JILA scientists have eliminated collisions between atoms in an atomic clock by packing the atoms closer together. The discovery could boost the performance of experimental atomic clocks made of thousands or tens of thousands of neutral atoms trapped by intersecting laser beams. The researchers demonstrated the new approach using their experimental clock comprising about 4000 strontium atoms – a class of particles also known as fermions. When they are in identical energy states, they cannot occupy the same place at the same time, meaning that they cannot collide. By packing atoms into thousands of horizontal optical tubes, the investigators improved the clock’s performance tenfold because the atoms interacted so strongly that they stopped hitting one another. Normally coexisting separately and in a relaxed state, the atoms were so perturbed from being forced close together that they froze in place. Physicist Jun Ye of NIST adjusts the laser setup for a strontium atomic clock in his laboratory at JILA. Courtesy of J. Burrus, NIST. Typically, the laser beam used to operate the clock interacts with atoms unevenly, leaving the atoms dissimilar enough to collide. However, because the interaction energy of the packed atoms in the optical tubes is higher than any energy shifts that could be caused by the laser, the atoms are prevented from differentiating enough to collide. With this knowledge, the scientists found that their device provides highly accurate time by measuring oscillations between energy levels in the atoms. Their discovery means also that the greater number of atoms in a clock, the better, and the greater the measurement precision. Their findings appeared online Feb. 3, 2011, in Science (doi: 10.1126/science.1196442). JILA is jointly operated by NIST (National Institute of Standards and Technology) and the University of Colorado at Boulder.