For a neutral 40Ca-based optical frequency standard we report a fractional frequency instability of 4 × 10-15 in 1 s, which represents a fivefold improvement over existing atomic frequency standards. Using the technique of optical Bordé–Ramsey spectroscopy with a sample of 107 trapped atoms, we have resolved linewidths as narrow as 200 Hz (FWHM). With colder atoms this system could potentially achieve an instability as low as 2 × 10-16 in 1 s. Such low instabilities are important for frequency standards and precision tests of fundamental physics.
© 2000 Optical Society of America
C. W. Oates, E. A. Curtis, and L. Hollberg, "Improved short-term stability of optical frequency standards: approaching 1 Hz in 1 s with the Ca standard at 657 nm," Opt. Lett. 25, 1603-1605 (2000)