Photonic platforms are an excellent setting for quantum technologies as weak photon–environment coupling ensures long coherence times. The second key ingredient for quantum photonics is interactions between photons, which can be provided by optical nonlinearities in the form of cross-phase modulation. This approach underpins many proposed applications in quantum optics^{1,2,3,4,5,6,7} and information processing⁸, but achieving its potential requires strong single-photon-level nonlinear phase shifts as well as scalable nonlinear elements. In this work we show that the required nonlinearity can be provided by exciton–polaritons in micropillars with embedded quantum wells. These combine the strong interactions of excitons^9,10 with the scalability of micrometre-sized emitters¹¹. We observe cross-phase modulation of up to 3 ± 1 mrad per polariton using laser beams attenuated to below the average intensity of a single photon. With our work serving as a stepping stone, we lay down a route for quantum information processing in polaritonic lattices.

光子平台是量子技术的绝佳环境，因为弱光子-环境耦合可确保长相干时间。量子光子学的第二个关键要素是光子之间的相互作用，这可以通过交叉相位调制形式的光学非线性提供。^{这种方法支持量子光学1、2、3、4、5、6、7}和信息处理⁸中的许多拟议应用，但实现其潜力需要强大的单光子级非线性相移以及可扩展的非线性元件。在这项工作中，我们证明了所需的非线性可以通过嵌入量子阱的微柱中的激子-极化子来提供。这些结合了激子的强相互作用^9,10具有微米大小的发射器¹¹的可扩展性。我们使用衰减到单个光子平均强度以下的激光束观察到每个极化子高达 3 ± 1 mrad 的交叉相位调制。以我们的工作为垫脚石，我们为极化晶格中的量子信息处理开辟了一条路线。