Quest towards ultimate performance in superconducting nanowire single photon detectors

Abstract

Citation:S. Miki, Quest towards ultimate performance in superconducting nanowire single photon detectors, Sci. China-Phys. Mech. Astron. 61, 020331 (2018), https://doi.org/10.1007/s11433-017- In 2007, superconducting nanowire single photon detectors (SSPD or SNSPD) [1] made an outstanding impact in the field of quantum information technology by demonstrating quantum key distribution (QKD) over a 200-km optical fiber with a 42-dB optical loss using a practical SNSPD system [2]. This successful demonstration was realized thanks to its extremely low dark count rate (DCR) of a few Hz and short timing jitter of 60 ps, while the system detection efficiency (SDE) showed a poor value of 0.7% at a wavelength of 1550 nm. Afterwards, significant efforts have been devoted to the improvement of SDE in practical systems, and remarkable advancements have been continually achieved in the past decade. The SDE of SNSPD is primarily determined by the product of three factors: optical coupling efficiency (η oc ), absorption efficiency (η abs ), and intrinsic detection efficiency (η int ). Therefore, the advancements to increase each factor cumulatively resulted in the improvement of SDE.In the initial stage of the SNSPD research, the nanowire device consists only of an NbN superconducting nanowire on the substrate. In this case, the nanowire cannot absorb the incident photons efficiently, because the thickness of the superconducting nanowire is only few nanometers. To achieve high η abs , Rosfjord et al. [3] demonstrated an integrated λ/4 optical-cavity structure on the superconducting nanowire, and this approach could be successfully adapted in practically available devices with relatively large sensitive area [4,5]. The optical cavity design has evolved through various ap-*Corresponding author (email: s-miki@nict.go.jp) proaches aimed at obtaining higher η abs : double-side cavity for back side illumination [6,7], optimized optical cavity for front side illumination [8], and cavity with distribution Bragg reflector (DBR) [9][10][11]. It is to be noted that such interesting approaches also made additional attempts to obtain the flexible wavelength spectrum by employing nonperiodic DBR [12], and to reduce the polarization sensitivity by optimizing the optical cavity design [13].Considerable attention has also been focused on obtaining an η int that is as high as possible. Among various approaches aimed at increasing η int , exploration of the optimal superconducting material is one of the most interesting topic. SNSPD with various superconducting materials have demonstrated excellent SDEs exceeding 80% at a wavelength of 1550 nm. For example, SNSPD with amorphous WSi nanowire showed the highest SDE of 93% thus far, with widely saturated bias current dependencies at an operation temperature of 120 mK [8]. However, an operation temperature of 120 mK is not favorable from a practical point of view, because a relatively large and complicated cryocooler system such as an adiabatic demagnetization (ADR) cryocooler or a 3 He cryocooler...