**Nanomedicine,
Volume I: Basic Capabilities**

**©
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 (Cs^{133}) 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 (~10^{4} watts/m^{3}),
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 10^{6} Cs atoms (~0.001 micron^{3} 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
P^{31} 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