nonperturbing

Nonperturbing is an adjective used to describe actions, interactions, or measurements that do not perturb the system in a specified sense, or do so only negligibly within a given model or approximation. In perturbation theory, quantities are expanded in a small parameter; a nonperturbing interaction is one whose leading effect is zero or effectively negligible at the order considered.

In quantum physics, nonperturbing or non-invasive measurements are sought to extract information without significantly disturbing the system’s state or subsequent evolution. A rigorous example is a quantum non-demolition (QND) measurement, which measures an observable that commutes with the system’s Hamiltonian, allowing repeated measurements with minimal back-action on that observable.

Practically, no measurement is perfectly nonperturbing. The label is often relative, indicating that the perturbation falls below a chosen threshold or that the back-action is sufficiently small for the experiment’s purpose. Nonperturbing regimes frequently involve weak or dispersive coupling, where the interaction strength is reduced to limit disturbance while still yielding usable data.

Outside physics, the term is used more broadly to describe probes or techniques that aim to minimize perturbation to a sample or system, such as noninvasive imaging or spectroscopic methods designed to couple weakly to the subject of study.

Limitations include the fact that even minimal coupling can introduce some back-action, and the designation depends on the chosen theoretical framework and measurement goals. The concept is related to, but distinct from, perturbation theory and nonperturbative approaches. See also perturbation theory, nonperturbative methods, QND measurement, and weak measurement.