Rare events—such as chemical reactions, protein folding, and phase transitions—occur when thermal fluctuations push a system over an energy barrier. Although the multidimensional theory describing these processes was formulated decades ago by Kramers and Langer, direct experimental test of this theory in controllable many-body systems have been lacking.

In this work, the researchers realize such a test using a cluster of five identical, laser-cooled ions trapped by electromagnetic fields. The ions self-organize into a tiny pyramid that can spontaneously flip between two mirror-image orientations. Rather than inverting through the high-energy pathway familiar from ammonia molecules, the ions exploit their permutation symmetry to follow a much lower-energy collective rearrangement known as a pseudo-rotation. By tuning the trapping conditions, the researchers control the activation barrier and measure inversion rates over more than two orders of magnitude, finding quantitative agreement with multidimensional Kramers-Langer theory.

Control over the reaction pathway is demonstrated by replacing one ion with a heavier isotope. This breaks the symmetry that enables pseudo-rotation, forcing the system to pass through an energetically costly route and hence dramatically suppressing inversions—a giant kinetic isotope effect. This work establishes trapped-ion clusters as a precision platform for studying rare events and engineering reaction dynamics through symmetry and quantum control.

"Symmetry-controlled thermal activation in pyramidal Coulomb clusters: Testing Kramers-Langer theory", Akhil Ayyadevara, Anand Prakash, Shovan Dutta, Arun Paramekanti, and S. A. Rangwala, Physical Review Letters.

[Authors 1,2,3,5 from Raman Research Institute, Bangalore, India; Arun Paramekanti from University of Toronto].

More information here: https://journals.aps.org/prl/abstract/10.1103/xsdn-srjd