Up on the Jaynes–Cummings ladder of a quantum-dot/microcavity system

Abstract

In spite of their different natures, light and matter can be unified under the strong-coupling regime, yielding superpositions of the two, referred to as dressed states or polaritons. After initially being demonstrated in bulk semiconductors¹ and atomic systems², strong-coupling phenomena have been recently realized in solid-state optical microcavities³. Strong coupling is an essential ingredient in the physics spanning from many-body quantum coherence phenomena, such as Bose–Einstein condensation⁴ and superfluidity⁵, to cavity quantum electrodynamics. Within cavity quantum electrodynamics, the Jaynes–Cummings model^6,7,8 describes the interaction of a single fermionic two-level system with a single bosonic photon mode. For a photon number larger than one, known as quantum strong coupling, a significant anharmonicity is predicted for the ladder-like spectrum of dressed states. For optical transitions in semiconductor nanostructures, first signatures of the quantum strong coupling were recently reported⁹. Here we use advanced coherent nonlinear spectroscopy to explore a strongly coupled exciton–cavity system^10,11. We measure and simulate its four-wave mixing response^12,13, granting direct access to the coherent dynamics of the first and second rungs of the Jaynes–Cummings ladder. The agreement of the rich experimental evidence with the predictions of the Jaynes–Cummings model is proof of the quantum strong-coupling regime in the investigated solid-state system.