Clustered ferromagnetic Josephson junctions form ultralow energy synaptic elements.

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.

We describe our process for fabricating single-fluxquantum circuits for applications in communications metrology, advanced computing, and quantum information. The process flow includes four superconducting metal layers, one junction layer, one palladium-gold bias resistor layer, and a contact pad layer. Planarization using chemical-mechanical polishing is performed on the two lower insulating layers. This process is based on Josephson junctions with niobium superconducting electrodes and self-shunted, amorphous niobium-doped silicon barriers on the insulating side near the metal-insulator transition. The barrier material is deposited by co-sputtering silicon and niobium, with the junction properties controlled by the relative sputtering powers and the duration of the barrier deposition step. The profiles of the deposition rates across 3-in (76.2 mm) wafers give a corresponding radial profile of the critical current density $J_c$ that increases by ∼ 46 % from the center to the edge of the wafer. To characterize our process, we show data for niobium metal quality and magnetic penetration depth, wire and via inductances, and via critical currents. The uniformity of the junction critical current $I_c$ across a wafer was measured on test chips with 2.7-μm-diameter junctions. Within a 5 mm × 5 mm area, the standard deviation of $I_c$ was ∼ 2 % for chips near the center of the wafer and up to ∼ 7 % for chips near the edge. We also describe process improvements and innovations under development aimed at increasing circuit density and high-speed performance.

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.

We review our development of Josephson-junction-based programmable reference sources to synthesize spectrally-pure waveforms with quantum-based voltage accuracy for characterizing and improving future communication devices and systems. The goal is to provide sources that together span the entire radio frequency (RF) band, equivalent to seven orders of magnitude in frequency (20 kHz to 300 GHz). To synthesize waveforms in the microwave frequency band, we are extending the existing Josephson arbitrary waveform synthesizer (JAWS) technology used for ac voltage metrology at audio frequencies (<20 kHz). Using new microwave circuits and experimental techniques, we demonstrate JAWS-based synthesis of single-tone waveforms at 1 GHz, with -49 dBm output power and 79 dBc spur-free dynamic range (SFDR). To synthesize waveforms in the millimeter-wave frequency band, we are exploiting single flux quantum (SFQ) superconducting digital technology to design circuits that amplify SFQ pulses using a network of pulse splitters. With our initial SFQ-based circuit, we demonstrate synthesis of a 4 GHz sine wave with -74 dBm output power and 70 dBc SFDR.