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光量子存储器在量子计算、量子传感、量子通信等领域有非常重要的地位。冷原子系统因其优异的量子相干特性、可控性和极佳的弱光场处理能力，成为实现高质量光量子态存储的重要平台之一。其中雪茄型结构的冷原子系综由于其光学深度可达 100 以上而具备高效的存储性能。然而外界不均匀的剩余磁场，使其存储寿命在实用过程中受到了极大地限制。本文研究了由囚禁线圈关断残余及环境涡旋电流产生的非均匀磁场引发的原子自旋退相干问题。理论和实验表明，直流磁场能提供量子化轴而且可抑制非均匀磁场的影响，并调控自旋退相干与重相干周期。进一步地，本文演示了在磁子能级的光学泵浦过程中泵浦光功率可有效控制原子布居占比，从而精准控制退相干和重相干发生的强度。基于以上磁场调控结果，本文提出了一种双时间点的光子纠缠态的产生、存储和测量方案。基于冷原子系综制备的光子对是窄线宽的，其在时间点上编码的光子纠缠态在长距离传输中更稳定。利用外加磁场的方式调控原子自旋波退相干重相干的周期时间，可以选择性地将双时间点的原子自旋波转化为对应时间的读光子，从而构建正交的时间点测量基矢。Optical quantum memory plays a critical role in fields such as quantum computing, quantum sensing, and quantum communication. Cold atomic systems, owing to their excellent quantum coherence, controllability, and exceptional capability in handling weak optical fields, have emerged as one of the key platforms for faithful optical quantum state storage. Among these, cigarette-shaped, with up to 2 cm or more, cold atomic ensembles exhibit over 85 % storage effciency due to their optical depth reaching 100 or more. However, further applications are significantly hindered by the limited storage lifetimes caused by inhomogeneous residual magnetic fields along the long atomic cloud. This study analyzes the issue of atomic spin decoherence induced by non-uniform magnetic field with linear gradient, and obtain the result that storage lifetime dramatically decreases with this increasing linear gradient. Further, we demonstrate that in our two-dimensional magneto-optical trap system with a longitudinal atom-light interaction length of 2.7 cm, a DC magnetic field can provide a quantization axis, suppress the effects of inhomogeneous fields,and regulate the cycles of spin dephasing and rephasing. With the proper setting for optical pumping process of magnetic quantum levels, adjusting the pump laser power effectively controls the atomic population distribution, thereby precisely optimizes the light storage effciency at different time bins, as shown in Fig. 7(a). Based on these findings, we propose a scheme for storage of time-bin entangled photon pairs, who are prepared at two different time slots of DLCZ process. A bias magnetic field on the generation MOT (left panel of Fig. 7) induces modulation on the storage time as (a), so that read pulse exerted on rj reads only wj (j= 1, 2). Therefore, the two photonic time bins becomes distingushable and orthogonal. The retrieved photon pairs thus have fully controllable time bins for both photons. Compared to other degrees of freedom, the time encrypted photonic entanglement remains robust in long-distance network.
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Keywords:
- Optical quantum storage /
- Cold atomic ensemble /
- Larmor precession /
- Atomic spin wave
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