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Atomic Clocks
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We describe the development of a small Hg+ ion clock suitable for space use.
The breadboard physics package is 1–2 liters in volume, produces frequency stability
of 10–13⁄ √τ and should significantly advance the state of space-qualified
atomic clocks.
A space-based clock with frequency stability better than 10–14 over a several day
period would enable one-way deep space navigations, where Doppler data is accumulated
in a down-link only fashion. Currently, deep space navigation is implemented by
measuring the Doppler frequency shift of a 2-way link from a ground station to a
spacecraft (s/c) and the coherent return link. Typically, these links are maintained
for 7-8 hours per s/c track, requiring full use of a 34-meter antenna in the Deep Space
Network (DSN) for the time the s/c is sufficiently above the horizon.
- Atom-ion collisions
We are developing an apparatus for investigation of collisions between cold atoms
and cold ions. Cold collisions for laser cooled atoms in a MOT have been previously
investigated for a number of systems, such as Cs + Cs and Rb + Rb, as well as Rb + Cs,
and others. The collisions between Cs atoms or Rb atoms in particular are of great
interest in atomic clock development, where large collisional shifts in the clock
frequency reduce the attainable uncertainties. Despite these studies, the collisional
dynamics between laser cooled atoms and ions has not been as yet investigated. This is
partially due to the challenge of combining trapped ions with cold atoms in a single
apparatus.
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Important developments both in theoretical and observational cosmology have fueled
considerable interest in searches for variations of the fine structure constant.
Experimentally, Webb et al have found evidence for a cosmological variation of the
fine structure constant through an analysis of the absorption lines in galactic halos
from quasar-emitted light. Recently developed small ion atomic clocks enable Solar
System tests for equivalence principle (EP) violating α-variations by way of
rate-comparisons of three ultra-stable atomic clocks near-to and far-from the sun
where gravitational red-shift changes are more than 104 larger than in low Earth orbit.
No space tests of the EP have been made in nearly 30 years, since the GP-A hydrogen
maser reached a 10,000 km apogee confirming EP red-shift predictions to ∼1 part in 104.
Today’s small ion clocks, nearly 100x more stable and 100x smaller than the GP-A H-maser,
could probe for EP violating scalar fields near the sun for mission costs comparable to
low Earth orbiters and improve the GP-A sensitivity by 5 to 6 orders of magnitude.
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One of the most important features of the EIT resonance is its very narrow line.
Moreover, this resonance can be centered at a magnetically insensitive transition
of a hyperfine manifold whose frequency is one of the best Nature's frequency etalons.
Combination of these two factors suggests using EIT as an optical bandpass filter
in and optoelectronic oscillator, which then will become a very precise clock.
We have demonstrated the operation of a proof-of-principle table top device and
now focus on building a miniature, ultimtely chip-size, version.
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