Frequency biases in a cold-atom coherent population trapping clock

A compact cold-atom clock based on coherent population trapping (CPT) has been developed. The clock typically demonstrates a short-term fractional frequency stability of 4×10^-11/√τ, limited by frequency noise on the interrogation lasers. The clock interrogates a sample of 2×10⁵ atoms under free-fall with typical cycle and Ramsey periods of 50 ms and 16 ms, respectively. The largest two systematic frequency shifts that can limit the clock´s long-term stability are the light shift and the Doppler shift.