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领域。由于精细的量子现象，可以设想许多新的技术应用。虽然经典的信息编码依赖于比特，它可以是0和1，量子比特（或量子位）与量子物体的状态有关，例如单原子，单自旋或单光子。由于量子叠加原理，量子位可以是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，即量子物体之间的强相关性，作为快速和安全的量子通信协议的资源。为了实现量子通信网络，量子存储器是基本的器件，因为它们作为光子之间的接口，用作信息载体，和原子，利用信息存储和处理。为了在量子网络中发挥作用，量子存储器必须满足按需读出、高效保真、长存储时间和多模态等特定要求。虽然原子气体实现了第一个非凡的量子存储实验，但固态系统也提供了有趣的前景。其中，稀土掺杂晶体最近成为有吸引力的候选者，因为它们是在惰性介质中自然捕获的光学活性离子的集合，不需要外部捕获场和超高真空室。在效率和存储时间方面，它们的性能已经达到或超过了捕获原子或冷原子集合的性能。这些晶体在光学、无线电和微波范围内都表现出跃迁，因此它们可以作为光子或微波存储器，也可以作为光学和微波频率之间的接口，从而为采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景，并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满足理想光子量子存储器的所有要求。该项目正是解决了这个问题，旨在开发一种新的电信兼容集成量子设备平台，其中包含具有前所未有功能的固态量子存储器。其核心思想是不采用稀土掺杂晶体，而是采用化学计量晶体，即稀土离子完全取代晶体基质的一种元素，具有增加光的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：-优化块状晶体的相干特性，这将使量子存储协议的实现成为可能，这在这类材料中从未被证明；-探索密闭环境，例如激光写入波导，以实现集成量子存储器。我们期望波导的制造能够促进光纤耦合器件的实现和利用电场对原子跃迁的有效操纵，并提高光与稀土离子之间的相互作用强度。这可能会使电信光的存储利用光学跃迁，在稀释的散装样品会太弱。因此，提议的平台可能允许同时演示高效、长寿命和多路存储设备，这些设备也与现有的电信光纤网络兼容。这种量子存储器将优于现有的量子存储设备，它们的演示将为使用固态技术进行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.