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REVIEW 2 major objections 5 minor 26 references

Charge sensitivity in the transmon regime

T0 review · 2 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read Ramsey measurements with embedded parity detection show quasiparticle-induced parity flips, not charge offset, set the dephasing limit of a tantalum transmon at EJ/EC≈50.

desk verdict A useful experimental result with a genuine gap: charge-parity noise measurably dephases a transmon at EJ/EC~50, and the protocol is clever, but the missing parity-readout fidelity and multi-flip bound should be requested before publication. read the letter →

arxiv 2508.03973 v2 pith:QZHBEPFO submitted 2025-08-05 quant-ph cond-mat.mes-hallcond-mat.supr-con

Rocio Gonzalez-Meza , Vito Iaia , Anika Zaman , Hiu-Yung Wong , Yujin Cho , Kristin Beck , Yaniv J. Rosen This is my paper · ORCID
classification quant-phcond-mat.mes-hallcond-mat.supr-con PACS 03.67.Lx85.25.Cp
keywords transmonqubitcharge-paritynoisequasiparticlepoisoningdephasingRamseyinterferometryspinechochargedispersionsuperconductingqubits
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

The paper tries to show that transmons, even at the canonical EJ/EC≈50 operating point, are not as charge-insensitive as assumed: residual charge dispersion (~6 kHz on the 0-1 transition) plus quasiparticle-induced parity switches produce measurable T2* fluctuations. The authors embed single-shot parity detection into every Ramsey shot, categorizing shots as parity-flipped or not. Unflipped shots yield T2*=43.0±1.0 μs versus 23.4±0.6 μs for the unsorted set, and T2* decreases monotonically with charge dispersion while spin-echo times stay flat. This establishes charge-parity noise as a dominant decoherence channel in this regime and motivates parity flip rate as a characterization metric.

What carries the argument

Embedded parity detection: a single-shot measurement of the 1-2 transition, using a frequency-selective long π12 pulse tuned to one parity band (f+12 or f−12), performed before and after each Ramsey shot, with the two outcomes (pi, pf) classifying each shot as flipped or unflipped. The supporting machinery is a Lindblad master-equation simulation in which the Hamiltonian switches between parity-dependent detunings ±2πΔ01 at a random time tf, averaged over realizations.

What would settle it

Run the same Ramsey sequence with a parity detector that can catch two or more flips per shot, e.g., repeated or continuous parity readout during the delay. If the apparent T2* difference between unsorted and single-parity subsets vanishes, or if single-parity shots are found to contain more than one flip at the claimed ~1 kHz rate, the central mechanism would be contradicted.

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Extended reading notes

Core claim

The central discovery is that the dephasing of a transmon in the charge-insensitive regime is limited by fast parity switches (quasiparticle tunneling events) rather than by the slow charge-offset drift itself. By resolving the two parity bands on the 1-2 transition (≈150 kHz charge dispersion) and using long selective π12 pulses to read out parity at the start and end of each Ramsey shot, the authors directly observe individual parity flips. Post-selecting shots that stayed in one parity recovers a dephasing time T2*=43.0±1.0 μs, nearly double the 23.4±0.6 μs obtained when averaging over both parities. The monotonic decrease of T2* with charge offset, the absence of this trend in spin-echo,

Load-bearing premise

The entire parity classification rests on the assumption that at most one quasiparticle tunneling event happens during a single Ramsey shot and that the one-shot readout of the 1-2 transition never mislabels which parity is occupied; the paper does not quantify the readout fidelity.

Editorial extensions

If this is right

  • Apparent T2* of a transmon can understate the intrinsic coherence by up to a factor of two when parity flips are averaged in; parity-resolved post-selection recovers the single-parity value.
  • Parity flip rate (≈1 kHz here) should be reported as a device characterization metric alongside T1 and T2*, since it sets a dephasing floor even at EJ/EC≈50.
  • Spin-echo sequences screen out the low-frequency noise from millisecond-timescale parity switches, so a Ramsey-versus-echo comparison can serve as a diagnostic for charge-parity noise.
  • In higher-coherence devices where other decoherence channels are weaker, charge-parity noise will be relatively more important, not less.
  • Devices deeper into the transmon regime or with reduced quasiparticle poisoning should show suppressed parity-related T2* variation; the protocol gives a direct way to verify this.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The paper's protocol assumes one flip per shot; a natural extension would be repeated or continuous parity readout during the delay to measure flip statistics within a shot, which would validate or bound the correction.
  • If the readout fidelity of the parity detector were quantified and accounted for, the inferred 'ideal' T2* would likely shift, since misclassification would mix flipped and unflipped populations; this correction is absent from the reported values.
  • The same embedded-detection logic could be applied to other telegraphic noise sources, such as two-level-system defects, by choosing a probe transition whose frequency distinguishes the noise state, effectively turning Ramsey decay into a noise-state-resolved measurement.
  • Because the 1-2 parity readout itself is a measurement on the qubit's higher levels, one could test for backaction of the parity detection on 0-1 coherence by varying the detection parameters; the paper does not report such a control.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 5 minor

Summary. The paper presents an experimental study of charge-parity (quasiparticle-poisoning) noise in a tantalum transmon with E_J/E_C ≈ 50. The authors resolve the two parity states through the 1-2 transition, interleave single-shot parity detection before and after each Ramsey interrogation, and post-select shots that did not undergo a parity flip. They report T2* = 23.4 ± 0.6 µs for unsorted Ramsey data versus T2* = 43.0 ± 1.0 µs for parity-stable shots, and show that Ramsey T2* decreases with increasing charge dispersion while spin-echo T2* remains flat. A Lindblad master-equation simulation with a random parity-flip time is used to compare with the measured dispersion dependence. The conclusion is that charge-parity noise, not the usual assumption of charge insensitivity, is a dominant source of T2* fluctuations in this transmon regime.

Significance. If the post-selection and simulation withstand scrutiny, the result is an important experimental demonstration that transmons at the standard E_J/E_C ≈ 50 operating point can still be limited by charge-parity noise. The protocol—embedding single-shot parity detection within Ramsey and spin-echo measurements—is direct and transferable, and the reported contrast between unsorted (23.4 ± 0.6 µs) and parity-stable (43.0 ± 1.0 µs) T2* is a striking falsifiable observation. The paper also states its own limitation: the effect is only visible when T2* is sufficiently long (Appendix A.2), and longer-term drifts remain unexplained. These admissions strengthen the credibility of the central claim rather than weakening it.

major comments (2)
  1. [Section 2.2, Fig. 3] The post-selection step is load-bearing: the 43.0 vs 23.4 µs difference is what establishes parity flips as the dominant dephasing mechanism. The manuscript does not report the assignment fidelity of the single-shot parity readout, and it treats “one flip per shot” as the only classification error. At the quoted ~1 kHz flip rate, the total per-shot exposure includes two ~10 µs parity-detection/readout windows plus the Ramsey delay (~100 µs), so a non-negligible fraction of shots will have a flip inside a parity-detection window; such events are exactly those misclassified by the (pi, pf) comparison. Please quantify the parity-readout contrast/fidelity and the conditional misclassification probability as a function of delay, and show that the extracted T2* improvement survives a correction for these errors.
  2. [Appendix A.1, Fig. 5(b)] The Lindblad simulation is offered as confirmation, but the input values of T1 and Tphi are not stated. If Tphi (or the no-flip T2*) was taken from the same Ramsey data, the comparison is partly circular. Report the exact T1 and Tphi values, state whether they are independently measured (e.g., T1 from inversion recovery and Tphi from Hahn echo), and quantify the sensitivity of the simulated T2*(Δ01) curve to these inputs. In addition, the simulation averages over exactly one parity flip; shots with zero and two flips should be included with their Poisson weights to match the experiment at the quoted flip rates.
minor comments (5)
  1. [Section 3.1, Fig. 4(b)] The improved T2* for parity-stable shots is shown only for the f+12 subgroup. To rule out an asymmetry in the parity-detection protocol, show the f−12 subgroup or a combined analysis.
  2. [Section 2.2] The statement that 1 kHz flip rates make multi-flip events unlikely is heuristic. Replace it with a quantitative Poisson estimate that includes the finite duration of the parity-detection and reset windows.
  3. [Section 3.2, Fig. 5] The axes in Fig. 5 are not fully defined: the bottom and top x-axes presumably correspond to offset charge and Δ01, respectively. Specify the conversion, the number of repetitions per point, and how error bars are obtained.
  4. [Appendix A.1, Eq. (3)] Define δω and the sign convention for α in the Hamiltonian, and specify whether α is the measured anharmonicity or the full 1-2 detuning.
  5. [Section 2.1] Several device parameters are attributed to Ref. [8]. Clarify which values (e.g., charge dispersion of the 1-2 transition, T1) are measured in the present work and which are taken from prior LLNL publications.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the central T2* comparison is direct post-selection of measured data, and the simulation is a forward model rather than a fit of the claimed result.

full rationale

The paper's central quantitative claim is a direct experimental comparison: unsorted Ramsey shots give T2* = 23.4 +/- 0.6 us while shots that remained in a single parity give T2* = 43.0 +/- 1.0 us. This is not a fitted parameter renamed as a prediction; it is a post-selection of the same measured Ramsey data. The protocol's stated assumption of at most one parity flip per shot and the absence of a quantified parity-readout fidelity are experimental-correctness risks, not circularity. The simulation in Appendix A.1 is a forward Lindblad-master-equation model: it takes a known Hamiltonian with parity-dependent detuning, a uniform distribution of a single flip time, and independently specified relaxation/dephasing rates, then computes Ramsey decay and extracts T2*. Nothing in the quoted text shows that the simulation was tuned to reproduce the measured T2* versus charge-dispersion trend; it is used as a consistency check. The LLNL self-citations (Refs. [8], [13], [19], [20]) are not load-bearing in a circular sense: the 1-2 transition charge dispersion is also directly measured in Fig. 2; the phase method is described in the text and is a standard technique; the beating pattern is directly visible in Fig. 4. The paper itself flags its limitations (e.g., faster parity flips making detection unreliable, and longer-term T2* drifts not explained by charge-parity noise), which further shows the argument is not structured to force its own conclusion. No equation reduces to its own input, and no fitted parameter is renamed as a prediction. Therefore no significant circularity is present.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

The central claim rests on standard transmon theory plus three domain assumptions about parity correspondence, flip rarity, and quasi-static charge offset. No new particles or forces are introduced. The only under-specified inputs are the dissipation rates in the supporting simulation.

free parameters (1)
  • T1 and Tphi in the Lindblad simulation = not specified
    Appendix A.1 defines Gamma1 = 1/T1 and Gammaphi = 2/Tphi, but the numerical values used to produce the red curve in Fig. 5(b) are not stated. If these were adjusted to match the Ramsey data, the simulation becomes a fit rather than an independent prediction.
assumptions (5)
  • standard math Transition frequencies follow the charge dispersion relation f±ij ~= fbar_ij +/- eps_ij cos(2 pi ng), Eq. (1).
    Standard transmon theory from Koch et al. (Ref. [6]); used to connect the measured 1-2 dispersion to the 0-1 dispersion and to define parity bands.
  • domain assumption The parity state detected in the 1-2 manifold is the same parity state that affects the 0-1 Ramsey evolution.
    Parity detection is performed on the 1-2 transition while coherence is measured in the 0-1 manifold. This assumes the electron parity is a global property of the island, which is standard but not explicitly tested.
  • domain assumption At most one parity flip occurs within a single Ramsey shot.
    Section 2.2: 'We measured parity flip rates on the order of 1 kHz, which is unlikely to have more than one parity flip during this sequence.' If multiple flips occur, the pi vs pf classification fails.
  • domain assumption Charge offset ng is quasi-static during the minutes-long Ramsey measurement.
    Section 2 cites charge drift on the order of 0.5 e/hour, much slower than the experimental timescale, so ng and hence Delta_ij are treated as constant.
  • standard math Lindblad master equation with Markovian noise captures the qubit dynamics including parity flips.
    Appendix A.1 uses QuTiP Lindblad evolution; this assumes Born-Markov and secular approximations, which are standard for superconducting qubit simulations.

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Cite this review

Pith. "Pith review of Charge sensitivity in the transmon regime." pith.science (2026). https://pith.science/paper/QZHBEPFO

@misc{pith2026250803973,
  author       = {Pith},
  title        = {Pith review of: Charge sensitivity in the transmon regime},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/QZHBEPFO}},
  note         = {Machine review of arXiv:2508.03973}
}
abstract

Transmons are widely adopted in quantum computing architectures for their engineered insensitivity to charge noise and correspondingly long relaxation times. Despite this advantage, transmons often exhibit large fluctuations in dephasing times across different devices and also within qubits on the same device. Existing transmon qubits are assumed to be insensitive to charge noise. However, very little recent attention has been paid to the dependence of dephasing on the local charge environment. In this study, we see fluctuations in the dephasing time, $T_{\phi}$, which correlate to charge offset. While charge offset fluctuations are slow, parity switches are fast processes tied to the charge offset and can affect $T_{\phi}$ in Ramsey experiments. We implement a protocol to detect parity switching events using single-shot methods, which are interleaved within a Ramsey measurement. We find that events that remain in the same parity state have a higher $T_2$ than measurements averaged over both parities. Our results show that transmons can be limited by charge-noise, even with $E_\text{J}/E_\text{C} \approx 50$. Consequently, parity flip rates must be considered as a device characterization metric.

Figures

Figures reproduced from arXiv: 2508.03973 by the authors.

Figure 1
Figure 1. A diagram of a transmon’s three lowest en [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Parity frequencies detection and pulse duration optimization using high-resolution spectroscopy. (a) [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Embedded parity detection protocol. (a) Schematic of the Ramsey decay experiment with embedded parity detection: For each shot at a fixed delay time, parity detection sequences are performed at the beginning (pi) and end (pf ) of the Ramsey se￾quence. This allows identification of parity flips during the experiment by comparing pi and pf . (b) Detailed view of the parity detection sequence, labeled as pk (k = i, f):… view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: shows a Ramsey decay experiment with embedded parity detection measurements. For the unsorted dataset ( [PITH_FULL_IMAGE:figures/full_fig_p004_4.png]
Figure 5
Figure 5. Figure 5: Dephasing time T ∗ 2 extracted from Ramsey experiments in the 0-1 subspace. (a) Results from experiments performed with embedded parity measurements. We observe no variation in T ∗ 2 with charge offset for events with no parity switch (blue cross). However, Measurement…

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