Abstract
This paper describes the construction of a cryostat and an optical system with a free-space coupling efficiency of 56.5% ± 3.4% to a superconducting nanowire single-photon detector (SNSPD) for infrared quantum communication and spectrum analysis. A 1K pot decreases the base temperature to T = 1.7 K from the 2.9 K reached by the cold head cooled by a pulse-tube cryocooler. The minimum spot size coupled to the detector chip was 6.6 ± 0.11 µm starting from a fiber source at wavelength, λ = 1.55 µm. We demonstrated photon counting on a detector with an 8 × 7.3 µm2 area. We measured a dark count rate of 95 ± 3.35 kcps and a system detection efficiency of 1.64% ± 0.13%. We explain the key steps that are required to improve further the coupling efficiency.
Highlights
Free-space quantum optical communication in the mid-infrared [1] is an important technology for applications such as naval operations that cannot rely on optical fibers
This demonstration was achieved not by increasing the devices detection efficiency but by maximizing the coupling efficiency, i.e. the fraction of photons emitted by the source that are coupled to the superconducting nanowire single-photon detector (SNSPD)
Free-space coupling demonstration In Section 4 we proved that we could couple near-IR light with free-space optics on an 8 × 7.3 μm2 area NbN SNSPD based on 100 nm wide nanowires
Summary
Free-space quantum optical communication in the mid-infrared (mid-IR) [1] is an important technology for applications such as naval operations that cannot rely on optical fibers. To ensure high coupling efficiency, the minimum dimension of the detector has to be larger than the beam diameter; this dimension is limited This requirement becomes a non-trivial issue for experiments in the mid-IR, which require optical fibers with a mode-field diameter MFD > 20 μm versus the typical active area of an SNSPD which is ≤ 15×15 μm. We propose here a cryogenic set-up for superconducting single-photon detectors built to achieve high-efficiency (> 50%) free-space coupling. The use of two separate optical systems allowed us to verify that Lens 1 was at the focal distance from the SNSPD chip; in particular, we were able to verify that the image was focused on the same plane where the beam from Source 2 was focused
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