Single-photon source, driven by composite pulse sequences for applications in a doped-solid quantum memory

量子存储器是任何量子信息架构的关键组件。由于光子之间的相互作用可以忽略不计，它们是量子信息的理想载体。因此，需要用于单光子的光学存储器。存储时间和效率是量子存储协议最相关的基准。在过去几年中，申请人对Pr：YSO晶体中通过电磁感应透明（EIT）的光存储进行了彻底的研究。我们展示了EIT存储时间长达一分钟，存储效率高达76%。然而，这些实验仍然在经典光的水平上进行。为了实现EIT量子存储器，我们需要一个窄带单光子源。该方案基于Duan-Lukin-Cirac-Zoller（DLCZ）量子存储方法，在一个专门设计的磁光阱（MOT）中的冷铷原子系综中实现窄带单光子源，以提供大的光学深度。作为DLCZ方法的新颖而强大的变体，我们提出了基于复合脉冲（CP）和受激拉曼绝热通道（STIRAP）的方案，以驱动集体原子相干性，该集体原子相干性对应于单个光子的鲁棒且非常精确定义的激发。重要的优点是：（i）较高的占空比和基本上确定性的操作的源绝热刺激，而不是自发辐射，（ii）更大的亮度，通过限制单光子发射体积朝向前向方向，（iii）更大的鲁棒性方面的实验参数的波动。作为DLCZ的替代方案，我们提出了一种基于EIT的量子滤波器的实现，它只传输单光子态。这些方法对量子技术也远远超出光存储的重要性。作为一项新的技术发展，我们还将在冷铷原子加载到空芯光纤中实现这些概念。最近，我们证明了巨大的光学深度OD > 1000在这样的设置，即数量级超过典型值在以前的DLCZ实验。这使得进一步的重大改进朝着更低的噪音和更高的亮度。最后，我们将通过与极化锂酸盐（PPLN）波导中2.55 µm的强中红外辐射进行和频混合，有效地转换从铷发射的795 nm的单光子，以获得606 nm的单光子，即与Pr：YSO存储器匹配。

Quantum memories are a key component of any quantum information architecture. As photons have negligible interaction with each other, they are ideal carriers for quantum information. Thus, optical memories for single photons are required. The storage time and efficiency are the most relevant benchmarks for quantum memory protocols. In previous years, the applicant conducted thorough investigations on light storage by electromagnetically-induced transparency (EIT) in Pr:YSO crystals. We demonstrated EIT storage times up to the regime of one minute, and storage efficiencies of up to 76 %. However, these experiments still operated at the level of classical light. To push towards an EIT quantum memory, we require now a source for narrow-band single photons.The proposal deals with the experimental setup of a narrow-band single-photon source, based on the Duan-Lukin-Cirac-Zoller (DLCZ) quantum memory approach, implemented in an ensemble of cold Rubidium atoms from a specifically designed magneto-optical trap (MOT) to provide large optical depth.As novel and powerful variants of the DLCZ approach, we propose schemes based on composites pulses (CP) and stimulated Raman adiabatic passage (STIRAP) to drive collective atomic coherences corresponding to a robust and very precisely defined excitation by a single photon. Important advantages are (i) higher duty cycle and essentially deterministic operation of the source by adiabatically-stimulated rather than spontaneous emission, (ii) larger brightness by confining the single-photon emission volume towards the forward direction, (iii) larger robustness with regard to fluctuations of experimental parameters. As an alternative to DLCZ, we propose the implementation of an EIT-based quantum filter, which transmits only single-photon states. The approaches are of significant interest for quantum technologies also well beyond light storage. As a new technical development, we will implement the concepts also in cold Rubidium atoms loaded into a hollow-core fiber. Recently we demonstrated huge optical depths OD > 1000 in such a setup, i.e. orders of magnitude beyond typical values in previous DLCZ experiments. This enables further major improvements towards lower noise and higher brightness. Finally, we will efficiently convert the single photons at 795 nm emitted from Rubidium by sum-frequency mixing with intense mid-infrared radiation at 2.55 µm in a periodically-poled lithium-niobate (PPLN) waveguide, to obtain single photons at 606 nm, i.e. matched to a Pr:YSO memory.