Josephson Voltage Research Articles

Microwave properties of microstrip line circuits used for Josephson voltage standard arraysat 70 GHz

The microwave properties of microstrip lines used for Josephson voltage standard circuitsat frequencies from 70 to 75 GHz were investigated at a temperature of 4.2 K. Ourmeasurements quantify the different loss sources. The excitation of a parasitic substratemode is postulated, demonstrated by microwave modelling and confirmed byexperiments. These modes dominate the losses for wafers with a thickness above500 µm. The substrate modes can induce an inhomogeneous microwave distribution in theJosephson junction array, thereby causing poor operation of the voltage standard circuits.Silicon dioxide fabricated in various PECVD processes was used as the dielectric layer inthe microstrip line circuits. Permittivity and losses of silicon dioxide and thesilicon substrate were determined, as well as the surface resistance of niobium.

Practical Operation of Cryogen-Free Programmable Josephson Voltage Standards

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.

10 Volt Programmable Josephson Voltage Standard Circuits Using NbSi-Barrier Junctions

Programmable Josephson voltage standard (PJVS) circuits were developed that operate at 16 GHz to 20 GHz with operating margins larger than 1 mA. Two circuit designs were demonstrated, each having a total of ~ 300,000 junctions, which were divided into either 16 or 32 sub-arrays. Triple-stacked junctions were used in order to fit the ~300,000 junctions on each circuit. The amorphous NbxSi1-x-barrier junction technology provided a high degree of uniformity of the barrier and junction electrical properties, which was necessary to achieve the 1 mA operating margin. Although the margins on both the 16-array and 32-array circuits were much greater than 1 mA, the circuit yield for the 16-array design was lower because the longer arrays are more sensitive to defects. The use of lumped-element microwave splitters and tapered arrays significantly reduced the microwave input power and increased the operating margins of these designs. In addition, the broadband microwave response of the designs allowed the PJVS output voltage to be continuously adjusted by using the microwave frequency while remaining on margins.

Improved Characteristics of Intrinsically Shunted ${\rm Nb/Al}-{\rm AlO}_{x}-{\rm Nb}$ Josephson Junctions

Josephson junctions with non-hysteretic current-voltage characteristics form the basis of most superconducting electronic circuits including RSFQ logic and programmable Josephson voltage standards. In contrast to conventional SIS devices, Nb/Al-AlOx-Nb (SNIS) junctions with an additional comparatively thick Al interlayer and a comparatively thin barrier AlOx are intrinsically shunted at T ≥ 4.2 K . In this contribution, we provide experimental and theoretical arguments proving that this finding is mainly explained by a broad distribution of highly-conductive barrier transparencies with a significant effect from nearly ballistic modes. An additional advantage of the proposed SNIS junctions is possibility to tune the critical voltage value by modifying Nb and/or Al film thicknesses. With observations of wide Shapiro steps up to 1.25 V at 6.3 K we show that this type of Josephson junctions can be successfully used at temperatures above 4.2 K. The presence of well-developed quantized voltage features even at 7.2 K means that Nb/Al-AlOx-Nb devices can successfully operate far above the liquid helium temperature and, in principle, are compatible with two-stage cryocoolers.

Superconducting Neural Network for Solving a Combinatorial Optimization Problem

We propose a neural network using coupled-SQUIDs to solve the N-Queens problem, a combinatorial optimization problem. The N-Queen problem consists of placing N queens on an N × N chess board such that none of the queens are able to capture any other using standard chess moves for a queen. We run a numerical simulation to show that a network consisting of a combination of coupled-SQUIDs can arrive at the solution. However, conditions of the network may be trapped in incorrect answers due to the existence of local minima on the energy function of the network. The Josephson voltage oscillation effect is effective for escaping from such conditions due to the existence of local minima. We investigate network dynamics and discuss the performance of the network on the basis of the parameters of the Nb integration circuit.

Materials for superconducting nanowires for quantum phase-slip devices

Quantum phase-slip processes in superconducting nanowires of suitably small cross-section have been proposed as the basis for a new current standard, based on physics dual to that for the Josephson voltage standard. The practical realisation of such devices presents several challenges. We consider the requirements which need to be met in constructing a nanowire quantum-phase-slip device and in particular the need to maximise Rξ, the normal-state resistance of a length of nanowire equal to the superconducting coherence length. Titanium and niobium-silicon are promising materials for the nanowires.

Automatic Protocol for the Direct Comparison of Josephson Voltage Standards

Automatic Protocol for the Direct Comparison of Josephson Voltage Standards

1 V programmable voltage standards based on SNIS Josephson junction seriesarrays

Binary-divided 1 V arrays were fabricated for programmable Josephson voltage standards inclose cooperation between INRIM and PTB. The arrays, consisting of 8192 overdampedNb/Al–AlOxNb SNIS Josephson junctions, were successfully operated at microwave frequenciesaround 70 GHz. While the characteristic voltage of the junctions ranged from0.25 to 0.6 mV, the current margins were larger than 1 mA for the Shapiro stepn = 1. In addition, flat Shapiro steps of higher order (n = 3 and 4) were generated at increased microwave power levels due to the large characteristicvoltage, demonstrating the small parameter spread of the SNIS junctions. Operation attemperatures up to 6–7 K has also been verified, measuring quantized voltages up to 1.25 V on then = 1 step.

Comparison of the Josephson voltage standards of the VNIIM and the BIPM (part of the ongoing BIPM key comparison BIPM.EM-K10.b)

A comparison of the 10 V Josephson array voltage standard of the Bureau International des Poids et Mesures (BIPM) was made with that of the D I Mendeleyev Institute for Metrology (VNIIM), Russian Federation, in November 2010. For this exercise, option A of the BIPM.EM-K10.b comparison protocol was applied. This option required the VNIIM to provide a reference voltage for measurement by the BIPM using its Josephson standard with its own measuring device. The BIPM array is kept floating from ground.The final results are in very good agreement with the combined relative standard uncertainty of 2.0 parts in 1010 for the nominal voltage of 10 V.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Bilateral comparison of 10 V standards between the NSAI–NML (Ireland) and the BIPM, March to April 2011 (part of the ongoing BIPM key comparison BIPM.EM-K11.b)

As a part of the ongoing BIPM key comparison BIPM.EM-K11.b, a comparison of the 10 V voltage reference standards of the BIPM and the National Standards Authority of Ireland–National Metrology Laboratory (NSAI–NML), Dublin, Ireland, was carried out from March to April 2011. Two BIPM Zener diode-based travelling standards (Fluke 732B) were transported by freight to NSAI–NML. At NSAI–NML, the 10 V output EMF of each travelling standard was measured by direct comparison with a group of characterized Zener diode-based electronic voltage standards. At the BIPM, the travelling standards were calibrated before and after the measurements at NSAI–NML, with the Josephson Voltage Standard. Results of all measurements were corrected for the dependence of the output voltages on internal temperature and ambient pressure. The comparison results show that the voltage standards maintained by NSAI–NML and the BIPM were equivalent, within their stated expanded uncertainties, on the mean date of the comparison.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Comparison of the Josephson voltage standards of the NMC, A*STAR and the BIPM (part of the ongoing BIPM key comparisons BIPM.EM-K10.a and BIPM.EM-K10.b)

A comparison of the 10 V and 1 V Josephson array voltage standards of the Bureau International des Poids et Mesures (BIPM) was made with those of the National Metrology Centre (NMC), A*STAR, Singapore, in September 2010. For this exercise, option B of the comparison protocol was applied for the two voltages. The BIPM provided a reference voltage for measurement by the NMC, A*STAR using its Josephson standard with its own measuring device (option B of the comparison protocol). The BIPM array is floating from ground.The final results are in very good agreement within the combined relative standard uncertainty of 9.6 parts in 1011 and 0.9 parts in 109 for the nominal voltages of 10 V and 1.018 V respectively.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

The 2008 NCSLI Josephson Voltage Standards Interlaboratory Comparison

Josephson voltage standards (JVS) provide a highly accurate representation of the volt. Although the Josephson Effect provides an intrinsic standard of voltage, intercomparisons between different s...

Evaluation of Low-Frequency Characteristic of a Thermal Converter Using Programmable Josephson Voltage Standard

We are developing a programmable Josephson voltage standard (PJVS) system based on a compact cryocooler. The system is capable of generating two arbitrary waveforms by use of a dual-output PJVS (DO-PJVS) device. Owing to this dual-output feature of the PJVS system, the low-frequency characteristic in the ac-dc transfer difference of a thermal converter has been investigated using the 90° addition method. The evaluation of the low-frequency characteristic of a thermal converter is of great importance for connecting conventional ac-dc transfer standards and new Josephson-based ac voltage standards. To fulfill this purpose, a dual-heater thermal voltage converter (DH-TVC) with improved low-frequency performance has been developed. The low-frequency ac-dc transfer difference of the DH-TVC was evaluated to be smaller than 1 μV/V at frequencies of as low as 1 Hz.

Analog-to-digital conversion for low-frequency waveforms based on the Josephson voltage standard

A waveform synthesizer adopting a superconductor–normal metal–superconductor junction array has been developed, which can generate arbitrary stepwise waveforms with a number of quantum-voltage steps up to 1 V level amplitude. As an application of the synthesizer, we have built a sampling voltmeter that measures the differential voltages between a sinusoidal waveform produced by a semiconductor-based ac source and the Josephson waveforms. We carried out extensive sampling measurements for a 50 Hz sine wave with 1 V amplitude, applying sampling apertures in the range of 55 µs ⩽ta ⩽ 130 µs and using Josephson waveforms with 32, 60, 80 and 100 quantum steps. From the measurements, the amplitude of the ac waveform was determined with a type A uncertainty (k = 2) of 0.15 µV. Also, we elucidated how the phase jitter in the ac waveform is reflected in the overall uncertainty for the measurements. The type B uncertainty due to the jitter is at least one order of magnitude smaller than the type A uncertainty.

Strong Attenuation of the Transients' Effect in Square Waves Synthesized With a Programmable Josephson Voltage Standard

This paper addresses the effect of transients on the operation of a 1-V programmable Josephson voltage standard for frequencies ranging between 125 Hz and 4 kHz. A detrimental effect of the transients occurring during the transition between different voltage steps is to make the ac Josephson voltage dependent on the bias current in the junctions. In other words, the current margins where the array behaves as a quantum standard are reduced to zero: the voltage steps have a slope. However, by using square waveforms, the effect of the transients can be reduced to a level where this slope is no longer measurable. This observation will probably lead to a simplification in the development of a new type of high-precision Josephson-based waveform synthesizer.

Improved Fabrication Yield for 10-V Programmable Josephson Voltage Standard Circuit Including 524288 NbN/TiN/NbN Josephson Junctions

A 10-V programmable Josephson voltage standard (PJVS) circuit including 524 288 vertically stacked NbN/TiN/NbN Josephson junctions was successfully fabricated without any defects. The chip was cooled with a compact cryocooler at a temperature of 9.8 K and generated an output voltage of 17.3 V with the first Shapiro step height of 1.0 mA under microwave irradiation at 16 GHz. Furthermore, the circuit design having the high output voltage increased the number of available chips for the 10-V PJVS. The fabrication yield for the chip, which generates greater than 10 V, was 36%, which is ten times greater than that for a perfect chip without any defects. This method was not effective for the chip that had serious defects, such as a disconnection across the array or short between the array and ground; thus, efforts to reduce such defects, e.g., frequent maintenance inside vacuum chambers for film preparation and etching, were found to be essential.

Single-Chip 10-V Programmable Josephson Voltage Standard System Based on a Refrigerator and Its Precision Evaluation

We precisely evaluated a single-chip 10-V programmable Josephson voltage standard (PJVS) system based on a closed-cycle refrigerator by directly comparing it with a conventional Josephson voltage standard system. The PJVS chip fabricated using NbN/TiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /NbN junction technology was used. The result of the measurement showed a good agreement with a combined standard uncertainty of 3.1 parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> at 10 V.

Comparison of the Josephson voltage standards of the EIM and the BIPM

A comparison of the 10 V Josephson array voltage standard of the Bureau International des Poids et Mesures (BIPM) was made with that of the Hellenic Institute of Metrology (EIM), Greece, in March 2010. For this exercise, the option B of the BIPM.EM-K10.b comparison protocol was applied, in which the BIPM only provides a reference voltage that has to be measured by the EIM using its Josephson voltage standard and its own measuring device. The results of the two participants are in very good agreement and the overall relative standard uncertainty is 2.0 parts in 1010.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Comparison of the Josephson voltage standards of the SMD and the BIPM (part of the ongoing BIPM key comparison BIPM.EM-K10.b)

A comparison of the 10 V Josephson array voltage standard of the Bureau International des Poids et Mesures (BIPM) was made with that of the Service Métrologie – Metrologische Dienst (SMD), Belgium, in November 2009. For this exercise, the option B of the BIPM.EM-K10.b comparison protocol was applied. The results of both participants are in very good agreement and the overall relative standard uncertainty is 1.3 parts in 1010.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Comparison of the INMETRO and NIST Josephson Voltage Standards (part of the ongoing regional key comparison SIM.EM.BIPM-K10.b.1)

A direct comparison of the 10 V Josephson Voltage Standards (JVS) of the National Institute of Standards and Technology (NIST), USA, and the Instituto Nacional de Metrologia, Normalização e Qualidade Industrial (INMETRO), Brazil, was made in June 2009. The process consisted of two comparisons, first using the NIST CJVS's measuring system (hardware and software) to measure the 10 V reference voltage provided by the INMETRO JVS and then using the INMETRO JVS's measuring system to measure the 10 V reference voltage provided by the NIST CJVS. The results of the two comparisons were in agreement to within 1.1 nV and their mean indicated that the difference between the two JVSs at 10 V was 0.54 nV with a combined standard uncertainty of 1.48 nV or a relative standard uncertainty of 1.48 parts in 1010.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).