This research follows up on work first begun on the 657 nm intercombination line in neutral calcium (natural linewidth = 375 Hz) by Barger and Bergquist at NIST in 1979. Using a four-beam optical Ramsey technique we perform high resolution spectroscopy on calcium atoms in a thermal beam with a pre-stabilized probe laser. We then stabilize the frequency of the probe laser to the atomic resonance to yield a stable optical frequency. Thanks in part to state-of-the-art optical cavities, we have used this system to achieve stabilities 1-2 orders of magnitude better than those of hydrogen masers for time scales up to several hundred seconds. Previous versions of the Ca standard in our laboratory used magneto-optic traps to generate samples of laser-cooled atoms that demonstrated one-second fractional-frequency instabilities of 4 x 10-15 and an absolute fractional frequency uncertainty of 7.5 x 10-15. More recently we have found that we can achieve a similar instability with a much simpler, thermal beam-based system, albeit with increased uncertainty. As a result, one can envision a compact, single-laser system that could achieve an instability approaching 1 Hz in 1 s. When combined with a femtosecond frequency comb, such calcium clocks could find application in a variety of precision timing applications.
Future research will include: (1) the addition of a blue detection laser to the system in order to improve the clock stability by up to an order of magnitude or more, especially on timescales of seconds and below; (2) evaluating potential shifts and optimizing the performance of the beam clock on longer timescales, for hours or days.