Abstract After the birth of quantum mechanics, the notion in physics that the frequency of light is the only factor that determines the energy of a single photon has played a fundamental role. However, under the assumption that the theory of Lewis--Riesenfeld invariants is applicable in quantum optics, it is shown in the present work that this widely accepted notion is valid only for light described by a time-independent Hamiltonian, i.e., for light in media satisfying the conditions, , , and simultaneously. The use of the Lewis--Riesenfeld invariant operator method in quantum optics leads to a marvelous result:the energy of a single photon propagating through time-varying linear media exhibits nontrivial time dependence without a change of frequency.
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