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IEEE Transactions on Applied Superconductivity

Side-Channel Leakage in SFQ Circuits and Related Attacks on Qubit Control and Readout Systems

Yerzhan Mustafa, Selçuk Köse

Published May 19, 2023• 10 citations
Publisher

Abstract

Single flux quantum (SFQ) technology is currently used as a cryogenic control and readout circuitry of superconducting quantum computers. The low power operation and high switching frequency of SFQ technology are instrumental to efficiently scale a quantum computer. With the growing interest in the hardware security of SFQ circuits, a novel side-channel leakage mechanism in SFQ-to-dc converters is uncovered in this article. The leakage mechanism is investigated for two logic styles, namely, rapid SFQ (RSFQ) and energy-efficient RSFQ (ERSFQ). By observing the fluctuations in the power supply provided by the room temperature electronics, a malicious adversary can extract critical information about the applied input signals. This type of side-channel leakage can lead to serious security vulnerabilities for superconducting electronics and quantum computers. Two potential side-channel attacks that exploit this vulnerability and target SFQ-based qubit control and readout systems are proposed.

DOI
10.1109/tasc.2023.3277864

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Single flux quantum (SFQ) technology is currently used as a cryogenic control and readout circuitry of superconducting quantum computers. The low power operation and high switching frequency of SFQ technology are instrumental to efficiently scale a quantum computer. With the growing interest in the hardware security of SFQ circuits, a novel side-channel leakage mechanism in SFQ-to-dc converters is uncovered in this article. The leakage mechanism is investigated for two logic styles, namely, rapid SFQ (RSFQ) and energy-efficient RSFQ (ERSFQ). By observing the fluctuations in the power supply provided by the room temperature electronics, a malicious adversary can extract critical information about the applied input signals. This type of side-channel leakage can lead to serious security vulnerabilities for superconducting electronics and quantum computers. Two potential side-channel attacks that exploit this vulnerability and target SFQ-based qubit control and readout systems are proposed.
With the growing interest in emerging and upcoming quantum computers, it is important to consider possible hardware security vulnerabilities and countermeasures. This work proposes side-channel attacks targeting classical-quantum interface circuits in superconducting quantum computers. Particularly, a single flux quantum (SFQ) technology-based classical controller, which is a promising candidate for large-scale in-fridge qubit control and readout circuitry, is considered. We have uncovered a substantial side-channel leakage from superconducting interface circuits through the power supply variations. An attacker, who has access to room temperature electronics where the power is generated, can successfully decode certain information about various components within quantum computers.
Abstract Scalable superconducting quantum computing systems require ultrafast and low-latency readout architectures. A Josephson phase digital detector (JDPD) converts the phase of an incoming analog microwave signal into a constant digital current state, enabling ultrafast, fully cryogenic phase discrimination. Thus, a JDPD is a critical component for scalable single-flux quantum (SFQ)-based qubit readout architectures and quantum computation. The flux bias driver (FBD) is a key element of the JDPD system, as it dynamically adjusts the detector’s operating point and enables the combined FBD-JDPD setup to function as a scalable SFQ-based qubit readout solution. Previous work has focused on detector operational and bias control, while information leakage through the shared bias infrastructure has received limited attention. Accordingly, this work investigates side-channel leakage due to the fluctuations in the bias current of an FBD-JDPD system. Extensive simulations demonstrate that phase-dependent switching events in a JDPD may cause bias current fluctuations that propagate backward through the inductively coupled bias network. When temporally aligned with the FBD set and reset operations, these fluctuations manifest as measurable perturbations on the shared bias line. The results show that the peak probabilities of 43.8 μ A for phase θ r = 0 and of 46.7 μ A for phase θ r = π appear due to the interface between the room temperature electronics and the device’s bias network. This leakage presents a serious security and privacy concern, as the shared bias network, which interfaces with electronics operating at room temperature, can serve as an easy access point for a malicious adversary.
Abstract 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. This paper deals with a scalable SFQ qubit control circuit (SQCC) structure that requires only low-frequency input. The circuit mainly consists of a pulse generator and a counter, that output the SFQ pulse train with adjustable frequency and a controllable number of pulses, which is applicable to control single-qubit Clifford operations. The design of low-voltage rapid single flux quantum (LV-RSFQ) and energy-efficient rapid single flux quantum (ERSFQ) for the SQCC achieves low power consumption and provides a basis for scaling up SQCC to control more qubits. The proposed circuits are fabricated under the SIMIT-Nb03 process and successfully pass test verification. The achieved test results reveal that the adjustable output frequency ranges of the SQCC based on the LV-RSFQ and ERSFQ designs in order are [2.40, 8.11] GHz and [4.81, 5.14] GHz. In the operating frequency range, the circuit is able to generate the correct number of SFQ pulses under control. The controllable number range is from 1 to 128. When the circuits operate at 5 GHz, the total power consumptions of the above circuits in order are 23.88 μW and 6.2 μW. All input signals are low-frequency signals, which frees the control of large-scale qubits from limitations caused by high-frequency inputs produced by room-temperature microwave sources.
Abstract Superconducting qubits have emerged as the leading qubit modality for quantum computing. Recent efforts by both academic and industrial researchers aim to enable high-speed, low-noise, and potentially scalable control and measurement of these qubits with superconducting digital electronics. Because the technology is still in an early phase of development, the classical control plane that communicates with quantum systems represents an attack surface that has not yet been thoroughly examined. This work exposes a critical vulnerability by presenting a frequency-activated hardware Trojan—termed as the data transmission Trojan (DT-Trojan)—embedded within a single-flux-quantum (SFQ) digital logic path. The DT-Trojan is functionally indistinguishable from a nominal flip-flop during conventional low-frequency bias-margin testing, but gets triggered at higher frequencies through a magnetically coupled feedback loop that introduces deterministic phase perturbations. Once triggered, the Trojan corrupts high-frequency SFQ pulse streams by disrupting phase coherence, which reduces quantum gate fidelity and leaks information covertly through spectral sidebands. Comprehensive circuit-level simulations using the JoSIM superconducting circuit simulator validate this dual-mode behavior. The results demonstrate a greater than 75% reduction in control fidelity and measurable spectral leakage once activated, underscoring the difficulty of detecting frequency-dependent Trojans in cryogenic environments. This work highlights an emerging class of security threats in SFQ-based quantum control and calls for verification strategies that account for high-frequency switching dynamics and magnetic-coupling effects in superconducting electronics.
The rapidly growing interest in quantum computing also increases the importance of securing these computers from various physical attacks. Constantly increasing qubit counts and improvements to the fidelity of the quantum computers hold great promise for the ability of these computers to run novel algorithms with highly sensitive intellectual property. However, in today's cloud-based quantum computer setting, users lack physical control over the computers. Physical attacks, such as those perpetrated by malicious insiders in data centers, could be used to extract sensitive information about the circuits being executed on these computers. This work shows the first exploration and study of power-based side-channel attacks in quantum computers. The explored attacks could be used to recover information about the control pulses sent to these computers. By analyzing these control pulses, attackers can reverse-engineer the equivalent gate-level description of the circuits, and the algorithms being run, or data hard-coded into the circuits. This work introduces five new types of attacks, and evaluates them using control pulse information available from cloud-based quantum computers. This work demonstrates how and what circuits could be recovered, and then in turn how to defend from the newly demonstrated side-channel attacks on quantum computing systems.
Design and manufacturing of superconductive electronics have been evolving over the past three decades with significant progress made in related fields. Rapid single flux quantum (RSFQ) logic circuits have become popular among superconductive logic families and its energy-efficient variants (ERSFQ and eSFQ) have shown promise as an ultra lowpower and high-speed circuit fabric. SFQ circuits have been demonstrated at tens of GHz with an energy consumption of an attojoule per gate. There are many differences between SFQ and conventional CMOS circuits. SFQ logic circuits are based on the manipulation of the quantized magnetic flux pulses. Most of the logic gates are sequential in nature requiring the clock to be distributed to every logic gate. SFQ logic gates have no gain and hence splitters are needed to drive multiple fanouts. Design and successful demonstration of a controllable superconducting switch and a compact reliable memory element have evaded researchers so far. This paper starts by describing key differences between SFQ logic and conventional CMOS and concludes by listing key challenges that must be overcome to achieve the very large scale integration of SFQ circuits and make the demonstration of a superconductive CPU a reality.