All-Optical Atomic Clock Based on Dark States of 85Rb

An all-optical RF standard based on dark states of ⁸⁵Rb atoms has been developed. With this system we were able to measure ultra-narrow optically induced hyperfine dark resonances below 20 Hz (Q-value > 1.5 × 10⁸). The frequency of a signal generator was stabilized to the dark resonance giving a relative frequency stability (square root of Allan variance) of 3.5 × 10^-11τ^-1/2 (1S < t < 2000s). The best stability reached at an integration time of t = 2000s was 6.4 × 10^-13, which is sufficient for many high-precision applications. The frequency shifts caused by various experimental parameters were also studied.