Quantum optical metrology

We review recent progress in the field of optical quantum metrology, with a focus on the analysis of the current level of theoretical and experimental research on the generation, transformation, and measurement of nonclassical states of light, such as ${N}$00$N$, squeezed, and hybrid states, which combine transformations of both discrete and continuous variables of a quantized light field. We show how such states can be used to improve the measurement accuracy and to estimate unknown phase parameters in both linear and nonlinear metrology. Significant attention is paid to the description of actual quantum metrology schemes that take the loss of particles, the limited fidelity of photon detectors, and other factors into account. We therefore identify both the ultimate (fundamental) bounds imposed by quantum mechanical uncertainties of the quantities being measured and the bounds due to the effect of classical noise on the propagation and measurements of a quantized field. Of special importance are quantum metrology options based on spontaneous parametric light scattering, which, for more than 50 years, has been an indispensable tool for key accomplishments in quantum optics and related areas of photonics: quantum cryptography, quantum computing, and quantum sensing. In this regard, we analyze the current status of the use of the well-known Hong–Ou–Mandel photon anticorrelation effect and biphoton interference in various quantum metrology approaches in measuring temperature, length, material concentration, and so on. We also discuss the use of biphotons in photometry, radiometry, and sensing for the absolute calibration of modern photon-count detectors, as well as for measurements of the brightness temperature of hot radiation sources. The quantum metrology phenomena, methods, and approaches discussed here in light of the most recent progress on sources and detectors of quantum radiation will be an important tool in developing and practically implementing new schemes and algorithms for quantum processing and information transmission.