AC metrology applications of the Josephson effect.

The performance of programmable voltage signals that exploit the quantum behavior of superconducting Josephson junctions continues to improve and enhance measurements in metrology, communications, and quantum control. We review advances in pulse-driven digital synthesis techniques with Josephson-junction-based devices. Quantum-based synthesis of voltage waveforms has been demonstrated at frequencies up to 3 GHz and rms amplitudes up to 4 V.

Nb/-Si/Nb-junction Josephson arbitrary waveform synthesizers for quantum information.

We demonstrate Josephson arbitrary waveform synthesizers (JAWS) with increased operating temperature range for temperatures below 4 K. These JAWS synthesizers were fabricated with externally-shunted Nb/-Si/Nb junctions whose critical current exhibits improved temperature stability compared to the self-shunted Nb/NbSi/Nb junctions typically used. Vertical stud resistors made of 230 nm of PdAu were developed to provide the milliohm shunt resistance required for junction overdamping while maintaining a small footprint suitable for high-density series arrays embedded in a coplanar waveguide.

Single-Flux-Quantum Multiplier Circuits for Synthesizing Gigahertz Waveforms With Quantum-Based Accuracy.

We designed, simulated, and experimentally demonstrated components for a microwave-frequency digital-to-analog converter based on single flux quantum (SFQ) circuits and an amplifier based on superconducting-quantum-interference-device (SQUID) stacks. These are key components for a self-calibrated programmable waveform reference for communications metrology capable of synthesizing high-frequency signals with quantum-based output accuracy. The amplifier is an SFQ voltage multiplier circuit that consists of a network of SFQ-splitters and SQUID transformers that provides output signals consisting of quantized pulses.

1 GHz Waveform Synthesis With Josephson Junction Arrays.

We synthesize single- and multiple-tone waveforms at gigahertz frequencies from arrays of Josephson junctions and demonstrate their quantum-locked operation over a range of experimental input parameters. We first use a lumped-element circuit to synthesize 1 and 2 GHz single-tone waveforms with -71 dBm output power and in-band spurious-free dynamic range (SFDR) of -66 dBc. We then introduce a narrow-band diplexer circuit and synthesize a 1 GHz sinusoid with higher power (-49 dBm) and in-band SFDR of -79 dBc.

Digital Control of a Superconducting Qubit Using a Josephson Pulse Generator at 3 K.

Scaling of quantum computers to fault-tolerant levels relies critically on the integration of energy-efficient, stable, and reproducible qubit control and readout electronics. In comparison to traditional semiconductor-control electronics (TSCE) located at room temperature, the signals generated by rf sources based on Josephson-junctions (JJs) benefit from small device sizes, low power dissipation, intrinsic calibration, superior reproducibility, and insensitivity to ambient fluctuations. Previous experiments to colocate qubits and JJ-based control electronics have resulted in quasiparticle poisoning of the qubit, degrading the coherence and lifetime of the qubit.

Synaptic weighting in single flux quantum neuromorphic computing.

Josephson junctions act as a natural spiking neuron-like device for neuromorphic computing. By leveraging the advances recently demonstrated in digital single flux quantum (SFQ) circuits and using recently demonstrated magnetic Josephson junction (MJJ) synaptic circuits, there is potential to make rapid progress in SFQ-based neuromorphic computing. Here we demonstrate the basic functionality of a synaptic circuit design that takes advantage of the adjustable critical current demonstrated in MJJs and implement a synaptic weighting element.

Dual Josephson Impedance Bridge: Towards a Universal Bridge for Impedance Metrology.

This paper presents a full characterization of a Dual Josephson Impedance Bridge (DJIB) at frequencies up to 80 kHz by using the DJIB to compare the best available impedance standards that are (a) directly traceable to the quantum Hall effect, (b) used as part of international impedance comparisons, or (c) believed to have calculable frequency dependence. The heart of the system is a dual Josephson Arbitrary Waveform Synthesizer (JAWS) source that offers unprecedented flexibility in high-precision impedance measurements. The JAWS sources allow a single bridge to compare impedances with arbitrary ratios and phase angles in the complex plane.

Quantized Pulse Propagation in Josephson Junction Arrays.

We present time-domain electrical measurements and simulations of the quantized voltage pulses that are generated from series-connected Josephson junction (JJ) arrays. The transmission delay of the JJ array can lead to a broadening of the net output pulse, depending on the direction of the output pulse propagation relative to the input bias pulse. To demonstrate this, we compare time-domain measurements of output pulses from radio-frequency Josephson Arbitrary Waveform Synthesizer (RF-JAWS) circuits fabricated with two different output measurement configurations, so that the backward-propagating and forward-propagating pulses can be measured.

Characterization of Uniformity in Nb/NbSi/Nb Josephson Junctions.

The uniformity of the barriers in Josephson junctions (JJs) is a critical parameter in determining performance and operating margins for a wide variety of superconducting electronic circuits. We present an automated measurement system capable of measuring individual JJs across a 1 × 1 cm die at both ambient temperature and 4 K. This technique allows visualization of the spatial variation over a large area of the critical electrical properties of the junctions and allows for the direct correlation between room-temperature (RT) resistance and low temperature properties.

Jitter Sensitivity Analysis of the Superconducting Josephson Arbitrary Waveform Synthesizer.

We present the first jitter sensitivity analysis of a superconducting voltage reference waveform synthesizer with fundamentally accurate output pulses. Successful deployment of a reference waveform source at microwave frequencies will represent a new paradigm for radio frequency metrology. The programmable waveform synthesizer considered in this paper contains a 1.

Ultralow power artificial synapses using nanotextured magnetic Josephson junctions.

Neuromorphic computing promises to markedly improve the efficiency of certain computational tasks, such as perception and decision-making. Although software and specialized hardware implementations of neural networks have made tremendous accomplishments, both implementations are still many orders of magnitude less energy efficient than the human brain. We demonstrate a new form of artificial synapse based on dynamically reconfigurable superconducting Josephson junctions with magnetic nanoclusters in the barrier.

Josephson Arbitrary Waveform Synthesis With Multilevel Pulse Biasing.

We describe the implementation of new commercial pulse-bias electronics that have enabled an improvement in the generation of quantum-accurate waveforms both with and without low-frequency compensation biases. We have used these electronics to apply a multilevel pulse bias to the Josephson arbitrary waveform synthesizer and have generated, for the first time, a quantum-accurate bipolar sinusoidal waveform without the use of a low-frequency compensation bias current. This uncompensated 1 kHz waveform was synthesized with an rms amplitude of 325 mV and maintained its quantum accuracy over a1.

Two-Volt Josephson Arbitrary Waveform Synthesizer Using Wilkinson Dividers.

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.

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.