Registration of infrared single photons by a two-channel receiver based on fiber-coupled superconducting single-photon detectors O. Okunev 1 , G. Chulkova 1 , I. Milostnaya 1 , A. Antipov 1 , K. Smirnov 1 , D. Morozov 1 , A. Korneev 1 , B. Voronov 1 , G. Gol’tsman 1 , W. Slysz 2 , M. Wegrzecki 2 , J. Bar 2 , P. Grabiec 2 , M. Górska 2 , A. Pearlman 3 , A. Cross 3 , J. Kitaygorsky 3 , Roman Sobolewski 3 1 Moscow State Pedagogical University, Moscow 119992, Russia, chulkova@mspu-phys.ru 2 Institute of Electron Technology, PL-02-668 Warsaw, Poland 3 University of Rochester, Rochester, NY 14627-0231, USA, roman.sobolewski@rochester.edu Abstract-Single-photon detectors (SPDs) are the foundation of all quantum communications (QC) protocols. Among different classes of SPDs currently studied, NbN superconducting SPDs (SSPDs) are established as the best devices for ultrafast counting of single photons in the infrared (IR) wavelength range. The SSPDs are nanostructured, 100 µ m 2 in total area, superconducting meanders, patterned by electron lithography in ultra-thin NbN films. Their operation has been explained within a phenomenological hot-electron photoresponse model. We present the design and performance of a novel, two-channel SPD receiver, based on two fiber-coupled NbN SSPDs. The receivers have been developed for fiber-based QC systems, operational at 1.3 µ m and 1.55 µ m telecommunication wavelengths. They operate in the temperature range from 4.2 K to 2 K, in which the NbN SSPDs exhibit their best performance. The receiver unit has been designed as a cryostat insert, placed inside a standard liquid-helium storage dewar. The input of the receiver consists of a pair of single-mode optical fibers, equipped with the standard FC connectors and kept at room temperature. Coupling between the SSPD and the fiber is achieved using a specially designed, precise micromechanical holder that places the fiber directly on top of the SSPD nanostructure. Our receivers achieve the quantum efficiency of up to 7% for near-IR photons, with the coupling efficiency of about 30%. The response time was measured to be <1.5 ns and it was limited by our read-out electronics. The jitter of fiber-coupled SSPDs is <35 ps and their dark-count rate is below 1s -1 . The presented performance parameters show that our single-photon receivers are fully applicable for quantum- correlation-type QC systems, including practical quantum cryptography. Keywords : single-photon detectors, superconductors, superconducting nanostructures, two-channel SPD receiver, cryogenic insert. 1. Introduction The main trend of the telecommunications is the development of the quantum cryptography systems for information transmission. In the recent years a considerable achievement in the development of optical emitters has been made to implement the quantum cryptography systems. The signal sources used in this case are the single photon sources based on the quantum dots or photon pairs emitters. On the other hand the single photon registration system still requires further development. The common types of single-photon detectors (SPD) at the fiber-optic wavelengths for telecommunication (1.3 μm and 1.55μm ) are Si, Ge and InGaAs avalanche photodiodes (APDs) and photomultiplier tubes (PMT) [1,2]. Unfortunately, the single-photon avalanche photodiodes exhibit low detection rates. The practical counting rate limit of the InGaAs SPDs is well below 1 MHz. We have recently introduced a new type of SPD, based on ultrathin, submicron-width NbN superconducting structures. These superconducting SPDs (SSPD) are ultrafast and sensitive for both visible and infrared (IR) photons. SSPDs are mostly applied for studying the kinetics of luminescence processes in molecular compounds. Another field of application of such detectors is testing and debugging CMOS circuits by registering very weak optical pulses generated during MOSFET switches. Thus, one can obtain information about performance of practically any transistor in a circuit [3]. Success in the development of the quantum dots, as well as sources of photon pairs [4,5] allow one to carry on research in quantum cryptography and quantum computing [6, 7]. A single photon detector is a key Second International Conference on Advanced Optoelectronics and Lasers, edited by Igor A. Sukhoivanov, Vasily A. Svich, Yuriy S. Shmaliy, Proc. of SPIE Vol. 7009, 70090V, (2008) ∙ 0277-786X/08/$18 ∙ doi: 10.1117/12.793402 Proc. of SPIE Vol. 7009 70090V-1