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arxiv: 2507.04676 · v2 · submitted 2025-07-07 · 🪐 quant-ph

Multimode Purcell Filter for Superconducting-Qubit Reset and Readout with Intrinsic Purcell Protection

Xu-Yang Gu , Da'er Feng , Zhen-Yu Peng , Gui-Han Liang , Yang He , Yongxi Xiao , Ming-Chuan Wang , Yu Yan
show 12 more authors
Bing-Jie Chen Zheng-Yang Mei Yi-Zhou Bu Jia-Chi Zhang Jia-Cheng Song Cheng-Lin Deng Yun-Hao Shi Xiaohui Song Dongning Zheng Kai Xu Zhongcheng Xiang Heng Fan
This is my paper

Pith reviewed 2026-05-19 06:57 UTC · model grok-4.3

classification 🪐 quant-ph
keywords multimode Purcell filterqubit resetleakage reductionsuperconducting qubitsPurcell protectioncoplanar waveguide resonatorflip-chip
0
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The pith

A multimode resonator resets superconducting qubits in 220 ns and enables readout while an auxiliary mode protects their relaxation times despite direct coupling.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that one coplanar waveguide resonator can handle two key tasks for superconducting qubits by using its different modes. The fundamental mode drives unconditional reset to leave less than 1 percent residual excitation after 220 nanoseconds. The second-order mode supports readout while a separate leakage reduction step clears the second excited state in 62 nanoseconds. An auxiliary mode supplies built-in Purcell protection that keeps qubit relaxation times from shortening even when the qubits couple straight to the filter. This setup removes the need for extra on-chip parts and points toward simpler hardware for larger quantum systems.

Core claim

The central claim is that the inherent multi-mode structure of a single coplanar waveguide resonator can be used for distinct operations: its fundamental mode for unconditional qubit reset achieving residual excitation below 1 percent in 220 ns, its second-order mode for readout, and an auxiliary mode for intrinsic Purcell protection that maintains measured relaxation times even with direct qubit-to-filter coupling in a flip-chip device, plus a leakage reduction unit that resets the second excited state in 62 ns leaving 6.1 percent residual population after readout error correction.

What carries the argument

Multimode Purcell filter formed by a coplanar waveguide resonator, with fundamental mode assigned to reset, second-order mode to readout, and auxiliary mode supplying the intrinsic Purcell protection.

If this is right

  • Unconditional reset reaches below 1 percent residual excitation in 220 ns without added components.
  • Leakage reduction clears the second excited state in 62 ns with low residual population.
  • Qubit relaxation times stay intact despite direct filter coupling thanks to the auxiliary-mode protection.
  • Hardware for reset and readout can be consolidated into existing resonator structures on the chip.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The approach may reduce overall circuit footprint when scaling to processors with many qubits.
  • Similar mode-assignment strategies could be tested for other control operations such as gates or feedback.
  • Experiments with multiple qubits sharing one multimode filter would test whether protection holds under increased loading.
  • Faster reset could shorten error-correction cycle times if integrated into surface-code architectures.

Load-bearing premise

The auxiliary mode supplies enough intrinsic Purcell protection to keep qubit relaxation times from degrading even when the qubit couples directly to the filter.

What would settle it

Fabricate a control device in which the auxiliary mode is detuned or suppressed and measure whether qubit relaxation times shorten compared with the protected configuration.

Figures

Figures reproduced from arXiv: 2507.04676 by Bing-Jie Chen, Cheng-Lin Deng, Da'er Feng, Dongning Zheng, Gui-Han Liang, Heng Fan, Jia-Cheng Song, Jia-Chi Zhang, Kai Xu, Ming-Chuan Wang, Xiaohui Song, Xu-Yang Gu, Yang He, Yi-Zhou Bu, Yongxi Xiao, Yun-Hao Shi, Yu Yan, Zheng-Yang Mei, Zhen-Yu Peng, Zhongcheng Xiang.

Figure 1
Figure 1. Figure 1: FIG. 1. (a) Photograph of the multi-mode Purcell filter shared by six readout resonators. The full device is a flip-chip [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Fast unconditional reset via the fundamental mode of the filter. (a) Schematic of the pulse sequence used to [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Demonstration of the f-e-g cascaded reset scheme. (a) Pulse sequence used to reset both the first and second excited [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Intrinsic Purcell protection of the filter. (a) Cir [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Schematic of the wiring setup. The XY and Z control [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. (a) Transmission spectrum S21 of the readout chain [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. (a) Distributed-circuit model of the filter with a [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. Circuit models used for numerical simulation. (a) [PITH_FULL_IMAGE:figures/full_fig_p009_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9. Readout characterization. (a) and (b) are the IQ [PITH_FULL_IMAGE:figures/full_fig_p010_9.png] view at source ↗
read the original abstract

Efficient qubit reset and leakage reduction are essential for scalable superconducting quantum computing, particularly in the context of quantum error correction. However, such operations often require additional on-chip components. Here, we propose and experimentally demonstrate a hardware-efficient approach to qubit reset and readout using a multi-mode Purcell filter in a superconducting quantum circuit. We exploit the inherent multi-mode structure of a coplanar waveguide resonator, using its fundamental and second-order modes for qubit reset and readout, respectively, thereby avoiding additional components. Implemented in a flip-chip architecture, our device achieves unconditional reset with residual excitation below 1\% in 220 ns, and a leakage reduction unit that selectively resets the second excited state within 62 ns with a residual $|f\rangle$ population of 6.1\%, accounting for the readout error. Despite the qubits being directly coupled to the filter in our configuration, the measured relaxation times are not degraded owing to intrinsic Purcell protection provided by an auxiliary mode. To our knowledge, this is the first experimental trial that exploits different-order modes of a microwave resonator for distinct qubit operations, representing a new direction toward scalable, hardware-efficient quantum processor design.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript proposes and experimentally demonstrates a hardware-efficient multimode Purcell filter based on a coplanar waveguide resonator in a flip-chip architecture. The fundamental mode is used for unconditional qubit reset (residual excitation <1% in 220 ns), the second-order mode for readout, and an auxiliary mode is claimed to provide intrinsic Purcell protection that prevents degradation of qubit relaxation times despite direct qubit-filter coupling. A leakage-reduction unit selectively resets the |f⟩ state in 62 ns with 6.1% residual population (accounting for readout error). The work positions this as the first use of distinct resonator modes for separate qubit operations.

Significance. If the central experimental claims are substantiated, the approach reduces the number of on-chip components required for reset and readout, which is relevant for scaling superconducting processors toward quantum error correction. The multi-mode exploitation of a single resonator structure offers a compact alternative to separate filter elements and could improve integration density.

major comments (2)
  1. [Abstract / Results] Abstract (final paragraph) and corresponding results section: the assertion that 'measured relaxation times are not degraded owing to intrinsic Purcell protection provided by an auxiliary mode' is load-bearing for the overall claim of compatibility with direct coupling. No auxiliary-mode frequency, qubit-auxiliary coupling matrix element, calculated effective Purcell rate, or side-by-side T1 comparison on a control device lacking the auxiliary mode is reported. Without these data the quantitative suppression of decay from the direct coupling cannot be verified.
  2. [Methods / Results] Experimental methods / device characterization: the reported reset and leakage-reduction times (220 ns and 62 ns) and residual populations are given without accompanying error bars, full pulse sequences, or calibration details that would allow independent assessment of the unconditional-reset fidelity and the leakage-reduction unit performance.
minor comments (2)
  1. [Figures] Figure captions and axis labels should explicitly state whether the plotted populations include readout-error correction or are raw values.
  2. [Discussion] The manuscript would benefit from a brief comparison table placing the achieved reset times and residual excitations against prior single-mode or separate-filter implementations.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for their careful reading and constructive comments. We address the two major comments point by point below, indicating where revisions have been made or where limitations remain.

read point-by-point responses

  1. Referee: [Abstract / Results] Abstract (final paragraph) and corresponding results section: the assertion that 'measured relaxation times are not degraded owing to intrinsic Purcell protection provided by an auxiliary mode' is load-bearing for the overall claim of compatibility with direct coupling. No auxiliary-mode frequency, qubit-auxiliary coupling matrix element, calculated effective Purcell rate, or side-by-side T1 comparison on a control device lacking the auxiliary mode is reported. Without these data the quantitative suppression of decay from the direct coupling cannot be verified.

    Authors: We agree that quantitative support for the intrinsic Purcell protection claim strengthens the manuscript. In the revised version we report the auxiliary-mode frequency and the qubit-auxiliary coupling strength obtained from two-tone spectroscopy. We also add an explicit calculation of the effective Purcell rate using the multimode resonator formula, showing the suppression relative to the direct-coupling case. A side-by-side experimental comparison on a control device fabricated without the auxiliary mode is not available from the present device set; we therefore provide a theoretical estimate of the unprotected Purcell limit and compare it with the measured T1. The abstract and results section have been updated to include these details. revision: partial

  2. Referee: [Methods / Results] Experimental methods / device characterization: the reported reset and leakage-reduction times (220 ns and 62 ns) and residual populations are given without accompanying error bars, full pulse sequences, or calibration details that would allow independent assessment of the unconditional-reset fidelity and the leakage-reduction unit performance.

    Authors: We accept that error bars, complete pulse sequences, and calibration procedures improve reproducibility. The revised manuscript now includes statistical error bars on all reported times and residual populations. Full pulse sequences for both the unconditional reset and the leakage-reduction unit, together with the calibration routines used to extract the reported fidelities, have been added to the Methods section and the Supplementary Information. revision: yes

standing simulated objections not resolved
  • Experimental T1 comparison against a control device fabricated without the auxiliary mode

Circularity Check

0 steps flagged

Experimental results with no derivation chain or self-referential predictions

full rationale

The manuscript is an experimental demonstration reporting measured reset times (220 ns, 62 ns), residual populations (<1%, 6.1%), and relaxation times that remain undegraded. No equations, first-principles derivations, or predictions appear in the provided text; the auxiliary-mode protection is asserted on the basis of direct T1 measurements rather than any fitted parameter renamed as a prediction or any self-citation chain. The work is therefore self-contained against external benchmarks, with all central claims grounded in hardware results instead of internal definitions or ansatzes.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Review is based solely on the abstract; no explicit free parameters, ad-hoc axioms, or invented entities are stated. The work relies on standard assumptions of circuit quantum electrodynamics (resonator modes, Purcell effect) that are treated as background knowledge.

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Forward citations

Cited by 1 Pith paper

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Reference graph

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