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Microclocks at NIST - Rb87 CSAC
Improved Frequency Stability with a Chip-Scale Atomic Clock Based on 87Rb and Excited on the D1 Transition

The first 87Rb-based CSAC physics package has been demonstrated in a volume of 12 mm3, dissipating 195 mW of power at an ambient temperature of 22 °C. Excited using light resonant with the D1 line, it exhibited a much better short-term stability than CSACs operating on the D2 line of cesium.

A chip-scale atomic magnetometer

The chip-scale atomic clock based on 87Rb. (a) Schematic of the clock.  The components are: 1–the VCSEL, 2–the optics package including (from bottom to top) a glass spacer, a neutral-density filter, a refractive microlens surrounded by an SU-8 spacer, a quartz l/4 waveplate, and a neutral-density filter, 3–the 87Rb vapor cell with transparent ITO heaters above and below it, and 4–the photodiode assembly.  (b) Photograph of the clock physics package. 

A fractional frequency instability of 4 ×10-11 / t1/2 was achieved for integration times between 1 s and 10 s. Using noble gases of neon or argon, a residual long-term drift of -5 ×10-9 / day was measured. When using the D1 line, optical Zeeman pumping and the existence of a dark trapping state cause atoms to accumulate in the “stretched state”. As a result of this a higher cell temperature is required, which increases the power consumption of the CSAC as well as the broadening due to spin-exchange collisions.

Allan deviation of the 87Rb physics package, which is excited on the D1 optical transition, as compared with the Cs physics package, which is excited on the D2 optical transition.

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).

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References:

Requires Adobe Acrobrat Reader S. Knappe, P. D. D. Schwindt, V. Shah, L. Hollberg, J. Kitching, L. Liew and J. Moreland, "A chip-scale atomic clock based on 87Rb with improved frequency stability," Opt. Exp. 13, 1249, 2005.

Requires Adobe Acrobrat Reader M. Stahler, R. Wynands, S. Knappe, J. Kitching, L. Hollberg, A. Taichenachev, and V. Yudin, "Coherent population trapping resonances in thermal Rb-85 vapor: D-1 versus D-2 line excitation," Opt. Lett., 27, 1472, 2002.