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Quantum Science and Technology

Side-channel attacks on digital readout circuits for superconducting qubits

Gulnur Kulzhanova, Yerzhan Mustafa, Selçuk Köse

Published Jul 15, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1088/2058-9565/ae8b57

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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.
Quantum computation requires high-fidelity qubit readout, preserving the quantum state. In the case of superconducting (SC) qubits, readout is typically performed using a complex analog experimental setup operated at room temperature, which poses significant technological and economic barriers to large system scalability. An alternative approach is to perform a cryogenic on-chip qubit readout based on a Josephson Digital Phase Detector (JDPD): a flux switchable device capable of digitizing the phase sign of a coherent input. The readout operation includes the flux excitation of the JDPD to evolve from a single to a double-minima potential. In this work, the effect of the flux bias characteristics on the JDPD performances is studied numerically. To meet the identified requirements that maximize detection fidelity and tackle the engineering challenges, a cryogenic on-chip Single Flux Quantum based flux bias driver is proposed and discussed.
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.
Scientific efforts and financial investments in quantum computers have increased in recent years, with superconducting architectures as one of the most promising platforms. To reach the true potential of quantum computers, the current systems need to be significantly scaled up. Current methods of superconducting qubit readout require routing between room-temperature electronics and the qubit which is operated in cryogenic conditions. When systems that use this approach are scaled up, problems arise; the numerous cables required for the routing to room temperature electronics become even more problematic as the system size increases. This method also suffers from increased measurement feedback latency which is worsened further by the scale-up. A novel architecture of on-chip qubit readout that addresses both of these problems is proposed by the Quantum Computing and Devices (QCD) group. The proposition involves a sensitive calorimeter which measures the qubit state. This measurement manifests as a change in the calorimeter frequency which can be further sensed by a probe-signal and encoded into the phase value of the reflected signal. To analyse the qubit state and determine the next action, the signal is required to be transferred to a control unit which determines the feedback to other microwave components. This requires digitisation of the reflected phase value. The method of digitisation that is proposed in the architecture utilises the Josephson digital phase detector (JDPD) which is a superconducting quantum circuit device that is also placed on chip. The JDPD is able to effectively digitise the phase value of an input signal owing to its non-linear potential, the configuration of which can be switched on command. However, the power of the reflected signal is extremely low which negatively affects the sensitivity of the JDPD and leads to higher detection error. The aim of this thesis is to increase the sensitivity of the JDPD with respect to low-power signals. This is achieved by optimising the critical current of the two Josephson junctions present in the system as well as the inductance of the central inductor. Successful optimisation makes the device more compatible with the proposed calorimetric qubit readout. The optimisation is conducted by simulating the system in Quantum Toolbox in Python (QuTiP) and analysing the resulting detection error. Before optimisation, the theory behind the system is discussed, and key operators are derived to later justify the parameters and assumptions that are chosen when optimising. Based on the simulations, this thesis found a low detection error space forinductance and critical current. The parameter pairs within this space yield adetection error of ∼ 41% which is an improvement from the, essentially random, baseline of ∼ 48%. The system has many free param.
As superconducting quantum processors increase in size and complexity, the scalability of standard techniques for qubit control and readout becomes a limiting factor. Replacing room temperature analog components with cryogenic digital components could allow for the realization of systems well beyond the current state-of-the-art qubit arrays with tens of qubits. The standard technique for performing a qubit measurement with heterodyne readout uses a quantum-limited cryogenic amplifier chain and requires bulky microwave components inside the refrigerator with multiple control lines and pump signals. Additionally, the result is only accessible in software at room temperature. An alternative method for measuring qubits involves mapping the qubit state onto the photon occupation in a microwave cavity, followed by subsequent photon detection using a Josephson photomultiplier (JPM). The JPM measures the qubit and stores the result in a classical circulating current. To make use of this result, we can leverage existing single flux quantum (SFQ) circuitry. An underdamped Josephson transmission line (JTL) can be coupled to the JPM and fluxons traveling along the JTL are accelerated or delayed, depending on the circulating current state of the JPM. This fluxon delay can then be converted to an SFQ logic signal resulting in a digital qubit readout with a proximal microfabricated device, paving the way for cryogenic digital feedback necessary for error-correcting codes.
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.