Quantum Data Ruler
quant-ph
Submitted: 2026-10-06
Updated: 2026-10-06
License: http://creativecommons.org/licenses/by/4.0/
The gist: A ruler reads an unknown length against a calibrated scale.
Abstract
A ruler reads an unknown length against a calibrated scale. The quantum data ruler reads a matrix element of an unknown state against the signal of a known reference, from their ratio. It is exact for any Hamiltonian and any time, since transition operators change by a state-dependent constant times one function of time, removing the vanishing-signal limit. The readout is unbiased under dephasing and amplitude damping, and recovers the coherent ergotropy of a quantum battery that two-point measurement erases. The structure survives the classical limit as action--angle Fourier modes.
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