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Quantum Science and Technology

Compromising qubit control in quantum computing via hardware Trojans in multitone generators

Ayisat Adedokun, Yerzhan Mustafa, Selçuk Köse

Published Mar 9, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1088/2058-9565/ae4eb8

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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.
Abstract Hardware Trojans that exploit the unique characteristics of superconducting electronic (SCE) circuits are explored in this paper. Two types of hardware Trojan circuits are proposed: a magnetically-coupled data transmission Trojan embedded in single flux quantum full adder circuit and a pulse-interleaved Trojan implemented within a frequency divider. The working principles of how these Trojan circuits exploit the frequency-sensitive nature of SCE circuits are explained. The proposed Trojan circuits stay hidden during typical low-frequency testing, and only triggered and become active (and potentially dangerous) at higher frequencies. Extensive analysis demonstrate that these Trojans have a high chance of staying stealthy during testing, underscoring the need for more comprehensive security measures that account for the full range of operating frequencies for the protection of both classical superconducting systems and future quantum technologies.
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.
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.
Compared to traditional semiconductor control electronics (TSCE) located at room temperature, cryogenic single flux quantum (SFQ) electronics can provide qubit measurement and control alternatives that address critical issues related to scalability of cryogenic quantum processors. Single-qubit control and readout have been demonstrated recently using SFQ circuits coupled to superconducting qubits. Experiments where the SFQ electronics are co-located with the qubit have suffered from excess decoherence and loss due to quasiparticle poisoning of the qubit. A previous experiment by our group showed that moving the control electronics to the 3 K stage of the dilution refrigerator avoided this source of decoherence in a high-coherence 3D transmon geometry. In this paper, we also generate the pulses at the 3 K stage but have optimized the qubit design and control lines for scalable 2D transmon devices. We directly compare the qubit lifetime T1T_1T1​, coherence time T2∗T_2^*T2∗​ and gate fidelity when the qubit is controlled by the Josephson pulse generator (JPG) circuit versus the TSCE setup. We find agreement to within the daily fluctuations for T1T_1T1​ and T2∗T_2^*T2∗​, and agreement to within 10% for randomized benchmarking. We also performed interleaved randomized benchmarking on individual JPG gates demonstrating an average error per gate of 0.460.460.46% showing good agreement with what is expected based on the qubit coherence and higher-state leakage. These results are an order of magnitude improvement in gate fidelity over our previous work and demonstrate that a Josephson microwave source operated at 3 K is a promising component for scalable qubit control.
Quantum computing has demonstrated superior efficiency compared to classical computing. Quantum circuits are essential for implementing functions and achieving correct computational outcomes. Quantum circuit compilers, which translate high-level quantum operations into hardware-specific gates while optimizing performance, serve as the interface between the quantum software stack and physical quantum machines. However, untrusted compilers can introduce malicious hardware Trojans into quantum circuits, altering their functionality and leading to incorrect results. In the world of classical computing, effective hardware Trojans are a critical threat to integrated circuits. This process often involves stealthily inserting conditional logic gates that activate under specific input conditions. In this paper, we propose a novel advanced quantum Trojan that is controllable, allowing it to be activated or deactivated under different circumstances. These Trojans remain dormant until triggered by predefined input conditions, making detection challenging. Through a series of benchmark experiments, we demonstrate the feasibility of this method by evaluating the effectiveness of embedding controlled trojans in quantum circuits and measuring their impact on circuit performance and security.
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.