Controlled Strong Coupling and Absence of Dark Polaritons in Microcavities with Double Quantum Wells.

We demonstrate an efficient switching between strong and weak exciton-photon coupling regimes in microcavity-embedded asymmetric double quantum wells, controlled by an applied electric field. We show that a fine-tuning of the electric field leads to drastic changes in the polariton properties, with the polariton ground state being redshifted by a few meV and having acquired prominent features of a spatially indirect dipolar exciton. We study the properties of dipolar exciton polaritons, called dipolaritons, on a microscopic level and show that, unlike recent findings, they are not dark polaritons but, owing to the finite size of the exciton, are mixed states with a comparable contribution of the cavity photon, bright direct, and long-living indirect exciton modes.

Microcavity controlled coupling of excitonic qubits.

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits--like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots--is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton-photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample's coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling.