Josephson Voltage Research Articles

AC–DC Calibrations With a Pulse-Driven AC Josephson Voltage Standard Operated in a Small Cryostat

AC–DC measurements have been performed on a commercial thermal transfer standard (TTS) using a pulse-driven ac Josephson voltage standard with a small cryostat enabling shorter voltage leads. A 20 mV RMS output voltage was chosen, close to the maximum value that can be obtained without low-frequency compensation (at a sigma-delta code amplitude of about 10 %) and close to the upper limit of the TTS in its 22 mV range. The measured deviation at higher frequencies, which is due to the length of the voltage leads, was an order of magnitude smaller than was observed before with a normal cryostat. The observed square dependence on the signal frequency and cable length follows exactly the expected behavior. All results agree well with the calibrations performed using a micropot reference system. Furthermore, the influence of other cables and equipment was investigated. The measurement results can be quantitatively understood both in terms of a lumped circuit description and in terms of reflected waves.

On-Chip Temperature Distribution of Josephson Voltage Standard Devices

A substrate surface temperature characterization for the National Institute of Standards and Technology Josephson voltage standard has been performed by fabricating and measuring Josephson junction arrays with on-chip thermometry. Circuits were designed with a single voltage standard style array consisting of 12 810 junctions. 31 banks of thermometer junctions were placed around the array and microwave termination resistor to monitor temperature via a junction critical current measurement. Temperature rise in devices with on-chip termination resistors were compared to that of devices terminated off-chip. The thermal profiles on several different substrates were measured and compared to determine which substrate exhibited the best thermal transport. Finally, the thermal properties of junctions deposited directly on silicon were compared to those of the junctions deposited on an interface layer. We found that devices with off-chip terminations had half the temperature rise compared to the devices with on-chip termination, and the devices with remotely located terminations exhibited larger constant-voltage steps over a larger frequency/power range than the devices with termination resistors very close to array junctions. Comparing substrates, we found that sapphire was best for transporting heat from voltage standard circuits and that an interface layer did not affect heat transport in the substrate.

Cryocooled Josephson standards for AC voltage metrology

The Josephson effect is worldwide used as a basis for constant reference voltages in national metrological institutes and in calibration laboratories of industry. Research on Josephson voltage standards is aiming at a fundamental change also in the metrology of the volt for AC and arbitrary waveforms: programmable Josephson voltage standards converting a digital code into a quantum-accurate stepwise waveform are already available in primary laboratories and even more advanced standards for converting sub-nanosecond binary coded pulses into any arbitrary signal with quantum accuracy are now actively developed and tested. A new experimental setup based on a two-stage Gifford-McMahon cryocooler has been developed at INRiM for the operation of AC-Josephson voltage standards. Among its distinct features, the possibility of employing both the aforementioned techniques (programmable and pulsed Josephson voltage standards) is particularly interesting. Quantum-based AC voltage sine waves have been synthesized with both programmable and pulse-driven arrays, although their accuracy is still limited by thermal oscillations due to the cryocooler piston motion.

Comparison of the Josephson voltage standards of the Republic of Belarus and Russian Federation

Comparison of the Josephson voltage standards of the Republic of Belarus and Russian Federation

Practical Quantum Realization of the Ampere from the Elementary Charge

One major change of the future revision of the International System of Units (SI) is a new definition of the ampere based on the elementary charge \emph{e}. Replacing the former definition based on Amp\`ere's force law will allow one to fully benefit from quantum physics to realize the ampere. However, a quantum realization of the ampere from \emph{e}, accurate to within $10^{-8}$ in relative value and fulfilling traceability needs, is still missing despite many efforts have been spent for the development of single-electron tunneling devices. Starting again with Ohm's law, applied here in a quantum circuit combining the quantum Hall resistance and Josephson voltage standards with a superconducting cryogenic amplifier, we report on a practical and universal programmable quantum current generator. We demonstrate that currents generated in the milliampere range are quantized in terms of $ef_\mathrm{J}$ ($f_\mathrm{J}$ is the Josephson frequency) with a measurement uncertainty of $10^{-8}$. This new quantum current source, able to deliver such accurate currents down to the microampere range, can greatly improve the current measurement traceability, as demonstrated with the calibrations of digital ammeters. Beyond, it opens the way to further developments in metrology and in fundamental physics, such as a quantum multimeter or new accurate comparisons to single electron pumps.

Stepwise compensation waveform generation by using Josephson voltage standards for joule balance

Flux linkage difference in the new joule balance can be obtained by the time integration of the induced voltage u(t) from the suspended coil relative to a moving magnet driven by a dc motor translation platform. Due to the finite acceleration of the dc motor, the transition and waveform of u(t) are finite and not regular. To accurately measure the time integration of u(t), a compensation waveform with precision time integration should be synthesized. In this paper, a stepwise compensation waveform is synthesized by a programmable Josephson voltage standard (PJVS) according to u(t). The accuracy of measuring the stepwise waveform with multiple transitions can be improved by reducing the ratio of the time integrated value of the total transitions to the total waveform less than one part in 102 in the joule balance. The time integration of the rise/fall transition is measured by a synchronized reference square wave generated by another PJVS system. With the total time integration more than 20 Vs, the uncertainty of the generated stepwise waveform is within 5.2 × 10−8 VsV−1s−1. The result confirms that the PJVS has the capability to generate a stepwise compensation voltage for flux linkage difference measurement.

A dry-cooled AC quantum voltmeter

The paper describes a dry-cooled AC quantum voltmeter system operated up to kilohertz frequencies and 7 V rms. A 10 V programmable Josephson voltage standard (PJVS) array was installed on a pulse tube cooler (PTC) driven with a 4 kW air-cooled compressor. The operating margins at 70 GHz frequencies were investigated in detail and found to exceed 1 mA Shapiro step width. A key factor for the successful chip operation was the low on-chip power consumption of 65 mW in total. A thermal interface between PJVS chip and PTC cold stage was used to avoid a significant chip overheating. By installing the cryocooled PJVS array into an AC quantum voltmeter setup, several calibration measurements of dc standards and calibrator ac voltages up to 2 kHz frequencies were carried out to demonstrate the full functionality. The results are discussed and compared to systems with standard liquid helium cooling. For dc voltages, a direct comparison measurement between the dry-cooled AC quantum voltmeter and a liquid-helium based 10 V PJVS shows an agreement better than 1 part in 1010.

The quantum vector digital voltmeter of INMETRO

The paper describes the quantum vector digital voltmeter developed at INMETRO, which is based on a programmable Josephson voltage synthesizer. The system employs digital regulation for phase-alignment and frequency synchronization of signals, is fully automated and allows calibration of ac sources and analog-to-digital converters with uncertainties bearing some parts of 10-07 up to frequencies of around 500 Hz.

Using a Programmable JVS for evaluation of Zener voltage standards stability and secondary uncertainty under controlled temperature variation

A study was carried out with two 732B model Zener-diode based electronic voltage standards, in order to analyze the influence of external temperature variation in their outputs. The temperature was controlled and the external variations were monitored. Inmetro Programmable Josephson Voltage Standard (PJVS) and Secondary systems were used for this purpose. Studies have shown that, despite the internal temperature control, there is a strong correlation between the external temperature and the internal thermistor value, for both Zeners. Inmetro Zener calibration uncertainty budget was revisited and the achieved uncertainties are ±0.09 μV (at 1.018 V) and ±0.4 μV (at 10 V), k=2.

Superconducting NbN Nanowires and Coherent Quantum Phase-Slips in DC Transport

A quantum current standard dual to the Josephson voltage standard has been trumpeted as a primary application for the phenomenon of coherent quantum phase-slips (CQPS) in superconducting nanowires, but it requires a dc transport geometry, distinct from the ring geometry in which CQPS have been probed by microwave spectroscopy measurements. We present measurements on NbN nanowires with a width of 15 nm and including integrated thin-film resistors to provide a high-impedance environment. Analysis shows that the nanowire behavior is in line with expectations. The results demonstrate that the process is promising both for future dc transport CQPS measurements and for the fabrication of other circuits in which low-width superconducting nanowires are required.

Uncertainty estimation of non-ideal analog switches using programmable Josephson voltage standards for mutual inductance measurement in the joule balance

Measurement of the mutual inductance is one of the key techniques in the joule balance to determine the Planck constant h, where a standard-square-wave compensation method was proposed to accurately measure the dc value of the mutual inductance. With this method, analog switches are used to compose an analog-switch signal generator to synthesize the excitation and compensation voltages. However, the accuracy of the compensation voltage is influenced by the non-ideal behaviors of analog-switches. In this paper, the effect from these non-ideal switches is analyzed in detail and evaluated with the equivalent circuits. A programmable Josephson voltage standard (PJVS) is used to generate a reference compensation voltage to measure the time integration of the voltage waveform generated by the analog-switch signal generator. Moreover, the effect is also evaluated experimentally by comparing the difference between the mutual inductance measured with the analog-switch signal generator and the value determined by the PJVS-analog-switch generator alternately in the same mutual inductance measurement system. The result shows that the impact of analog switches is 1.97 × 10−7 with an uncertainty of 1.83 × 10−7 (k = 1) and confirms that the analog switch method can be used regularly instead of the PJVS in the mutual inductance measurement for the joule balance experiment.

Comparison of the Josephson Voltage Standards of the DMDM and the BIPM (part of the ongoing BIPM key comparison BIPM.EM-K10.b)

A comparison of the Josephson array voltage standards of the Bureau International des Poids et Mesures (BIPM) and the Directorate of Measures and Precious Metals (DMDM), Belgrade, Serbia, was carried out in June 2015 at the level of 10 V. For this exercise, options A and B of the BIPM.EM-K10.b comparison protocol were applied. Option B required the BIPM to provide a reference voltage for measurement by the DMDM using its Josephson standard with its own measuring device. Option A required the DMDM to provide a reference voltage with its Josephson voltage standard for measurement by the BIPM using an analogue nanovoltmeter and associated measurement loop. Since no sufficiently stable voltage could be achieved in this configuration, a digital detector was used. In all cases the BIPM array was kept floating from ground.The final results were in good agreement within the combined relative standard uncertainty of 1.5 parts in 1010 for the nominal voltage of 10 V.Main textTo 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 (CIPM MRA).

Bilateral comparison of 1 V and 10 V standards between the DEFNAT (Tunisia) and the BIPM February to March 2016 (part of the ongoing BIPM key comparison BIPM.EM-K11.a and b)

As part of the ongoing BIPM key comparison BIPM.EM-K11.a and b, a comparison of the 1 V and 10 V voltage reference standards of the BIPM and the Laboratoire de Métrologie Electrique, DEFNAT (Tunisia), was carried out from February to March 2016. Two BIPM Zener diode-based travelling standards (Fluke 732B), BIPM_C (ZC) and BIPM_6 (Z6), were transported by freight to DEFNAT and back to BIPM. In order to keep the Zeners powered during their transportation phase, a BIPM in-house voltage stabiliser was connected in parallel to the internal battery. The voltage stabiliser consists of a set of two batteries, electrically protected from surcharge-discharge, easy to recharge and is designed to power two transfer standards for 10 consecutive days. At DEFNAT, the reference standard for DC voltage is a Josephson Voltage Standard. The output EMF (Electromotive Force) of each travelling standard was measured by direct comparison with the primary standard.At the BIPM, the travelling standards were calibrated, before and after the measurements at DEFNAT, with the Josephson Voltage Standard. Results of all measurements were corrected for the dependence of the output voltages of the Zener standards on internal temperature and ambient atmospheric pressure.The final result of the comparison is presented as the difference between the values assigned to DC voltage standards by DEFNAT, at the level of 1.018 V and 10 V, at DEFNAT, U DEFNAT, and those assigned by the BIPM, at the BIPM, U BIPM, at the reference date of the 26th of February 2016. U DEFNAT - U BIPM = + 0.07 μV; u c = 0.04 μV, at 1.018 V U DEFNAT - U BIPM = + 0.38 μV; u c = 0.10 μV, at 10 Vwhere u c is the combined standard uncertainty associated with the measured difference, including the uncertainty of the representation of the volt at the BIPM and at NSAI-NML, based on KJ-90, and the uncertainty related to the comparison.Main textTo 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 (CIPM MRA).

Bilateral comparison of 10 V standards between the NSAI - NML (Ireland) and the BIPM, February 2016 (part of the ongoing BIPM key comparison BIPM.EM-K11.b)

As 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 in January and February 2016. Two BIPM Zener diode-based travelling standards (Fluke 732B), BIPM_7 (Z7) and BIPM_9 (Z9), were transported by freight to NSAI-NML. At NSAI-NML, the reference standard for DC voltage at the 10 V level consists of a group of characterized Zener diode-based electronic voltage standards. The output EMF (Electromotive Force) of each travelling standard was measured by direct comparison with the group standard.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 of the Zener standards on internal temperature and ambient atmospheric pressure.The final result of the comparison is presented as the difference between the values assigned to DC voltage standards by NSAI - NML, at the level of 10 V, at NSAI - NML, U NML, and those assigned by the BIPM, at the BIPM, U BIPM, at the reference date of the 31 of January 2016. U NML - U BIPM = + 0.22 μV; u c = 1.35 μV , at 10 Vwhere u c is the combined standard uncertainty associated with the measured difference, including the uncertainty of the representation of the volt at the BIPM and at NSAI-NML, based on KJ-90, and the uncertainty related to the comparison.Main textTo 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 (CIPM MRA).

Comparison of the Josephson Voltage Standards of the NIMT and the BIPM (part of the ongoing BIPM key comparison BIPM.EM-K10.b)

A comparison of the Josephson array voltage standards of the Bureau International des Poids et Mesures (BIPM) and the National Institute of Metrology - (Thailand), NIMT, was carried out in November 2015 at the level of 10 V. For this exercise, options A and B of the BIPM.EM-K10.b comparison protocol were applied. Option B required the BIPM to provide a reference voltage for measurement by the NIMT using its Josephson standard with its own measuring device. Option A required the NIMT to provide a reference voltage with its Josephson voltage standard for measurement by the BIPM using an analogue nanovoltmeter and associated measurement loop. In all cases the BIPM array was kept floating from ground.The final results were in good agreement within the combined relative standard uncertainty of 2.6 parts in 1010 for the nominal voltage of 10 V.Main textTo 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 (CIPM MRA).

Application of a 10 V Programmable Josephson Voltage Standard in Direct Comparison With Conventional Josephson Voltage Standards

This paper briefly describes the working principle of the 10 V programmable Josephson voltage standard (PJVS) that was developed at the National Institute of Standards and Technology and how to use it in a direct comparison with a conventional Josephson voltage standard (CJVS). Manual and automatic comparison methods were developed to verify the agreement between the two types of Josephson standards. A 10 V PJVS provided by the National Aeronautics and Space Administration (NASA) was used as a transfer standard in the 2014 Josephson voltage standard Interlaboratory Comparison that is organized by the National Conference of Standards Laboratories International. The results of automatic direct comparisons between a NASA PJVS and three CJVSs are reported. Allan variance is applied to analyze the large number of correlated data for Type A uncertainty.

The 2014 North American Josephson Voltage Interlaboratory Comparison

Abstract:The 10th North American Josephson voltage standard (JVS) interlaboratory comparison (ILC) at 10 V was completed in 2014. This ILC was unique because it consisted of two parts. An on-site comparison was conducted between the National Institute of Standards and Technology (NIST) transportable JVS and the pivot laboratory's conventional JVS (CJVS) system. A set of four traveling Zener voltage standards then served to transfer traceability from the pivot laboratory to the 12 other participants. In addition to the regular ILC activities, a second on-site comparison was conducted between the NIST transportable JVS and the programmable JVS (PJVS) provided by the National Aeronautics and Space Administration (NASA). Due to limited availability of the PJVS, only two laboratories were selected to make direct comparisons between their CJVS systems and NASA's PJVS. The method has been used for the first time in the JVS ILC and has the advantage of using the PJVS as a transfer standard. This allowed the parti...

Uncertainty Evaluation of a 10 V RMS Sampling Measurement System Using the AC Programmable Josephson Voltage Standard

We have evaluated the uncertainty in the differential sampling measurement of the rms amplitude of 10 V waveform synthesis based on an ac programmable Josephson voltage standard system with a compact cooler and a modified digital voltmeter (DVM). The error due to a sampling DVM timer is much reduced with an external 20-MHz frequency reference for the sampling DVM. The thermal electromotive force voltage of our reverse-polarity relay switch is sufficiently small and stable for the application of a differential sampling system. We have also performed a variety of measurements to evaluate this differential sampling technique. The evaluation results show that the overall expanded uncertainty is 1.3 parts in $10^{6}$ ( $k = 2$ ) for the frequency of 62.5 Hz at an rms amplitude of 10 V.

Precision square waves synthesized by programmable Josephson voltage standards for induced voltage compensation in a Joule balance

A programmable Josephson voltage standard (PJVS) can be used to generate a precision square wave for induced voltage compensation to measure the mutual inductance between the coils in a joule balance. In this paper, the influence of the transitions between quantized voltages in the synthesized square waves is analyzed in detail. The ratio of the time-integrated value of the transitions to the total waveform is reduced to several parts in 104 to improve the measurement accuracy. The influence of different configurations of the integrating digitizer is discussed. The result shows that when the voltages are in a quantum state, the time-integrated agreement between the measured and theoretical differences for two PJVS systems is within 4 × 10−9 V s V−1 s−1. For the total time integration of a voltage waveform larger than 2 V s, the combined relative uncertainty is less than 5.9 × 10−8 V s V−1 s−1. The result confirms the capability of the PJVS to generate a precision square wave for the joule balance.

Zero-Compensation Method and Reduced Inductive Voltage Error for the AC Josephson Voltage Standard

In the pulse-driven ac Josephson voltage standard, the low frequency (LF) component of a drive signal increases the system complexity and induces unwanted voltages through on-chip inductances. A novel zero compensation (ZC) method for pulse-driven ac waveform synthesis is presented in this paper. A pulse train is obtained through a two-level $\Delta{-}\Sigma$ modulation and then reconstructed by replacing each pulse with three pulses of a negative–positive–negative pattern. The amplitudes of the reconstructed pulse train are carefully adjusted so that the positive pulses drive the Josephson junctions to the first quantum state, whereas the negative pulses keep the junctions on the zero quantum state and cancel the contribution of the positive pulses to the LF inductive voltage component. This new method eliminates the need for LF signal reinjection required by the conventional bias method and eliminates the need for a direct-current bias required by the recently proposed ZC method. An 8-kHz sinusoidal waveform with an amplitude of 1 mV rms is synthesized to demonstrate the feasibility of the approach. We also demonstrate that the inductive voltage error is significantly reduced for waveforms synthesized with the new method.