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Readout limits
How gently can a protected quantum memory be read, and what does that cost?
Why this direction
A quantum error-correcting code protects information by hiding it from the environment. The same information has to stay reachable by the experimenter. Those two requirements pull against each other, and the trade has mostly been treated as an engineering detail rather than something the code's own structure decides.
Our preprint bounds it: the logical decoherence induced by weak continuous readout separates into a conjugate cost and a self cost, and the self cost has a floor set by the code's Knill–Laflamme structure. The consequence is that readout quality becomes a code-design objective rather than a downstream problem — engineer the readout so its back-action is an error the code already corrects.
The direction is young and the open questions below are the honest edge of it.
Published
How gently can a protected quantum memory be read? Rate limits on logical readout from the structure of the code
Separates the logical decoherence induced by weak continuous readout into a conjugate cost and a self cost, and lower-bounds the self cost from the code's own Knill–Laflamme structure. Verified cell-by-cell across cat and GKP codes.
Where it stops
Leading order in the measurement rate, single-mode Markovian limit, and the framework cannot express canonical phase measurement — which is how number-phase codes are conventionally read.
Findings
1- Aug 2026
Most of the cat-over-GKP readout advantage is a choice of meter, not a property of the code
The 7.8x raw readout advantage of cat over GKP qubits at matched photon number splits into a meter-scale factor and a residual ratio of about 1.5x. The large factor comes from the cat's protected variable being an unbounded quadrature whose signal grows with photon number, against GKP's bounded modular one — a units effect, not an intrinsic one.
Still open
What this direction has not answered, and what would settle each.
How tight is the structural floor on logical error for dissipative cats?
The ideal-code floor is a valid lower bound but runs 6–14× low against the dissipative cat. The missing factor is set by the confinement ratio κ₂/γ and buffer adiabaticity g₂/κ_b.
Settled if
An effective gap for the dissipative cat closes the floor-to-rate distance across the κ₂/γ sweep — or is shown not to exist.
Does the readability limit extend to codes read by canonical phase measurement?
Our preprint bounds logical decoherence from a code's own structure, but the framework cannot express canonical phase measurement — which is how number-phase codes are conventionally read. So the result carries a scope restriction we state rather than hide.
Settled if
An extension of the bound to phase measurement, or a proof that the restriction is essential.