Resolution of photon-recoil structure of the 6573-Å calcium line in an atomic beam with optical Ramsey fringes

The photon-recoil components of the Ca 6573-Å line have been resolved using the three-zone optical Ramsey interference technique with an atomic beam. Linewidths as narrow as 3 kHz HWHM (line Q of 8×1010) are reported for zone separations up to 7 cm. An indication of the light-shift-induced contraction of the recoil splitting predicted by C.J. Bordé is obtained. Techniques are discussed which should lead to an optical wavelength/frequency standard with an accuracy of better than 10−14.