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arXiv:2610.10944

Decoupling ultrastrong Rabi splitting from quantum vacuum dressing in a chiral cavity

Ayoub El-Amrani, Nasir Boubekraoui, Zakaria Mzaouali, Houssam Sabri, Herschel Rabitz, Abdelouahed El Fatimy, Dukhyung Lee

quant-phcond-mat.mes-hallSubmitted Oct 7, 2026• Announced Oct 9, 2026• Revised Oct 21, 2026 (v2)

v1 · Oct 7, 2026 v2 · Oct 21, 2026

Abstract

Polariton formation and quantum vacuum dressing are usually regarded as intertwined consequences of ultrastrong light--matter coupling. Here we show that they can be selectively controlled through the relative direction of rotation of the cavity field and the matter excitation. We consider cyclotron resonance coupled to a chiral cavity supporting a single circularly polarized mode. We quantize the electromagnetic field within the cavity, derive from first principles the Hamiltonian, and investigate how mode hybridization and vacuum dressing are rendered as we tune the magnetic field. When the cavity field co-rotates with the cyclotron motion, energy is exchanged resonantly between light and matter, producing a large Rabi splitting and conventional polaritons, while the ground state remains the bare vacuum. Reversing the magnetic field makes the cyclotron motion counter-rotating with the cavity field. The same cavity mode then couples through anomalous processes that create and annihilate light and matter excitations in pairs: the conventional polaritonic anticrossing disappears, while the ground state acquires squeezing and virtual excitations. Our results show that spectral hybridization and quantum vacuum dressing are distinct manifestations of ultrastrong coupling, offering an additional degree of freedom for engineering cavity quantum materials.

Comments: 38 pages, 7 figures

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Dressing quantum states of matter with virtual photons can create exotic effects ranging from vacuum-field modified transport to polaritonic chemistry, and may drive strong squeezing or entanglement of light and matter modes. The established paradigm of cavity quantum electrodynamics focuses on resonant light-matter interaction to maximize the coupling strength

Ω_\mathrm{R}/ω_\mathrm{c}

, defined as the ratio of the vacuum Rabi frequency and the carrier frequency of light. Yet, the finite oscillator strength of a single electronic excitation sets a natural limit to

Ω_\mathrm{R}/ω_\mathrm{c}

. Here, we demonstrate a new regime of record-strong light-matter interaction which exploits the cooperative dipole moments of multiple, highly non-resonant magnetoplasmon modes specifically tailored by our metasurface. This multi-mode coupling creates an ultrabroadband spectrum of over 20 polaritons spanning 6 optical octaves, vacuum ground state populations exceeding 1 virtual excitation quantum for electronic and optical modes, and record coupling strengths equivalent to

Ω_\mathrm{R}/ω_\mathrm{c}=3.19

. The extreme interaction drives strongly subcycle exchange of vacuum energy between multiple bosonic modes akin to high-order nonlinearities otherwise reserved to strong-field physics, and entangles previously orthogonal electronic excitations solely via vacuum fluctuations of the common cavity mode. This offers avenues towards tailoring phase transitions by coupling otherwise non-interacting modes, merely by shaping the dielectric environment.

We explore the nonlinear response of tailor-cut light-matter hybrid states in a novel regime, where both the Rabi frequency induced by a coherent driving field and the vacuum Rabi frequency set by a cavity field are comparable to the carrier frequency of light. In this previously unexplored strong-field limit of ultrastrong coupling, subcycle pump-probe and multiwave mixing nonlinearities between different polariton states violate the normal-mode approximation while ultrastrong coupling remains intact, as confirmed by our mean-field model. We expect such custom-cut nonlinearities of hybridized elementary excitations to facilitate nonclassical light sources, quantum phase transitions, or cavity chemistry with virtual photons.

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_{0.36}

Ga

_{0.64}

As two-dimensional electron gas with Fabry-Perot cavity modes. By employing cross-polarized spectroscopy and magnetic-field differential, we resolve the temporal beating of the cyclotron resonance oscillations. Remarkably, we find that the Rabi oscillations do not start immediately after excitation of the cyclotron resonance, but after a delay corresponding to one cavity round-trip time. This demonstrates that the strong-coupling regime sets up only after the formation of the cavity mode field. Our results provide direct insight into the dynamics of hybrid light-matter states in the THz regime.

We investigate the nonlinearities of custom-tailored ultrastrongly light-matter coupled states by driving them with both the vacuum electromagnetic field and strong coherent THz waveforms. In this novel regime, where both the Rabi frequency induced by the coherent field and the vacuum Rabi frequency set by the cavity field are on the order of the carrier frequency of light, we observe strong subcycle pump-probe and multiwave mixing nonlinearities and nonlinear polariton correlations, violating the normal-mode approximation. Such custom-cut nonlinearities of hybridized excitations set the stage for nonlinear optical control of deep-strong coupling, exploring quantum phase transitions or nonclassical light sources.