# CMSA Topological Quantum Matter: Vacuum fluctuations in cavities: breakdown of the topological protection in the integer Quantum Hall effect

SEMINARS, CMSA EVENTS

##### Speaker:

Jerome Faist *- Institute of Quantum Electronics, ETH Zurich*

When a collection of electronic excitations are strongly coupled to a single mode cavity, mixed light-matter excitations called polaritons are created. The situation is especially interesting when the strength of the light-matter coupling Ω_{R} is such that the coupling energy becomes close to the one of the bare matter resonance ω_{0}. For this value of parameters, the system enters the so-called ultra-strong coupling regime, in which a number of very interesting physical effects were predicted caused by the counter-rotating and diamagnetic terms of the Hamiltonian.

In a microcavity, the strength of the electric field caused by the vacuum fluctuations, to which the strength of the light-matter coupling Ω_{R}is proportional, scales inversely with the cavity volume. One very interesting feature of the circuit-based metamaterials is the fact that this volume can be scaled down to deep subwavelength values in all three dimension of space.^{1} Using metamaterial coupled to two-dimensional electron gases under a strong applied magnetic field, we have now explored to which extend this volume can be scaled down and reached a regime where the stability of the polariton is limited by diffraction into a continuum of plasmon modes^{2}.

We have also used transport to probe the ultra-strong light-matter coupling^{3}, and show now that the latter can induce a breakdown of the integer quantum Hall effect^{4}. The phenomenon is explained in terms of cavity-assisted hopping, an anti-resonant process where an electron can scatter from one edge of the sample to the other by “borrowing” a photon from the cavity^{5}. We are also evaluating a proposal suggesting that the value of the quantization voltage can be renormalized by the cavity^{6}.

- Scalari, G.
*et al.*Ultrastrong Coupling of the Cyclotron Transition of a 2D Electron Gas to a THz Metamaterial.*Science*335, 1323–1326 (2012). - Rajabali, S.
*et al.*Polaritonic Nonlocality in Light Matter Interaction.*Nat Photon*15, 690–695 (2021). - Paravicini-Bagliani, G. L.
*et al.*Magneto-Transport Controlled by Landau Polariton States.*Nat. Phys.*15, 186–190 (2019). - Appugliese, F.
*et al.*Breakdown of topological protection by cavity vacuum fields in the integer quantum Hall effect.*Science*375, 1030–1034 (2022). - Ciuti, C. Cavity-mediated electron hopping in disordered quantum Hall systems.
*Phys. Rev. B*104, 155307 (2021). - Rokaj, V., Penz, M., Sentef, M. A., Ruggenthaler, M. & Rubio, A. Polaritonic Hofstadter butterfly and cavity control of the quantized Hall conductance.
*Phys. Rev. B*105, 205424 (2022).

For more information on how to join, please see: https://cmsa.fas.harvard.edu/event_category/topological-quantum-matter-seminar/