Closely interleaved self-comparison method applied to precise measurement

A self-comparison method with closely interleaved switching states is analyzed and used to evaluate some type-B uncertainties of an 87Rb atomic fountain clock. Free from additional frequency reference, the method can be applied to a running fountain to reach a precision beyond its uncertainty. A verification experiment proves an uncertainty of 9.2×10-16 at an averaging time of 242500 s. Further, the method is applied to measure light shift, and no visible relative frequency shift is found in the fountain within the uncertainty of 2.1×10-15. When applied to the evaluation of a cold collisional shift, the result gives a -2.2×10-15 shift with a 9.5×10-16 uncertainty.