Evaluation of the accuracy of the optically pumped caesium beam primary frequency standard of BNM-LPTF

The primary frequency standard LPTF-JPO employs a caesium thermal beam, and optical pumping techniques. We present the main parts of the design. The short-term stability is reported, σy(τ) = 3.5 × 10−13 τ−1/2, where τ is the averaging time. The general expression for frequency shift for a square-wave frequency modulation is given. We discuss the determination of each frequency shift, and we present new methods to evaluate some of them. A relative combined standard uncertainty of 6.4 × 10−15 (1 σ) is obtained.