In quantum physics, a virtual state is a very short-lived, unobservable quantum state.[1]
In many quantum processes a virtual state is an intermediate state, sometimes described as "imaginary"[2] in a multi-step process that mediates otherwise forbidden transitions. Since virtual states are not eigenfunctions of any operator,[3] normal parameters such as occupation, energy and lifetime need to be qualified. No measurement of a system will show one to be occupied,[4] but they still have lifetimes derived from uncertainty relations.[5][6] While each virtual state has an associated energy, no direct measurement of its energy is possible[7] but various approaches have been used to make some measurements (for example see[8] and related work[9][10] on virtual state spectroscopy) or extract other parameters using measurement techniques that depend upon the virtual state's lifetime.[11] The concept is quite general and can be used to predict and describe experimental results in many areas including Raman spectroscopy,[12]non-linear optics generally,[5] various types of photochemistry,[13] and nuclear processes.[14]
^Robinson AL (February 1985). "Tunable Far IR Molecular Lasers Developed: Stimulated Raman scattering associated with a series of closely spaced rotational states is the key to wavelength tunability". Science. 227 (4688). New York, N.Y.: 736–7. doi:10.1126/science.227.4688.736. PMID17796721.
^Lee DI, Goodson III T (2007). Nunzi JM (ed.). "Quantum spectroscopy of an organic material utilizing entangled and correlated photon pairs". Linear and Nonlinear Optics of Organic Materials VII. 6653. International Society for Optics and Photonics: 66530V. Bibcode:2007SPIE.6653E..0VL. doi:10.1117/12.745492. S2CID122068309.