Sympathetic cooling of a trapped proton mediated by an LC circuit.

Efficient cooling of trapped charged particles is essential to many fundamental physics experiments, to high-precision metrology and to quantum technology. Until now, sympathetic cooling has required close-range Coulomb interactions, but there has been a sustained desire to bring laser-cooling techniques to particles in macroscopically separated traps, extending quantum control techniques to previously inaccessible particles such as highly charged ions, molecular ions and antimatter. Here we demonstrate sympathetic cooling of a single proton using laser-cooled Be ions in spatially separated Penning traps.