Spectrally narrow, long-term stable optical frequency reference based on   a Eu$^{3+}$:Y$_{2}$SiO$_{5}$ crystal at cryogenic temperature
  Using an ultrastable continuous-wave laser at 580 nm we performed spectral hole burning of Eu$^{3+}$:Y$_{2}$SiO$_{5}$ at very high spectral resolution. Essential parameters determining the usefulness as a "macroscopic" frequency reference: linewidth, temperature sensitivity, long-term stability were characterized, using a H-maser stabilized frequency comb. Spectral holes with linewidth as low as 6 kHz were observed and the upper limit of the drift of the hole frequency was determined to be on the order of 5$\pm$3 mHz/s. We discuss necessary requirements for achieving ultra-high-stability in laser frequency stabilization to these spectral holes. 
Qun-Feng Chen, Andrei Troshyn, Ingo Ernsting, Steffen Kayser, Sergey   Vasilyev, Alexander Nevsky, Stephan Schiller
physics.optics physics.atom-ph