Quantum Light in van der Waals Heterostructures: From Ultrastrong Coupling in the Terahertz Regime to Quantum Sensing with Atomic Resolution

Two-dimensional (2D) materials provide a versatile platform for engineering light–matter interactions across vastly different regimes, paving the way toward quantum systems with tailored properties. In this talk, I will present results that outline a unified framework in which 2D materials bridge condensed matter physics and quantum optics, opening new directions for cavity-controlled electronic phases, collective emission, and quantum sensing at the atomic scale.

I will begin by describing the development of a sub-diffraction terahertz (THz) spectroscopy technique [1], which enabled the first demonstration of ultrastrong coupling between THz cavity modes and a tunable interband transition in bilayer graphene. The observed cavity-induced resonance emerges from the interband continuum and mimics a Coulomb-bound exciton. These experiments reveal a coupling strength exceeding 40% of the mode frequency, marking the onset of a non-perturbative regime of hybrid exciton–photon states in quantum materials.

Next, I will discuss how optically active defects in hexagonal boron nitride (hBN) provide a promising platform for quantum sensing with atomic-scale resolution [2]. Unlike NV centers in diamond, which lie about 10 nm below the surface and thus limit spatial resolution, B-centers in hBN can reside within only a few atomic layers of the target material. This enables their integration into van der Waals heterostructures and AFM-like tips to directly visualize Moiré patterns and strongly correlated electronic states with nanometer precision.

Finally, I will present our observation of superradiance in ensembles of quantum emitters in hBN, where emitters separated by sub-wavelength distances collectively emit photons in short, intense bursts—exhibiting the hallmark signatures of cooperative spontaneous emission.

[1] F. Helmrich, H.S. Adlong, I. Khanonkin, M. Kroner, G. Scalari, J. Faist, A. Imamoglu, T.F. Nova. “Cavity-Driven Attractive Interactions in Quantum Materials”. Preprint arXiv:2408.00189v3 (2024).

[2] L. Liu, I. Khanonkin, J. Eberle, B. Rizek, S. Falt, K. Watanabe, T. Taniguchi, A. Imamoglu and M. Kroner. “Quantum Emitters in Ultra-Thin Hexagonal Boron Nitride Layers”, Preprint  arXiv:2507.02633v1 (2025).