Scientists have achieved strong coupling between radiofrequency (RF) photons and plasmons of electrons confined on the surface of liquid helium. This breakthrough represents a significant step towards developing new quantum devices and exploring condensed matter phenomena in a low-dissipation environment. The observed interaction is robust enough to allow coherent energy exchange between both systems, opening the door to quantum manipulation of these collective electron excitations.

Electrons on liquid helium form a nearly ideal two-dimensional system, characterized by extremely high mobility and long coherence times, making them a promising platform for quantum computing and simulation. However, the interaction of these electrons with external electromagnetic fields has traditionally been weak. The present work overcomes this limitation by designing a resonant cavity that drastically enhances the coupling between RF photons and plasmons, which are the collective oscillations of the electrons.

The experiment was conducted at cryogenic temperatures, near absolute zero, to minimize decoherence. Researchers used a superconducting microwave cavity to confine RF photons and an electrostatic trap for electrons on the helium surface. By tuning the resonance frequencies of the cavity and the plasmons, they achieved a matching that maximizes energy transfer. The observation of a Rabi splitting in the cavity's transmission spectrum is unequivocal evidence of this strong coupling regime, where energy is reversibly exchanged between photons and plasmons.

This achievement not only advances the fundamental understanding of light-matter interactions in low-dimensional systems but also has practical implications. It could lead to the creation of new types of ultrasensitive quantum sensors, long-lived quantum memories, or even the implementation of plasmon-based qubits. The ability to control and manipulate these plasmons with RF photons opens new avenues for engineering quantum states in systems of electrons floating on helium.