New Sisyphean cooling technique could improve precision of atomic clocks

Researchers from the Neutral Atom Optical Clocks Group at the National Institute of Standards and Technology (NIST), the University of Colorado, and Pennsylvania State University recently developed a new sub-recoil Sisyphus cooling technique that could help improve the accuracy of atomic clocks.

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

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

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

Atomic clocks are timekeeping devices that reference a frequency to the oscillatory motion of atoms. The operation of these clocks depends on very precise spectroscopy techniques that target long-lived atomic states with an ultra-narrow transition linewidth between these states, typically in the sub-Hz range.

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

In essence, Chen and his colleagues strategically designed the energy shift of their excited clock state according to a periodically modulated pattern. This method allowed them to precisely control the location at which a clock line excitation occurs within their Sisyphus cooling process.

“More specifically, we configure 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 leave the potential landscape away from the minimum of the potential. The cooling is realized after repeatedly climbing the energy potential.”

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

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

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

The most recent efforts aimed at improving clock spectroscopy have therefore investigated strategies to minimize the trap potential difference between the ground state and the excited clock state. To address this challenge, Chen and his colleagues focused on improving the cooling of samples before performing high-precision clock spectroscopy.

“To achieve better cooling during sample preparation before performing clock spectroscopy, we temporarily introduced an engineered spatially dependent excited state shift, creating more, not less, trap potential difference for the two clock states,” Chen said.

“This allowed us to realize the Sisyphus cooling mechanism, which in turn improved the sample condition later for better clock spectroscopy with less trap potential difference. Furthermore, the cooler temperatures helped us to use shallower traps on the atoms, which also reduced this difference.”

The new Sisyphus cooling technique devised by this team of researchers could soon help improve the precision 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 next studies, these researchers plan to continue using their Sisyphus cooling technique to improve the precision of optical lattice clocks developed at NIST.

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

“Moreover, the lower temperatures allow us to keep 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 this will translate into improved clock accuracy.”

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