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 NOON, squeezed, and hybrid states, which com- bine 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. Sig- nificant 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 im- posed 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 quanti Of special importance are quantum metrology options spontaneous parametric light scattering, which, for m 50 years, has been an indispensable tool for key acc ments in quantum optics and related areas of photoni tum cryptography, quantum computing, and quantum In this regard, we analyze the current status of the u well-known Hong-Ou-Mandel photon anticorrelati and biphoton interference in various quantum metro proaches in measuring temperature, length, material co tion, and so on. We also discuss the use of biph photometry, radiometry, and sensing for the absolute tion of modern photon-count detectors, as well as for ments of the brightness temperature of hot radiation The quantum metrology phenomena, methods, aproaches 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.