{"id":1780,"date":"2013-07-25T18:32:09","date_gmt":"2013-07-25T18:32:09","guid":{"rendered":"https:\/\/mtlsites.mit.edu\/annual_reports\/2013\/?p=1780"},"modified":"2013-08-14T16:31:45","modified_gmt":"2013-08-14T16:31:45","slug":"free-space-coupled-superconducting-nanowire-single-photon-detectors","status":"publish","type":"post","link":"https:\/\/mtlsites.mit.edu\/annual_reports\/2013\/free-space-coupled-superconducting-nanowire-single-photon-detectors\/","title":{"rendered":"Free space Coupled Superconducting Nanowire Single photon Detectors"},"content":{"rendered":"

Superconducting nanowire single\u2011photon detectors (SNSPDs)[1<\/a>]<\/sup> are a technology that combines single\u2011photon sensitivity, high speed, and high time resolution in the visible and near\u2011infrared wavelength range[2<\/a>]<\/sup>. These features make SNSPDs suitable for optical communication and other applications[3<\/a>]<\/sup>. For SNSPDs to be used in quantum communication, quantum key distribution or failure analysis in VLSI circuits[4<\/a>]<\/sup> high system detection efficiency (SDE) is also required. \u00a0Recently high SDE has been demonstrated for SNSPDs based on WSi (SDE\u00a0>\u00a090%)[5<\/a>]<\/sup> and NbN (>\u00a076%)[6<\/a>]<\/sup> by employing passive or active fiber\u2011to\u2011detector alignment.<\/p>\n

The availability of standard optical fiber mounts on the market makes it easy to integrate an optical fiber even at cryogenic temperatures. However, the 1\u2011to\u20111 correspondence between the number of fibers and the number of SNSPDs makes it difficult to integrate several channels in the same cryogenic system. In addition, mid-infrared experiments are limited by the large diameter of mid\u2011IR optical fibers\u2019 cores. We propose a 4\u2011lens system to couple 1550-nm photons in free\u2011space from an optical fiber at room temperature to an SNSPD chip mounted inside a vibration\u2011isolated cryostat at T\u00a0=\u00a03.2\u00a0K. The system is designed to focus the beam into a 5\u2011\u00b5m\u2011diameter spot (Figure 1) that can be swept over a 4×4\u00a0mm2<\/sup> area, preserving its dimensions. The advantage of a free\u2011space coupling system is that the only optical element integrated in the cryostat is the lens that ultimately focuses the light on the chip.<\/p>\n

We have already built a preliminary version of the free\u2011space coupling setup with only 2 lenses (Figure 2) able to focus the beam to an 87\u2011\u00b5m\u2011diameter spot. Both lenses were mounted at room temperature outside the cryostat. We tested a large\u2011area circular\u2011shaped SNSPD (63.6 \u00b5m2<\/sup>) with maximum device detection efficiency DDEmax<\/sub>\u00a0=\u00a012%. For this detector we obtained a DCR\u00a0=\u00a014\u00a0k counts\/sec at bias current equal to 97.8% of the SNSPD maximum operating current and a maximum SDE = 0.2%.<\/p>\n\n\t\t