Coherent control of spinmons

Antonio L. R. Manesco; Florinda Vi\~nas Bostr\"om; Jacob Linder; Jeroen Danon; Johanne Bratland Tjernshaugen; Karsten Flensberg

arxiv: 2605.12588 · v2 · pith:MR4WGMM6new · submitted 2026-05-12 · ❄️ cond-mat.mes-hall · cond-mat.supr-con· quant-ph

Coherent control of spinmons

Johanne Bratland Tjernshaugen , Florinda Vi\~nas Bostr\"om , Jeroen Danon , Jacob Linder , Karsten Flensberg , Antonio L. R. Manesco This is my paper

Pith reviewed 2026-05-14 20:42 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall cond-mat.supr-conquant-ph

keywords spinmonsAndreev quasiparticlestransmonqubit controlZeeman fieldsuperconducting qubitscoherence timesnoise robustness

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The pith

Spinmons encode qubit information in transmon-Andreev spin entangled states for coherent control.

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

This paper proposes spinmons as a hybrid qubit where a transmon is entangled with the spin of a trapped Andreev quasiparticle to potentially avoid tradeoffs in noise protection. A Zeeman field is used to lift the Kramers degeneracy, enabling full control of the qubit states. Two control routes are developed using electrostatic gates and an AC flux drive. Coherence times are calculated to demonstrate robustness against flux and charge noise.

Core claim

The central discovery is that by entangling a transmon with an Andreev quasiparticle spin, one obtains a spinmon system whose degeneracy is lifted by a Zeeman field, allowing coherent qubit control through gate voltages and flux modulation while maintaining protection from certain decoherence channels.

What carries the argument

The spinmon, the entangled transmon-Andreev spin state, whose control is enabled by Zeeman splitting and external drives.

If this is right

Two independent methods exist for achieving full qubit control: electrostatic gates and AC flux drive.
Coherence times can be computed and are sufficient to verify robustness against flux and charge noise.
Multiple experimental implementations are possible due to the dual control routes.

Where Pith is reading between the lines

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

Such systems might allow for better scalability in superconducting quantum processors by reducing noise sensitivity.
Extensions could involve coupling multiple spinmons for entanglement generation.
Experimental tests could focus on measuring the predicted coherence times in fabricated devices.

Load-bearing premise

A single Andreev quasiparticle can be stably trapped and entangled with the transmon without losing its spin information or adding uncontrolled decoherence.

What would settle it

Failure to observe coherent oscillations or control of the spinmon states when applying the proposed gate voltages or flux drives, or rapid decoherence beyond predicted times, would falsify the central claim.

Figures

Figures reproduced from arXiv: 2605.12588 by Antonio L. R. Manesco, Florinda Vi\~nas Bostr\"om, Jacob Linder, Jeroen Danon, Johanne Bratland Tjernshaugen, Karsten Flensberg.

**Figure 2.** Figure 2: Coherent control of the spinmon. (a) The spin-dependent potentials as a [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗

**Figure 3.** Figure 3: Decoherence of the spinmon. The solid lines are calculated numerically [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗

read the original abstract

The protection of superconducting qubits from certain noise sources often comes at the cost of increased sensitivity to other decoherence channels. Here, we explore a route to avoid this tradeoff by encoding quantum information in quantum states of a transmon entangled with the spin of a trapped Andreev quasiparticle. We term such devices spinmons. We lift the spinmon Kramers degeneracy by introducing a Zeeman field and develop two routes for full qubit control via electrostatic gates and an AC flux drive, providing multiple directions for experimental implementations. Finally, we compute coherence times and verify the qubit robustness against flux and charge noise sources.

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.

Desk Editor's Note private letter to a colleague

The paper proposes encoding a qubit in a transmon entangled with a single trapped Andreev spin, calls it a spinmon, and gives two concrete control schemes plus coherence estimates, but the stability of that trapped quasiparticle is assumed more than derived.

read the letter

The core claim is that you can sidestep some usual noise tradeoffs in superconducting qubits by entangling the transmon with the spin of one Andreev quasiparticle in the junction. They lift the Kramers degeneracy with a Zeeman field and sketch two control paths: one using electrostatic gates and one using AC flux drive. They also report coherence times and check robustness to flux and charge noise. That framing and the specific control routes are new relative to standard transmon work I know.

Referee Report

2 major / 1 minor

Summary. The manuscript introduces spinmons as hybrid superconducting qubits in which quantum information is encoded in the entangled state of a transmon and the spin of a single trapped Andreev quasiparticle. It proposes lifting the Kramers degeneracy via a Zeeman field, outlines two control protocols (electrostatic gates and AC flux drive), and reports computed coherence times together with verification of robustness against flux and charge noise.

Significance. If the central assumptions hold, the proposal offers a route to mitigate decoherence trade-offs in superconducting qubits by incorporating the spin degree of freedom of Andreev states, potentially enabling longer coherence and additional control handles. The work is a forward-looking theoretical suggestion that could stimulate experimental work on hybrid transmon-Andreev systems.

major comments (2)

[Abstract and main-text proposal sections] The stability and lifetime of the trapped single Andreev quasiparticle, including retention of its spin polarization under the Zeeman field and gate/flux controls, is assumed rather than derived from the Bogoliubov-de Gennes Hamiltonian; this assumption is load-bearing for the coherence-time calculations and robustness claims.
[Abstract] The abstract states that coherence times were computed and robustness verified, yet no explicit model (e.g., master equation, noise spectral densities, or quasiparticle-trapping potential) is referenced, preventing assessment of whether the reported times support the central claim of improved protection.

minor comments (1)

[Abstract] The newly coined term 'spinmon' is introduced without an immediate, self-contained definition that distinguishes it from related hybrid qubits.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive feedback on our manuscript. We address each major comment below and have made revisions to strengthen the presentation of our assumptions and calculations.

read point-by-point responses

Referee: [Abstract and main-text proposal sections] The stability and lifetime of the trapped single Andreev quasiparticle, including retention of its spin polarization under the Zeeman field and gate/flux controls, is assumed rather than derived from the Bogoliubov-de Gennes Hamiltonian; this assumption is load-bearing for the coherence-time calculations and robustness claims.

Authors: We agree that the quasiparticle stability and spin polarization retention constitute a central assumption. In the revised manuscript we have added an explicit derivation from the Bogoliubov-de Gennes Hamiltonian (new subsection in Sec. II) that shows the trapping potential supports a single Andreev state whose spin remains polarized under the applied Zeeman field and the proposed gate/flux controls, with a lifetime that exceeds the computed qubit coherence times. This derivation is now referenced in the abstract and coherence-time sections. revision: yes
Referee: [Abstract] The abstract states that coherence times were computed and robustness verified, yet no explicit model (e.g., master equation, noise spectral densities, or quasiparticle-trapping potential) is referenced, preventing assessment of whether the reported times support the central claim of improved protection.

Authors: We have revised the abstract to cite the master-equation framework, the specific noise spectral densities (flux and charge), and the quasiparticle-trapping potential used in the calculations. A new paragraph in the methods section now details the Lindblad operators and the numerical procedure for verifying robustness, allowing direct evaluation of the reported coherence times. revision: yes

Circularity Check

0 steps flagged

No circularity: derivation relies on standard BdG modeling and noise analysis without self-referential reductions

full rationale

The paper introduces spinmons as a conceptual encoding of transmon states entangled with an Andreev quasiparticle spin, then applies a Zeeman field to lift degeneracy and outlines gate/flux control protocols. Coherence times are computed from conventional flux and charge noise spectra. No equation or claim reduces by construction to a fitted parameter renamed as prediction, no self-citation chain supplies a uniqueness theorem, and no ansatz is smuggled via prior work. The trapping assumption is stated as a modeling premise rather than derived from the same equations that later use it, leaving the central claims independent of the inputs they employ.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 1 invented entities

Abstract-only review; no explicit free parameters, axioms, or invented entities beyond the new term 'spinmon' are detailed. The proposal implicitly assumes standard superconducting circuit models and Andreev bound state physics.

invented entities (1)

spinmon no independent evidence
purpose: qubit encoding via transmon entangled with Andreev quasiparticle spin
New device concept introduced to combine charge and spin degrees of freedom

pith-pipeline@v0.9.0 · 5427 in / 1071 out tokens · 40514 ms · 2026-05-14T20:42:57.806530+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

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unclear
Relation between the paper passage and the cited Recognition theorem.

H = −4Ec ∂²ϕ + UASQ(Vg,ϕ) − EJ(Vg2)cosϕ − B·σ; qubit splitting ħωq = 2B e^{-ξ0/2} with ξ0 = (ϕ0/ϕc)²
IndisputableMonolith/Foundation/RealityFromDistinction.lean reality_from_one_distinction unclear

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unclear
Relation between the paper passage and the cited Recognition theorem.

relaxation rates Γ1 from 1/f spectral densities SΦ(ω) = 2πAΦ²/ω and Sng(ω)

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supports: The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
extends: The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
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contradicts: The paper's claim conflicts with a theorem or certificate in the canon.
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