Stoichiometric rare-earth crystals for novel integrated quantum memories

量子信息科学是研究量子系统中存在的信息的研究领域。多亏了精致的量子现象，许多新的技术应用被设想出来。经典的信息编码依赖于比特，比特可以是0或1，而量子比特(或量子比特)与量子对象的状态有关，例如单原子、单自旋或单光子。由于量子叠加原理，量子比特可以是0、1或两者的相干叠加，从而获得异常丰富的字母表。量子信息科学还利用量子纠缠，即量子对象之间的强关联，作为快速安全的量子通信协议的资源。从实现量子通信网络的角度来看，量子存储器是基本的器件，因为它们充当作为信息载体的光子和用于信息存储和处理的原子之间的接口。要在量子网络中使用，量子存储器必须满足特定的要求，如按需读出、高效率和保真度、长存储时间和多模式。虽然原子气体使第一次引人注目的量子存储实验成为可能，但固态系统也提供了有趣的前景。在这些实验中，稀土掺杂晶体最近成为有吸引力的候选材料，因为它们是自然捕获在惰性介质中的光学活性离子的系综，不需要外部陷阱场和超高真空室。他们已经展示了在效率和存储时间方面与被困原子或冷原子系综相等或克服的性能。这些晶体在光学和无线电和微波范围内都表现出跃迁，因此它们既可以用作光子或微波存储器，也可以作为光和微波频率之间的接口，从而为使用超导器件的混合系统开辟了道路。尽管它们的性能非常有前途，而且在过去十年中实现了里程碑式的实验，但目前还没有一种独特的稀土掺杂晶体来满足理想的光子量子存储器的所有要求。本项目正是解决这个问题，旨在开发一种新型的电信兼容集成量子器件平台，包含具有前所未有的功能的固态量子存储器。中心思想不是使用稀土掺杂晶体，而是化学计量晶体，即稀土离子完全取代晶体基质中的一种元素，具有增加光吸收和缩小电子跃迁的不均匀线宽的双重目标，这要归功于较低的局部机械应力。解决的挑战是：-优化大块晶体的相干特性，使量子存储协议得以实现，这在这类材料中从未展示过；-探索受限环境，即激光写入波导，以实现集成量子记忆。我们预计，这种波导的制造将有助于实现光纤耦合器件，并通过电场有效地操纵原子跃迁，并增强光与稀土离子的相互作用强度。这可能提供利用光学跃迁的电信光存储的途径，而在稀释的大宗样品中，光学跃迁太弱。因此，拟议的平台可能允许同时演示高效、长寿命和多路复用的存储设备，这些存储设备也与现有的电信光纤网络兼容。这种量子存储器的性能将超过现有的量子存储设备，它们的展示将为将固态技术用于真正的量子信息应用开辟新的途径。

Quantum information science is the field of research that studies the information present in a quantum system. A number of new technological applications can be envisaged thanks to exquisitely quantum phenomena. While classical information encoding relies on bits, which can be either 0s and 1s, the quantum bits (or qubits) are associated to the state of quantum objects, e.g. single atoms, single spins, or single photons. Because of the quantum superposition principle, the qubits can then be 0s, 1s, or coherent superposition of both, thus giving access to an exceptionally richer alphabet. Quantum information science also exploits quantum entanglement, i.e. strong correlation between quantum objects, as a resource for fast and secure quantum communication protocols.In view of realising networks for quantum communication, quantum memories are fundamental devices as they act as interfaces between the photons, used as information carriers, and atoms, exploited for information storage and processing. To be useful in quantum networks, the quantum memories must fulfil specific requirements, as on-demand read-out, high efficiency and fidelity, long storage time, and multimodality. While atomic gases enabled the first remarkable quantum storage experiments, solid-state systems also offer interesting perspectives.Among these, the rare-earth doped crystals recently emerged as attractive candidates because they are ensembles of optically active ions naturally trapped in inert media, which do not require external trapping fields and ultra-high vacuum chambers. They have already featured performances equalising or overcoming those of trapped atoms or cold atomic ensembles in terms of efficiency and storage times. These crystals exhibit transitions both in the optical and in the radio- and micro-wave range, thus they could serve as photonic or microwave memories, but also as interfaces between optical and microwave frequencies, thus opening the way to hybrid systems employing superconducting devices.Despite their very promising performances and the milestone experiments realised in the last decade, a unique rare-earth doped crystal that fulfils all the requirements of an ideal photonic quantum memory does not yet exist.This project exactly tackles this problem and aims at developing a novel platform for telecom-compatible integrated quantum devices, containing solid-state quantum memories with unprecedented functionalities. The central idea is to employ not rare-earth doped crystals but stoichiometric crystals, i.e. where the rare-earth ions fully substitute one element of the crystal matrix, with the two-fold aim of increasing the absorption of light and narrowing the inhomogeneous linewidth of the electronic transitions, thanks to a lower local mechanical stress.The challenges addressed are:- the optimisation of the coherence properties of bulk crystals that will enable the implementation of quantum storage protocols, never demonstrated in these kind of materials; - the exploration of confined environment, i.e. laser written waveguides, for the realisation of integrated quantum memories.We expect the waveguide fabrication to facilitate the realisation of fibre-coupled devices and the efficient manipulation of the atomic transitions by means of electric fields, and to boost the interaction strength between the light and the rare-earth ions. This might give access to the storage of telecom light exploiting optical transitions that in diluted bulk samples would be too weak. Therefore, the proposed platform might permit the simultaneous demonstration of efficient, long-lived and multiplexed storage devices, which are also compatible with existing telecom fibre network. Such quantum memories would outperform the existing quantum storage devices, and their demonstration would open new avenues for the use of solid-state technologies for real quantum information applications.