Single Photon Detection Using MgB2 Superconductor
Optical Science Laboratory
Quantum cryptography is the ideal way to achieve secure communications, since the security is guaranteed by the physical principle of quantum mechanics. To realize a quantum cryptography network, it is necessary to strongly increase the communications distance and speed, which are limited by the performance of conventional single photon detectors. Recently, a new type of single photon detector that uses NbN superconducting nanowire (SSPD) has been developed. In quantum key distribution (QKD) experiments, NbN-SSPD has substantially outperformed conventional semiconducting single photon detectors . Here, we are investigating single photon detection using MgB2 superconductor. Since Tc of MgB2 is 40 K, which is much higher than the 16 K of NbN, a MgB2-SSPD would have a higher operating temperature. Moreover, the short electron-phonon relaxation time of MgB2 means that the MgB2-SSPD would operate faster than a NbN-SSPD.
To fabricate the MgB2-SSPD, it is necessary to develop a technique for MgB2 utlta-thin film growth and to develop a MgB2 nano-fabrication process. We use the molecular-beam epitaxy (MBE) for the ultra-thin film growth, and have obtained the 10-nm-thick MgB2 film with Tc of 20 K. For nano-fabrication of NbN, reactive ion etching (RIE) with fluorine-based gas has been established, but no etching gas has been reported for MgB2. We have developed a new lift-off process using a Si/C bilayer mask, which can withstand the high temperatures needed for MgB2 deposition. Using the method, we have fabricated a MgB2 nanowire with a width of 200 nm. Figure 1 shows the MgB2 nanowire, which is illuminated by a laser pulse from an optical fiber. When the nanowire is biased by a dc current source, electrical signals with a repetition frequency of 100 MHz appear, corresponding to the laser pulse [Fig. 2(a)]. The signals become intermittent as the intensity of the laser pulse strongly decreases [Fig. 2(b), (c)], indicating that the MgB2 nanowire works in the photon detection regime .
 H. Takesue et al., Nature Photon. 1 (2007) 343.
 H. Shibata et al., IEEE Trans. Appl. Supercond. 19 (in press).
Fig. 1. Photograph of the MgB2 nanowire photon detector illuminated by laser light from an optical fiber.
Fig. 2. Optical power dependence of output signals.
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