AEROSPACE REPORT NO. TR-2007(8555)-5
The Rubidium Atomic Clock and Basic Research December 10,2007
James C. Camparo Electronics and Photonics Laboratory Physical Sciences Laboratories
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The Rubidium Atomic Clock and Basic Research
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J. C. Camparo
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The vapor-cell atomic clock finds application today in the global positioning system and telecommunications. To improve and miniaturize the humble device for future applications will require a deeper understanding of atomic and chemical physics.
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This work was supported under The Aerospace Corporation's Mission-Oriented Investigation and Experimentation program, funded by U.S. Air Force Space and Missile Systems Center under Contract No. FA8802-04-C-0001. This report is a reprint of a feature article in Physics Today. Reprinted with permission of The American Institute of Physics.
in
The rubidium atomic clock and basic research James Camparo The vapor-cell atomic clock finds application today in the global positioning system and telecommunications. To improve and miniaturize the humble device for future applications will require a deeper understanding of atomic and chemical physics. James Camparo is a senior scientist at The Aerospace Corporation in Los Angeles.
You're returning home from a conference, in a rental car on a dark, rainy road 50 miles from the airport; you're running out of time to catch your flight, and you're lost. As little as 10 years ago, this would have been the stuff of travel nightmares, to be shared with colleagues during a coffee break between conference sessions. Today it is less of a nightmare and more of an inconvenience. You pull your car to the side of the road, program your desired destination into the rental car's navigation system, and let it calculate a direct route to the airport from your present location using signals from orbiting GPS (global positioning system) satellites. Thankfully, you make it home without an exciting but harrowing story to tell family and friends. Unseen and forgotten in this now-routine use of GPS are the atomic clocks. The propagation times of signals traveling 20 000 km from space to Earth are measured at the nanosecond level, and from those measurements, positions are calculated to within, in some cases, a few centimeters. More generally, modern systems ranging from cell-phone communications to satellite navigation take advantage of atomic clocks, in particular the capabilities of small, lowpower, rubidium vapor-cell clocks. How the vapor-cell clock came to play such an important role in modern timekeeping is a story that goes back half a century, and one that highlights the close interplay between basic science and technological innovation.
The vapor-cetf atomic clock The vapor-cell atomic clock is an unpretentious device. Light from a small discharge lamp passes through a vapor of rubidium atoms housed in a glass cell and is detected by a photodiode. The light intensity transmitted by the vapor is used to lock the frequency of an RF signal to an atomic transition. The RF signal can then be down-converted to generate a train of one-second pulses for timekeeping. Given its conceptual simplicity, the extent to which the vapor-cell clock's realization relies on basic atomic and chemical physics is quite striking. Since time intervals are defined as the elapsed phase of some stable oscillation, all clocks are, at heart, frequency standards. Just like in your wristwatch, the timekeeping signal in an atomic clock is derived from a quartz crystal oscillator. ©2007 American Institute ol Physics, S-0031-9228-0711-010-3
However, a crystal oscillator's RF frequency vxt1, depends on a number of environmental factors, such as humidity, vibration, and radiation, that can cause the clock to either speed up or slow down. In the vapor-cell atomic clock, -i\till is electronically tied to an atomic resonance, thereby transferring the stability of atomic structure to the clock's tick rate. The atomic resonance that forms the basis of the Rb clock's operation is the transition between two hyperfine states of 87Rb. As in hydrogen, the ground state of Rb is split into two hyperfine levels by the magnetic-dipole interaction between the single valence electron and the nucleus. Those states are labeled by the total-angular-momentum quantum number F, as shown in figure la. Each hyperfine state is further split into Zeeman sublevels, characterized by the quantum number mF. The mF = 0 sublevels are unaffected to first order by stray magnetic fields, which can arise in devices from a host of internal and environmental sources. The socalled 0-0 transition between the F = 2, mF = 0 and F = 1, mF = 0 states, with frequency vhts = 6.8347 GHz, is unaffected by such fields and is ideal for stabilizing \\U1|. Figure lb shows the means by which vxlll is electronically tied to vhls. The first step is to multiply the output of a voltagecontrolled crystal oscillator (VCXO) by a factor M to create a microwave field with a frequency near vhrbr + can be expanded in terms of the unperturbed eigenstates—that is,
* = X cn|n>e-;^'.
(1!
n= 1
Shortly after the field is turned on, c, = Cj = 1 /\/2~j and it can be shown by time-dependent perturbation theory that
C-K- ~
ieE0 (r + r e'>), 31 32
(2)
V2*
If the phase of the field's b mode varies rapidly and randomly with respect to the a mode, then the cosine term of equation 4 averages to zero, and each excitation pathway acts independently. Alternatively, if
. 89, 472 (1953). 7. D. K. Walter, W. M. Griffith, W. Happer, Phys. Rev. Lett. 88, 093004 (2002); P. Oreto et al., Phys. Rev. A 69, 042716 (2004). 8. J. C. Camparo, S. B. Delcamp, Opt. Commun. 120, 257 (1995). 9. R. Frueholz, M. Wun-Fogle, /. Am. Ceram. Soc. 66, 605 (1983); R. Cook, R. Frueholz, Proceedings of the 42nd Annual Frequency Control Symposium, IEEE, Piscataway, NJ (1988), p. 525. 10. J. Camparo, C. Klimcak, S. Herbulock, IEEE Trans. Instrum. Meas. 54, 1873 (2005); I. Coffer, B. Sickmiller, J. Camparo, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 51,139 (2004); J. Camparo, IEEE Ultrason. Ferroelectr. Freq. Control 52, 1075 (2005). 11. I. Camparo, I. Coffer, Phys. Rev. A 59, 728 (1999); J. Coffer, M. Anderson, J. Camparo, Phys. Rev. A 65, 033807 (2002). 12. J. ]. Townsend, J. G. Coffer, I. C. Camparo, Phys. Rev. A 72, 033807 (2005); M. Huang, J. G. Coffer, J. C. Camparo, Opt. Commun. 265, 187 (2006). 13. }. Vanier, Appl. Phys. B 81, 421 (2005). 14. S. Knappe et al., Appl. Phys. Lett. 85, 1460 (2004). 15. J. Vanier, C. Audoin, The Quantum Physics of Atomic Frequency Standards, Adam Hilger/IOP, Bristol, UK (1989), chap. 3. 16. J. Bjorkholm, P. Liao, Lecture Notes in Physics: Volume 43, Springer, Berlin (1975), p. 176. 17. A. Kastler, /. Opt. Soc. Am. 53, 902 (1963). 18. J. Camparo, P. Lambropoulos, /. Opt. Soc. Am. B 19,1169 (2002). • November 2007
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