QND measurement: [A_obs, H_sys+H_int] = 0 → back-action-evasion

Braginsky 1974 / Caves-Thorne-Drever-Sandberg-Zimmermann 1980 quantum non-demolition (QND) measurement: an observable  whose measurement back-action does not disturb its own subsequent value. Formal criterion: the QND observable commutes…