Quantum non-demolition (QND) measurements are a special class of quantum observations designed to extract information about a system without disturbing the property being measured — that is, without collapsing the quantum state of interest.
Key Features:
- Repeatability: A QND measurement allows the same observable to be measured multiple times, yielding the same result, because it does not project the system into a different state.
- Preservation of Quantum Coherence: Since the system is not irreversibly disturbed, QND measurements are crucial for monitoring quantum states over time.
- Used in Quantum Technologies: QND techniques are important in quantum optics, gravitational wave detection, and quantum information processing, especially for feedback control and error correction.
Example:
In quantum optics, QND measurements can be made on the photon number in a cavity without absorbing the photons, by coupling the field to atoms and observing atomic phase shifts rather than directly measuring the light.
QND measurements provide a path to gentle observation, vital for developing non-invasive quantum sensors and robust quantum computers.