Improved short-term stability of optical frequency standards: approaching 1 Hz in 1 s with the Ca standard at 657 nm

For a neutral Ca-40-based optical frequency standard we report a fractional frequency instability of 4 x 10(-15) in 1 s, which represents a fivefold i mprovement over existing atomic frequency standards. Using the technique of optical Borde-Ramsey spectroscopy with a sample of 10(7) trapped atoms, we have resolved linewidths as narrow as 200 Bz (FWHM). With colder atoms thi s system could potentially achieve an instability as low as 2 x 10(-16) in 1 s. Such low instabilities are important for frequency standards and preci sion tests of fundamental physics. OCIS codes: 300.6320, 140.3320, 020.7010 .