Microcavity exciton polaritons are hybrid quasiparticles resulting from the quantum superposition of excitons and cavity photons inside semiconductor cavities. As half-light, half-matter quasiparticles, they inherit all the advantages from their excitonic and photonic components, such as low effective mass, strong nonlinearity, fast propagation, as well as enhanced sensitivity to external stimuli, such as electric and magnetic fields. These advantages allow them to be an exceptional connecting bridge between condensed matter and photonic systems, which plays important roles in not only fundamental sciences but also novel optoelectronic and quantum applications. In terms of fundamental physics, as interacting bosons with low effective masses, exciton polaritons are ideal candidates for investigating room temperature collective phenomena, such as non-equilibrium Bose–Einstein condensation, superfluidity[5,6], and quantum vortices. With the advances of potential trapping in polariton systems, exciton polaritons can be trapped to serve as artificial atoms, opening a way to effectively emulate electron Hamiltonians. This unique property generally allows exciton polaritons to be a solid-state analog of cold atoms in optical lattices, which have been shown to play important roles in topology and proposed quantum simulators with room temperature operation. In addition, being intrinsically lossy due to the limited lifetimes of their components, exciton polaritons are also promising platforms for the investigation of rich non-Hermitian physics and related applications[9,10]. In the quest for practical applications, possessing spontaneous coherence in the condensation process, exciton polaritons allow realizing polariton lasers with thresholds orders of magnitude lower than that of conventional photonic lasers, because of the exemption of the population inversion process[11,12]. In addition, due to the strong nonlinearity from their excitonic components, exciton polaritons are excellent building blocks for developing all-optical ultrafast switches and transistors[13–15], which lays the foundation for all-optical circuits and chips. This strongly interacting nature also allows to induce quantum effects possibly at the level of single particles[17,18]. This promising regime brings exciton polaritons into the rapidly rising field of quantum simulation, computing, and information processing with the possibility of room temperature operation.
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