Scientists have made a breakthrough in the development of strontium optical atomic clocks that rely on an ultra-high quality optical transition for the clock signal and which offer greater precision and accuracy than the current cesium-based timekeeping method. The strontium atomic clock uses thousand of atoms simultaneously, which helps to improve the measurement precision; an approach that could lead to applications in industry, navigation and communications.
Researchers at JILA, the National Institute of Standards and Technology, the University of Colorado and the National Institute of Metrology in Beijing, China, whose work has been published in Science [Swallows et al. Science (2011) doi: 10.1126/science.1196442], have developed a system whereby increasing the number of particles allows both the measurement precision and accuracy to increase accordingly. This is useful, as while precision improves with the number of particles because of the enhanced signal to noise ratio, the accuracy usually deteriorates with increased atom numbers because of atomic interactions and collisions.
The team came up with their breakthrough by investigating neutral strontium optical atomic clocks, finding tiny frequency shifts caused by colliding fermions, and realizing that the clock laser was interacting slightly differently with the strontium atoms inside a one-dimensional trap. Of course, the performance of the clock was compromised by these collisions, so the team developed a new two-dimensional trap designed to prevent the fermions from colliding, and ensuring strong interactions between the atoms.
Such strong interactions also work to suppress collisions among the strontium atoms, due to their being identical, and they cannot collide when they are first loaded into the clock’s optical traps. When the strontium atoms start to interact with the clock laser, some of them enter slightly different states and become distinguishable, which means they can then collide.
In the old trap, the atoms did not have a strong interaction and were able to easily move between identical and distinguishable states, but an increase in the strength of the atomic interactions results in a big energy gap appearing between the two states: a gap which stops identical atoms from becoming distinguishable and colliding. This trap squeezes the strontium atoms so much that they enter the strongly interacting regime, such that the majority of the frequency shifts are suppressed.
The more atoms there are for each site, the more effective the trap. This, and the increased interaction strength when more atoms are present, means that it should be possible to build an even better system using more atoms.
As researcher Jun Ye points out, “By turning up the atomic interaction, one would naively expect an increased magnitude for frequency shift…we are able to create an energy gap in the system such that inhomogeneous excitations can no longer be used to scramble fermionic atoms to make them distinguishable, and hence the frequency shifts are suppressed due to the Pauli Exclusion Principle.”


Laurie Donaldson