Temperature of laser-cooled 171Yb+ ions and application to a microwave frequency standard

Microwave frequency standards based on buffer gas-cooled Yb-171(+) ions have demonstrated high stability but are limited in accuracy by the second-order Doppler shift caused by thermal motion. We have previously obtained near shot noise-limited Ramsey fringes with a laser-cooled ion cloud. Here, we present measurements confirming that the ion temperature remains <1 K throughout the microwave interrogation period for a Ramsey pulse separation of up to 10 s and longer. The potential stability of the ions as a frequency standard is better than sigma(y)(tau) = 5 X 10(-14)tau(-1/2), and estimates of the systematic offsets to the clock frequency and their uncertainties indicate that a total uncertainty of four parts in 10(15) or better is achievable.