Coupling between quantum dots and photonic crystal cavities

Abstract

<p indent="0mm">Cavity quantum electrodynamics studies the interaction between light and matter at the single quanta level, i.e., cavity photons and quantum emitters, which has widespread applications in quantum information processing and optical devices, such as lasers, solar cells and light-emitting diodes. To study cavity quantum electrodynamics, a quantum emitter with excellent quantum properties and a cavity with high quality factor and small mode volume are preferred. The coupling between self-assembled quantum dots and photonic crystal cavities has attracted broad attention due to the advanced semiconductor fabrication techniques and excellent optical properties. Great progress has been made in the last two decades, which provides an efficient quantum interface between light and matter at the near-infrared domain. The self-assembled quantum dots have excellent optical properties, including high brightness, high single photon purity, and high indistinguishability, possessing great advantages in the applications of laser, single-photon source, entangled photon source and quantum bit. On the other hand, photonic crystal cavities have high quality factor and small mode volume, providing strong optical confinement. It can greatly enhance the interaction between light and matter. By coupling to photonic crystal cavities, the optical properties of quantum dots can be strongly modified. For example, in the weak coupling regime, where the coupling strength <italic>g</italic> is smaller than the decay rate of the system, the spontaneous emission rate of quantum dots is enhanced due to the increase of local density of states. The enhancement factor, known as Purcell factor, is proportional to the quality factor and inversely proportional to the mode volume of the cavity. The weak coupling can be used to optimize the performance of optical devices, such as reducing the laser threshold, increasing the efficiency of single-photon source and entangled photon source. In the strong coupling regime, where the coupling strength <italic>g</italic> is larger than the decay rate, the energy shifts back and forth between quantum dots and cavity modes at vacuum Rabi frequency, leading to the formation of exciton-polariton with Rabi splitting about 2<italic>g</italic>. The strong coupling has been widely proposed for the applications in quantum information processing and investigation of nonlinear optical effects at single photon level, such as the realization of quantum logical gate, the operation of qubit, the transfer of quantum states and single-photon blockade. Additionally, photonic crystal structures have a high level of integration. By integrating with waveguides, the coupled system, served as quantum nodes, can be connected to achieve the quantum internet. Besides the traditional photonic crystal cavities, recently a new type of cavity with topological origin has been proposed and demonstrated, providing a more robust platform to investigate cavity quantum electrodynamics and develop optical devices with built-in protection for topological nanophotonic circuitry. In this paper, we review the basic principle and recent progress of the coupling between self-assembled quantum dots and photonic crystal cavities, which show a great opportunity to realize various optical devices with diverse functionalities, providing an ideal platform to realize on-chip photonic circuitry.