Using f (t) = A exp(-τ /τ c ) cos(ωt + φ) and rearranging equation S2, we obtained expression S3. With a value of B chosen such that s 2 B/2τ c > τ the influence of τ in the integral is negligible. By taking the norm the dephasing rate is isolated since the complex phase cancels out and we can bin all the τ independent terms together as it is shown in equation S4. Beating Energy (meV) Delay time (ps) Delay time (ps) Delay time (ps)