Quantum change point metrology

arXiv:2610.38345v1 Announce Type: new Abstract: We quantify the extra Bayes risk of locating a quantum change point and estimating its amplitude with one measurement. The joint and marginal optimizations use the full record and the same joint prior, with both parameters unknown. For product records on a known pure state curve, position has clipped absolute loss in resolution units, and amplitude offset has squared loss in fixed calibrated units. Ideal steps exhibit certified conflict at finite signal strength, including in phase records of sixteen qubits, and recover both informative marginal optima at strong signal. For finite offset prior width, a retained quantum boundary region serves position, and the remaining suffix serves amplitude.