timing measurement based multi-photon quantum interference.