Future trends in fundamental physics and applications

As discussed in the previous chapter, with the present frequency stability of 6 · 10−16 and accuracy of 1.1 · 10−15, microwave frequency standards based on fountains are now in their adulthood. Also, within a few years, further affordable betterments could push the stability into the low 10−16 range (over one-day averaging time) and the accuracy to 1 · 10−16. We have also learnt that, when the quantum projection noise dominates over technical noise sources, the instability of an oscillator which is locked to an atomic transition with frequency ν0 and linewidth ∆ν can be expressed asσ(τ) = ∆νπν0√ TNτ (8.1)where T is the cycle time required to perform a single determination of the line center frequency and N denotes the number of detected atoms [764]. Note that this formula is valid for averaging times τ > T . Also, a signal contrast of 100% and the absence of dead time (between the cycles) are assumed. Now, while the processes that limit the linewidth of an atomic transition in the microwave region are essentially comparable to those in the optical domain, a 5-orders-of-magnitude enhancement is gained by moving from the operating frequency of the cesium primary standard to optical frequencies. Thus, potentially, a dramatic reduction in the instability can be obtained by establishing a frequency standard on an optical transition. At risk of becoming pedantic, before going on, we insist in elucidating the principle of operation of a passive atomic frequency standard (Figure 8.1) [715].