Single-Photon Detectors

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Photon counting modules with high efficiencies and low error probabilities are absolutely essential for optical quantum computing, scalable quantum information protocols, and fundamental physical tests (e.g., loophole-free Bell inequality experiments). Conventional single-photon detectors, such as avalanche photodiodes (APDs) and photomultiplier tubes (PMTs), only have quantum efficiencies of up to 75% and 10-20%, respectively. In contrast, VLPCs (Visible Light Photon Counters) and SSPMs (Solid-state Photomultipliers) are solid-state devices that utilize an avalanche multiplication effect, differing (from each other) only in their wavelength sensitivities. VLPCs are sensitive only in the visible range, while the SSPMs are sensitive from the visible to beyond 10 microns (excluding a range around microns.

Both these detectors feature unique capabilities that APDs and PMTs cannot offer, including high quantum efficiency, and low pulse-height dispersion, leading to multi-photon counting capability. The VLPCs have an inferred internal quantum efficiency of 94% +/- 5% (at 694 nm) and SSPMs 96% +/-3% (at 660 nm). However, the actual measured values for both the detectors were in fact limited to less than 88% due to in-coupling losses. We are currently improving these detectors by a) using custom anti-reflection (AR) coatings for the detectors, b) reducing losses and background due to the optical coupling fibers, c) using improved low-noise electronics, and d) incorporating novel cryogenic cooling designs. With these improvements, we anticipate actual detection efficiencies in excess of 90%.