© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

10.1.2.5 Atomic Frequency Standards

The most accurate oscillators are the Atomic Frequency Standards (AFS) used in "atomic clocks." In these "clocks," an atom flips between two slightly different configurations -- one in which the electron spin and the nuclear spin point in the same direction, and another configuration in which the two spins point in opposite directions. In its original form,^3028,3029 a beam of cesium (Cs¹³³) atoms is emitted from an oven and passes through an evacuated (~10^-11 atm) chamber, where the atoms are focused by one fixed magnet, then defocused by a second fixed magnet. Between the two fixed magnets, the beam passes through a microwave field. When the frequency of this oscillating field exactly matches a natural atomic resonance ground-state hyperfine transition frequency of Cs (~9.192,631,770 GHz), the spin energy state can switch polarity, allowing "flipped" atoms to focus, rather than defocus, at the second fixed magnet. Atoms arriving at the focus are ionized by a hot-wire ionizer target, then directed onto an electron multiplier by a mass spectrometer to be counted. The microwave frequency is adjusted until the electron multiplier output current is maximized, constituting the measurement of the atoms' resonance frequency.

Early cesium AFSs were used to recalibrate a quartz oscillator about once a day, achieving Dn / n ~ 3 x 10^-11. In 1998, the best non-cryogenic laboratory Cs or Rb (rubidium) AFS had Dn / n ~ 2 x 10^-14, with a temperature stability of ~10^-13 K^-1 between 263-313 K and a magnetic stability of ~10^-12/gauss.¹⁷⁰³ The smallest non-cryogenic Rb AFS was a space-qualified system with mass of 1.3 kg and power consumption of 11 watts (~10⁴ watts/m³), achieving Dn / n <~ 5 x 10^-13. Another system that achieved Dn / n <~ 5 x 10^-14 had mass ~5.5 kg and drew 39 watts.¹⁷⁰³ Laser-cooled low-temperature clocks using Bose-Einstein condensates were expected eventually to demonstrate Dn / n ~ 10^-18.

In 1998, the miniaturization of rubidium AFS for space applications was being actively studied,^1704,1705 along with newer approaches such as optically-pumped cesium AFS using solid state diode lasers (thus eliminating the bulky magnets),¹⁷⁰⁶ diode laser-pumped rubidium AFS in which the Rb discharge lamp is replaced with a ~100% efficient diode laser tuned to the correct transition frequency,^1707,1708 and Hg⁺ "optical clocks."¹⁷⁰⁹ Optical pumping methods using diode lasers defined the NIST-7 atomic-beam standard of Dn / n <~ 5 x 10^-15, starting in 1993.

A detailed analysis of micron-size atomic clocks is beyond the scope of this book. The possibility cannot be ruled out, but in 1998 the feasibility was unknown. From Eqn. 4.50, the minimum sensor capable of detecting a spin transition requires N_min ~ 7 x 10⁶ Cs atoms (~0.001 micron³ of Cs), taking transition frequency n_L = 9.192,631,770 GHz and electron spin angular momentum L_electron = L_proton. However, many problems must be overcome including interactions involving spontaneous decay, measurement-induced transitions, phase changes due to gas molecule collisions, spatial confinement effects, the use of symmetries and cancellation of couplings. J. Soreff [personal communication, 1998] suggests consideration of an oscillator structure in which a phosphorus atom is covalently bound to a tetrahedral support of carbyne rods extending to the ends of an evacuated chamber. The fifth valence electron on the phosphorus atom should have a hyperfine interaction with the P³¹ nucleus, but it is presently unknown how tightly coupled the thermal vibrations in the rods are to the hyperfine state transition, since many coupling modes may vanish by symmetry.

Last updated on 23 February 2003