Quantum Quirk: JILA scientists pack atoms together to prevent collisions in atomic clock accuracy

Physicists have measured and controlled seemingly forbidden collisions between neutral strontium atoms-a class of antisocial atoms known as fermions, which are not supposed to collide when in identical energy states. The advance makes possible a significant boost in the accuracy of atomic clocks based on hundreds or thousands of neutral atoms.

Described in the April 17 issue of the journal Science,* the research was performed at JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado (CU) at Boulder.

"This is one of the most precise measurements of collisional effects in a clock," says NIST/JILA Fellow Jun Ye, whose strontium atomic clock design enables scientists to "peek into very tiny effects."

The new techniques make JILA's strontium clock 50 percent more accurate than the results reported last year, so that it now would neither gain nor lose 1 second in more than 300 million years. The method could also be applicable to many other atomic clocks based on neutral atoms.

Co-authors of the paper include scientists from nearby NIST Boulder, who provided signals from a calcium atomic clock via fiber-optic cable to serve as the "ruler" for the JILA measurements, and a NIST theorist from the Joint Quantum Institute in Maryland, who helped explain why the collisions occur.

JILA's strontium clock is one of several next-generation atomic clocks under development around the world. These experimental clocks are based on a variety of different atoms and designs, from single ions (electrically charged atoms) to thousands of neutral atoms; it is not yet clear which design will emerge as the best and be chosen as the future international time standard. The latest JILA work helps eliminate a significant drawback to clock designs based on ensembles of neutral atoms. The presence of many atoms increases both the precision and signal of a clock based on the oscillations between energy levels, or "ticks," in those atoms. However, uncontrolled interactions between atoms can perturb their internal energy states and shift the number of clock ticks per second, reducing overall accuracy.