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IEEE International Symposium on Parallel & Distributed Processing, Workshops and Phd Forum

SFQ-Driven Pulse-Phase Sequence Generator for Superconducting Qubit Control

Meriam Gay Bautista, Patricia Gonzalez-Guerrero, Anastasiia Butko

Published Jun 3, 2025• 1 citation
Publisher

Abstract

The convergence of quantum computing and high-performance computing (HPC) necessitates innovative approaches to qubit control and coherence for scalable quantum systems. In this work, we explore the integration of superconducting Single-Flux Quantum (SFQ) digital circuits to enhance quantum state manipulation, leveraging their potential for high-speed, energy-efficient classical control. Our approach maps SFQ pulse trains and configurable SFQ-based switches to discrete phase shifts, enabling efficient qubit state transformations with minimal decoherence overhead. We introduce a classically driven SFQ-based quantum gate implementation, where SFQ pulses orchestrate the state evolution of qubits. To support dynamic and flexible qubit control within hybrid quantum-HPC architectures, we design and implement a multi-frequency SFQ pulse generator, capable of generating tailored pulse sequences that seamlessly interface with SQUID-based resonator-qubit cavities. This work demonstrates a promising path toward integrating quantum accelerators with classical HPC infrastructure, opti-mizing quantum-classical workflows, and enabling scalable, high-performance quantum computation.

DOI
10.1109/ipdpsw66978.2025.00182

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The convergence of quantum computing and high-performance computing (HPC) necessitates innovative approaches to qubit control and coherence for scalable quantum systems. In this work, we explore the integration of superconducting Single-Flux Quantum (SFQ) digital circuits to enhance quantum state manipulation, leveraging their potential for high-speed, energy-efficient classical control. Our approach maps SFQ pulse trains and configurable SFQ-based switches to discrete phase shifts, enabling efficient qubit state transformations with minimal decoherence overhead. We introduce a classically driven SFQ-based quantum gate implementation, where SFQ pulses orchestrate the state evolution of qubits. To support dynamic and flexible qubit control within hybrid quantum-HPC architectures, we design and implement a multi-frequency SFQ pulse generator, capable of generating tailored pulse sequences that seamlessly interface with SQUID-based resonator-qubit cavities. This work demonstrates a promising path toward integrating quantum accelerators with classical HPC infrastructure, opti-mizing quantum-classical workflows, and enabling scalable, high-performance quantum computation.
In the interest of alleviating qubit coherence constraints and making quantum control circuits more efficient, this paper explores qubit control using superconducting Single-Flux Quantum (SFQ) digital circuits. By using SFQ pulse trains and SFQ switches, we represent qubit state transformations through their equivalent phase changes, where each SFQ pulse represents a phase change of certain degrees. We propose classical-based unitary quantum gates represented by SFQ pulses. We create a classical equivalent model for quantum gates based on an SFQ-based parametric pulse sequence equivalent to quantum Pauli gates. To generate pulse sequences of configurable parameters, we implement a versatile multi-frequency pulse generator that seamlessly integrates with the qubit-resonator cavity.
An approach for scalable quantum computing infrastructure based on the use of low-power digital superconducting single flux quantum (SFQ) circuits is described. Rather than replicating the room-temperature microwave control and measurement infrastructure solutions dominating the current systems, we use the inherent to superconducting technology methods - the use of SFQ pulses directly at the base temperature. For qubit control, we irradiate qubits with the coherent SFQ pulse sequences computed using optical control theory. For qubit measurement, Josephson photon counter performs projective quantum measurement, the result of which is converted to digital SFQ output. These operations are aided by a high-speed digital SFQ coprocessor located at higher temperature stage (e.g., 3 K) to process the measurement results and load new control sequences to 20 mK SFQ quantum-classical interface circuits.
Abstract The development of quantum computers based on superconductors requires the improvement of the qubit state control approach aimed at the increase of the hardware energy efficiency. A promising solution to this problem is the use of superconducting digital circuits operating with single-flux-quantum (SFQ) pulses, moving the qubit control system into the cold chamber. However, the qubit gate time under SFQ control is still longer than under conventional microwave driving. Here we introduce the bipolar SFQ pulse control based on ternary pulse sequences. We also develop a robust optimization algorithm for finding a sequence structure that minimizes the leakage of the transmon qubit state from the computational subspace. We show that the appropriate sequence can be found for arbitrary system parameters from the practical range. The proposed bipolar SFQ control reduces a single qubit gate time by halve compared to nowadays unipolar SFQ technique, while maintaining the gate fidelity over 99.99%.
Single-flux-quantum (SFQ) digital logic is an attractive candidate for classical control and monitoring of large-scale quantum processors based on superconducting qubits. The authors describe a control approach based on repeated irradiation of the qubits with short registers of classical bits, which can be stored locally in compact SFQ registers and streamed to the quantum array at a high rate. Numerical simulations show achievable gate fidelity in excess of 99.99%; moreover, the approach allows control of many qubits resonating at different frequencies with a single, global SFQ clock. As a result, the scheme is well matched to the control of a scalable two-dimensional surface code.
Scalability of today’s superconducting quantum computers is limited due to the huge costs of generating/routing microwave control pulses per qubit from room temperature. One active research area in both industry and academia is to push the classical controllers to the dilution refrigerator in order to increase the scalability of quantum computers. Superconducting Single Flux Quantum (SFQ) is a classical logic technology with low power consumption and ultra-high speed, and thus is a promising candidate for in-fridge classical controllers with maximized scalability. Prior work has demonstrated high-fidelity SFQ-based single-qubit gates. However, little research has been done on SFQ-based multi-qubit gates, which are necessary to realize SFQ-based universal quantum computing.In this paper, we present the first thorough analysis of SFQ-based two-qubit gates. Our observations show that SFQ-based two-qubit gates tend to have high leakage to qubit non-computational subspace, which presents severe design challenges. We show that despite these challenges, we can realize gates with high fidelity by carefully designing optimal control methods and qubit architectures. We develop optimal control methods that suppress leakage, and also investigate various qubit architectures that reduce the leakage. After carefully engineering our SFQ-friendly quantum system, we show that it can achieve similar gate fidelity and gate time to microwave-based quantum systems. The promising results of this paper show that (1) SFQ-based universal quantum computation is both feasible and effective; and (2) SFQ is a promising approach in designing classical controller for quantum machines because it can increase the scalability while preserving gate fidelity and performance.
The use of high-frequency input signals from room-temperature microwave sources makes it difficult to scale up the number of quantum bits in universal quantum computers. To address this issue, superconducting single flux quantum (SFQ) integrated circuits are being explored as suitable candidates for qubit manipulation in universal quantum computers. Previously, we designed a low-power SFQ qubit control circuit (SQCC) to produce an equidistant SFQ pulse train without high-frequency input. It was reported that the fidelity of qubit control could be further improved if the pattern of SFQ pulse train could be adjustable. In this paper, we present optimized designs for different pattern requirements of qubit control, building upon the SQCC without high-frequency input. These designs feature segmented frequency modulation circuits, as well as circuits capable of outputting arbitrary patterns. The circuit capable of generating arbitrary patterns currently achieves a maximum adjustable number of 400, with a corresponding frequency output of 20 GHz. It has been successfully fabricated using the SIMIT Nb03 process and has passed functionality measurement tests. The optimized circuit opens up possibilities for improved qubit control and advancements in quantum computing applications.