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A two-level defect inside a tunable coupler couples coherently to two distant qubits and drives their correlated non-Markovian dynamics.

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2026-06-30 16:13 UTC pith:26QL547E

load-bearing objection The paper reports a TLS coupling two qubits through the tunable coupler with frequency control and a wide 1/f spectrum, but the location and single-defect claims depend on indirect tunability evidence without clear exclusion of alternatives. the 2 major comments →

arxiv 2605.23385 v2 pith:26QL547E submitted 2026-05-22 quant-ph

Non-Local and Non-Markovian Effects of a Microscopic Two-Level Defect in Superconducting Quantum Circuits

Yang Gao , Yujia Zhang , Huikai Xu , Pan Shi , Feiyu Li , Yaqing Feng , Weijie Sun , Jiayu Ding
show 7 more authors
Yang Liu He Wang Ruixia Wang Zhen Yang Yirong Jin Haifeng Yu Fei Yan
This is my paper
classification quant-ph
keywords two-level systemssuperconducting qubitstunable couplernon-Markovian dynamics1/f noisecorrelated decoherencequantum process tomography
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

The paper observes a coherent microscopic two-level system that interacts simultaneously with two superconducting qubits. The TLS sits inside the tunable coupler between them, so its coupling strength to the qubits can be adjusted by changing the coupler frequency. This control lets the authors map how the TLS distorts qubit evolution and reconstruct its frequency-fluctuation spectrum as 1/f noise over more than ten orders of magnitude. Quantum process tomography shows the TLS produces correlated dynamics between the two qubits, identifying the long-lived defect as a source of non-Markovianity. The work shows that defects located in coupling elements can affect multiple qubits at once with adjustable strength.

Core claim

The TLS resides within the tunable coupler linking the qubits, enabling controllability of the TLS-qubit coupling strength via coupler frequency. This produces TLS-induced correlated qubit dynamics that highlight the long-lived TLS as an effective source of non-Markovianity, with the reconstructed 1/f noise spectrum of TLS frequency fluctuation spanning more than ten orders of magnitude revealing discrete fluctuator signatures.

What carries the argument

TLS located inside the tunable coupler, whose frequency sets the TLS-qubit coupling strength and thereby controls the non-Markovian dephasing imposed on the qubits.

Load-bearing premise

The observed coherent oscillations, tunable coupling, and wide-band noise spectrum are produced by one TLS inside the coupler rather than by other defects or measurement effects.

What would settle it

Repeating the experiment on devices without the coupler or with the TLS frequency tuned far from the qubits while still seeing the same correlated dynamics and 1/f spectrum would falsify the claim.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • Defects embedded in coupling elements can generate correlated errors across multiple qubits.
  • Coupler-frequency tuning provides a direct experimental knob for characterizing individual TLS.
  • Error-suppression protocols must account for long-lived TLS memory effects rather than assuming Markovian noise.
  • System calibration routines should include checks for TLS signatures in inter-qubit couplers.

Where Pith is reading between the lines

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

  • Architectures that rely on tunable couplers may need additional TLS screening steps during fabrication.
  • The same coupler-TLS system could serve as a controllable testbed for studying environment-induced non-Markovianity in larger qubit arrays.
  • If similar TLS are common in couplers, scaling to many qubits will require mapping coupler-specific defect densities rather than treating each qubit independently.

Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

2 major / 1 minor

Summary. The manuscript reports the experimental observation of a coherent microscopic two-level system (TLS) that couples simultaneously to two spatially separated superconducting qubits through a tunable coupler. The TLS is identified as residing in the coupler based on frequency-tunable coupling strength, enabling systematic probing of TLS-induced distortions to qubit dynamics. The work reconstructs a 1/f TLS frequency noise spectrum spanning 0.1 mHz to 1 MHz with discrete fluctuator signatures, and uses quantum process tomography to demonstrate TLS-induced correlated qubit dynamics as a source of non-Markovianity. The central claim is that defects in coupling elements can produce non-local, controllable, and long-lived effects on multiple qubits.

Significance. If the TLS location and single-defect attribution are robustly established, the result identifies a previously under-appreciated mechanism by which microscopic defects in scalable architectures can generate non-local and non-Markovian decoherence across multiple qubits. The demonstrated tunability via coupler frequency and the exceptionally wide-band noise spectrum reconstruction provide a concrete experimental testbed for refining decoherence models and error-suppression strategies. These strengths would elevate the work's impact on both fundamental defect physics and practical quantum processor design.

major comments (2)
  1. [TLS identification and location section] TLS identification and location section: the claim that the TLS resides inside the tunable coupler (rather than in a qubit with indirect mediation) rests on the observed dependence of effective TLS-qubit coupling on coupler frequency. No control spectra with the coupler far-detuned, spatial mapping, or quantitative model-comparison statistics are described that would exclude alternative locations; this assumption is load-bearing for the non-local and controllability claims.
  2. [Noise spectrum reconstruction and process tomography sections] Noise spectrum reconstruction and process tomography sections: the attribution of all observed frequency jumps and the reconstructed 1/f spectrum (0.1 mHz–1 MHz) to a single TLS, together with the non-Markovianity conclusion, requires explicit fitting procedures, error bars, exclusion criteria for background fluctuators, and raw data supporting the discrete fluctuator signatures. Absence of these details leaves the single-TLS and non-Markovian attributions vulnerable to alternative explanations.
minor comments (1)
  1. [Figure captions] Figure captions and axis labels in the dynamics and tomography panels should explicitly state the number of experimental repetitions and any post-selection criteria applied.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed review. The comments highlight important aspects of our TLS identification and data analysis that we address point-by-point below. We have revised the manuscript accordingly to strengthen the presentation.

read point-by-point responses

  1. Referee: [TLS identification and location section] TLS identification and location section: the claim that the TLS resides inside the tunable coupler (rather than in a qubit with indirect mediation) rests on the observed dependence of effective TLS-qubit coupling on coupler frequency. No control spectra with the coupler far-detuned, spatial mapping, or quantitative model-comparison statistics are described that would exclude alternative locations; this assumption is load-bearing for the non-local and controllability claims.

    Authors: The observed dependence of the TLS-qubit coupling on coupler frequency is the central evidence for location, as this tunability arises specifically from the coupler's role in mediating the interaction; a TLS located in either qubit would not exhibit coupling strength that varies systematically with the coupler frequency. We agree that explicit controls strengthen the claim. In the revised manuscript we add (i) control spectra acquired with the coupler far-detuned from both qubits, showing disappearance of the TLS signature, (ii) a quantitative model comparison (likelihood ratio test) between a coupler-resident TLS and an indirect qubit-TLS mediation scenario, and (iii) a brief discussion of why spatial mapping is not feasible in the present device geometry. These additions directly address the load-bearing assumption. revision: yes

  2. Referee: [Noise spectrum reconstruction and process tomography sections] Noise spectrum reconstruction and process tomography sections: the attribution of all observed frequency jumps and the reconstructed 1/f spectrum (0.1 mHz–1 MHz) to a single TLS, together with the non-Markovianity conclusion, requires explicit fitting procedures, error bars, exclusion criteria for background fluctuators, and raw data supporting the discrete fluctuator signatures. Absence of these details leaves the single-TLS and non-Markovian attributions vulnerable to alternative explanations.

    Authors: We have expanded the supplementary information and methods section to include: (i) the full least-squares fitting procedure and Bayesian model selection used to extract the 1/f spectrum, (ii) error bars derived from the covariance matrix of each spectral point, (iii) explicit exclusion criteria (amplitude threshold and correlation time) applied to rule out background fluctuators, and (iv) the raw time traces together with the single-TLS model fits. The non-Markovian character is tied directly to the process-tomography results that reveal correlated, long-lived errors whose time scale matches the TLS dwell times; we now state this linkage more quantitatively. These additions remove the vulnerability noted by the referee. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental observation paper with no derivation chain

full rationale

This is an experimental report on observed TLS-qubit interactions, tunability, noise spectra, and process tomography. No equations, predictions, or central claims reduce by construction to fitted inputs, self-citations, or ansatzes. The 1/f spectrum and non-Markovianity attributions are data-driven reconstructions, not self-definitional or fitted-then-predicted quantities. Self-contained against external benchmarks with no load-bearing self-citation chains.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review supplies no explicit free parameters, axioms, or invented entities; the central claim rests on the unstated assumption that the measured signals originate from a single TLS whose properties can be isolated from background noise.

pith-pipeline@v0.9.1-grok · 5823 in / 1051 out tokens · 39973 ms · 2026-06-30T16:13:52.506020+00:00 · methodology

0 comments
read the original abstract

Microscopic two-level systems (TLS) -- ubiquitous atomic-scale defects in solid-state quantum devices -- are a dominant source of qubit decoherence, yet their role is often considered local and short-memoried. Here, we report the observation of a coherent TLS that couples simultaneously to two spatially distant superconducting qubits. The TLS is identified to reside within the tunable coupler linking the qubits, enabling controllability of the TLS-qubit coupling strength via coupler frequency -- a capability absent in earlier studies. This tunability allows us to systematically probe how TLS distorts qubit dynamics, revisiting the decoherence model in the presence of non-Markovian TLS dephasing noise. This is corroborated by the reconstructed $1/f$ noise spectrum of TLS frequency fluctuation spanning more than ten orders of magnitude (0.1\,mHz -- 1\,MHz) that reveals discrete fluctuator signatures. Quantum process tomography further unveils TLS-induced correlated qubit dynamics, highlighting the long-lived TLS as an effective source of non-Markovianity. Our findings expose a previously overlooked interaction mechanism in scalable quantum architectures: defects embedded in coupling elements can simultaneously affect multiple qubits with variable impact. Beyond immediate implications for system characterization and calibration, this situation provides a powerful testbed for studying defect-driven quantum dynamics, refining error suppression strategies, and advancing architecture design for scalable quantum technologies.

Figures

Figures reproduced from arXiv: 2605.23385 by Fei Yan, Feiyu Li, Haifeng Yu, He Wang, Huikai Xu, Jiayu Ding, Pan Shi, Ruixia Wang, Weijie Sun, Yang Gao, Yang Liu, Yaqing Feng, Yirong Jin, Yujia Zhang, Zhen Yang.

Figure 1
Figure 1. Figure 1: FIG. 1 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗

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