Abstract
Quantum information is a rapidly emerging field aiming at combining two of the greatest advances in science and technology of the twentieth century, that is, quantum mechanics and information science. To reliably generate, store, process, and transmit quantum information, diverse systems have been studied. While for specific tasks some of these systems are more suitable than others, no single system can meet all envisioned demands. Erbium doped crystal has optical transition at 1.5 μm and possesses long optical coherence time and spin coherence time, and thus is one of the best candidates in building several essential blocks for quantum information applications. In this review, we summarize the applications of erbium doped crystals in quantum memories, quantum transducers, quantum sources, and quantum manipulations based on erbium-erbium interactions. Finally, the outlooks for near term prospects of the mentioned topics are also given.
Highlights
Nuclear spins and electron spins feature with their long coherence time; superconducting qubits, quantum mechanical systems and microwave cavities can be strongly coupled to electromagnetic fields in radio or microwave frequencies; and photons at telecom wavelengths are unparalleled in sending information over long distance
On-chip erbium quantum memories: (a) Quantum memory based on erbium- and titanium-indiffused lithium-niobate waveguide[37]; (b) nanophotonic quantum memory by using focued-ion-beam to fabricate a one-dimensional photonic cavity in a YVO crystal[41]; (c) waveguide memory fabricated by femtosecond-laser micromachining on the surface of a YSO crystal[38]; (d) quantum memory comprised of an amorphous silicon waveguide on a YSO crystal[42]
Quantum memory and manipulation based on erbium doped crystals*(Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurements of Ministry of Education, Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Physics, Beijing Institute of Technology, Beijing 100081, China) ( Received 27 September 2021; revised manuscript received 1 November 2021 )
Summary
图 1 若干代表性量子体系的特征参数 [13]. 不同材料在图中的位置是根据该体系本身的相干时间 (x 轴) 和工作频率 (y 轴) 排列 的. 例如, 核自旋和电子自旋在低温下具有较长的相干时间; 超导量子比特、量子机械系统和微波光子腔与传统电磁波有很强的 耦合作用; 同时通信波段的光子在信息的长距离传输方面具有无可比拟的优势 Fig. 1. Nuclear spins and electron spins feature with their long coherence time; superconducting qubits, quantum mechanical systems and microwave cavities can be strongly coupled to electromagnetic fields in radio or microwave frequencies; and photons at telecom wavelengths are unparalleled in sending information over long distance. 这一 纪录由澳大利亚国立大学的 Sellars 研究小组 [16] 在 2015 年利用 ZEFOZ (zero first-order Zeeman effect) 技术在铕 (Eu) 掺杂的 YSO 晶体中实现. 目前 ZEFOZ 技术目前也只 在 Pr[21], Eu[16,22] 和镱 (Yb)[23] 少数几种稀土元素 中实现. 同 时, 由于超导电路对磁场的敏感性, 上文所述的需 要外加磁场的 ZEFOZ 技术也很难应用到超导体 系中. 目 前, 通过在零磁场下寻找铒离子的 ZEFOZ 跃迁, 已经实现的相干时间为 1.6 ms[31], 远高于之前在零 磁场下观测到的 50 μs 的相干时间 [32]. Hyperfine structure of 167Er:YSO as a function of applied magnetic field. (a) The ground state of 167Er:YSO consists of 16 hyperfine energy levels, all of which show nonlinear behaviour around B = 0. 目前, 基于稀土掺杂晶体可集成量子存储器的 研究还处于探索阶段 [34], 主要的技术思路包括两类, 见图 4. 一种方式是直接在晶体材料上进行微纳加 工. 主要采用的工艺方法包括离子扩散 [35−37]、激光 直写 [38,39] 和聚焦离子束刻蚀 [40,41] 等, 如图 4(a) — 图 4(c) 所示. 2011 年, 加拿大卡尔加里大学的研 究小组利用离子扩散技术, 在铌酸锂晶体材料上制 备出波导结构, 实现了对纠缠光子的量子存储 [36]; 2019 年, 美国加州理工的研究小组通过在 Er 掺杂 的 YSO 晶体上直接用聚焦离子束刻蚀的方法加 工出了一维光子晶体腔结构, 实现了单光子的量子 存储 [40]; 2020 年, 中国科学技术大学的研究小组通 过在铕 (Eu) 掺杂的 YSO 晶体上利用激光直写制 备波导的方式, 实现了按需读取的量子存储 [38,39]. 这种直接在晶体上进行加工的方式虽然可以用于 研究单个微纳光量子存储器的性能特性, 但是其局
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