Scientists at JILA, a joint institute of NIST and the University of Colorado Boulder, have made significant advancements in controlling collisions between neutral strontium atoms, a type of fermion that typically doesn’t collide. This breakthrough could improve the accuracy of atomic clocks based on neutral atoms.
The new techniques have made JILA’s strontium atomic clock 50% more accurate, with an error rate of only 1 second in 300 million years. The method could be applicable to other atomic clocks based on 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. Fermions, according to the rules of quantum physics, cannot occupy the same energy state and location in space at the same time. Therefore, fermions, such as a collection of identical strontium atoms, are not supposed to collide.
However, as Ye and his research group improved the performance of their strontium clock over the past two years, they began to observe small shifts in the frequencies of the clock ticks due to atomic collisions. The extreme precision of their clock unveiled in 2008 enabled the group to measure these minute interactions systematically, including the dynamic effect of the measurement process itself, and to significantly reduce the resulting uncertainties in clock operation.
The lattice is shaped like a tall stack of pancakes, or wells. About 30 atoms are grouped together in each well, and these neighboring atoms sometimes collide. Ye’s group discovered that two atoms located some distance apart in the same well are subjected to slight variations in the direction of the laser pulses used to boost the atoms from one energy level to another. The non-uniform interaction with light excites the atoms unevenly. Strontium atoms in different internal states are no longer completely identical, and become distinguishable enough to collide, if given a sufficient amount of time.
This differential effect can be suppressed by making the atoms even colder or increasing the trap depth. The probability of atomic collisions depends on the extent of the variation in the excitation of the ensemble of atoms. Significantly for clock operations, the JILA scientists determined that when the atoms are excited to about halfway between the ground state and the more energetic excited state, the collision-related shifts in the clock frequencies goes to zero. This knowledge enables scientists to reduce or even eliminate the need for a significant correction in the clock output, thereby increasing accuracy.
The discoveries described in Science also would apply to clocks using atoms known as bosons, which, unlike fermions, can exist in the same place and energy state at the same time. This category of clocks includes NIST-F1, which is operated by NIST Boulder as the U.S. civilian time and frequency standard. In the case of bosons, variations in light-matter interactions would reduce (rather than increase) the probability of collisions. Beyond atomic clocks, the high precision of JILA’s strontium lattice experimental setup is expected to be useful in other applications requiring exquisite control of atoms, such as quantum computing—potentially ultra-powerful computers based on quantum physics—and simulations to improve understanding of other quantum phenomena such as superconductivity.
Keywords: Strontium, Atomic, Collisions, Precision, Interactions, Fermions, Bosons
