Coherence and Dephasing

Abstract

The time-dependent perturbation theory that we have used to treat resonance energy transfer and absorption of light assumes that we know that a system is in a given state (state 1), so the coefficient associated with this state (C₁) is 1, while the coefficient for finding the system in a different state (C₂) is zero. The resulting expression for the rate of transitions to state 2 (Eq. (2.58) or Eq. (7.8)) neglects the possibility of a return to state 1. It can continue to hold at later times only if the transition to state 2 is followed by a relaxation that takes the two states out of resonance.