Dielectric rotary echoes in vitreous silica

Dielectric rotary echo experiments at microwave frequencies have been performed on vitreous silica at temperatures between 11 and 70 mK. The relaxation time $T_1$ of the tunneling states has been measured directly and found to be distributed in agreement with theoretical predictions. From a Fourier transformation of the echo envelope, the absolute value and the distribution of the dipole moments could be deduced. Significant differences between intrinsic tunneling states and those associated with ${\mathrm{OH}}^{\mathrm{-}}$ impurities were observed.