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IEEE Transactions on Quantum Engineering

Josephson Microwave Sources Applied to Quantum Information Systems

Adam J. Sirois, Manuel A. Castellanos-Beltran, Anna E. Fox, Samuel P. Benz, P. F. Hopkins

Published Jan 1, 2020• 26 citations
Publisher

Abstract

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.

DOI
10.1109/tqe.2020.3045682

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Scaling of quantum computers to fault-tolerant levels relies critically on the integration of energyefficient, 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 , and the thermal occupation P th when the qubit is controlled by the JPG circuit versus the TSCE setup.We find agreement to within the daily fluctuations of ±0.5 μs and ±2 μs for T 1 and T * 2 , respectively, and agreement to within the 1% error for P th .Additionally, we perform randomized benchmarking to measure an average JPG gate error of 2.1 × 10 -2 .In combination with a small device size (< 25 mm 2 ) and low on-chip power dissipation ( 100 μW), 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.
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We present the design and characterization of a broadband RF Josephson arbitrary waveform synthesizer (RF-JAWS) with a series array of 4500 Josephson junctions (JJs) and integrated low-pass/high-pass five-pole superconducting diplexers. The integrated diplexers enable broadband filtering of the feedthrough signal components in the drive-current pulses with decade-wide instantaneous bandwidth. The diplexers have at least 30 GHz passband with less than 0.8 dB insertion loss. The JJ array is driven with a delta-sigma pulse sequence that encodes 10 kHz and 1.005 GHz tones, and generates an open-circuit voltage of 22 mV rms (−26.18 dBm available power assuming a 50Ω\mathrm{\Omega }ΩThevenin equivalent source) at 1.005 GHz—a 25% increase compared to the state of the art. The drive current pulses undergo a three-step equalization to compensate for the linear distortion of the room-temperature electronics, the on-chip diplexers, and the remaining on- and off-chip components. The measured parasitic feedthrough voltages at 1.005 GHz are around −33 dBc, and their effect on the quantum locking ranges (QLRs) is quantified by measurements. The results demonstrated in this article show a significant step toward a broadband, integrated, quantum-based microwave voltage source with useful power above −30 dBm.
Microwave SQUID multiplexers (μMUX) have become a key technology for the readout of large arrays of low-temperature detectors, particularly in cosmic microwave background experiments. A critical element of this technique is the flux-ramp modulation signal, whose noise, especially at low frequencies, directly limits the readout performance. This work proposes and experimentally demonstrates the use of a Josephson Arbitrary Waveform Synthesizer (JAWS) as a low-noise, high precision source for generating the sawtooth-shaped flux-ramp signal required in μMUX readout systems. The JAWS, based on the AC Josephson effect, provides intrinsically quantized and spectrally pure waveforms with excellent signal-to-noise ratio. The operation of a 9000-junction JAWS array is characterized and integrated with a cryogenic μMUX device and a custom SDR-based readout system. With this setup, the noise floor level around the harmonic typically used for phase demodulation of the SQUID signal to recover the detector signal was −104dBc/Hz, and the signal-to-noise ratio of this harmonic was 73dB. These results demonstrate the feasibility of using JAWS-based waveform synthesis for μMUX flux-ramp generation.