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IEEE International Symposium on Circuits and Systems proceedings

Enabling Classical-Quantum Interface Using Digital SFQ for Pulse-Phase Driven Control for Superconducting Qubits

Meriam Gay Bautista, Patricia Gonzalez-Guerrero, George Michelogiannakis, Anastasiia Butko

Published May 25, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.1109/iscas56072.2025.11043402

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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.
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.
Superconducting qubits are conventionally controlled with shaped microwave pulses from a microwave carrier tone. This works well for small systems, but is difficult to scale up to the millions of qubits needed for a general-purpose quantum computer. Instead, the authors suggest irradiating a qubit with trains of quantized flux pulses derived from single flux quantum (SFQ) digital logic. The pulses are generated by an SFQ driver circuit cofabricated on the qubit chip. This work opens the door to tight integration of a quantum array with a classical coprocessor, to lower wiring heat load, latency, and overall system footprint.
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.
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%.
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.
Single-flux-quantum (SFQ) circuits have great potential in building cryogenic quantum-classical interfaces for scaling up superconducting quantum processors. SFQ-based quantum gates have been designed and realized. However, current control schemes are difficult to tune the driving strength to qubits, which restricts the gate length and usually induces leakage to unwanted levels. In this study, we design the scheme and corresponding pulse generator circuit to continuously adjust the driving strength by coupling SFQ pulses with variable intervals. This scheme not only provides a way to adjust the SFQ-based gate length, but also proposes the possibility to tune the driving strength envelope. Simulations show that our scheme can suppress leakage to unwanted levels and reduce the error of SFQ-based Clifford gates by more than an order of magnitude.