Ultracold Ions Tune Atomic Clockwork

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BRAUNSCHWEIG, Germany, Dec. 30, 2021 — Researchers at the QUEST Institute at the Physikalisch-Technische Bundesanstalt (PTB) have cooled charged ions down to 200 μK. The team combined methods — its established laser cooling of coupled ions method, and those from quantum computing — to achieve the result.

The application of quantum algorithms ensured that ions that are too dissimilar for traditional laser cooling to work effectively could be cooled together after all. According to the researchers, this is a step toward the realization of an optical atomic clock with highly charged ions that is more accurate than existing optical atomic clocks.
Mismatched partners are being cooled: A single beryllium ion (red, left) and a single highly charged argon ion (purple, right) are bombarded by lasers from various sides and are almost brought to a complete standstill. Courtesy of PTB.
Mismatched partners are being cooled: A single beryllium ion (red, left) and a single highly charged argon ion (purple, right) are bombarded by lasers from various sides and are almost brought to a complete standstill. Courtesy of PTB.

The established method requires the laser cooling of precisely arranged particles; to be effective, the particles must be as similar as possible. The team uses coupled ions in the work, in which one ion, called the “cooling ion” or “logic ion,” is cooled by lasers while the other is cooled simultaneously. The cooled atom can be investigated spectroscopically, giving it the name “spectroscopy ion.”

However, the method has always reached its limits when the two ions differ too much in their charge-to-mass ratios. 
To overcome this barrier, the QUEST team turned to quantum computing. Quantum algorithms are not just used to perform calculations impossible for ordinary computers; they can also help extract kinetic energy from the mismatched ion pair. During the process of so-called algorithmic cooling, quantum operations are used to transfer the energy from the barely coolable motion of the spectroscopy ion to the easily coolable motion of the logic ion.

“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 Lukas Spieß, a Ph.D. student at QUEST. “What is more, we also observed an unprecedentedly low level of electric-field noise.”

An atomic clock that incorporates this process could reach an uncertainty of less than 10−18, a level of performance that is only reached by the highest-performing optical atomic clocks in the world. The findings also hold implications for the development of quantum computers and precision spectroscopy.

The research was published in Physical Review X (

Published: December 2021
laser cooling
A process and method by which manipulation and orientation of a given number of directed laser beams decreases the motion of a group of atoms or molecules such that their internal thermodynamic temperatures reach near absolute zero. The 1997 Nobel Prize in Physics was awarded to Steven Chu, Claude Cohen-Tannoudji and William D. Phillips for the development of methods to cool and trap atoms with laser light.
atomic clock
An atomic clock is a highly precise timekeeping device that uses the vibrations or oscillations of atoms as a reference for measuring time. The most common type of atomic clock uses the vibrations of atoms, typically cesium or rubidium atoms, to define the length of a second. The principle behind atomic clocks is based on the fundamental properties of atoms, which oscillate at extremely stable and predictable frequencies. The primary concept employed in atomic clocks is the phenomenon of...
optical clock
An optical clock is a highly precise and advanced timekeeping device that relies on the oscillations of electromagnetic radiation in the optical or ultraviolet part of the electromagnetic spectrum. Unlike traditional atomic clocks, which use microwave frequencies, optical clocks operate at much higher frequencies, typically involving transitions in atoms or ions at optical wavelengths. Optical clocks have the potential to provide unprecedented accuracy and stability in timekeeping. Key points...
Metrology is the science and practice of measurement. It encompasses the theoretical and practical aspects of measurement, including the development of measurement standards, techniques, and instruments, as well as the application of measurement principles in various fields. The primary objectives of metrology are to ensure accuracy, reliability, and consistency in measurements and to establish traceability to recognized standards. Metrology plays a crucial role in science, industry,...
Research & TechnologyLaserslaser coolingatomic clockoptical clockoptical atomic clockionscharged ionsabsolute zerosuper cooledspectroscopyPDTQUEST InstitutePhysikalisch-Technische BundesanstaltmetrologyTest & MeasurementEurope

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