New Sisyphus cooling technique could improve accuracy of atomic clocks

Researchers from the National Institute of Standards and Technology (NIST) Optical Neutral Atom Clock Group, the University of Colorado, and Pennsylvania State University recently developed a new Sisyphean underrecoil cooling technique that could help improve the accuracy of atomic clocks.

This technique, described in an article published in Physical Exam Letterswas initially used to create a high-performance ytterbium optical lattice clock, but it could also aid in the development of other quantum clocks and metrology tools.

“Precision spectroscopy is a very broad field of research with a long history,” Chun-Chia Chen, a co-author of the paper, told Phys.org. “Atomic physicists perform spectroscopic studies on objects ranging from atoms and ions to molecules and beyond. Perhaps surprisingly, high-precision spectroscopy has also been performed on antimatter, an active area of ​​research currently being explored at CERN.”

While trying to improve the accuracy of atomic clocks, Chen and his NIST colleagues came across a paper that described a new scheme for the Sisyphus laser cooling of hydrogen and antihydrogen. Inspired by this scheme, they set out to design a similar cooling approach that could improve the performance of their atomic clocks.

Atomic clocks are time-measuring devices that calculate a frequency based on the oscillatory motion of atoms. The operation of these clocks relies on high-precision spectroscopy techniques that allow the measurement of atomic states with a long lifetime and an ultra-narrow transition line width between these states, generally less than a Hz.

“Traditionally, we use this ultra-narrow spectroscopy function for frequency stabilization purposes, which serves as the basic idea for current state-of-the-art frequency standards and optical atomic clocks,” Chen explains. “However, before performing high-precision spectroscopy, we use the ultra-narrow excitation with another quantum engineering tool for the implementation of Sisyphus cooling.”

Basically, Chen and his colleagues strategically designed the energy shift of their excited clock state by following a periodically modulated pattern. This method allowed them to precisely control where a clock line excitation occurs in their Sisyphus cooling process.

“Specifically, we set the excitation condition such that it preferentially occurs at the position corresponding to the bottom of the periodic potential landscape,” Chen said. “Once excited, atoms lose their kinetic energy by climbing the potential and preferentially exit the potential landscape away from the potential minimum. Cooling is achieved after repeated climbing of the energy potential.”

In their recent study, the researchers demonstrated their Sisyphus cooling system by exploiting the ultra-narrow transition of a Ytterbium-based optical lattice clock. However, the same approach should theoretically also be applicable to other systems equipped with narrow linewidth transitions.

“Over the past two decades, the goal of realizing high-precision clock spectroscopy of neutral atoms has been best achieved by creating identical trapping conditions for atoms in the ground state and the excited clock state,” Chen explained.

“This is done by transforming a trap formed by lasers into a standing wave that operates at what we call a magic wavelength. In this situation, a difference in the trapping potential felt by atoms in the two atomic states is essentially an enemy of achieving high-precision clock spectroscopy.”

Most recent efforts to advance clock spectroscopy have therefore explored strategies to minimize the trap potential difference between the ground state and the excited state of the clock. To address this challenge, Chen and colleagues focused on improving sample cooling before performing high-precision clock spectroscopy.

“To achieve enhanced cooling during sample preparation before running clock spectroscopy, we momentarily introduced a space-dependent excited-state shift, which introduced more, not less, trap potential difference for the two clock states,” Chen said.

“This allowed us to realize the Sisyphean cooling mechanism, which in turn improved the sample state subsequently for better clock spectroscopy with a smaller trap potential difference. Additionally, the colder temperatures helped us use shallower traps on the atoms, which also reduced this difference.”

The new Sisyphus cooling technique developed by this team of researchers could soon help improve the accuracy of other optical clock systems. In addition, it could be used to cool samples for other emerging technologies, including quantum information processing and computing systems. In their future studies, these researchers plan to continue using their Sisyphus cooling technique to improve the accuracy of optical lattice clocks developed at NIST.

“The additional cooling allows us to create atomic ensembles with more uniform conditions inside the magic wavelength standing-wave laser trap,” added researcher Andrew Ludlow. “This in turn allows us to more carefully and precisely characterize the small effects of the trapping laser on the clock frequency.”

“In addition, the lower temperatures allow us to hold the atoms in even weaker laser traps, where unwanted trapping effects are even smaller. After some careful measurements that we are currently working on, all of this will translate into improved clock accuracy.”

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