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EPJ Quantum Technology

Comparative study of high-$J_{c}$ and low-$J_{c}$ SFQ microwave generators for transmon control

Hongxiang Shen, Wenhui Luo, Nobuyuki Yoshikawa

Published Apr 24, 2026• 0 citations
Publisher

Abstract

Abstract Scalability of superconducting quantum processors is increasingly constrained by the thermal load and wiring overhead associated with room-temperature control hardware. Cryogenic control interfaces based on single-flux-quantum (SFQ) logic offer a potential route to alleviating these bottlenecks, although their dissipation depends sensitively on the Josephson-junction critical current and the underlying fabrication process. In this work, we comparatively investigate SFQ-synthesized microwave generators implemented in high- and low-critical-current processes, and evaluate their suitability for transmon qubit control through a combination of experimental characterization and hybrid QuTiP–JoSIM analysis. By modulating SFQ pulse intervals, we synthesize coherent microwave drives and obtain single-qubit gate fidelities above 99.9% at 10 mK in QuTiP-based simulations for both implementations. The low- $J_{\mathrm{c}}$ J c implementation exhibits reduced phase-noise-induced error, achieves higher gate fidelity within its valid bias window, and shows improved robustness against frequency detuning, while the high- $J_{\mathrm{c}}$ J c implementation maintains a lower broadband noise floor. Using an explicit model of thermal dissipation and circuit footprint, we further estimate the integration limits of cryogenic control channels. The results show that reducing the switching current substantially relaxes the cooling-budget constraint and increases the number of channels that can be deployed simultaneously. These results clarify the trade-offs associated with implementing SFQ microwave generators in different fabrication processes and provide practical guidance for the design of scalable cryogenic qubit-control interfaces.

DOI
10.1140/epjqt/s40507-026-00514-x

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Comparative study of high-JcJ_{c}Jc​ and low-JcJ_{c}Jc​ SFQ microwave generators for transmon control
Abstract Scalability of superconducting quantum processors is increasingly constrained by the thermal load and wiring overhead associated with room-temperature control hardware. Cryogenic control interfaces based on single-flux-quantum (SFQ) logic offer a potential route to alleviating these bottlenecks, although their dissipation depends sensitively on the Josephson-junction critical current and the underlying fabrication process. In this work, we comparatively investigate SFQ-synthesized microwave generators implemented in high- and low-critical-current processes, and evaluate their suitability for transmon qubit control through a combination of experimental characterization and hybrid QuTiP–JoSIM analysis. By modulating SFQ pulse intervals, we synthesize coherent microwave drives and obtain single-qubit gate fidelities above 99.9% at 10 mK in QuTiP-based simulations for both implementations. The low- JcJ_{\mathrm{c}}Jc​ J c implementation exhibits reduced phase-noise-induced error, achieves higher gate fidelity within its valid bias window, and shows improved robustness against frequency detuning, while the high- JcJ_{\mathrm{c}}Jc​ J c implementation maintains a lower broadband noise floor. Using an explicit model of thermal dissipation and circuit footprint, we further estimate the integration limits of cryogenic control channels. The results show that reducing the switching current substantially relaxes the cooling-budget constraint and increases the number of channels that can be deployed simultaneously. These results clarify the trade-offs associated with implementing SFQ microwave generators in different fabrication processes and provide practical guidance for the design of scalable cryogenic qubit-control interfaces.
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Abstract To achieve large-scale quantum processors, cryogenic quantum-bit (qubit) interface circuits that can control qubits inside a dilution refrigerator are indispensable. Superconducting single-flux quantum (SFQ) circuits are a promising building block for qubit interface circuits because SFQ circuits can operate with high clock frequencies and low power dissipation. In the present study, we developed an SFQ-based qubit interface circuit that we refer to as a pulse-pair microwave pulse generator (PP-MPG). The PP-MPG can generate microwave pulses for qubit control, with the microwave amplitude controlled based on the delay between paired SFQ pulses. By finely adjusting the delay with delay controlling circuits, the microwave amplitude can be controlled over a wide range. We fabricated a PP-MPG chip and tested it at 4.2 K, showing that the microwave amplitude can be controlled over a range of approximately 30 dB. Our results show that the PP-MPG has the potential for use in future large-scale quantum processors.
Abstract Realizing a practically useful superconducting quantum computer will require a dramatic scale-up of quantum control electronics to meet the growth in required qubit counts. Conventional rack-based room-temperature control electronics impose severe constraints on wiring density, cost, and system footprint, creating a barrier to large-scale and fault-tolerant quantum computing. Here we demonstrate a hybrid quantum control architecture that combines cryogenic complementary metal-oxide semiconductor (cryo‑CMOS) application-specific integrated circuits (ASICs) for two‑qubit flux control with room‑temperature radio‑frequency electronics for single‑qubit control and readout. The resulting system achieves high density and scalable quantum control without compromising two‑qubit gate fidelity. Deployed on a 156‑qubit quantum processor, the cryo-CMOS subsystem achieves two‑qubit gate errors at the {10}^{-3} level, with a best randomized benchmarking error per gate of 8.4\times{10}^{-4}, stable operation over multi‑day timescales, and no measurable performance degradation as we increase the temperature of the cryo-CMOS ASICs from 7 to 20 K. Thermal analysis identifies cryo-CMOS chip operation near 20 K as a favorable regime, enabling efficient cooling of a large number of channels per cryostat. Compared against rack‑based flux-control, the cryo-CMOS subsystem achieves statistically indistinguishable two-qubit gate fidelities under matched gate settings and enables a tenfold improvement in system spatial footprint. These results establish cryo‑CMOS flux control as a practical and scalable foundation for large‑scale and fault-tolerant superconducting quantum systems.
As superconducting and silicon-spin quantum processors scale from hundreds to thousands of physical qubits, the interconnect bottleneck between room-temperature classical electronics and mK cryogenic quantum processors poses a critical physical and architectural barrier. Low-latency control loops—essential for Quantum Error Correction (QED), fast reset protocols, and adaptive quantum algorithms—are strictly constrained by thermal load budgets, speed-of-light signal latency across thermal stages, and the bandwidth limits of coax lines. This paper presents a detailed architectural framework for next-generation quantum-classical interfaces. We investigate the transition from traditional, room-temperature arbitrary waveform generators (AWGs) connected via coaxial lines to co-integrated, cryo-CMOS, and single-flux-quantum (SFQ) digital-to-analog and digital signal processing control layers operating directly at the 4 K and 20 mK stages. We evaluate thermal transport, power dissipation, signal attenuation, and physical routing constraints, proposing a heterogeneous hybrid-control topology that satisfies latency budgets below 100 ns while remaining within the tight thermal limits of modern dilution refrigerators.
In the quest to build a fault-tolerant quantum computer, superconducting circuits based on Josephson junctions have emerged as a leading architecture. Coherence times have increased significantly over the last two decades, and processors with ∼ 50 qubits have been experimentally demonstrated. These systems traditionally utilize microwave frequency control signals, and heterodyne based detection schemes for measurement. Both of these techniques rely heavily on room temperature microwave generators, high-bandwidth lines from room temperature to millikelvin temperatures, and bulky non-reciprocal elements such as cryogenic microwave isolators. Reliance on these elements makes it impractical to scale existing devices up a single order of magnitude, let alone the 5-6 orders of magnitude needed for performing fault-tolerant quantum algorithms. Here, I present results that suggesting superconducting digital logic, namely Single Flux Quantum (SFQ) logic, can replace analog control and measurement techniques, avoiding the significant overhead involved. I describe a scheme for measuring qubits with a device known as a Josephson Photomultiplier (JPM), which crucially stores the result of a qubit measurement in a classical circulating supercurrent within the device and allows for integration with SFQ detection circuitry. This technique is experimentally demonstrated, with single-shot measurement fidelity of 92%. Two methods for accessing this measurement result are presented, one utilizing ballistic fluxons, and another utilizing flux comparison. Initial experimental results of the latter are presented. In addition, I describe a scheme for controlling qubits with sequences of digital SFQ pulses. This method is then used to control a qubit without a microwave signal generator, with results of an average single-qubit gate fidelity of around 95%. When combined, these techniques form a nearly fully digital interface to superconducting qubits, which could allow these systems to scale much more easily.
Abstract We propose a superconducting ring-oscillator-based SFQ microwave burst generator for scalable cryogenic qubit control. The scheme locally generates high-speed SFQ pulse trains on chip, reducing the need for high-frequency microwave input lines from room temperature. The implemented circuit occupies an area of approximately 0.7 mm×0.7 mm, excluding the TFF array used for low-speed verification, and its static power dissipation is estimated to be 1.14 μW. To suppress timing deviations accumulated in the ring oscillator, corrective pulses are inserted into the generated pulse sequence through low-speed control signals. Continuous-wave measurements verify basic microwave generation, while burst-mode experiments show that properly timed corrective pulses improve firstsideband symmetry and enhance the carrier peak power. Reference-based detrended TIE simulations further verify the existence of effective corrective timing windows in which the residual timing error is reduced below the without-corrective reference. These results indicate that corrective-pulse-assisted SFQ ring oscillators provide a compact and low-power approach for local microwave burst generation in cryogenic qubit-control systems.