Much of modern experimental physics relies on a counterintuitive principle- Under the right circumstances, zapping matter with a laser doesn’t inject energy into the system; rather, it sucks energy out. By cooling the system to a fraction of a degree above absolute zero, one can observe quantum effects that are otherwise invisible.

Laser cooling works like a charm, but only when a system’s ladder of quantum states is just right. Atoms of alkali metals and a few other elements are ideal. Molecules, with their multitudes of energy levels, pose a much greater challenge. And fundamental particles such as protons, which lack internal states altogether, can’t be laser cooled at all.

Nevertheless, there’s a lot of interest in experimenting on protons at low temperature—in particular, precisely testing how their mass, magnetic moment, and other properties compare with those of antiprotons. Toward that end, the Baryon Antibaryon Symmetry Experiment (BASE) collaboration has now demonstrated a method for using a cloud of laser-cooled beryllium ions to sympathetically cool a single proton, even when the proton and ions are too distant to directly interact.

The core of the apparatus is shown in the photo. At each end of the central cylinder, 9 cm apart, is an electromagnetic trap- one for the Be⁺ ions and one for the proton. When the charged particles move around in their respective traps, they induce tiny currents in the trap electrodes (the parallel rings in the photo). By wiring the electrodes together, the researchers caused the traps to exchange energy and thus approach a common thermal equilibrium.

Because the induced currents are so small, though, the rate of energy exchange is slow. The researchers’ innovation was to also wire in a superconducting inductor–capacitor (LC) circuit with an extraordinarily high quality factor of 15 000. At the resonant frequency, the current was amplified by the same amount, and the proton was cooled in just seconds.

In their proof-of-principle experiment, the researchers reduced the proton’s temperature to a rather unimpressive 2.6 K—about the same as can already be achieved by other methods. The problem is that the temperature of the LC circuit itself is a relatively balmy 15 K, and heat from the circuit leaks into the proton trap to compete with the cooling effect of the ions.

Still, the fact that the members of the BASE collaboration could maintain a proton temperature more than 80% below that of the circuit is promising, and they have ideas for ways to push the whole apparatus to even lower temperatures. If they can cool the proton to 10–100 mK, the technology will be well suited to the precision measurements the collaboration is interested in. The ions are far enough away from the proton not to interfere in the measurement, and the setup can easily be adapted to replace the proton with an antiproton. (M. Bohman et al., Nature 596, 514, 2021.)