Fiber-coupled NbN superconducting single-photon detectors for quantum correlation measurements

Abstract

We have fabricated fiber-coupled superconducting single-photon detectors (SSPDs), designed for quantum-correlationtype experiments. The SSPDs are nanostructured (~100-nm wide and 4-nm thick) NbN superconducting meandering stripes, operated in the 2 to 4.2 K temperature range, and known for ultrafast and efficient detection of visible to nearinfrared photons with almost negligible dark counts. Our latest devices are pigtailed structures with coupling between the SSPD structure and a single-mode optical fiber achieved using a micromechanical photoresist ring placed directly over the meander. The above arrangement withstands repetitive thermal cycling between liquid helium and room temperature, and we can reach the coupling efficiency of up to ~33%. The system quantum efficiency, measured as the ratio of the photons counted by SSPD to the total number of photons coupled into the fiber, in our early devices was found to be around 0.3 % and 1% for 1.55 µm and 0.9 µm photon wavelengths, respectively. The photon counting rate exceeded 250 MHz. The receiver with two SSPDs, each individually biased, was placed inside a transport, 60-liter liquid helium Dewar, assuring uninterrupted operation for over 2 months. Since the receiver's optical and electrical connections are at room temperature, the set-up is suitable for any applications, where single-photon counting capability and fast count rates are desired. In our case, it was implemented for photon correlation experiments. The receiver response time, measured as a second-order photon cross-correlation function, was found to be below 400 ps, with timing jitter of less than 40 ps.

Multimode Fiber Coupled Superconductor Nanowire Single-Photon Detector

“…However, the single-mode fiber coupled SNSPD restricted the viable applications [12], [13]. In the application of fluorescence detection [14], it is difficult to collect the light into a single mode fiber that has a spot ð$9 mÞ and low divergence angle in practical applications.…”

Review of Devices, Packaging, and Materials for Cryogenic Optoelectronics

“…Although most devices described here use single-mode fibers, multimode optical fibers are used in some applications: to achieve coupling of visible light from free space, a 50-mm multimode fiber terminated with a gradient index (GRIN) lens was coupled to an SNSPD in [40]. Compared with a coupling by a single-mode fiber, multimode fibers result in higher jitter in SNSPDs [40,41], caused by modal dispersion of light pulses due to the difference in propagation velocities among modes.…”

“…In addition, direct coupling between optical fiber and detectors, bonded with adhesives, has been demonstrated at cryogenic temperatures for devices, such as photoconductive switches [43,44], MSM photodiodes [37], and SNSPDs [8,41].…”

“…Alignment of the single-mode and multimode optical fibers to small-area (10 mm 3 10 mm) SNSPD with a micromechanical photoresist ring is described in [41,60]. The ring is aligned with marks in the substrate allowing better than 1-mm alignment of the fiber, as pictured in Fig.…”