  The stability of valley polarized electron states is crucial for the
development of valleytronics. A long relaxation time of the valley polarization
is required to enable operations to be performed on the polarized states. Here
we investigate the stability of valley polarized states in diamond, expressed
as relaxation time. We have found that the stability of the states can be
extremely long when we consider the symmetry determined electron-phonon
scattering. By Time-of-Flight measurements and Monte Carlo simulations, we
determine electron-phonon coupling constants and use these data in order to map
out the relaxation time temperature dependency. The relaxation time can be
microseconds or longer below 100K and 100 V/cm for diamond due to the strong
covalent bond, which is highly encouraging for valleytronic applications.
