Quantum optical frequency standards (review)

When the optical absorption of gases is saturated by a coherent field, resonance peaks of widths several orders of magnitude greater than the Doppler width appear at the centers of the lines. These resonance peaks are used as high-precision frequency standards. The present review is concerned with various optical frequency standards based onresonance peaks of saturated gas lines. These methods include stabilization of a laser with the aid of the Lamb dip, the absorption peak in internal and external cells, etc. It is shown that frequency stability better than 10^–14 in 100 sec and a reproducibility of 5·10^–13 can be achieved. It is reported that the quadratic Doppler effect in the emission of optical-frequency photons influences the precision of the frequency reproducibility. A review is given of the measurements of an infrared frequency (approximately 88.386.2 GHz, wavelength 3.39 μ) by comparison with a klystron harmonic. Potentialities of quantum optical frequency standards are discussed and future tasks are outlined.