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arXiv:2505.03689 · IEEE Transactions on Applied Superconductivity

Variable Frequency Pulse Generation From Breathers in Josephson Transmission Lines

Gregory Cunningham, Yufeng Ye, Kaidong Peng, Alec Yen, Jessica Kedziora, Kevin P. O’Brien

Published Jul 21, 2025• 1 citation
arXivHTMLPDF

Abstract

Single flux quantum technology has the potential to enhance readout and control of superconducting quantum systems due to their low energy consumption, high speed, and cryogenic operating temperatures. Current cryogenic readout and control typically requires microwave pulses of specific frequencies to travel between the room temperature control electronics and the cryogenic setup. Latency in control and readout can be improved by generating pulses within the dilution refrigerator. In this work, we consider a protocol for generating gigahertz frequency microwave tones from trains of DC-centered fluxons and fluxoids in Josephson transmission lines using the dynamics of breather formation, without room temperature synthesis or shunt / bias resistors. Simulations show that pulses with frequencies in the range of 15.2 to 21.5 GHz can be generated with maximal energy efficiency of 97% and bandwidth from 40 to 365 MHz. This protocol can also be used to generate gigahertz frequency Gaussian pulses. We detail metrics relevant to the control and readout of quantum systems such as input power, output power, and footprint

DOI
10.1109/tasc.2025.3591399

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Single flux quantum technology has the potential to enhance readout and control of superconducting quantum systems due to their low energy consumption, high speed, and cryogenic operating temperatures. Current cryogenic readout and control typically requires microwave pulses of specific frequencies to travel between the room temperature control electronics and the cryogenic setup. Latency in control and readout can be improved by generating pulses within the dilution refrigerator. In this work, we consider a protocol for generating gigahertz frequency microwave tones from trains of DC-centered fluxons and fluxoids in Josephson transmission lines using the dynamics of breather formation, without room temperature synthesis or shunt / bias resistors. Simulations show that pulses with frequencies in the range of 15.2 to 21.5 GHz can be generated with maximal energy efficiency of 97% and bandwidth from 40 to 365 MHz. This protocol can also be used to generate gigahertz frequency Gaussian pulses. We detail metrics relevant to the control and readout of quantum systems such as input power, output power, and footprint
Superconducting quantum computing is a promising path towards achieving fault-tolerant quantum computation. Control and readout of signals at mK temperatures and subsequent amplification to room temperature electronics has allowed for impressive feats such as the first demonstration of quantum supremacy / advantage. The scalability of current systems that enable quantum computation are hindered by signal latency, excess heat loads, and overcrowding of cables in the dilution refrigerator. In this thesis, we propose cryogenic solutions using magnetic flux that improve scalability of superconducting quantum processors. In an effort to bridge the energy gap between the mK and 4 K stages of the dilution refrigerator, we simulate a flux soliton amplifier that can provide up to 10x gain to flux soliton pulses with low-loss in a resistance free traveling-wave bias scheme. To address latency and spatial cable considerations, we simulate a flux soliton cryogenic pulse generator that uses breather oscillations to create microwave pulses in the range of 15 - 24 gigahertz with over 97\% energy efficiency. In addition, we present an experimental investigation of transmission properties for resonantly phase-matched Josephson traveling-wave amplifiers in magnetic fields to develop useful intuition towards the challenge of operating cryogenic parametric amplifiers in high magnetic fields. Our work presents paths for utilizing magnetic flux as a resource to advance large-scale high-fidelity quantum computing.
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.
Frequency combs represent exceptionally precise measurement tools due to the coherence of their spectral lines. While optical frequency comb sources constitute a well-established technology, superconducting circuits provide a relatively unexplored on-chip platform for low-dissipation comb emitters able to span from gigahertz to terahertz frequencies. We demonstrate coherent microwave frequency comb generation by leveraging the ac Josephson effect in a superconducting quantum interference device. A time-dependent magnetic drive periodically generates voltage pulses, which in the frequency domain correspond to a comb with dozens of spectral modes here reported up to mode 46. The emitted power at the device level ranges from -170 dBm to -130 dBm per harmonic, corresponding to 40 dB dynamic range in the 4-8 GHz bandwidth. The micrometer-scale footprint and minimal dissipation inherent to superconducting systems foster the integration of our comb generator with advanced cryogenic electronics. Transferring optical techniques to the solid-state domain may enable new applications in quantum technologies.
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.
Comparative study of high-JcJ_{c}Jc​ and low-JcJ_{c}Jc​ SFQ microwave generators for transmon control
Abstract Scalability of superconducting quantum processors is increasingly constrained by the thermal load and wiring overhead associated with room-temperature control hardware. Cryogenic control interfaces based on single-flux-quantum (SFQ) logic offer a potential route to alleviating these bottlenecks, although their dissipation depends sensitively on the Josephson-junction critical current and the underlying fabrication process. In this work, we comparatively investigate SFQ-synthesized microwave generators implemented in high- and low-critical-current processes, and evaluate their suitability for transmon qubit control through a combination of experimental characterization and hybrid QuTiP–JoSIM analysis. By modulating SFQ pulse intervals, we synthesize coherent microwave drives and obtain single-qubit gate fidelities above 99.9% at 10 mK in QuTiP-based simulations for both implementations. The low- JcJ_{\mathrm{c}}Jc​ J c implementation exhibits reduced phase-noise-induced error, achieves higher gate fidelity within its valid bias window, and shows improved robustness against frequency detuning, while the high- JcJ_{\mathrm{c}}Jc​ J c implementation maintains a lower broadband noise floor. Using an explicit model of thermal dissipation and circuit footprint, we further estimate the integration limits of cryogenic control channels. The results show that reducing the switching current substantially relaxes the cooling-budget constraint and increases the number of channels that can be deployed simultaneously. These results clarify the trade-offs associated with implementing SFQ microwave generators in different fabrication processes and provide practical guidance for the design of scalable cryogenic qubit-control interfaces.
For superconducting quantum processors, microwave signals are delivered to each qubit from room-temperature electronics to the cryogenic environment through coaxial cables. Limited by the heat load of cabling and the massive cost of electronics, such an architecture is not viable for millions of qubits required for fault-tolerant quantum computing. Monolithic integration of the control electronics and the qubits provides a promising solution, which, however, requires a coherent cryogenic microwave pulse generator that is compatible with superconducting quantum circuits. Here, we report such a signal source driven by digital-like signals, generating pulsed microwave emission with well-controlled phase, intensity, and frequency directly at millikelvin temperatures. We showcase high-fidelity readout of superconducting qubits with the microwave pulse generator. The device demonstrated here has a small footprint, negligible heat load, great flexibility to operate, and is fully compatible with today's superconducting quantum circuits, thus providing an enabling technology for large-scale superconducting quantum computers.