HTML5 Webook

61/84

1IntroductionA superconducting nanowire single photon detector (SSPD) is sensitive to a broad wavelength region from deep ultraviolet to mid-wavelength infrared and has advantages in many characteristics such as detection eciency (value of output count divided by incident photon number), maximum count rate (countable photon number in a cer-tain time interval), dark count rate (output count without incident photon) and jitter (temporal deviation of output signal) over an avalanche photo diode (Avalanche Photo Diode: APD) that is a semiconductive photon detector sensitive to the communication wavelength band of 1,550 nm (1 nm is 1/1000000 of 1 mm) [1]–[5]. We started re-search and development of SSPD aiming at practical use in a quantum key distribution system (QKD: Quantum Key Distribution) and have developed a multi-channel SSPD system that is mounted on a small mechanical cryocooler operable with 100 V power on which a 6-channel SSPD can be mounted [6]. e detection eciency of the SSPD system we have developed reached 80% at 1,550 nm [7] and the system has been contributing to many excellent results such as an experiment to verify the Tokyo QKD network [8] and fundamental experiments in quantum optics [9][10]. On the other hand, the application eld of photon detectors is widespread, from communication and measurement to biology and medical treatment. In most cases of these applications, the wavelength of the detection target is less than 1,000 nm, so the silicon APD and photo multiplier tube (Photo Multiplier Tube: PMT) have been used as photon detectors. e detection eciency of a silicon APD in the visible wavelength range reaches 70%, which means that it is necessary for SSPD to compete with these photon detectors in the future and to extend its ap-plication eld to achieve total performance in not only detection eciency but also in maximum count rate, dark count rate, and jitter, etc. that are superior to those of other detectors.In this paper, we summarize the research and develop-ment of SSPD we have conducted focusing on the light of 1,550 nm, and introduce other research we are implement-ing now such as broadening of the wavelength band and a multi-pixel detector for the purpose of extension of the application eld and higher performance.2Development of multi-channel SSPD system2.1Structure of SSPD device and its functional principlee structure of the SSPD device (a) and the principle of photon detection (b) are shown in Fig. 1. Briey explain-ing the principle of photon detection of the SSPD, one incident photon destroys the superconducting state. In order to realize the phenomenon, it is necessary to mini-mize the volume of the superconductive substance to its limit. Hence, a superconducting nanowire manufactured from a superconducting membrane of 5 nm thickness to a wire of 100 nm width is used. When this superconducting nanowire absorbs a photon, a hot spot where the super-4-2 Development of Superconducting Nanowire Single-Photon DetectorHirotaka TERAISuperconducting nanowire single-photon detector (SSPD) has attractive features such as high detection efficiency, high maximum count rate, low dark count rate, low jitter, and so on. Due to those excellent features, SSPD have been employed in many experiments in quantum information field such as fundamental researches on quantum optics and system demonstrations of quantum key distribution. In this paper, we will introduce our current status on the development of SSPD system mainly for the application in quantum information field and also introduce our recent research activities toward further expansion of application fields and improvement of its performance.574 Quantum Node Technology