Supercooling coupled ions for more accurate atomic clocks

The journey towards even more accurate optical atomic clocks.


The well-known use of laser beams is to heat things. But cooling with the help of laser beams is also commonly known to physicists who study precision spectroscopy and the development of optical atomic clocks.

Now, the researchers at the QUEST Institute at the Physikalisch-Technische Bundesanstalt (PTB) have reached extremely low temperatures (200 µK) with highly charged ions.

The team working on this succeeded by combining their established methods, including the laser cooling of coupled ions and methods from the field of quantum computing.

The application of quantum algorithms ensured that different ions for traditional laser cooling could work if they cooled down together.

It means that we are getting closer to an optical atomic clock with highly charged ions, which might have the potential to be more accurate than existing optical atomic clocks. The researchers published results in the current issue of “Physical Review X.”

The majority of ions and other charged particles of spectroscopic interest lack the fast, cycling transitions necessary for direct laser cooling.

To investigate particles – such as ions – extremely accurately (say, using precision spectroscopy or measuring their frequency in an atomic clock), it is necessary to bring them as close to a standstill.

The most extreme standstill is the lowest possible temperature – meaning cooling them efficiently. One of the established high-tech cooling methods is so-called laser cooling.

Lasers slowed down skillfully arranged particles. Not every particle is suited to this method. That is why pairs of coupled ions have been used at the QUEST Institute for a long time to overcome this: One ion (called the “cooling ion” or the “logic ion”) is cooled by lasers; simultaneously, its coupled partner ion is also cooled and can then be investigated spectroscopically (hence, it is called the “spectroscopy ion”).

Limitations of this method are reached when the two ions have differed by too much in their charge-to-mass ratios – that is, when they have been very different in mass and very differently charged.

“But it is now these very ions that are particularly interesting for our research, for instance, for developing novel optical clocks,” explains QUEST physicist Steven King.

He and his team are very experienced in applying the laws of quantum mechanics (coupled cooling is, based on quantum laws); they have made use of the toolkit of the quantum computing researcher.

Quantum algorithms – i.e., computer operations based on manipulating individual quanta – cannot only be used to perform calculations faster than ever with a quantum computer. They can also help to extract kinetic energy from the mismatched ion pair. During the process of so-called algorithmic cooling, quantum operations are used to do just that: to transfer the energy from the barely coolable motion of the spectroscopy ion to the easily coolable motion of the logic ion.

And they managed to do this extremely well: “We were able to extract so much energy from the pair of ions – consisting of a singly charged beryllium ion and a highly charged argon-ion – that their temperature finally dropped to only 200 µK,” said one of QUEST’s Ph.D. students Lukas Spieß. Such an ensemble has never been so close to absolute zero (as in: so motionless).

“What is more, we also observed an unprecedentedly low level of electric-field noise,” he expanded.

Noise normally leads to the ions being heated when the cooling stops, but this turns out to be particularly low in their apparatus. Combining these two things means that the final major hurdle in their way has now been overcome, and an optical atomic clock that is based on highly charged ions can be built.

This atomic clock could reach an uncertainty of less than 10–18. Only the best optical atomic clocks in the world can get this kind of performance. These findings are also of great significance for the development of quantum computers and precision spectroscopy.

“Since our technique is universal, applications extend across a multitude of fields, such as the development of ultraprecise clocks based on trapped highly charged atoms and molecules, localization and motional control over macroscopic particles such as nanodiamonds, precision measurements of the properties of antiprotons, and improving the fidelity of quantum computation.” Research quote.

Journal Reference

  1. Steven A. King, Lukas J. Spieß, Peter Micke, Alexander Wilzewski, Tobias Leopold, José R. Crespo López-Urrutia, Piet O. Schmidt: Opens external link in new windowAlgorithmic Ground-state Cooling of Weakly-Coupled Oscillators using Quantum Logic. Physical Review X 11, 041049 (2021) DOI: 10.1103/PhysRevX.11.041049
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