  The anharmonic properties of the low-frequency E2 phonon in ZnO were measured
using impulsive stimulated Raman scattering. At 5 K, the frequency and lifetime
are (2.9787 +/- 0.0002) THz and (211 +/- 7) ps. The unusually long lifetime and
the high accuracy in the determination of the frequency hold promise for
applications in metrology, quantum computation and materials characterization.
The temperature dependence of the lifetime is determined by two-phonon
up-conversion decay contributions, which vanish at zero temperature. Results
suggest that the lifetime is limited by isotopic disorder and that values in
the nanosecond range may be achievable in isotopically-pure samples.
