Strong Coupling Between RF Photons and Plasmons of Electrons on Liquid Helium

Plasmons, arising from the collective motion of electrons, can interact strongly with electromagnetic fields or photons; this capability has been exploited across a broad range of applications, from chemical reactivity to biosensing. Recently, there has been growing interest in plasmons for applications in quantum information processing. Electrons floating on liquid helium provide an exceptionally clean, disorder-free system and have emerged as a promising platform for this purpose. In this work, we establish this system as a tunable plasmon-photon hybrid platform. We demonstrate strong coupling between floating-electron plasmons and radio-frequency (RF) photons confined in an LC resonator. Time-resolved measurements reveal coherent oscillatory energy exchange between the plasmonic and photonic modes, providing direct evidence of their coherent coupling. These results represent a step towards cavity quantum electrodynamics with a floating-electron plasmon coupled to a resonator. Furthermore, the LC resonator serves as a sensitive probe of electron-on-helium physics, enabling the observation of the Wigner crystal transition and a quantitative study of the temperature-dependent plasmon decay arising from ripplon-induced scattering.

💡 Research Summary

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The paper reports the first demonstration of strong coupling between the collective plasmon mode of electrons floating on the surface of liquid helium and radio‑frequency (RF) photons confined in a lumped‑element LC resonator. Electrons on helium form an exceptionally clean two‑dimensional electron system (2DES) with virtually no disorder; consequently, plasmonic losses are extremely low compared with metallic or semiconductor 2DESs. By employing a Corbino‑type electrode geometry, the authors can electrostatically confine the electron sheet, tune its density (n₀), radius (R*), and radial mode index (μ) with three independent DC voltages (V_BC, V_BM, V_BO). The LC resonator (L = 708 nH, C ≈ 2.13 pF) has a bare resonance frequency ω₀/2π ≈ 120.9 MHz, and its coupling to the plasmonic mode is described by an input‑output model where the reflection coefficient Γ_ref depends on the resonator decay rates (κ_ext, κ_int), the plasmon decay rate γ_p, and the coupling strength g.

In the frequency domain, sweeping V_BM (or V_BO) tunes the plasmon frequency ω_p according to the dispersion relation
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