Single-Flux Quantum Digital Electronics Readout for Superconductor Nanostripe Single-Photon Detectors

Superconductor nanostripe single-photon detectors (SNSPDs) [1] are currently the most popular research-class single-photon detectors, since they exhibit near-unit quantum efficiency, up to gigahertz count rates, picosecond jitter, sub-hertz dark counts, and a relatively high operating temperature (2-4 K). However, single-pixel meander-type SNSPDs typically operate in a threshold mode as simple photon counters and implementing advanced functionalities is challenging. We report here a concept of digitally-assisted SNSPDs, where a photon sensing element is directly integrated with a Josephson-junction–based single-flux quantum (SFQ) digital circuitry to provide processing, readout, or even timing and control of the detector. Depending on a type of the SFQ circuit used, they can offer enhanced digital processing capabilities such as, e.g., directly compute higher-order photon correlation functions for characterization of single-photon sources to test the purity and indistinguishability of single-photon sources. Another area of growing interest is the SNSPD photon-number or photon-energy resolving capability for such fields of quantum measurement and quantum metrology. Finally, on-chip extraction of a time stamp of the photon detection event is critical in the quantum key distribution (quantum cryptography) and security verification systems, and, additionally, in biological applications, such as time-correlated fluorescence spectroscopy. The merge of SNSPDs with the SFQ circuitry is a natural choice, since they are both superconductor-based cryogenic technologies and can be relatively easily integrated via, e.g., flip-chip technology.