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Laser tweezers help stabilize atomic clock

12 Sep 2019

Preliminary data suggest that trapping and controlling individual strontium atoms with light beams yields better performance.

A team of physicists in the US has reported that an atomic clock based on laser tweezers offers some “unique possibilities” for a future generation of super-precise timepieces.

Demonstrated at JILA (formerly the Joint Institute for Laboratory Astrophysics) at the University of Colorado in Boulder, the new design uses a grid-like array of laser light to trap and control an individual strontium atom in each beam.

The approach, described by Adam Kaufman and colleagues in the latest issue of the journal Science appears to offer a combination of near-continuous operation with both strong signals and high stability – a set of features not previously found together.

Isolated atoms
Part of the reason for the good stability is the isolation of individual atoms made possible by the laser tweezers, which makes them less likely to interfere with each other. That, along with good uptime, is a property it shares with state-of-the-art clocks that are based on a single ion - but the tweezer clock manages to combine these features with the strong signals and stability of a multi-atom lattice clock.

“The tweezer design’s long-term promise as a competitive clock is rooted in its unique balancing of these capabilities,” Kaufman said in a JILA release.

Next-generation optical atomic clocks are able to stabilize a laser frequency to atoms “ticking” between two energy levels, with their extraordinary precision equivalent to gaining or losing less than one second over the known age of the universe.

The tweezer clock looks to improve performance even more. By trapping and controlling atoms individually, to maintain ticking stability, they can reuse the same atoms several times without needing a constant supply of new ones.

“Using our technique, we can hold onto atoms and reuse them for as long as 16 seconds,” Kaufman added. “[This] improves the duty cycle - the fraction of time spent using the atoms’ ticking to correct the laser frequency - and precision. The tweezer clock can also get a single atom very rapidly into a trap site, which means there is less interference and you get a more stable signal for a longer time.”

Redefining the second
Ultimately, the JILA team and others working on similar technology at physical standards laboratories around the world are preparing for a future redefinition of the second, which since 1967 has been based on a microwave transition within a cesium atom.

In the new JILA tweezer clock, an infrared laser is aimed into a microscope and focused to a small spot. Radio waves at ten different frequencies are applied sequentially to a special deflector that creates ten spots of light to trap individual atoms. The traps are refilled every few seconds from a pre-chilled cloud of atoms overlapped with the tweezer light.

The strontium atoms held by the tweezers are then excited by another laser, which is stabilized by a silicon crystal cavity. Described by the team as “clock laser” light, this was provided by co-author Jun Ye’s JILA laboratory.

However, if there are too many atoms in the system, collisions between them can destabilize the clock. To get rid of extra atoms, a pulse of light is applied to create weakly bound molecules, which then break apart and escape the trap.

According to JILA, this means that tweezer sites are left with either a single atom or empty, meaning that during an experimental “run”, each tweezer has about a 50/50 chance of being empty or containing a single atom. “Having at most one atom per site keeps the ticking stable for longer time periods,” explains the team.

They are now planning to build a larger clock featuring about 150 atoms, and to evaluate the performance formally. Kaufman also wants to add entanglement, which as well as improving clock sensitivity and performance, could provide a new platform for quantum computing and simulation.

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