High-energy neutron irradiation of superconducting compounds

The effect of high-energy neutron irradiation (E greater than 1 MeV) at ambient reactor temperatures on the superconducting properties of a variety of superconducting compounds is reported. The materials studied include the A-15 compounds Nb$sub 3$Sn, Nb$sub 3$Al, Nb$sub 3$Ga, Nb$sub 3$Ge and V$sub 3$Si, the C-15 Laves phase HfV$sub 2$, the ternary molybdenum sulfide Mo$sub 3$Pb$sub 0$.$sub 5$S$sub 4$ and the layered dichalcogenide NbSe$sub 2$. The superconducting transition temperature has been measured for all of the above materials for neutron fluences up to 5 x 10$sup 19$ n/cm$sup 2$. The critical current for multifilamentary Nb$sub 3$Sn has also been determined for fields up to 16 T and fluences between 3 x 10$sup 17$ n/cm$sup 2$ and 1.1 x 10$sup 19$ n/ cm$sup 2$. (auth

Low-frequency phase locking in high-inductance superconducting nanowires

Niobium nitride nanowires show considerable promise as high-speed single-photon detectors. We report the observation of an anomalous low-frequency ( ∼ 10 MHz) response in long, superconducting NbN nanowires (100 nm wide, 4 nm thick, and 500 μm long). This behavior, although strikingly reminiscent of the ac Josephson effect, can be explained by a relaxation oscillation resulting from the high kinetic inductance of the type II nanowire. We simulate all of the observed effects using a simple resistive-hotspot/series-inductor model. The voltage pulses observed are indistinguishable from the pulses induced by visible photons, and our observations suggest noise-induced relaxation oscillations are one mechanism for the dark counts in photon detectors

Quantum dot single photon sources studied with superconducting single photon detectors

We report the observation of photon antibunching from a single, self-assembled InGaAs quantum dot (QD) at temperatures up to 135 K. The second-order intensity correlation, <formula formulatype="inline"><tex>$g^{(2)}$</tex> </formula>(0), is less than 0.260 <formula formulatype="inline"><tex>$pm$</tex></formula> 0.024 for temperatures up to 100 K. At 120 K, <formula formulatype="inline"><tex>$g^{(2)}$</tex></formula>(0) increases to about 0.471, which is slightly less than the second-order intensity correlation expected from two independent single emitters. In addition, we characterize the performance of a superconducting single photon detector (SSPD) based on a nanopatterned niobium nitride wire that exhibits 68 <formula formulatype="inline"><tex>$pm$</tex></formula> 3-ps timing jitter and less than 100-Hz dark count rate with a detection efficiency (DE) of up to 2% at 902 nm. This detector is used to measure spontaneous emission lifetimes of semiconductor quantum wells (QWs) emitting light at wavelengths of 935 and 1245 nm. The sensitivity to wavelengths longer than 1 <formula formulatype="inline"><tex>$mu$</tex></formula>m and the Gaussian temporal response of this superconducting detector present clear advantages over the conventional detector technologies. We also use this detector to characterize the emission from a single InGaAs QD embedded in a micropillar cavity, measuring a spontaneous emission lifetime of 370 ps and a <formula formulatype="inline"><tex>$g^{(2)}$</tex> </formula>(0) of 0.24 <formula formulatype="inline"><tex>$pm$</tex></formula> 0.03

Quantum key distribution at 1550 nm with twin superconducting single-photon detectors

The authors report on the full implementation of a superconducting detector technology in a fiber-based quantum key distribution (QKD) link. Nanowire-based superconducting single-photon detectors (SSPDs) offer infrared single-photon detection with low dark counts, low jitter, and short recovery times. These detectors are highly promising candidates for future high key rate QKD links operating at 1550 nm. The authors use twin SSPDs to perform the BB84 protocol in a 1550 nm fiber-based QKD link clocked at 3.3 MHz. They exchange secure key over a distance of 42.5 km in telecom fiber and demonstrate that secure key can be transmitted over a total link loss exceeding 12 dB