Quantum metrology

Quantum metrology uses quantum states and effects to estimate physical quantities. It exploits coherence, entanglement, and measurement statistics to improve precision or sensitivity.

Quantum Fisher information: A quantity that bounds how much information a quantum state carries about an unknown parameter. It sets the quantum Cramér–Rao bound on estimation precision.
Atomic clock: A clock that uses an atomic transition frequency as its timekeeping reference. Quantum control of atomic transitions enables exceptionally precise time measurement.
Standard quantum limit: A precision scaling limit typically obtained from independent probes with uncorrelated measurement noise. It is the benchmark that entangled or squeezed probes can surpass under suitable conditions.
LIGO: A pair of gravitational-wave observatories in the United States that use laser interferometry. LIGO demonstrated squeezed-light enhancement in precision interferometric measurement.
Quantum Cramér–Rao bound: A lower bound on the variance of an unbiased parameter estimate from quantum measurements. It expresses the best precision allowed by a chosen quantum probe state.
Gravitational-wave detection: The measurement of spacetime disturbances produced by accelerating masses, using observatories such as LIGO. Squeezed light reduces measurement noise in gravitational-wave interferometers.
Shot noise: Statistical fluctuation caused by the discrete, random arrival or detection of particles. Its inverse-square-root scaling with particle number defines a common classical benchmark.
Carlton M. Caves: A physicist known for foundational work on quantum limits in interferometry and quantum optics. His analyses showed how squeezed states could reduce interferometric measurement noise.
Quantum entanglement: A form of correlation in which a composite quantum state cannot be separated into states of its parts. Entangled probes can improve phase-estimation precision beyond independent probes.
Optical interferometry: A measurement technique that uses interference between light waves to infer differences in optical path. Quantum states of light can sharpen interferometric phase measurements.

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