Yige Lin

Suppression of Collisional Shifts in a Strongly Interacting Lattice Clock

Keeping Time Optical lattice clocks are comprised of atoms placed in an optical lattice formed by opposing laser beams and can be more precise than traditional microwave atomic clocks because of the higher frequency at which they operate, and the number of atoms available for interrogation. However, interactions between the atoms may lead to shifts in the frequency of the clock transition, usually proportional to the atomic density. Swallows et al. (p. 1043, published online 3 February) demonstrate an opposite and unexpected effect of interactions: For sufficiently strongly interacting systems, the frequency shift is suppressed. Indeed, in a strontium-based fermionic lattice clock, the shift and its associated spread were reduced by an order of magnitude. Increasing atomic interactions improved the accuracy and precision of a clock formed from atoms trapped in an optical lattice. Optical lattice clocks with extremely stable frequency are possible when many atoms are interrogated simultaneously, but this precision may come at the cost of systematic inaccuracy resulting from atomic interactions. Density-dependent frequency shifts can occur even in a clock that uses fermionic atoms if they are subject to inhomogeneous optical excitation. However, sufficiently strong interactions can suppress collisional shifts in lattice sites containing more than one atom. We demonstrated the effectiveness of this approach with a strontium lattice clock by reducing both the collisional frequency shift and its uncertainty to the level of 10−17. This result eliminates the compromise between precision and accuracy in a many-particle system; both will continue to improve as the number of particles increases.

Operating a 87 Sr optical lattice clock with high precision and at high density

We describe recent experimental progress with the JILA Sr optical frequency standard, which has a systematic uncertainty at the 10-16 fractional frequency level. An upgraded laser system has recently been constructed in our lab which may allow the JILA Sr standard to reach the standard quantum measurement limit and achieve record levels of stability. To take full advantage of these improvements, it will be necessary to operate a lattice clock with a large number of atoms, and systematic frequency shifts resulting from atomic interactions will become increasingly important. We discuss how collisional frequency shifts can arise in an optical lattice clock employing fermionic atoms and describe a novel method by which such systematic effects can be suppressed.

Resolved Atomic Interaction Sidebands in an Optical Clock Transition

We report the observation of resolved atomic interaction sidebands (ISB) in the (87)Sr optical clock transition when atoms at microkelvin temperatures are confined in a two-dimensional optical lattice. The ISB are a manifestation of the strong interactions that occur between atoms confined in a quasi-one-dimensional geometry and disappear when the confinement is relaxed along one dimension. The emergence of ISB is linked to the recently observed suppression of collisional frequency shifts. At the current temperatures, the ISB can be resolved but are broad. At lower temperatures, ISB are predicted to be substantially narrower and useful spectroscopic tools in strongly interacting alkaline-earth gases.

Supression of collisional frequency shifts in an optical lattice clock

By strongly confining atoms in a two-dimensional optical lattice, we have suppressed collisional frequency shifts in a 87Sr optical lattice clock.