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arxiv: 2604.04454 · v1 · submitted 2026-04-06 · 🪐 quant-ph

Efficient direct quantum state tomography using fan-out couplings

Jaekwon Chang , Guedong Park , Hyunseok Jeong , Yong Siah Teo , Yosep Kim This is my paper

Pith reviewed 2026-05-10 20:12 UTC · model grok-4.3

classification 🪐 quant-ph
keywords direct quantum state tomographyfan-out couplingconstant circuit depthquantum error mitigationGHZ state fidelitysuperconducting qubitsmutually commuting interactions
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The pith

A fan-out coupling to one meter qubit enables direct quantum state tomography at constant circuit depth independent of system size.

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

The paper introduces a direct quantum state tomography scheme that pairs strong-measurement estimation with a fan-out coupling architecture. This setup creates mutually commuting interactions between any number of system qubits and a single meter qubit, so the total circuit depth stays fixed instead of growing with system size. The coupling is involutory, meaning two applications restore the initial state exactly, which makes noise scaling for error mitigation direct and reliable. Experiments on a superconducting processor show successful four-qubit density-matrix reconstruction and single-circuit fidelity estimation for GHZ states as large as 20 qubits, matching conventional tomography while using fewer resources. A reader would care because standard full tomography quickly becomes impractical for large devices, whereas this method supports selective element access and scalable verification tasks.

Core claim

We introduce a direct quantum state tomography scheme combining strong-measurement estimation with a fan-out coupling architecture. It enables mutually commuting interactions between system qubits and a single meter qubit, thereby achieving constant circuit depth, independent of system size. Notably, the involutory fan-out coupling reduces to the identity under repetition, enabling straightforward noise scaling for quantum error mitigation. We experimentally validate the scheme on a superconducting quantum processor via the IBM Quantum Platform, demonstrating four-qubit state reconstruction and single-circuit GHZ-state fidelity estimation up to 20 qubits with error mitigation.

What carries the argument

The involutory fan-out coupling architecture, which supplies mutually commuting interactions from many system qubits to one meter qubit and returns exactly to the identity after two applications.

Load-bearing premise

The fan-out coupling remains perfectly involutory and all interactions stay mutually commuting under real hardware noise and control imperfections without introducing non-commuting errors.

What would settle it

An experiment on systems beyond 20 qubits in which the effective depth grows with qubit number or repeated applications deviate from identity scaling would falsify the constant-depth and noise-mitigation claims.

Figures

Figures reproduced from arXiv: 2604.04454 by Guedong Park, Hyunseok Jeong, Jaekwon Chang, Yong Siah Teo, Yosep Kim.

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: shows density matrices reconstructed via DQST and standard QST, requiring 31 and 81 distinct circuits, respectively. To ensure physicality, they are pro￾jected onto the closest physical density matrix by mini￾mizing the Frobenius-norm distance (see Methods). As summarized in Table I, both methods achieve high fi￾delity with the ideal states, and QREM further improves the reconstruction accuracy. Notably, D… view at source ↗
Figure 3
Figure 3. Figure 3: a shows the circuit used for fidelity estimation of an n-qubit GHZ state. To mitigate errors, we combine QREM with zero-noise extrapolation (ZNE) applied to the controlled-U 1 ES operations, while leaving the target state unchanged [47–49]. Due to implementation con￾straints of U 1 ES, digital ZNE is performed at the level of individual CNOT gates [49, 57, 58] (see Supplemen￾tary Note 3 for details). Figur… view at source ↗
read the original abstract

Characterizing quantum states is essential for validating quantum devices, yet conventional quantum state tomography becomes prohibitively expensive as system size grows. Direct tomography offers a distinct route by enabling selective access to individual complex density-matrix elements, with a particular advantage for sparse target states and some verification tasks. Here we introduce a direct quantum state tomography scheme combining strong-measurement estimation with a fan-out coupling architecture. It enables mutually commuting interactions between system qubits and a single meter qubit, thereby achieving constant circuit depth, independent of system size. Notably, the involutory fan-out coupling reduces to the identity under repetition, enabling straightforward noise scaling for quantum error mitigation. We experimentally validate the scheme on a superconducting quantum processor via the IBM Quantum Platform, demonstrating four-qubit state reconstruction and single-circuit GHZ-state fidelity estimation up to 20 qubits with error mitigation. Consistent results with standard tomography and improved efficiency establish our scheme as a promising approach to reconstructing full quantum states and scalable verification tasks.

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 introduces a direct quantum state tomography scheme that combines strong-measurement estimation with a fan-out coupling architecture between system qubits and a single meter qubit. This enables mutually commuting interactions, yielding constant circuit depth independent of system size. The involutory property of the fan-out coupling (reducing to the identity upon repetition) is used to enable straightforward noise scaling for quantum error mitigation. Experimental validation on IBM superconducting hardware demonstrates 4-qubit state reconstruction consistent with standard tomography and single-circuit GHZ-state fidelity estimation up to 20 qubits with error mitigation, claiming improved efficiency.

Significance. If the constant-depth scaling and noise-mitigation assumptions hold under realistic conditions, the approach could meaningfully advance scalable state characterization for large quantum systems, offering advantages for sparse states and verification tasks where conventional tomography scales poorly. The experimental demonstrations on real hardware provide practical grounding, though the headline efficiency gains depend on the robustness of the architectural claims.

major comments (2)
  1. [Fan-out coupling and noise mitigation section] Fan-out coupling and noise mitigation section (abstract and methods): The central claim that the involutory fan-out coupling reduces exactly to the identity under repetition, enabling straightforward noise scaling for QEM, is load-bearing for the 20-qubit GHZ fidelity results. The manuscript provides no analysis, simulation, or bounds on how control errors, residual ZZ interactions, or decoherence on superconducting hardware deviate from U² = I, which could turn the two-copy operation into a non-identity channel and undermine the extrapolation argument.
  2. [Experimental validation section] Experimental validation section: The reported 4-qubit reconstruction consistency with standard tomography and 20-qubit GHZ results lack full circuit diagrams, explicit error-bar details, and data exclusion criteria. This prevents full auditing of whether the constant-depth property was achieved in practice and whether the noise-scaling QEM delivered the claimed improvement.
minor comments (2)
  1. [Abstract] The term 'strong-measurement estimation' appears in the abstract without a concise definition or reference in the main text; adding this would improve accessibility.
  2. [Figures] Figure captions should explicitly distinguish mitigated versus unmitigated data and note the circuit depth for each method to better support the constant-depth claim.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed comments, which help clarify key aspects of our work. We address each major comment point by point below and indicate the corresponding revisions.

read point-by-point responses

  1. Referee: [Fan-out coupling and noise mitigation section] Fan-out coupling and noise mitigation section (abstract and methods): The central claim that the involutory fan-out coupling reduces exactly to the identity under repetition, enabling straightforward noise scaling for QEM, is load-bearing for the 20-qubit GHZ fidelity results. The manuscript provides no analysis, simulation, or bounds on how control errors, residual ZZ interactions, or decoherence on superconducting hardware deviate from U² = I, which could turn the two-copy operation into a non-identity channel and undermine the extrapolation argument.

    Authors: We agree that a quantitative assessment of deviations from the ideal involutory property (U² = I) under realistic superconducting hardware noise is essential to support the noise-scaling QEM for the 20-qubit results. Although the fan-out coupling is exactly involutory in the ideal case, we will add a dedicated analysis subsection in the revised manuscript. This will include numerical simulations based on IBM hardware noise models (incorporating control errors, residual ZZ couplings, and decoherence) to provide bounds on the deviation from identity and evaluate its effect on the extrapolation procedure. revision: yes

  2. Referee: [Experimental validation section] Experimental validation section: The reported 4-qubit reconstruction consistency with standard tomography and 20-qubit GHZ results lack full circuit diagrams, explicit error-bar details, and data exclusion criteria. This prevents full auditing of whether the constant-depth property was achieved in practice and whether the noise-scaling QEM delivered the claimed improvement.

    Authors: We concur that full transparency in the experimental section requires additional documentation. In the revised manuscript, we will include complete circuit diagrams for the 4-qubit direct tomography and the 20-qubit GHZ fidelity estimation circuits, confirming the constant-depth implementation. We will also add explicit error-bar calculations based on finite-shot statistics, along with a clear statement of any data exclusion criteria applied during post-processing. These changes will enable independent verification of the results and the QEM improvement. revision: yes

Circularity Check

0 steps flagged

No significant circularity in the derivation chain

full rationale

The paper proposes a direct quantum state tomography scheme based on a fan-out coupling architecture between system qubits and a meter qubit. The constant-depth claim follows directly from the stated mutual commutativity of interactions, and the noise-scaling QEM follows from the involutory (U² = I) property of the coupling; both are design features of the architecture rather than quantities derived from data or prior results. Experimental validation on IBM hardware (4-qubit tomography and 20-qubit GHZ fidelity) is compared against standard tomography, providing external benchmarks. No equations reduce by construction to fitted inputs, no uniqueness theorems are imported via self-citation, and no ansatz is smuggled in. The scheme is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on the physical realizability of an involutory fan-out coupling in superconducting hardware and the assumption that the meter-system interactions remain mutually commuting under realistic control.

axioms (2)
  • domain assumption The fan-out coupling operator is involutory (U squared equals identity), allowing exact cancellation upon repetition for noise scaling.
    Invoked to justify straightforward error mitigation without additional calibration.
  • domain assumption All system-meter interactions commute, permitting simultaneous execution in constant depth.
    Stated as the architectural feature enabling size-independent circuit depth.

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