室温下存储GHz带宽极化量子纠缠态实验研究

The preparation and storage of photonic entanglement are central to the achievement of scalable photonic quantum communication, computation and metrology. The most widely used photonic entanglement source, a spontaneous parametric down conversion source, is not directly suitable for storage, because its working frequency bandwidth is significantly larger than any available quantum memory. To remedy this problem, we have successfully, for the first time, stored cavity-enhanced narrow-band down-conversion sources into cold atomic ensembles, supported by the project NSFC (No. 11004183). However, MHz-level bandwidth limits overall brightness of sources and achievable feedback numbers within memory lifetime be further improved. Bandwidth therefore becomes the remained bottleneck for the scalability of quantum information technology. Here we propose to develop broadband quantum memory technique as a solution. We explore two kinds of far off-resonance Raman memory, stimulated Raman absorption and spontaneous Raman scattering. By optimizing the performance of both, we will be able to select the best candidate of broadband memory scheme. We will combine the techniques and experiences obtained in the last project with room-temperature and broadband quantum memory, aiming at building memory-enabled large-scale quantum states. Our GHz-Bandwidth quantum interface between entangled photons and atomic quantum memory, will provide a key element towards the achievement real-world photonic quantum information processing.

光子纠缠的制备和存储是大尺度量子通信，计算和精密测量的核心问题。作为最为广泛应用的光子纠缠源，自发参量下转换源的工作频率带宽远大于可获得的量子存储的带宽，因此并不能直接对其进行量子存储。为了解决这一问题，我们在自然科学基金青年项目（No. 11004183）的支持下，首次成功将腔增强的窄带参量下转换源存储进冷原子系综中。尽管如此，MHz量级的的带宽仍然限制了光子源整体可获得的亮度和存储寿命内可进行的反馈操作次数。带宽因素因此变成制约量子信息技术发展的瓶颈。在本项目中，我们采用宽带量子存储技术作为解决方案。我们将围绕两种远失谐存储机制展开探索，即受激拉曼吸收和自发拉曼散射。通过仔细优化，我们可以比较性能从而选择最适合的量子存储方案。我们会把上一个研究项目中积累的技术和经验与宽带量子存储相融合，争取构建更大的基于存储的量子态。我们的GHz宽带纠缠－原子存储界面将为实际光量子信息处理提供关键要素。

目前最为广泛应用的光子纠缠源，是通过自发参量下获得的，这类转换源的工作频率带宽远大于可获得的量子存储的带宽，因此并不能直接对其进行量子存储。在本项目中，围绕远失谐自发拉曼散射，将室温量子存储与宽带量子存储相融合，构建更大的基于存储的量子态。相比冷原子中的量子存储，噪声的控制是热原子存储的关键，为了降低系统的噪声，我们利用两组串联的滤波腔，每组滤波腔的透过率达到70%，信噪比抑制达到10^5:1，透过率带宽700MHz，同时利用时间逻辑符合系统，将噪声抑制到原来的1/10000。通过精确控制原子温度和周围的磁场，增加原子的存储寿命。最终，我们首次在实验室实现了室温条件下的量子宽带存储，获得了强关联光子对，关联光子的互相关达到20，显示出了强的量子关联特性，存储寿命达到2微妙，存储带宽600M, 比目前冷原子中的带宽高一个数量级，该结果不仅对研究基础量子物理有重要的意义，而且可以为实际的光量子信息处理提供关键支持。