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. In this paper, we digitally control a 0.01-K transmon qubit with pulses from a Josephson pulse generator (JPG) located at the 3-K stage of a dilution refrigerator. We directly compare the qubit lifetime $T_1$, the coherence time $T_2^{\ast }$, and the thermal occupation $P_{\mathrm{th}}$ when the qubit is controlled by the JPG circuit versus the TSCE setup. We find agreement to within the daily fluctuations of $\pm 0.5\mu \mathrm{s}$ and $\pm 2\mu \mathrm{s}$ for $T_1$ and $T_2^{\ast }$, respectively, and agreement to within the 1% error for $P_{\mathrm{th}}$. Additionally, we perform randomized benchmarking to measure an average JPG gate error of $2.1\times {10}^{-2}$. In combination with a small device size ($<25{\mathrm{mm}}^2$) and low on-chip power dissipation ($\ll 100\mu \mathrm{W}$), these results are an important step toward demonstrating the viability of using JJ-based control electronics located at temperature stages higher than the mixing-chamber stage in highly scaled superconducting quantum information systems.

Quantum computers with thousands or millions of qubits will require a scalable solution for qubit control and readout electronics. Colocating these electronics at millikelvin temperatures has been proposed and demonstrated, but there exist significant challenges with power dissipation, reproducibility, fidelity, and scalability. In this article, we experimentally demonstrate the use of a Josephson arbitrary waveform synthesizer (JAWS) to generate control signals at 4 K and perform spectroscopy of two components of a typical superconducting quantum information system: a linear resonator and a (nonlinear) transmon qubit. By locating the JAWS chip at 4 K and a qubit at 0.1 K, the direct path for quasi-particle poisoning from the JAWS chip to the qubit is broken. We demonstrate the stable, self-calibrated, and reproducible output signal of the JAWS when operated in its quantum locking range, a feature that allows these synthesizers to be replicated and scaled in the cryostat, all with identical on-chip, quantized, outputs. This is a proof-of-concept demonstration to generate signals at 4 K using driven superconducting electronics to control qubits at lower temperatures.

Large-scale integration of single flux quantum (SFQ) circuits requires components as compact as possible. In this paper, we study the feasibility of integrating vertically stacked Josephson inductors (VSJIs) fabricated with self-shunted Josephson junctions into our SFQ-based circuits, and use both dc and RF electrical measurements to verify the performance of these junction stacks as compact inductors. The VSJIs consist of three Josephson junctions stacked vertically and are inserted into test structures to validate their potential as compact alternatives to geometrical inductors. The VSJIs are designed to be non-switching junctions and are combined with smaller area, switching Josephson junctions fabricated with the same junction barrier process. The VSJIs have a designed critical current of approximately 50% greater than that of the switching junctions, and their observed behavior can be modeled correctly by including the known nonlinearity of the Josephson inductance. To confirm that this nonlinearity does not degrade the expected bias margins, we simulate two common types of SFQ cells with the geometrical inductors replaced by VSJIs. The results verify that incorporating VSJIs in SFQ circuits would increase circuit density with minimal impact on circuit margins.

Compared to traditional semiconductor control electronics (TSCE) located at room temperature, cryogenic single flux quantum (SFQ) electronics can provide qubit measurement and control alternatives that address critical issues related to scalability of cryogenic quantum processors. Single-qubit control and readout have been demonstrated recently using SFQ circuits coupled to superconducting qubits. Experiments where the SFQ electronics are co-located with the qubit have suffered from excess decoherence and loss due to quasiparticle poisoning of the qubit. A previous experiment by our group showed that moving the control electronics to the 3 K stage of the dilution refrigerator avoided this source of decoherence in a high-coherence three-dimensional transmon geometry. In this paper, we also generate the pulses at the 3 K stage but have optimized the qubit design and control lines for scalable two-dimensional transmon devices. We directly compare the qubit lifetime T1, coherence time T2*, and gate fidelity when the qubit is controlled by the Josephson pulse generator (JPG) circuit vs the TSCE setup. We find agreement within the daily fluctuations for T1 and T2*, and agreement within 10% for randomized benchmarking. We also performed interleaved randomized benchmarking on individual JPG gates demonstrating an average error per gate of 0.46% showing good agreement with what is expected based on the qubit coherence and higher-state leakage. These results are an order of magnitude improvement in gate fidelity over our previous work and demonstrate that a Josephson microwave source operated at 3 K is a promising component for scalable qubit control.

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 quantumbased 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. The circuits were fabricated using our Nb/NbxSi1-x/Nb Josephson-junction (JJ) fabrication process, which produces selfshunted JJs with Nb-doped silicon barriers. In order to demonstrate quantum-based reproducibility, stability and performance at 4 K, we synthesized single-tone and multitone waveforms at gigahertz frequencies and demonstrated their operation over a range of synthesizer output and experimental bias parameters. We also propose circuit designs for achieving higher synthesis frequencies and higher output power with improved power accuracy and spectral purity, and discuss the potential limitations of these circuits.

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. Josephson pulse generators have also been used to control and characterize superconducting qubits.