Quantum Nondemolition Measurement
Quantum nondemolition (QND) measurement is a measurement of a quantum system, which preserves the integrity of the system and the value of the measured observable. This allows exactly the same system to be measured repeatedly.
The term nondemolition does not imply that the wave function fails to collapse. In fact, QND measurement is best thought of as the ideal quantum projective measurement.
Most devices capable of detecting a single particle and measuring its position destroy the particle in the measurement process. Less dramatically, the measurement may simply perturb the particle so that it is not in the measured eigenstate even immediately after the measurement. A perfect QND measurement of a particle's position, in contrast, would leave the particle in its measured position.
QND measurements are extremely difficult to carry out experimentally.
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