Phonon echoes, the acoustic analog of spin and photon echoes, are observed in fused silica glass at ultralow temperatures. From the phonon echo decay, we infer a homogeneous transverse lifetime of 14×${10}^{-6\mathrm{}}$ sec and a longitudinal relaxation time of 200×${10}^{-6\mathrm{}}$ sec for tunneling states at 0.68 GHz at a temperature of 20 mK. The coupling between the two-level systems and phonons is estimated to be 1.6 eV.

• Received 2 August 1976

DOI:https://doi.org/10.1103/PhysRevLett.37.852