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arxiv: 2605.14325 · v1 · pith:PI2J2PT4new · submitted 2026-05-14 · 🪐 quant-ph · cs.CR

Toward Covert Quantum Computing

Evan J. D. Anderson , Kaushik Datta , Boulat A. Bash This is my paper

Pith reviewed 2026-06-30 21:00 UTC · model grok-4.3

classification 🪐 quant-ph cs.CR
keywords covert quantum computingisoperimetric inequalitiesquantum crosstalkmulti-tenant quantum computingquantum privacyplanar graphsside channels
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The pith

Discrete isoperimetric inequalities limit adversary detection to O(sqrt(n)) border qubits in planar n-qubit circuits.

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

The paper introduces covert quantum computing, a privacy model in which an adversary controlling all other quantum computational units cannot detect computation performed on an inaccessible subset. It adopts the quantum-strategy framework from game theory and memory channel discrimination to analyze detection when the adversary can use quantum memories and adaptive operations. Under the standard model of planar circuit layouts with nearest-neighbor crosstalk, the authors derive discrete isoperimetric inequalities showing that detection information is confined to a border whose size scales as the square root of total qubits. Experiments on 54-qubit and 156-qubit processors confirm nearest-neighbor effects but also detect long-range couplings that create exploitable side channels.

Core claim

We derive discrete isoperimetric inequalities to show that, for an n-qubit circuit under this model, only O(sqrt(n)) border qubits provide detection information to the adversary.

What carries the argument

discrete isoperimetric inequalities applied to planar graphs with nearest-neighbor crosstalk, which bound the surface-to-volume ratio that controls information leakage to the adversary

If this is right

  • Adversary information scales with the square root of circuit size rather than linearly.
  • Larger circuits become relatively more private under the assumed crosstalk model.
  • Real devices exhibit additional side channels from long-range effects beyond the border qubits.
  • Mitigating drive-line leakage could restore the theoretical scaling.

Where Pith is reading between the lines

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

  • If long-range crosstalk can be suppressed through better spatial isolation, multi-tenant quantum clouds could support private computation at scale.
  • The border-qubit analysis could guide placement of decoy circuits or shielding structures.
  • The same isoperimetric approach may extend to other shared-resource quantum privacy settings such as blind quantum computation.

Load-bearing premise

Crosstalk is strictly nearest-neighbor and every circuit layout is a planar graph.

What would settle it

Measure whether long-range crosstalk on current hardware can be reduced to nearest-neighbor levels; if it cannot, the O(sqrt(n)) bound does not describe real detection risk.

Figures

Figures reproduced from arXiv: 2605.14325 by Boulat A. Bash, Evan J. D. Anderson, Kaushik Datta.

Figure 1
Figure 1. Figure 1: (a) The square lattice is shown with black vertices and edges. The shape that optimizes the vertex-isoperimetric inequality is a diamond: the dark red diamond encloses 5 vertices, and the light red diamond encloses 13. (b) The hexagonal lattice is shown with black vertices and edges; the heavy-hex lattice is obtained by adding the red vertices. The shaded regions show the disk-construction vertex subsets o… view at source ↗
Figure 2
Figure 2. Figure 2: Experiment circuits. The gate sequence on qubit [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Ramsey experiment results for qubit 35 in [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Experiment layouts. Top row: IQM Emerald (square lattice, n = 2, 4, 9, 16, 25). Bottom row: IBM ibm_fez (heavy-hex lattice, n = 2, 12, 54, 108). Spectator qubits in black, CZ gate qubits in red; edge sets indicated by like colorm with gray reserved for inactive couplings during experiments. 1 2 3 4 5 Experiment (Emerald) 0 10 20 30 Total detections 1/6 3/16 8/26 23/39 25/43 3/45 7/82 17/150 32/109 24/69 Ne… view at source ↗
Figure 5
Figure 5. Figure 5: Detectable frequency shifts by experiment totaled over all edge sets. Red bars indicate nearest-neighbor detections, blue [PITH_FULL_IMAGE:figures/full_fig_p009_5.png] view at source ↗
read the original abstract

As quantum computers become available through multi-tenant cloud platforms, ensuring privacy against adversaries sharing the same quantum processing unit becomes critical. We introduce and explore \emph{covert quantum computing}, a new concept that ensures an adversary with access to all other quantum computational units (QCUs) of a quantum computer cannot detect computation on the subset that they cannot access. Analogous to covert communication, we employ information theory. However, since here the adversary controls the systems used for detection, we require a richer framework for covertness analysis that accounts for the use of quantum memories and adaptive operations. Thus, we adopt the \emph{quantum-strategy} framework used in quantum game theory and memory channel discrimination. Current quantum computers use planar graph circuit layouts and typically assume nearest-neighbor crosstalk. We derive discrete isoperimetric inequalities to show that, for an $n$-qubit circuit under this model, only $\mathcal{O}(\sqrt{n})$ border qubits provide detection information to the adversary. We then explore this scaling law on IQM's 54-qubit \emph{Emerald} processor and IBM's 156-qubit \emph{ibm\_fez} machine employing the Heron 2 architecture. We implement Ramsey experiments on qubits not used in computation, and detect nearest-neighbor crosstalk, as expected. However, we also observe long-range coupling effects beyond the border qubits, revealing a side channel that the adversary can exploit. We hypothesize that this long-range crosstalk is induced by leakage from the drive and control lines. Beyond weakening covertness, it exposes co-tenants to both adversarial and unintended crosstalk and degrades circuits that span spatially distributed qubits, motivating further work on spatial isolation and crosstalk characterization.

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

1 major / 2 minor

Summary. The manuscript introduces covert quantum computing to protect n-qubit computations from detection by an adversary controlling other qubits on a shared device. Under a nearest-neighbor crosstalk model on planar graphs, it derives discrete isoperimetric inequalities showing that only O(sqrt(n)) border qubits suffice to provide the adversary with detection information. Experiments on IQM Emerald (54 qubits) and IBM ibm_fez (156 qubits) confirm nearest-neighbor effects but also observe long-range coupling, hypothesized to stem from drive-line leakage, which the authors note weakens covertness.

Significance. The parameter-free isoperimetric derivation supplies a concrete scaling law for an idealized model and is a clear strength. The experimental data on real hardware, even while deviating from the model, supplies useful observations on crosstalk that could inform hardware isolation efforts. The adoption of the quantum-strategy framework for handling adaptive adversaries with memories is appropriately chosen for the setting.

major comments (1)
  1. [Experimental exploration] Experimental section (paragraph beginning 'We then explore this scaling law...'): The reported long-range coupling effects directly contradict the strictly nearest-neighbor crosstalk and planar-graph assumptions used to derive the O(sqrt(n)) bound. Because this modeling premise is load-bearing for the central security claim, the experimental results leave the practical applicability of the bound unsupported without additional analysis of how the isoperimetric inequality changes under long-range interactions.
minor comments (2)
  1. The abstract introduces the acronym 'QCUs' before expanding it; a parenthetical definition on first use would improve readability.
  2. The transition between the theoretical bound and the experimental observations could be clarified by an explicit statement that the O(sqrt(n)) result is model-specific and does not extend to the observed long-range regime.

Simulated Author's Rebuttal

1 responses · 1 unresolved

We thank the referee for highlighting the relationship between our experimental observations and the modeling assumptions in the theoretical analysis. We address the comment below and clarify the scope of our results.

read point-by-point responses

  1. Referee: The reported long-range coupling effects directly contradict the strictly nearest-neighbor crosstalk and planar-graph assumptions used to derive the O(sqrt(n)) bound. Because this modeling premise is load-bearing for the central security claim, the experimental results leave the practical applicability of the bound unsupported without additional analysis of how the isoperimetric inequality changes under long-range interactions.

    Authors: The O(sqrt(n)) bound is derived under the explicit assumption of nearest-neighbor crosstalk on a planar interaction graph, which is the standard model for analyzing crosstalk in current superconducting processors. The experimental section is presented as an exploration of this scaling law on real hardware rather than a validation that the bound holds in practice. As already stated in the manuscript, the observed long-range coupling (hypothesized to originate from drive-line leakage) constitutes an additional side channel that weakens covertness beyond the border-qubit analysis. We agree that this deviation means the bound does not directly establish practical security on existing devices. In revision we will add explicit language stating that the central theoretical claim applies only to the idealized nearest-neighbor model and that the experimental results demonstrate the need for improved spatial isolation before the bound can be considered practically relevant. A quantitative re-derivation of the isoperimetric inequality under arbitrary long-range interactions lies outside the present scope. revision: partial

standing simulated objections not resolved
  • Quantitative analysis of how the isoperimetric inequality and resulting O(sqrt(n)) scaling change under long-range coupling models

Circularity Check

0 steps flagged

No significant circularity; derivation is self-contained

full rationale

The paper's central claim is a direct derivation of discrete isoperimetric inequalities under an explicit nearest-neighbor planar-graph model for circuit layouts, yielding the O(sqrt(n)) border-qubit scaling. This mathematical step is presented as following from the model assumptions stated upfront and does not reduce to any fitted parameters, self-citations, or renaming of known results. The experimental section on real hardware (observing long-range crosstalk) is separate and does not tune or validate the theoretical bound; the claim is not asserted to hold outside the model. No load-bearing step matches any enumerated circularity pattern.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on the domain assumption of nearest-neighbor-only crosstalk in planar layouts; no free parameters are fitted and no new entities are postulated.

axioms (2)
  • domain assumption Crosstalk occurs only between nearest-neighbor qubits on a planar graph layout.
    Invoked to apply discrete isoperimetric inequalities to the border-qubit count.
  • domain assumption The adversary has access to all qubits outside the user's computational subset and can perform adaptive quantum operations and memory-assisted measurements.
    Required to justify the richer quantum-strategy framework over classical information theory.

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discussion (0)

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