Magneto -Transport in Engineered Vacuum Fields

Abstract

Quantum electrodynamics predicts a non-trivial ground state for an electromagnetic mode. In the absence of photons, the so called zero-point energy of $\frac{1}{2}\hbar\omega$ remains. It gives rise to vacuum electric field fluctuations and important physical effects such as the spontaneous emission, the Lamb shift and the Casimir effect. Experimentally, it remains difficult to tune vacuum field modes and directly observe their physical consequences. By engineering vacuum fields in cavities one can reach a peculiar situation: an electronic excitation of matter can be revived after its decay by photon emission. In this so called strong light-matter coupling regime, the hybrid light-matter excitations (polaritons) are mostly probed with photonic excitations. Such an approach hides, that the coupling arises already from the vacuum field fluctuations in absence of photons. In this work, we develop an experimental platform allowing to probe the electronic part of the polaritonic ground state. Intriguingly, we can tune vacuum field modes, while observing the response in the matter part. It is implemented with a cavity-embedded 2D electron gas in the ultrastrong coupling regime and probed by magneto-transport. Transport - depending on virtual transitions to excited states - is modified, as these transitions become the polaritons in presence of a vacuum Rabi splitting. After a theoretical discussion, we experimentally show that few polariton excitations and also vacuum fields alone modify transport. This opens the way to vacuum-field-controlled many-body states in quantum Hall systems.