Crowding out collisions: close quarters boost atomic clock's accuracy

In a paradox typical of the quantum world, JILA scientists have eliminated collisions between atoms in an atomic clock by packing the atoms closer together. The surprising discovery, described in the Feb. 3 issue of Science Express, can boost the performance of experimental atomic clocks made of thousands or tens of thousands of neutral atoms trapped by intersecting laser beams. (M.D. Swallows, M. Bishof, Y. Lin, S. Blatt, M.J. Martin, A.M. Rey and J. Ye. Suppression of collisional shifts in a strongly interacting lattice clock. Science Express. Posted online 3 February.)

Such clocks work by using the interaction of light from lasers and magnetic fields with atoms to ''cool'' the atoms until they are almost motionless and hold them in small regions defined by the light beams, a technique called optical trapping. The devices provide highly accurate time by measuring oscillations (which serve as ''ticks'') between the energy levels in the atoms. Things that perturb or alter those energy levels, such as collisions, limit the accuracy of the clocks.

JILA  scientists demonstrated the new approach using an experimental clock holding about 4,000 strontium atoms. Instead of configuring the trap as a stack of pancake-shaped regions as in their previous work, the scientists packed the atoms into thousands of optical traps shaped like horizontal tubes. (JILA is jointly operated by the National Institute of Standards and Technology (NIST), an agency of the US Commerce Department and the University of Colorado Boulder)

The result was a more than 10-fold improvement in clock performance because the atoms interacted so strongly that, against all odds, they stopped hitting each other. The atoms, which normally like to hang out separately and relaxed, get so perturbed from being forced close together that the ensemble is effectively frozen in place.

''The atoms used to have the whole dance floor to move around on and now they are confined in alleys, so the interaction energy goes way up,'' says NIST/JILA Fellow Jun Ye, leader of the experimental team.

The trick works because strontium atoms are fermions, a class of particles that cannot occupy the same place at the same time in the same energy state-energetically identical strontium atoms cannot collide. Normally the laser beam used to operate the clock interacts with the atoms unevenly, leaving the atoms dissimilar enough to collide. But the interaction energy of atoms packed in optical tubes is now higher than any energy shifts that might be caused by the laser, preventing the atoms from differentiating enough to collide.

Ye believes this clock and others based on neutral atoms will become competitive in terms of the accuracy with world-leading experimental clocks. The JILA strontium clock is currently the best performing experimental clock based on neutral atoms and, along with several NIST ion and neutral atom clocks, a possible candidate for a future international time standard. The research was supported by the Defense Advanced Research Projects Agency (grant administered by the Army Research Office), NIST, National Science Foundation and Air Force Office for Scientific Research. For more details, Quantum Quirk: JILA scientists pack atoms together to prevent collisions in atomic accuracy