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arXiv:2412.11961 · IEEE Transactions on Quantum Engineering

Control of a Josephson Digital Phase Detector via an SFQ-Based Flux Bias Driver

Laura Di Marino, Luigi Di Palma, Michele Riccio, F. Fienga, M. Arzeo, Oleg A. Mukhanov

Published Jan 1, 2025• 5 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1109/tqe.2025.3583570

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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.
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.
We propose a new phase detection technique based on a flux-switchable superconducting circuit, the Josephson digital phase detector (JDPD), which is capable of discriminating between two phase values of a coherent input tone. When properly excited by an external flux, the JDPD is able to switch from a single-minimum to a double-minima potential and, consequently, relax in one of the two stable configurations depending on the phase sign of the input tone. The result of this operation is digitally encoded in the occupation probability of a phase particle in either of the two JDPD wells. In this work, we demonstrate the working principle of the JDPD up to a frequency of 400 MHz with a remarkable agreement with theoretical expectations. As a future scenario, we discuss the implementation of this technique to superconducting qubit readout. We also examine the JDPD compatibility with the single-flux-quantum architecture, employed to fast-drive and measure the device state.
Abstract Controlling a large number of superconducting qubits is a key challenge for realizing a fault-tolerant quantum computer. Implementing a qubit control system operating at the proximal millikelvin stage of the dilution refrigerator is a promising approach to reduce the number of coaxial cables required for qubit control. Single-flux-quantum (SFQ) logic circuits are one candidate for realizing such cryogenic control systems; however, their power dissipation must be minimized to avoid any negative impact on qubit performance. In this study, we demonstrate the operation of a flux-bias-free (FBF) flux qubit controlled by an SFQ driver on a monolithic chip. Here, the critical current ( I C ) of the Josephson junctions in the SFQ driver was set to a few µA, which is two orders of magnitude smaller than that in a typical SFQ circuit operating at 4 K, significantly reducing power consumption. Using this low-power SFQ driver, we successfully demonstrate single-qubit operation without any reduction of the energy relaxation time of the FBF qubit.
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.
Scaling superconducting quantum processors is increasingly constrained by the wiring, heat load, and calibration overhead associated with delivering high-resolution analog signals from room temperature to qubits at millikelvin temperature. Here we demonstrate a superconducting digital-to-analog converter (DAC) integrated with high-coherence fluxonium qubits in a multi-chip module architecture. The DACs generate persistent analog flux signals for tuning qubit parameters and are programmed deterministically using single-flux-quantum (SFQ) pulses, providing a digital interface compatible with established SFQ routing and demultiplexing technologies. Operating at millikelvin temperature, the DACs enable in-situ tuning of fluxonium qubits without measurable degradation of qubit coherence. The presented device provides a static control primitive for flux-tunable qubits, enabling parameter homogenization and eliminating the need for individual room-temperature DC bias lines. These results establish SFQ-programmable millikelvin DACs as a building block for digitally controlled superconducting quantum processors.