Quantum nature of a strongly coupled single quantum dot-cavity system

Cavity quantum electrodynamics (QED) studies the interaction between a quantum emitter and a single radiation-field mode. When an atom is strongly coupled to a cavity mode, it is possible to realize important quantum information processing tasks, such as controlled coherent coupling and entanglement of distinguishable quantum systems. Realizing these tasks in the solid state is clearly desirable. We coupled semiconductor based solid state atomic-like emitters, or artificial atoms, to monolithic optical cavities in a lattice matched semiconductor material. We choose self-assembled InAs quantum dots in GaAs host material as quantum emitters, and photonic crystal resonators built around it as the optical cavity. This nanocavity type shows the largest figures of merit in terms of quality-factor to mode volume ratio for an optical solid state resonator. Validating the efficacy of quantum dots in quantum information applications requires confirmation of the quantum nature of the quantum-dot–cavity system in the strong coupling regime. We have developed and applied an original and deterministic protocol, based on a combination of theoretical design/modelling, e-beam lithography, etching, and atomic force microscopy, to position one, and only one, quantum dot at the field antinode of the photonic crystal resonator. Then, we have found confirmation of the quantum nature of this system by measuring quantum correlations in photoluminescence from the strongly coupled quantum dot emission and the cavity mode resonance. When off-resonance, photon emission from the cavity mode and quantum-dot excitons is anti-correlated at the level of single quanta, proving that the mode is driven solely by the quantum dot despite an energy mismatch between cavity and excitons. When tuned to resonance, the exciton and cavity enter the strong-coupling regime of cavity QED and the quantum-dot exciton lifetime reduces by a factor of 145. The generated photon stream becomes anti-bunched (i.e., emits streams of single photon states), proving that the strongly coupled exciton/photon system is in the quantum regime. Our observations unequivocally show that quantum information tasks are achievable in solid-state cavity QED. These results are likely to have strong impact for applications of quantum information and communication science and technology in integrated solid state devices.