We have developed a new superconducting digital technology, Reciprocal Quantum Logic, that uses ac power carried on a transmission line, which also serves as a clock. Using simple experiments, we have demonstrated zero static power dissipation, thermally limited dynamic power dissipation, high clock stability, high operating margins, and a low bit-error rate. These features indicate that the technology is scalable to far more complex circuits at a significant level of integration. On the system level, Reciprocal Quantum Logic combines the high speed and low-power signal levels of single-flux-quantum signals with the design methodology of semiconductor digital logic, including low static power dissipation, low latency combinational logic, and efficient device count.

Ultra-low-power superconductor logic

The measurement of the Planck constant, h, is entering a new phase. The CODATA 2010 recommended value is 6.626 069 57 × 10(-34) J s, but it has been a long road, and the trip is not over yet. Since its discovery as a fundamental physical constant to explain various effects in quantum theory, h has become especially important in defining standards for electrical measurements and soon, for mass determination. Measuring h in the International System of Units (SI) started as experimental attempts merely to prove its existence. Many decades passed while newer experiments measured physical effects that were the influence of h combined with other physical constants: elementary charge, e, and the Avogadro constant, N(A). As experimental techniques improved, the precision of the value of h expanded. When the Josephson and quantum Hall theories led to new electronic devices, and a hundred year old experiment, the absolute ampere, was altered into a watt balance, h not only became vital in definitions for the volt and ohm units, but suddenly it could be measured directly and even more accurately. Finally, as measurement uncertainties now approach a few parts in 10(8) from the watt balance experiments and Avogadro determinations, its importance has been linked to a proposed redefinition of a kilogram unit of mass. The path to higher accuracy in measuring the value of h was not always an example of continuous progress. Since new measurements periodically led to changes in its accepted value and the corresponding SI units, it is helpful to see why there were bumps in the road and where the different branch lines of research joined in the effort. Recalling the bumps along this road will hopefully avoid their repetition in the upcoming SI redefinition debates. This paper begins with a brief history of the methods to measure a combination of fundamental constants, thus indirectly obtaining the Planck constant. The historical path is followed in the section describing how the improved techniques and discoveries in quantum mechanics steadily reduced the uncertainty of h. The central part of this review describes the technical details of the watt balance technique, which is a combination of the mechanical and electronic measurements that now determine h as a direct result, i.e. not requiring measured values of additional fundamental constants. The first technical section describes the basics and some of the common details of many watt balance designs. Next is a review of the ongoing advances at the (currently) seven national metrology institutions where these experiments are pursued. A final summary of the recent h determinations of the last two decades shows how history keeps repeating itself; there is again a question of whether there is a shift in the newest results, albeit at uncertainties that are many orders of magnitude less than the original experiments. The conclusion is that there is room for further development to resolve these differences and find new ideas for a watt balance system with a more universal application. Since the next generation of watt balance experiments are expected to become kilogram realization standards, the historical record suggests that there is yet a need for proof that Planck constant results are finally reproducible at an acceptable uncertainty.

https://iopscience.iop.org/article/10.1088/0034-4885/76/1/016101/pdf

History and progress on accurate measurements of the Planck constant

For four decades semiconductor electronics has followed Moore's law: with each generation of integration the circuit features became smaller, more complex and faster. This development is now reaching a wall so that smaller is no longer any faster. The clock rate has saturated at about 3-5 GHz and the parallel processor approach will soon reach its limit. The prime reason for the limitation the semiconductor electronics experiences is not the switching speed of the individual transistor, but its power dissipation and thus heat. Digital superconductive electronics is a circuit- and device-technology that is inherently faster at much less power dissipation than semiconductor electronics. It makes use of superconductors and Josephson junctions as circuit elements, which can provide extremely fast digital devices in a frequency range - dependent on the material - of hundreds of GHz: for example a flip-flop has been demonstrated that operated at 750 GHz. This digital technique is scalable and follows similar design rules as semiconductor devices. Its very low power dissipation of only 0.1 mu W per gate at 100 GHz opens the possibility of three-dimensional integration. Circuits like microprocessors and analogue-to-digital converters for commercial and military applications have been demonstrated. In contrast to semiconductor circuits, the operation of superconducting circuits is based on naturally standardized digital pulses the area of which is exactly the flux quantum Phi(0). The flux quantum is also the natural quantization unit for digital-to-analogue and analogue-to-digital converters. The latter application is so precise, that it is being used as voltage standard and that the physical unit 'Volt' is defined by means of this standard. Apart from its outstanding features for digital electronics, superconductive electronics provides also the most sensitive sensor for magnetic fields: the Superconducting Quantum Interference Device (SQUID). Amongst many other applications SQUIDs are used as sensors for magnetic heart and brain signals in medical applications, as sensor for geological surveying and food-processing and for non-destructive testing. As amplifiers of electrical signals. SQUIDs can nearly reach the theoretical limit given by Quantum Mechanics. A further important field of application is the detection of very weak signals by 'transition-edge' bolo-meters, superconducting nanowire single-photon detectors, and superconductive tunnel junctions. Their application as radiation detectors in a wide frequency range, from microwaves to X-rays is now standard. The very low losses of superconductors have led to commercial microwave filter designs that are now widely used in the USA in base stations for cellular phones and in military communication applications. The number of demonstrated applications is continuously increasing and there is no area in professional electronics, in which superconductive electronics cannot be applied and surpasses the performance of classical devices. Superconductive electronics has to be cooled to very low temperatures. Whereas this was a bottleneck in the past, cooling techniques have made a huge step forward in recent years: very compact systems with high reliability and a wide range of cooling power are available commercially, from microcoolers of match-box size with milli-Watt cooling power to high-reliability coolers of many Watts of cooling power for satellite applications. Superconductive electronics will not replace semiconductor electronics and similar room-temperature techniques in standard applications, but for those applications which require very high speed, low-power consumption, extreme sensitivity or extremely high precision, superconductive electronics is superior to all other available techniques. To strengthen the European competitiveness in superconductor electronics research projects have to be set-up in the following field: - Ultra-sensitive sensing and imaging. - Quantum measurement instrumentation. - Advanced analogue-to-digital converters. - Superconductive electronics technology.

European roadmap on superconductive electronics – status and perspectives☆

Over the last 30 years, metrology laboratories have used the quantum behavior of the Josephson effect to greatly improve voltage metrology. The following article reviews the history and present status of the research and development for Josephson voltage standards. Specifically, the technology and performance of voltage standards that have quantum accuracy is explained in detail, as is their impact on a wide range of electrical metrology applications, primarily those for dc and ac voltage measurements. The physics of the Josephson effect will be presented and the importance of quantum-based electrical standards will be discussed. A detailed explanation of the operation of the conventional Josephson voltage standard and its use for dc applications will be presented, including a description of the most important results. The latter sections of this paper describe recent efforts to apply the Josephson effect to ac voltage and other electrical metrology applications. Advanced voltage standard systems have been developed that provide new features such as stable, programmable dc voltages and quantum-accurate ac waveform synthesis. The superconducting technology and integrated circuit designs for these systems will be described. Two different systems have dramatically improved precision measurements for audio-frequency voltages and for electric power metrology.

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Application of the Josephson effect in electrical metrology

A new method for calibrating ac voltage waveforms based on measuring the difference relative to a synchronously synthesized waveform supplied from a programmable Josephson array is presented. In order to test the accuracy of this approach, the differences between two Josephson waveforms have been measured at the 1.2-V level for frequencies up to 5 kHz. The uncertainties achieved are at least a factor of ten lower than those from the direct sampling method. Below 400 Hz, this new method gives an uncertainty of 5 10-8 (k = 1), while at 1 kHz, the uncertainty is about 2 107 (k = 1)

Direct Comparison of Josephson Waveforms Using an AC Quantum Voltmeter

A new method for generating and measuring active, reactive, and apparent power at power frequencies has been devised. It makes use of digital signal synthesis and discrete Fourier transform (DFT) evaluation based on a single master clock. This results in a significant reduction of synchronizing errors and thus in an uncertainty of only 2.5/spl times/10/sup -6/ (k=1).

A new scheme for generating and measuring active, reactive, and apparent power at power frequencies with uncertainties of 2.5/spl times/10/sup -6/

Josephson series arrays have been operated as digital-to-analog converters with fundamental accuracy to generate precision waveforms with accuracies better than one part in 107 at power frequencies. These waveforms have been used in the past to calibrate the sampling voltmeter in the primary standard for electrical power at the Physikalisch-Technische Bundesanstalt (PTB). The present contribution reports the integration of such a Josephson waveform synthesizer into the PTB primary power standard in order to continuously calibrate the sampling voltmeter and thus reduce the uncertainty for the measurement of active, reactive, and apparent power at the 120-V and 5-A levels

Primary AC Power Standard Based on Programmable Josephson Junction Arrays

A Josephson AC voltage source was employed to characterize the dynamic behavior of a high-resolution 28-bit (8.5-digit) integrating analog-to-digital converter of a digital sampling voltmeter (DSV), which is widely used in AC metrology at the Physikalisch-Technische Bundesanstalt. Extensive measurements were carried out to validate previous mathematical models of the DSV when sampling AC signals. The characterization method is based on the discrete Fourier transform applied on sampled data of the ADC and on the known Josephson plateau values for quantifying the ratio of AC quantities and the nonlinearities of the DSV. The method shown allows considerable improvement of the accuracy of sampling techniques at low frequencies (DC up to some kilohertz) to be attained.

Characterization of a High-Resolution Analog-to-Digital Converter with an Ac Josephson Voltage Source

The simultaneous synchronous generation and sampling technique allows alternating current (AC) quantities (root mean square (RMS) values of voltage, voltage ratios, and power) to be determined with uncertainties of the order of a few parts in 10/sup 6/. Mathematical models for estimating measurement uncertainties and experimental comparisons of such a system against existing primary standards at the PTB were carried out.

Günther Ramm

Papers

A new scheme for generating and measuring active, reactive, and apparent power at power frequencies with uncertainties of 2.5/spl times/10/sup -6/

Direct Comparison of Josephson Waveforms Using an AC Quantum Voltmeter

Primary AC Power Standard Based on Programmable Josephson Junction Arrays

Characterization of a High-Resolution Analog-to-Digital Converter with an Ac Josephson Voltage Source

Evaluation of the synchronous generation and sampling technique