Qimet

Qimet is a term used in theoretical discussions of quantum measurement to denote a hypothetical unit of information gained from a measurement that is constrained by back-action on the measured system. In this usage, a qimet represents the smallest meaningful increment of knowledge about a quantum state that is compatible with a specified level of disturbance. The concept is employed to analyze trade-offs between information gain and measurement-induced disturbance in metrology and quantum information processing.

Origin and terminology: The term is not widely standardized. It appears in informal discussions and in a small number of papers since the mid-2020s; the coinage seems to derive from "quantum information" and "measurement."

Definition and properties: The qimet value is model-dependent; it is not an intrinsic property of a state but a function of the measurement protocol, prior information, and the disturbance budget. It is often discussed alongside the quantum Fisher information and the quantum Cramér–Rao bound as a way to gauge the efficiency of information extraction. In some formulations, qimet is bounded between 0 and 1, with 1 representing a fully efficient measurement under the allowed disturbance.

Applications: Researchers use qimet in thought experiments on precision limits, in comparative studies of measurement schemes (projective versus weak measurement), and in proposals for optimizing quantum sensors subject to back-action constraints.

Criticism: Because the concept is not standardized, different authors define qimet differently, which complicates cross-study comparisons. Observability depends on the chosen measurement model, and the quantity is not directly measurable without specification.

See also: Quantum metrology, Quantum Fisher information, Heisenberg limit, Quantum back-action, Information-disturbance trade-off.