The use of quantum effects to achieve high-precision measurements.Quantum metrology is the science of using quantum mechanical properties — such as superposition, entanglement, and squeezing — to push measurement precision beyond the limits of classical physics. In classical measurements, precision improves with the square root of the number of particles used, but quantum correlations can break this barrier, achieving what is called the Heisenberg limit, where precision scales linearly with particle number. This makes it possible to measure time, frequency, phase, and other quantities with extraordinary accuracy.
The theoretical foundations were laid in the 1980s by Carlton Caves, who showed how squeezed light could improve gravitational wave detectors, and the field has since grown with advances in quantum optics and atomic physics.
The key idea is to prepare a quantum state that is highly sensitive to the parameter being measured. For example, in an interferometer, a beam of light can be split into two paths, and the phase difference between them reveals the quantity of interest. By injecting a 'squeezed' state of light — where the uncertainty in one quadrature is reduced at the expense of the other — the phase measurement can be made more precise than with ordinary laser light. Similarly, entangled particles can be used to measure a collective property with less noise than independent particles.
Quantum metrology is already used in atomic clocks that define the second, and it is a critical component of gravitational wave observatories like LIGO, where squeezed light enhances sensitivity to ripples in spacetime.
The most precise measurements ever made — of atomic transition frequencies — rely on quantum metrology, achieving uncertainties of less than one part in 10^18, equivalent to losing or gaining one second over the age of the universe.
