Virtual 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.