Six barium ions lined up in a flat hexagon inside a Paul trap, a device that holds charged particles with oscillating electric fields. But a slight change in the shape of the trapping potential made the crystal rearrange: one ion moved to the center, and the other five formed a ring around it. This is isomerization, the rearrangement of a molecule between different spatial forms, except that instead of a tiny molecule, we have a crystal about ten micrometers across, a hundred thousand times larger than a benzene ring.

Physicists from Florence and Turin used this chameleon crystal as a tabletop model of molecular transformations. By varying the ratio of the trap's oscillation frequencies along two axes, they smoothly shifted the system from one stable shape to the other. At certain parameters, a bistable region emerged, where two stable states coexisted, and the crystal could be either a hexagon or a pentagon with an ion at the center. The energy barrier between these forms was tens of millikelvin, and the transition time ranged from 0.2 to 1.4 milliseconds.

To observe these transitions, the researchers used fluorescence microscopy: the ion at the center contributed noticeably to the signal, so the photon count rate could distinguish one configuration from the other. They then built a potential energy surface, a map of the system's energy as a function of coordinates, using the Monte Carlo method: by randomly displacing ions and keeping configurations with minimal energy, they obtained an energy profile along the reaction coordinate. Comparing the experimental state populations with the calculation, they extracted the crystal temperature: 8.8 millikelvin with an uncertainty of +3.5/−2.6 mK. This was the first time the temperature of an ion crystal was measured from the populations of isomeric states.

The work shows that an ion crystal can serve as a precise model of molecular isomerization. Unlike real molecules, where transitions last femtoseconds and are nearly impossible to observe in real time, here the dynamics are stretched to milliseconds and the sizes to micrometers. This allows isolating the contributions of thermal fluctuations and quantum tunneling. The next step could be cooling the crystal to its ground vibrational state and observing quantum tunneling between isomers. Such systems could be useful for calibrating temperature in ion traps and for preparing initial states in quantum simulators.