A superconducting qubit is driven through a control line whose coupling, by the fluctuation-dissipation theorem, simultaneously opens a channel for radiative decay (Purcell loss): stronger coupling gives faster gates but a shorter qubit lifetime, while weaker coupling preserves lifetime but demands high microwave power that heats the cryogenic environment, stresses control electronics, and causes crosstalk. Linear Purcell filters cannot protect the drive line, because the qubit's decay frequency coincides with the control-signal frequency. Kono et al. (2020) broke this trade-off for a single qubit using a Josephson quantum filter - a saturable ancillary qubit that reflects single photons but becomes transparent under strong control pulses - and later work demonstrated on-chip stopband filters with subharmonic drives and Purcell-protected flux lines, all at the single-qubit level. The open problem is the scalable version: suppress radiative decay into the control line at the control frequency while transmitting the large-amplitude pulses needed for fast, high-fidelity single- and two-qubit gates in multi-qubit processors, without per-qubit component, wiring, power, or crosstalk overhead that negates the benefit at scale.

Background

Superconducting qubits, most commonly the transmon (a weakly anharmonic superconducting oscillator built from Josephson junctions and a capacitance), are manipulated by microwave pulses delivered through a dedicated drive (control) line. The quality of gates is set by the Rabi rate - the frequency at which the driven qubit oscillates between its two states, which grows with the drive amplitude and with the qubit-line coupling - while the qubit's energy-relaxation time T1T_{1} (the mean time for the qubit to lose an excitation) is degraded by that same coupling: by the fluctuation-dissipation theorem, any line through which control fields enter is also a channel into which the qubit can spontaneously emit a microwave photon. This radiative, or Purcell, decay creates a fundamental engineering trade-off. Coupling the control line strongly enables fast gates but caps T1T_{1}; coupling it weakly preserves T1T_{1} but requires larger injected microwave power to keep gate speed, which heats the cryogenic environment of the qubit package, strains the output power of control electronics, and increases microwave crosstalk (unwanted drive of neighboring qubits) - problems that compound as processors scale to many qubits.

The standard countermeasure on readout lines is the Purcell filter, a frequency-selective network introduced by Reed et al. (2010) that transmits signals at the readout frequency while presenting a stopband at the qubit frequency, so fast readout and long T1T_{1} coexist. As Kono, Koshino, Lachance-Quirion, van Loo, Tabuchi, Noguchi and Nakamura emphasize in 'Breaking the trade-off between fast control and long lifetime of a superconducting qubit' (Nature Communications, 2020), this frequency-selective strategy fails for the drive line whenever the qubit's radiative-decay frequency and the control-signal frequency are identical, as they are for resonant qubit driving - a linear filter cannot simultaneously block and pass the same frequency.

Kono et al. therefore distinguished the two by amplitude rather than frequency: they strongly coupled an ancillary qubit (external coupling rate 2π×1122\pi \times 112 MHz) to the control line, forming a 'Josephson quantum filter' (JQF). In the single-photon (linear) regime the JQF reflects, suppressing the data qubit's radiative decay by roughly three orders of magnitude relative to the weak coupling that a state-of-the-art qubit with millisecond T1T_{1} would otherwise require; under the strong fields of control pulses the JQF saturates and becomes transparent, preserving fast gates. Subsequent work established alternative routes, each demonstrated at the single-qubit level: on-chip λ/4\lambda/4 and λ/2\lambda/2 filters with a stopband at the qubit frequency combined with subharmonic drives (control tones at a fraction of the qubit frequency, up-converted by the qubit's nonlinearity), giving up to 200-fold T1T_{1} improvement with 12-ns π\pi pulses (Sah et al., 2024) and 37.4-ns single-qubit gates at 99.91% fidelity via driving the transmon's Kerr nonlinearity (the anharmonic, power-dependent frequency shift of its oscillator) at one third of the qubit frequency (Xia et al., 2025); and fluxonium qubits (a superconducting qubit whose Josephson junction array inductance makes it highly anharmonic) driven at up to 11-photon subharmonics through a low-pass-filtered flux line, with 5-fold longer T1T_{1} and 10-fold longer T2T_{2}-echo times than in the unfiltered configuration (Schirk et al., 2025). Passive suppression through engineered interference of decay pathways has also been demonstrated (Bakr et al., 2025), and reviews of 'nonradiating qubit' concepts continue to treat radiation suppression as an open frontier (Basharin, 2026).

None of these demonstrations, however, has been carried into a multi-qubit processor. Each known route carries scale-up burdens: the JQF adds an ancillary qubit per data qubit with protection bandwidth of order one percent of its carrier frequency; subharmonic control involves large AC Stark shifts (drive-induced qubit-frequency shifts) that complicate calibration; strong-drive power on chip still dissipates heat and can crosstalk to non-targeted qubits; and every per-qubit filter or ancilla adds fabrication and wiring overhead. Adjacent scaling efforts, such as multiplexed control lines shared by many qubits (Shi et al., 2023) and Purcell-filtered multiconductor readout (Spring et al., 2025), address wiring and readout but not the drive-line decay-versus-drive trade-off itself.

Problem Statement

Realize, in a scalable superconducting-qubit architecture, the simultaneous achievement of (i) suppression of a qubit's radiative decay into its control line at the frequency where that decay occurs - so that the idle qubit's energy-relaxation time T1T_{1} is not limited by the control line even when the line is strongly coupled - and (ii) transmission of the large-amplitude control pulses required for fast, high-fidelity gates, including entangling and parametric gates where they are routed through the same protected lines, while (iii) keeping the per-qubit cost of the protection - added components such as ancilla qubits or filters, extra wiring and calibration, dissipated control power in the cryogenic environment, and microwave crosstalk to non-targeted qubits - small enough that it does not reintroduce, at processor scale, the very heating, crosstalk, and wiring burdens the protection was meant to remove. Single-qubit-level demonstrations of this trade-off breaking (Josephson quantum filter; on-chip stopband filters with subharmonic drives; Purcell-protected flux lines) already exist and constitute the baseline, not a solution; the unresolved target is the demonstration or quantitative design, with overhead accounting, of such protection operating across the multiple strongly driven qubits of a scalable processor.

References

  1. S. Kono, K. Koshino, D. Lachance-Quirion, A. F. van Loo, Y. Tabuchi, A. Noguchi, Y. Nakamura (2020), 'Breaking the trade-off between fast control and long lifetime of a superconducting qubit', Nature Communications, DOI: 10.1038/s41467-020-17511-y, https://doi.org/10.1038/s41467-020-17511-y
  2. Kazuki Koshino, Shingo Kono, Yasunobu Nakamura (2020), 'Protection of a Qubit via Subradiance: A Josephson Quantum Filter', Physical Review Applied, DOI: 10.1103/PhysRevApplied.13.014051, https://doi.org/10.1103/PhysRevApplied.13.014051
  3. Shumpei Masuda, Kazuki Koshino (2021), 'Effects of higher levels of qubits on control of qubit protected by a Josephson quantum filter', New Journal of Physics, DOI: 10.1088/1367-2630/abd809, https://doi.org/10.1088/1367-2630/abd809
  4. Ivan Iakoupov, Kazuki Koshino (2023), 'Saturable Purcell filter for circuit quantum electrodynamics', Physical Review Research, DOI: 10.1103/PhysRevResearch.5.013148, https://doi.org/10.1103/PhysRevResearch.5.013148
  5. Aashish Sah, Suman Kundu, Heikki Suominen, Qiming Chen, Mikko Möttönen (2024), 'Decay-protected superconducting qubit with fast control enabled by integrated on-chip filters', Communications Physics, DOI: 10.1038/s42005-024-01733-3, https://doi.org/10.1038/s42005-024-01733-3
  6. Mingkang Xia, Chao Zhou, Chenxu Liu, Param Patel, Xi Cao, Pinlei Lu, Boris Mesits, Maria Mucci, David Gorski, David Pekker, Michael Hatridge (2025), 'Fast superconducting qubit control with subharmonic drives', Nature Communications, DOI: 10.1038/s41467-025-67766-6, https://doi.org/10.1038/s41467-025-67766-6
  7. J. Schirk, F. Wallner, L. Huang, I. Tsitsilin, N. Bruckmoser, L. Koch, D. Bunch, N. J. Glaser, G. B. P. Huber, M. Knudsen, G. Krylov, A. Marx, F. Pfeiffer, L. Richard, F. A. Roy, J. H. Romeiro, M. Singh, L. Södergren, E. Dionis, D. Sugny, M. Werninghaus, K. Liegener, C. M. F. Schneider, S. Filipp (2025), 'Subharmonic Control of a Fluxonium Qubit via a Purcell-Protected Flux Line', PRX Quantum 6, 030315, DOI: 10.1103/yx15-jyl7, https://doi.org/10.1103/yx15-jyl7
  8. Mustafa Bakr, Mohammed Alghadeer, Simon Pettersson Fors, Simone D. Fasciati, Shuxiang Cao, Atharv Mahajan, Smain Amari, Anton Frisk Kockum, Peter Leek (2025), 'Intrinsic Multi-Mode Interference for Passive Suppression of Purcell Decay in Superconducting Circuits', arXiv:2507.09715, https://arxiv.org/abs/2507.09715
  9. M. D. Reed, B. R. Johnson, A. A. Houck, L. DiCarlo, J. M. Chow, D. I. Schuster, L. Frunzio, R. J. Schoelkopf (2010), 'Fast reset and suppressing spontaneous emission of a superconducting qubit', Applied Physics Letters 96, 203110, DOI: 10.1063/1.3435463, https://doi.org/10.1063/1.3435463
  10. Y. Sunada, S. Kono, J. Ilves, S. Tamate, T. Sugiyama, Y. Tabuchi, Y. Nakamura (2022), 'Fast Readout and Reset of a Superconducting Qubit Coupled to a Resonator with an Intrinsic Purcell Filter', Physical Review Applied, DOI: 10.1103/PhysRevApplied.17.044016, https://doi.org/10.1103/PhysRevApplied.17.044016
  11. Peter A. Spring, Luka Milanovic, Yoshiki Sunada, Shiyu Wang, Arjan F. van Loo, Shuhei Tamate, Yasunobu Nakamura (2025), 'Fast Multiplexed Superconducting-Qubit Readout with Intrinsic Purcell Filtering Using a Multiconductor Transmission Line', PRX Quantum, DOI: 10.1103/PRXQuantum.6.020345, https://doi.org/10.1103/PRXQuantum.6.020345
  12. Ryo Hanai, Alexander McDonald, Aashish Clerk (2021), 'Intrinsic mechanisms for drive-dependent Purcell decay in superconducting quantum circuits', Physical Review Research, DOI: 10.1103/PhysRevResearch.3.043228, https://doi.org/10.1103/PhysRevResearch.3.043228
  13. Pan Shi, Jiahao Yuan, Fei Yan, Haifeng Yu (2023), 'Multiplexed control scheme for scalable quantum information processing with superconducting qubits', arXiv:2312.06911, https://arxiv.org/abs/2312.06911
  14. Seung-Young Seo, Jaehyeong Jo, Jungjae Park, Ji Wan Song, Jiman Choi, Yong-Ho Lee, Kibog Park, Jae-Yoon Sim (2026), 'Cryogenic High-Q Passive Hanger Type Drive-Line Notch Filtering for Leakage-Resilient Qubit Control', EPJ Quantum Technology, DOI: 10.1140/epjqt/s40507-026-00508-9, https://doi.org/10.1140/epjqt/s40507-026-00508-9
  15. Alexey Basharin (2026), 'Toward Large Nonradiating Qubits: When Quantum Engineering Meets Photonics', Advanced Quantum Technologies, DOI: 10.1002/qute.202500587, https://doi.org/10.1002/qute.202500587