Burm Baek

Detecting single infrared photons with 93% system efficiency

Researchers develop a fiber-coupled single-photon-detection system using amorphous tungsten silicide superconducting nanowire single-photon detectors. The system detection efficiency is higher than 90% in the wavelength range between 1520 nm and 1610 nm. The device dark-count rate, timing jitter and reset time are 1 cps, 150 ps and 40 ns, respectively.

Superconducting a-WxSi1−x nanowire single-photon detector with saturated internal quantum efficiency from visible to 1850 nm

We have developed a single-photon detector based on superconducting amorphous tungsten–silicon alloy (a-WxSi1−x) nanowire. Our device made from a uniform a-WxSi1−x nanowire covers a practical detection area (16 μm×16 μm) and shows high sensitivity featuring a plateau of the internal quantum efficiencies, i.e., efficiencies of generating an electrical pulse per absorbed photon, over a broad wavelength and bias range. This material system for superconducting nanowire detector technology could overcome the limitations of the prevalent nanowire devices based on NbN and lead to more practical, ideal single-photon detectors having high efficiency, low noise, and high count rates.

Large sensitive-area NbN nanowire superconducting single-photon detectors fabricated on single-crystal MgO substrates

We report on the performance of large area NbN nanowire superconducting single-photon detectors (SSPDs). 20×20μm2 area SSPDs with 80 and 100nm linewidths and 50% fill factor were fabricated in 4-nm-thick NbN films grown on single-crystal MgO substrates. The high quality of the devices was verified by electrical and optical testing and compares favorably to measurements of 10×10μm2 area SSPDs. Measurements of kinetic inductance versus bias current indicate that the constriction density is low. The fiber-coupled detection efficiency of the devices was 0.4%–3.5% at 100Hz dark count rate.

Enhanced telecom wavelength single-photon detection with NbTiN superconducting nanowires on oxidized silicon

Superconducting nanowire single-photon detectors (SNSPDs) have emerged as a highly promising infrared single-photon detector technology. Next-generation devices are being developed with enhanced detection efficiency (DE) at key technological wavelengths via the use of optical cavities. Furthermore, new materials and substrates are being explored for improved fabrication versatility, higher DE, and lower dark counts. We report on the practical performance of packaged NbTiN SNSPDs fabricated on oxidized silicon substrates in the wavelength range from 830 to 1700 nm. We exploit constructive interference from the SiO2/Si interface in order to achieve enhanced front-side fiber-coupled DE of 23.2 % at 1310 nm, at 1 kHz dark count rate, with 60 ps full width half maximum timing jitter.

Hybrid superconducting-magnetic memory device using competing order parameters

In a hybrid superconducting-magnetic device, two order parameters compete, with one type of order suppressing the other. Recent interest in ultra-low-power, high-density cryogenic memories has spurred new efforts to simultaneously exploit superconducting and magnetic properties so as to create novel switching elements having these two competing orders. Here we describe a reconfigurable two-layer magnetic spin valve integrated within a Josephson junction. Our measurements separate the suppression in the superconducting coupling due to the exchange field in the magnetic layers, which causes depairing of the supercurrent, from the suppression due to the stray magnetic field. The exchange field suppression of the superconducting order parameter is a tunable and switchable behaviour that is also scalable to nanometer device dimensions. These devices demonstrate non-volatile, size-independent switching of Josephson coupling, in magnitude as well as phase, and they may enable practical nanoscale superconducting memory devices.

Ultra fast quantum key distribution over a 97 km installed telecom fiber with wavelength division multiplexing clock synchronization

We demonstrated ultra fast BB84 quantum key distribution (QKD) transmission at 625 MHz clock rate through a 97 km field-installed fiber using practical clock synchronization based on wavelength-division multiplexing (WDM). We succeeded in over-one-hour stable key generation at a high sifted key rate of 2.4 kbps and a low quantum bit error rate (QBER) of 2.9%. The asymptotic secure key rate was estimated to be 0.78- 0.82 kbps from the transmission data with the decoy method of average photon numbers 0, 0.15, and 0.4 photons/pulse.

Practical long-distance quantum key distribution system using decoy levels

Quantum key distribution (QKD) has the potential for widespread real-world applications, but no secure long-distance experiment has demonstrated the truly practical operation needed to move QKD from the laboratory to the real world due largely to limitations in synchronization and poor detector performance. Here, we report results obtained using a fully automated, robust QKD system based on the Bennett Brassard 1984 (BB84) protocol with low-noise superconducting nanowire single-photon detectors (SNSPDs) and decoy levels to produce a secret key with unconditional security over a record 140.6 km of optical fibre, an increase of more than a factor of five compared with the previous record for unconditionally secure key generation in a practical QKD system.

Co-Sputtered Amorphous Nb

Co-sputtered amorphous NbxSi1-x has been developed as a barrier material for Josephson-junction array circuits. This material is quite promising as a normal-metal barrier for state-of-the-art Josephson voltage standards. In addition, the capability of tuning the barrier resistivity over a wide range that includes the metal-insulator transition could lead to applications in high-speed superconductive electronics. The electrical characteristics and uniformity of amorphous NbxSi1-x-barrier junctions are similar to those of other normal-metal barriers, but the superior etching properties makes this barrier material especially promising for tall, stacked junctions that are required for high-junction-density applications. Under appropriate deposition conditions, the reproducibility of devices with co-sputtered amorphous NbxSi1-x is sufficient to produce high-quality stacked-junction superconductive devices

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We demonstrate a distributed fiber Raman sensor for absolute temperature measurement with spatial resolution on the order of 1 cm at 1550 nm wavelength in a single-mode fiber using superconducting nanowire single-photon detectors. Rapid measurements are shown, with less than 60 s integration period, allowing the demonstration of temperature evolution in an optical fiber recorded at over 100 resolvable, 1.2 cm spaced positions along the fiber simultaneously. This distributed sensor has potential application as a primary reference standard, in which high-accuracy, high-spatial-resolution temperature measurements can be obtained without the need for a separate temperature calibration standard.

Si

We report the first entanglement-based quantum key distribution (QKD) experiment over a 100-km optical fiber. We used superconducting single photon detectors based on NbN nanowires that provide high-speed single photon detection for the 1.5-mum telecom band, an efficient entangled photon pair source that consists of a fiber coupled periodically poled lithium niobate waveguide and ultra low loss filters, and planar lightwave circuit Mach-Zehnder interferometers (MZIs) with ultra stable operation. These characteristics enabled us to perform an entanglement-based QKD experiment over a 100-km optical fiber. In the experiment, which lasted approximately 8 hours, we successfully generated a 16 kbit sifted key with a quantum bit error rate of 6.9 % at a rate of 0.59 bits per second, from which we were able to distill a 3.9 kbit secure key.