Quantum Beats of Recoil-FreeγRadiation

Recoil-free $\gamma$ rays from the decay of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ are frequency modulated by vibrating the source with a piezoelectric crystal and one of the lines of the resulting multiplet emission spectrum is absorbed. The remaining radiation displays a time-dependent counting rate whose harmonic composition and relative phases are sensitive to small energy shifts and can be used for their measurement.