Coherent ferrons, the quanta of polarization waves, can potentially be hybridized with many other quasiparticles for achieving novel control modalities in quantum communication, computing, and sensing. Here, we theoretically demonstrate a new hybridized state resulting from the strong coupling between fundamental-mode (wavenumber is zero) coherent ferrons and cavity bulk acoustic phonons. Using a van der Waals ferroelectric ${\text{CuInP}}_2{\mathrm{S}}_6$ membrane as an example, we predict an ultrastrong ferron-phonon coupling at room temperature, where the coupling strength $g_c$ reaches over 10% of the resonant frequency ${\omega }_0$. We also predict an in situ bistable electric-field control of mode-specific ferron-phonon hybridization via ferroelectric switching. We further show that ${\text{CuInP}}_2{\mathrm{S}}_6$ allows for reaching the fundamentally intriguing but challenging deep-strong coupling regime (i.e., $g_c/{\omega }_0>1$) near the ferroelectric-to-paraelectric phase transition. Our findings establish the theoretical basis for exploiting coherent ferron as a new contender for a hybrid quantum system with strong and highly tunable coherent coupling.

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• Acoustic phonons
• Ferroelectricity
• Piezoelectricity
• Quantum information with hybrid systems
• Quasiparticles & collective excitations
• Transducers
• Elastic wave theory
• Landau-Ginzburg-Devonshire theory