Lystilstande

Lystilstande, or light states, are the quantum states of the electromagnetic field used in quantum optics to describe the statistical and phase properties of light. They differ from classical descriptions by exhibiting non-classical features such as specific photon statistics or phase-dependent measurements, and they form the basis for understanding how light behaves at the quantum level.

Common lystilstande include coherent states, which resemble classical laser light; Fock (number) states, with a definite photon number; and squeezed states, where uncertainty is reduced in one quadrature at the expense of the other. Thermal and mixed states describe light with statistical mixtures of such pure states.

Mathematically, lystilstande are represented by state vectors in a Hilbert space or by density operators. They can be depicted in phase space with Wigner or Q-functions. Non-classical light shows features such as sub-Poissonian statistics, quadrature squeezing, or negativity in the Wigner function.

Generation and detection rely on optical devices and measurement techniques. Coherent states arise from stable lasers; squeezed states from nonlinear optical processes; Fock states from heralded single-photon sources. Measurements include photodetection, homodyne or heterodyne detection, and quantum tomography to reconstruct the state.

Applications of lystilstande span quantum communication, quantum computation, and precision metrology, where quantum features of light enable capabilities beyond classical limits.