ULTRASLOW OPTICAL DEPHASING IN EU-3+-Y2SIO5

We report the measurement of what we believe to be the narrowest known optical resonance in a solid. This was made on the F-7(0) --> 5D0 tra nsition of Eu3+:Y2SiO5 at 1.4 K using photon echoes with sufficiently low laser power to almost eliminate contributions from instantaneous d iffusion. The optical resonance widths measured for Eu3+ ions in the t wo crystallographic sites of Y2SiO5 and in a magnetic field of 100 G a re 122 Hz (site 1) and 167 Hz (site 2). These widths are dominated by population decay, but up to 20 Hz is attributed to Y-89 spin fluctuati ons which have been reduced by the applied magnetic field.