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arXiv:2609.01334

Fast Microwave-free State Preparation and Measurement of Superconducting Qubits

R. Abraham, F. Amet, P. Anderson, M. Arrigo, J. Arteaga, C. J. Ballard, C. Barker, T. Barnes, P. Bechmann, R. Bhatt, K. Blaine, T. M. Borman, J. Botimer, G. R. Boyd, P. Bradley, A. D. Brandon, T. K. Bristol, A. Bulkos, R. M. Burnett, D. Burrowes, D. N. Cakan, N. Carniero, T. Chamberlin, D. Chen, M. Chilcote, B. G. Christensen, I. Christie, J. Clark, D. Clarke, J. M. Cochran, J. C. Collini, K. Connolly, G. Costa, D. Cowger, B. Dalfort, R. D. Dawson, D. Davies, S. Deitemeyer, N. DeNigris, F. Densmore, S. J. Di Giacomo, S. G. Diamond, R. DiCiro, S. M. Disseler, K. Dixon, J. Donnelly, E. Donohue, M. Downing, B. Eastin, N. Edwards, M. Ehsani, S. Ellis, R. Epstein, D. Ferguson, P. Fischer, S. Friedensen, D. Gabriel, M. A. Gettelman, A. Gillam, G. Gilmore, J. Goode, E. Goodwin, M. Gottschalk, A. L. Graninger, T. Graves-Abe, J. Hackley, N. Hartman, B. Heacock, B. Heischmidt, P. Helms, R. Hinkey, B. Hong, S. T. Howard, D. Jensen, N. Johns, D. R. Johnson, J. Johnson, H. Kaplan, Z. Keane, S. Keebaugh, C. Kegerreis, K. Kelcourse-Oquendo, M. S. Khalil, D. Killeen, A. S. Knutson, T. Kohler, M. Kornecki, F. Koutsouli, A. Krick, K. L. Krycka, J. Kuan, D. Lad, J. R. Lane, N. J. Laurita, A. C. Lee, A. R. Lemmon, E. M. Leonard, L. M. D. Leonard, J. Leventis, M. P. Lilly, A. Lisiewski, L. Llano, M. Longo, C. Lostoski, Z. Lou, M. G. Loving, D. Ludwig, N. Luhrs, J. L. Lund, K. A. Maddock, R. J. Magyar, K. Mahmud, T. A. Manning, A. I. Marakov, A. D. McCreary, P. F. McLaughlin, J. R. Medford, E. Metz, A. L. Middleton, A. Miklich, R. Miller, J. Mlack, M. Mucci, N. Mungo, T. Murphy, R. E. Murray, O. Naaman, J. Nakamura, M. Noevere, S. Novikov, M. E. Nowakowski, C. Nunez, K. N. Ogg, B. Oshokoya, D. Paz, A. Pesetski, T. Pillsbury, C. Pinion, A. D. Pitcock, A. J. Przybysz, P. Quarterman, D. Queen, I. Ramos, S. L. Reed, D. Reitz, M. Rennie, S. Reza, B. Richman, K. H. Rigdon, C. Rotella, M. Rudolph, E. Sadler, T. Safford, D. Saha, A. E. Saia, D. I. Santiago, R. Schwartz, A. Schwarzkopf, M. Scott-Jones, S. L. Sendelbach, S. J. Shapiro, A. N. Sharma, M. E. Sherwin, A. Sierakowski, J. N. Sills, R. Simha, N. Siwak, J. Smith, C. Snyder, H. Solomon, N. S. Sperlein, L. St. Marie, K. Stalpes, S. F. Steers, Z. Stegen, R. M. Stein, T. I. Stephenson, M. Stoutimore, J. D. Strand, J. A. Strong, E. Swain, S. Swami, A. Tamang, I. Thompson, A. B. Tokarchik, N. Tralshawala, D. Tran, S. Tyler, L. Upton, G. Van Dyke, J. Vannucci, K. Vempati, J. S. Vogel, R. Vyas, M. Wagner, P. Warner, N. Washington, B. Way, M. A. Wayne, D. Wehella-Gamage, M. E. Weippert, T. Weitzel, J. Wenner, L. White, S. Whitsitt, A. Wilkinson, R. Willey, I. Yandow, J. Yang, L. Yu, F. Yumiceva, V. Zheng, R. Zimmerman, C. Zinn, J. Ziskind

quant-phSubmitted Sep 1, 2026• Announced Sep 2, 2026• 0 citations
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Abstract

Fast, high-fidelity, scalable state preparation and measurement is critical to the realization of a quantum computing system. The state-of-the-art methods for preparation and readout of superconducting qubits require finely tuned microwave signals and ~100 ns of measurement time, which are major obstacles to the scalability and performance of superconducting quantum computers. Here, we have demonstrated novel, microwave-free methods for both preparation and readout of superconducting qubits with >99% fidelity in only 10 ns for either operation while maintaining qubit coherence. This technology is compatible with scalable superconducting digital control systems, and using quantum flux parametrons for amplification, we demonstrated full quantum-to-digital conversion in only 15 ns, which is an order of magnitude faster than state-of-the-art microwave-based techniques.

Comments
13 pages, 7 figures, to be submitted to Science Magazine, authors listed alphabetically

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Fast, high-fidelity, scalable state preparation and measurement is critical to the realization of a quantum computing system. The state-of-the-art methods for preparation and readout of superconducting qubits require finely tuned microwave signals and ~100 ns of measurement time, which are major obstacles to the scalability and performance of superconducting quantum computers. Here, we have demonstrated novel, microwave-free methods for both preparation and readout of superconducting qubits with >99% fidelity in only 10 ns for either operation while maintaining qubit coherence. This technology is compatible with scalable superconducting digital control systems, and using quantum flux parametrons for amplification, we demonstrated full quantum-to-digital conversion in only 15 ns, which is an order of magnitude faster than state-of-the-art microwave-based techniques.
In the quest to build a fault-tolerant quantum computer, superconducting circuits based on Josephson junctions have emerged as a leading architecture. Coherence times have increased significantly over the last two decades, and processors with ∼ 50 qubits have been experimentally demonstrated. These systems traditionally utilize microwave frequency control signals, and heterodyne based detection schemes for measurement. Both of these techniques rely heavily on room temperature microwave generators, high-bandwidth lines from room temperature to millikelvin temperatures, and bulky non-reciprocal elements such as cryogenic microwave isolators. Reliance on these elements makes it impractical to scale existing devices up a single order of magnitude, let alone the 5-6 orders of magnitude needed for performing fault-tolerant quantum algorithms. Here, I present results that suggesting superconducting digital logic, namely Single Flux Quantum (SFQ) logic, can replace analog control and measurement techniques, avoiding the significant overhead involved. I describe a scheme for measuring qubits with a device known as a Josephson Photomultiplier (JPM), which crucially stores the result of a qubit measurement in a classical circulating supercurrent within the device and allows for integration with SFQ detection circuitry. This technique is experimentally demonstrated, with single-shot measurement fidelity of 92%. Two methods for accessing this measurement result are presented, one utilizing ballistic fluxons, and another utilizing flux comparison. Initial experimental results of the latter are presented. In addition, I describe a scheme for controlling qubits with sequences of digital SFQ pulses. This method is then used to control a qubit without a microwave signal generator, with results of an average single-qubit gate fidelity of around 95%. When combined, these techniques form a nearly fully digital interface to superconducting qubits, which could allow these systems to scale much more easily.
High-fidelity, efficient quantum nondemolition readout of quantum bits is integral to the goal of quantum computation. As superconducting circuits approach the requirements of scalable, universal fault tolerance, qubit readout must also meet the demand of simplicity to scale with growing system size. Here we propose a fast, high-fidelity, scalable measurement scheme based on the state-selective ring-up of a cavity followed by photodetection with the recently introduced Josephson photomultiplier (JPM), a current-biased Josephson junction. This scheme maps qubit state information to the binary digital output of the JPM, circumventing the need for room-temperature heterodyne detection and offering the possibility of a cryogenic interface to superconducting digital control circuitry. Numerics show that measurement contrast in excess of 95% is achievable in a measurement time of 140 ns. We discuss perspectives to scale this scheme to enable readout of multiple qubit channels with a single JPM.
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.
Superconducting qubits are pivotal in advancing quantum computing, poised for scale but limited by the complexity and fidelity of their control and readout systems, relying on RF and signal processing infrastructure. This survey serves as a comprehensive and technically grounded review of control and readout architectures tailored for superconducting qubits. Synthesizing insights from device physics, circuit design, microwave engineering, signal processing, and cryogenic integration, this work details the practicalities of RF pulse generation, signal synthesis, and readout signal analysis for quantum systems. it covers key requirements, parameters, and pulse engineering techniques, including commonly used envelopes like Gaussian and DRAG designs. Moving to the system level, this survey systematically classifies and critically analyses current architectural strategies (covering key areas like frequency conversion, waveform management, and system infrastructure) and technology platforms (including adaptive classical control stacks, cryogenic CMOS circuits, and novel interconnects and interfaces), evaluating their trade-offs in performance. Extensive literature analysis identifies prevailing limitations such as wiring complexity, thermal budget constraints, latency, and power consumption, while highlighting underexplored opportunities for on-chip signal processing and novel interconnects, drawing analogies to advanced communication systems design. By consolidating diverse control paradigms and critically evaluating their trade-offs, this survey provides a unified foundation for designing next-generation quantum control stacks. Finally, a forward-looking roadmap outlines key trends in monolithic integration, cryo-compatible digital architectures, and physics-informed hardware co-design, offering both a retrospective synthesis and a prospective vision for quantum hardware engineering beyond the NISQ era.
As the field of quantum computing progresses to larger-scale devices, multiplexing will be crucial to scale quantum processors. While multiplexed readout is common practice for superconducting devices, relatively little work has been reported about the combination of flux and microwave control lines. Here, we present a method to integrate a microwave line and a flux line into a single “XYZ line.” This combined control line allows us to perform fast single-qubit gates as well as to deliver flux signals to the qubits. The measured relaxation times of the qubits are comparable to state-of-the-art devices employing separate control lines. We benchmark the fidelity of single-qubit gates with randomized benchmarking, achieving a fidelity as high as 99.93±0.04%, and we demonstrate that XYZ lines can in principle be used to run parametric entangling gates.
Superconducting qubits are Josephson junction-based circuits that exhibit macroscopic quantum behavior and can be manipulated as artificial atoms. Benefiting from the well-developed technology of microfabrication and microwave engineering, superconducting qubits have great advantages in design flexibility, controllability, and scalability. Over the past decade, there has been rapid progress in the field, which greatly improved our understanding of qubit decoherence and circuit optimization. The single-qubit coherence time has been steadily raised to the order of 10 to 100 μs, allowing for the demonstration of high-fidelity gate operations and measurement-based feedback control. Here we review recent progress in the coherence and readout of superconducting qubits.