This paper presents precision measurements with a prototype cryogen-liquid-free DC Josephson voltage standard that produces a 1 V maximum output. Its cryostat is sufficiently compact that it can be operated on a bench close to devices under test. Compared to the National Institute of Standards and Technology (NIST) programmable Josephson voltage standard (PJVS), this bench-top Josephson voltage standard (BJVS) has a smaller cryostat (only 25 % of the volume), the Josephson junction array circuit is less complex and has only 10 % as many junctions, simpler bias electronics, and reduced mechanical cooling requirements. Direct voltage comparison between the BJVS and the PJVS achieved a relative agreement of 5×10 -10 at 1 V. With the recent redefinition of the International System of Units (SI), the BJVS becomes a primary realization the unit volt, requiring only access to a standard single-phase power outlet and a GPS signal (SI second).

We have recently created a 4 V rms cryocooled JAWS (Josephson Arbitrary Waveform Synthesizer) using 204,960 nearly identical Josephson junctions (JJs) that are embedded in coplanar-wave guides. The JJs are pulse-biased at repetition rates up to 16×10 9 pulses per second to create quantum-accurate, calculable AC waveforms at frequencies from DC to greater than 1 MHz. This system has metrological applications including in precision ac voltage calibrations, comparisons of arbitrary impedances, and ac power measurements.

We present the first dc comparison of a programmable Josephson voltage standards and a pulse-driven Josephson arbitrary waveform synthesizer (JAWS) at 3 V. Both circuits are mounted side-by-side on the cold stage of a cryocooler. The relative agreement achieved was better than 1 part in 10 8 . This measurement allowed us to identify systematic errors of the JAWS system. An undesired current injection from the JAWS isolation amplifier into the measurement circuit was responsible for an error voltage of a few nanovolts.

We have performed direct dc comparisons between two cryocooled 10 V programmable Josephson voltage standards utilizing an automated synchronization scheme for the voltage reversals, which enables the use of a high sensitivity null detector on its 3 μν range by preventing any overload condition. No switches or manual operations are necessary to protect the null detector. Comparing the two systems under various test conditions provides robust verification of ideal system performance, and enables verification of the key components of the uncertainty budget for both the measurement methods and system operations.

The root-mean-square (rms) output voltage of the NIST Josephson arbitrary waveform synthesizer (JAWS) has been doubled from 1 V to a record 2 V by combining two new 1 V chips on a cryocooler. This higher voltage will improve calibrations of ac thermal voltage converters and precision voltage measurements that require state-of-the-art quantum accuracy, stability, and signal-to-noise ratio. We achieved this increase in output voltage by using four on-chip Wilkinson dividers and eight inner-outer dc blocks, which enable biasing of eight Josephson junction (JJ) arrays with high-speed inputs from only four high-speed pulse generator channels. This approach halves the number of pulse generator channels required in future JAWS systems. We also implemented on-chip superconducting interconnects between JJ arrays, which reduces systematic errors and enables a new modular chip package. Finally, we demonstrate a new technique for measuring and visualizing the operating current range that reduces the measurement time by almost two orders of magnitude and reveals the relationship between distortion in the output spectrum and output pulse sequence errors.

The performance of the NIST Josephson arbitrary waveform synthesizer has been improved such that it generates a root-mean-square (rms) output voltage of 1 V with an operating current range greater than 2 mA. Our previous 1 V JAWS circuit achieved this same maximum voltage over a current range of 0.4 mA by operating every Josephson junction in its second quantum state. The newest circuit synthesizes 1 V waveforms with the junctions operating in the first quantum state. The voltage per array is doubled because the number of junctions in each array was doubled through the use of improved microwave circuit designs that increased the bias uniformity to the junctions. We describe the circuit improvements and device operation, and we demonstrate the system capabilities by showing measured spectra of a 1 Hz sine wave and a dual-tone waveform. With only two arrays of the new circuit, we also synthesized a 128 mV sine wave without a compensation bias signal, which is one of the bias signals required for achieving 1 V. This is the same rms output voltage achieved with the previous circuit using four arrays.

A quantum-accurate waveform with an rms output amplitude of 1 V has been synthesized for the first time. This fourfold increase in voltage over previous systems was achieved through developments and improvements in bias electronics, pulse-bias techniques, Josephson junction array circuit fabrication, and packaging. A recently described ac-coupled bipolar pulse-bias technique was used to bias a superconducting integrated circuit with 25 600 junctions, which are equally divided into four series-connected arrays, into the second quantum state. We describe these advancements and present the measured 1 V spectra for 2 Hz and 10 Hz sine waves that remained quantized over a 0.4 mA current range. We also demonstrate a 2 kHz sine wave produced with another bias technique that requires no compensation current and remains quantized at an rms voltage of 128 mV over a 1 mA current range. Increasing the clock frequency to 19 GHz also allowed us to achieve a maximum rms output voltage for a single array of 330 mV.

The recent shortage and increasing cost of liquid helium provides motivation for cryogen-free operation of superconducting devices such as NIST programmable Josephson voltage standard (PJVS) systems. However, operation on closed-cycle cryocoolers must not compromise the performance of the PJVS system. New cryogenic packaging and cryostat integration are presented that have been optimized for the NIST 10 V PJVS, demonstrating improved attenuation of coldhead temperature oscillations, and improvement of thermal conductances in the junction-to-coldhead path. When combined with an improved design of 10 V PJVS devices employing Nb/Nb x Si 1-x /Nb junctions with increased operating margins, we have operated a NIST PJVS at 10 V with over 1.32 mA current margins on a nominal 200 mW cryocooler using a 3.0 kW water-cooled compressor. The new cryo-package, in conjunction with the improved generation of chips, eliminates the need for liquid cryogens in applications using NIST 10 V PJVS systems.

The two main challenges to operating a programmable Josephson voltage standard (PJVS) on a cryocooler are the available cooling power and the temperature oscillations of the cold head. We overcame these challenges and successfully operated a PJVS circuit on a cryocooler by employing one supercritical helium buffer that damps the temperature oscillations, developing a new cryogenic package that increases the thermal conductivity between the chip and the cold head, and increasing overall device performance with fabrication improvements. A 1.32 mA step width of the quantized voltage produced with all of the subarrays of the PJVS circuit biased was achieved at an operating temperature of 4.3 K. The quantum accuracy of the PJVS is maintained at temperatures up to 4.8 K. This result was obtained with a cryocooler that employs a 3 kW water-cooled compressor to produce at the chip about 270 mW of net cooling power at 4.3 K.

Millimeter wavelength radiation holds promise for detection of security threats at a distance, including suicide bomb belts and maritime threats in poor weather. The high sensitivity of superconducting Transition Edge Sensor (TES) detectors makes them ideal for passive imaging of thermal signals at these wavelengths. We have built a 350 GHz video-rate imaging system using a large-format array of feedhorn-coupled TES bolometers. The system operates at a standoff distance of 16m to 28m with a spatial resolution of 1:4 cm (at 17m). It currently contains one 251-detector subarray, and will be expanded to contain four subarrays for a total of 1004 detectors. The system has been used to take video images which reveal the presence of weapons concealed beneath a shirt in an indoor setting. We present a summary of this work.

Given the recent shortages of liquid helium, cryogen-free operation of superconducting devices, such as programmable Josephson voltage standard (PJVS) systems, has become preferable worldwide, and a necessity in some locations. However, reliable operation on a cryocooler is heavily dependent on the ability to create a constant temperature that is low enough to allow the PJVS junctions to operate uniformly. In this work, we systematically investigated as a function of temperature the performance of NIST 10 V PJVS chips employing Nb/NbxSi1-x/Nb superconducting junctions. Additionally, we addressed the major factors limiting the performance of a cryocooled PJVS: adequate attenuation of the coldhead temperature oscillations and the minimization thermal gradients between the chip and the cryocooler. Through the development of a robust and reproducible method for soldering chips to a Cu carrier (package), we increased the thermal conductances within the packaging to their practical maximum values. This, in addition to the incorporation of a passive two-stage thermal filter, allows us to confidently predict that the required cooling power for the successful cryogen-free operation of the NIST 10 V PJVS is ~ 0.5 W at 4 K.

Cryogen-free operation is rapidly becoming the preferred implementation of most superconducting electronics systems including programmable Josephson voltage standard (PJVS) systems. There are strong operational incentives for using the smallest possible cryocooler in order to minimize acoustic noise, system footprint, and power consumption. In addition, Nb/NbxSi1-x/Nb junction technology, which operates near 4 K, offers better yield than NbN/TiNx/NbN technology, which can operate at 8.5 K, thus making lower temperature operation near 4 K desirable. As junction density increases, however, self-heating of the junctions can create significant thermal gradients between the arrays and coldhead. Thus careful design of the overall system is required to maintain acceptable operating margins. We have developed a calorimetric measurement technique to characterize the system variables and used it to evaluate several different PJVS configurations. This technique uses the PJVS subarrays as both heat sources and temperature sensors, in conjunction with a time gated measurement technique, to characterize the thermal response of the system. A passive thermal filter incorporating a Pb thermal mass is used to reduce the temperature oscillations of the cryocooler. Our results suggest that, with appropriate system design, operation of a practical 10 V PJVS on a small (nominally 100 mW capacity at 4.2 K) cryocooler may be possible.

We are developing a 350 GHz cryogenic passive video imaging system for use in standoff security applications. This demonstration system uses 800 photon-noise-limited superconducting transition edge sensor bolometers, read out using a time-division multiplexed readout system. It will image a 1 m x 1 m field of view at a standoff distance of 16 m to a resolution of approximately 1 cm at video frame rates (20 frames per second). High spatial resolution is achieved by the use of an f/2.0 Cassegrain optical system with 1.3 m primary mirror. Preliminary dark and optical testing of prototype detectors indicates that we can achieve a noise equivalent temperature difference (NETD) below 100 mK for the fully sampled 1 m x 1 m image at 20 frames per second. We report on the current status of development of this system.

Mass Spectrometry is widely used for protein characterization, structural virology, drug discovery, and clinical chemistry. However, the detection efficiency of existing detectors for mass spectrometry degrades rapidly as mass is increased, and is only ~ 10 -5 at 10 6 Da. Superconducting detectors provide detection efficiency that is essentially independent of mass, and previous efforts have explored the use of superconducting tunnel junctions (STJs) and normal-insulator-superconductor (NIS) microcalorimeters as detectors. Both STJ and NIS detectors, however have active areas limited to ~ 1 mm 2 , well below the ~ 1 cm 2 required for a viable system. Microwave-interrogated microstripline meander detectors have the potential to provide the necessary area and speed. We describe such a design and present initial spectra of representative ionized biological molecules obtained from simple prototype detectors mounted on a cryocooler and interfaced to a commercial mass spectrometer.

NIST is designing a 10 V Programmable Josephson Voltage Standard (PJVS) system with an improved microwave design and arrays of stacked NbxSi1-x-barrier Josephson junctions. For this new design a "ground-up" approach was used that takes into account all system issues in order to produce a robust 10 V system. By improving the uniformity of the microwave drive along the length of each array, constant-voltage steps with a larger current range are generated allowing the use of smaller critical current junctions. Smaller critical currents are important for lowering the total overall power dissipated on chip. Reducing power dissipation also increases the operating margins. Thus, all aspects of the design are interrelated and important for an optimized system.

We present the outline of the optical design of a TeraHertz (THz) imager for the detection of shrapnel-loaded improvised explosive devices (IED) devices at “stand-off” distances of 14–26 meters. The system will use 4 antenna-coupled TES detector arrays of 16 by 16 pixels cooled in a cryogen-free system with microwave readout to see beneath clothing at non-lethal detonation distances. A spatial resolution of ∼10 mm and close to video frame rates is anticipated.

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, g(2)(0), is less than 0.260 plusmn 0.024 for temperatures up to 100 K. At 120 K, g(2) (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 plusmn 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 mum 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 g(2)(0) of 0.24 plusmn 0.03

We provide a direct comparison between the InGaAs avalanche photodiode (APD) and the NbN superconducting single photon detector (SSPD) for applications in fiber-based quantum cryptography. The quantum efficiency and dark count rate were measured for each detector, and used to calculate the quantum bit error rate (QBER) and shared key rate for a QKD link. The results indicate that, despite low quantum efficiency, the speed of the SSPD makes it a superior detector for quantum information applications. Finally, we present results of an initial integration of an SSPD into a receiver node of the DARPA quantum network to perform quantum key distribution.

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 1550nm. The authors use twin SSPDs to perform the BB84 protocol in a 1550nm fiber-based QKD link clocked at 3.3MHz. They exchange secure key over a distance of 42.5km in telecom fiber and demonstrate that secure key can be transmitted over a total link loss exceeding 12dB.

We use a superconducting single photon detector with ~65 ps jitter and < 40 Hz dark count rate to measure spontaneous emission lifetimes of quantum wells emitting light in the 900-1300 nm wavelength range.