Recent advances in laser cooling and trapping of neutral atoms in magneto-optic traps (MOTs) would be exploited in ion-trap-based atomic clocks, according to a proposal. Beams of laser-cooled neutral atoms would be used to: (1) put the ions in the required quantum states (e.g., spin-polarize the ions) in preparation for quantum state transitions that define the clock frequencies, (2) cool the trapped ions to reduce clock instabilities from second-order Doppler shifts of clock frequencies, and (3) monitor the clock quantum state transitions. The cooling, state preparation, and monitoring would occur via ion/atom collisions.

Lithium Atoms Would Be Cooled by laser beams in the LVIS. The resulting beam of lithium atoms would be spin-polarized by optical pumping on its way to the linear ion trap, where it would cool and spin-polarize the199Hg+ ions. Li+ ions generated as a byproduct of the microwave-induced transition in the199Hg+ ions would be ejected and detected. Depolarization of the beam of lithium atoms could be detected in a spin-state analyzer by optical fluorescence following excitation by a diode laser.

The proposal is based on a complex of interdependent physical phenomena that include elastic and inelastic collisions among ions and neutral atoms, electric polarization of neutral atoms by nearby ions, spin-exchange and charge-exchange reactions among neutral atoms and ions, and radiative state transitions. For the sake of brevity, the proposal is described below by way of an example: how it would be applied to an atomic clock based on 199Hg+ ions in a linear ion trap.

Atomic clocks of this type were described in several previous articles in NASA Tech Briefs. To recapitulate: a clock of this type includes a microwave local oscillator, the frequency of which is stabilized by comparison with the frequency (about 40.5 GHz) of a ground-state hyperfine transition of 199Hg+ ions. The ions are held in a linear ion trap to obtain a long interrogation time and thus a high resonance quality factor. Heretofore, the cooling of ions and the comparison of frequencies has involved a combination of ultraviolet (wavelength = 194 nm) and microwave excitation and interrogation of the ions. The generation of the excitatory ultraviolet light is difficult and expensive because one must use multiple visible lasers that generate watts of power, together with delicate frequency-doubling crystals.

The proposal would eliminate the need for the ultraviolet light. The proposal is based partly on the observation that it would be easier and less expensive to use small diode lasers to generate visible light and use this light to cool neutral Li atoms in an MOT. The light from the laser diodes would be carried by optical fibers and split into three intersecting, retroreflected, circularly polarized beams that would effect the cooling in the MOT; this would all be accomplished by use of off-the-shelf components.

The figure schematically depicts an apparatus that would be used to test the proposal. The MOT would be the heart of a low-velocity intense source (LVIS) of a cold beam of neutral Li atoms. The beam would be directed into the linear ion trap of an atomic clock. In the trap, the 199Hg+ ions would be cooled and state-prepared via collisions with the neutral Li atoms. Li+ ions would be generated as byproducts of a spin-dependent charge-transfer interaction that would occur as a result of the microwave-induced clock transition in the 199Hg+ ions. In the radio-frequency electric field that traps the 199Hg+ ions, the motion of the Li+ ions would be so great that the Li+ ions would be immediately ejected from the trap. The ejected Li+ ions could thus be used to indicate the clock transition; for this purpose, the ejected Li+ ions could be detected by channeltron electron multipliers surrounding the trap. Assuming detection of all ejected Li+ ions, the short-term clock stability has been estimated to be ≈10 -14t -1/2, where t is the averaging time in seconds.

This work was done by John Prestage of Caltech for NASA's Jet Propulsion Laboratory.


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Exploiting Ion/Atom Cold Collisions in Atomic Clocks

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This article first appeared in the April, 2000 issue of Electronics Tech Briefs Magazine.

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