A novel technique for microfabricating alkali atom vapor cells is described, in which alkali atoms are evaporated into a micromachined cell cavity through a glass nozzle. A cell of interior volume 1 mm3, containing 87Rb and a buffer gas, was made in this way and was integrated into an atomic clock based on coherent population trapping. A fractional frequency instability of 6 × 10-12 at 1000 s of integration time was measured. The long-term drift of the F = 1, mF = 0 → F = 2, mF = 0 hyperfine frequency of atoms in these cells is below 5 × 10-11 / day. This high stability results from an improvement of both the linear frequency drift and temperature-related effects. The measurements presented here show that it is possible to reach long-term stabilities below 10-11 at 1 hour of integration in microfabricated cells. This makes chip-scale atomic clocks a viable technology for applications that require small size and low power consumption in combination with long-term frequency stability.
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Measured Allan deviation for the 87Rb D1 CSAC (red dots). A clear improvement in stability is evident over the first CSAC, which was based on the Cs D2 line (black squares).
Fractional frequency instability measured in the Cs D2 CSAC (squares) and the cell made with the and the clock using the cell made with the new evaporation method, based on the D1 line of 87Rb (triangles).
