Solid-state coherent interface between microwaves and Telecom-C band light

量子通信网络承诺以绝对安全的方式分发信息，它基于大规模计算体系结构上的量子纠缠态的传播。未来量子网络的核心元素，即量子中继器和网络节点，可以通过使用不同物理性质的量子比特和量子存储器来实现。今天，连接两个遥远的单原子的基本量子网络已经被展示出来。与基于单原子方法的系统相比，超导量子电路、纳米机械设备和自旋掺杂固体等固态系统可能提供更大的可扩展性和更快的操作时间。然而，这种固态器件工作在微波和射频下，由于电缆的损耗和用于无线电接力通信的天线的高噪声温度(约100K)，这两种方式不太适合远距离量子通信。为了在它们之间建立光纤链路，人们必须使用量子介质转换器，即一种将物质与光子量子比特相干连接的装置。在这个项目中，我们将专注于基于掺Er离子同位素的同位素富集型四氟化钇锂(YLiF4)的频率转换。使用稀土离子掺杂晶体的优点是能够实现多模转换协议。同位素浓缩晶体以其超窄的非均匀展宽而闻名，这被认为是实现Telecom-C约1.54微米光子和超导量子电路之间的量子接口的关键。在我们的项目中，我们将专注于利用这种晶体实现相干微波到光场的双向转换。为了达到项目的目标，只需完成几个中间步骤。由于对这类晶体的实验很少，我们将首先探索它在毫秒级温度和小磁场下的相干性质，即在超导量子比特的工作条件下。然后，我们将利用电磁感应透明效应将微波场和光场转换为掺杂稀土离子的自旋激发，并测量存储时间。最后，利用光泵浦或自旋回波技术将自旋波激发转换为相干光场或微波场，并测量转换效率。

Quantum communication networks promises to distribute information in absolute secure way and it is based on dissemination of quantum entangled states over a large scale computing architecture. The core elements of future quantum networks, i.e. quantum repeaters as well as network nodes, can be realized by using qubits and quantum memories of diverse physical nature. Today, elementary quantum networks linking two remote single atoms have been demonstrated. Solid-state systems such as superconducting quantum circuits, nano-mechanical devices, and spin doped solids potentially offer larger scalability and faster operation time compared to systems based on the single atom approach. However, such solid-state devices operate at microwave and RFs, which are less suitable for long-range quantum communication than fiber-optical channels due to losses in cables and the high noise temperature of antennas (about 100 K) for radio-relay communication. To establish a fiber-optical link between them, one has to use a quantum media converter, i.e. a device which coherently interfaces matter and photonic qubits.In this project we will focus on frequency converted based on isotopically enriched Yttrium-Lithium tetrafluoride (YLiF4) doped with isotopes of erbium ions. The advantage of using rare-earth-ion doped crystal is the ability to implement multi-mode conversion protocol. Isotopically enriched crystals are known for their ultra-narrow inhomogeneous broadening, which is believed to be crucial for the implementation of quantum interfaces between Telecom-C around 1.54 um photons and superconducting quantum circuits. In our project we will focus on implementation of bi-directional conversion of coherent microwave to optical fields by using such a crystal. In order to reach the aim of the project few intermediate steps shall be fulfilled. Since there were only few experiments with such crystals, we will initially explore its coherent properties at millikelvin temperatures and small magnetic field, i.e. at operating conditions of superconducting qubits. Then we are going to implement conversion of microwave and optical fields to a spin excitation of doped rare-earth ions by using electromagnetically-induced-transparency and measure storage time. Finally, by using optical pumping or spin echo technique the spin wave excitation will be converted into coherent optical fields or microwave field and the conversion efficiency will be measured.