Quantum Electric Transport Meets Quantum Optics: Josephson Photonics with Strong Charge-Light Coupling Specifically for the continuation: Strong Light-Matter Coupling in Josephson Photonics:Multi-photon Resonances, Quantum Locking and Synchronization

光子学和量子电子学是目前物理学中发展最快的领域之一，因为它们与未来的信息处理、通信和传感有着至关重要的关系。这些场处理自然界中的两个基本量子，即光子和载流子，根据量子电动力学理论结合在一起。然而，探索量子导体的量子光学的发展直到最近才成为一个跨学科领域，在通过量子电子学创造和控制量子微波辐射方面具有新的前景。特别强大的是基于超导量子电路关键组件的设备，即直流电压偏置约瑟夫森结（JJ）和微波谐振器。它们允许达到强电荷-光相互作用的领域，并结合基本完美的电能到光子的转换。在第一个资助期（2017-2021年），我们与CEA Saclay的法国合作伙伴密切合作，大力推进了约瑟夫森光子学的这一新领域。描述和开发了用于单光子和纠缠光子的明亮多用途光源，并探索了超出量子电阻的高阻抗状态下的基本特性。然而，可能的技术应用仍然有一个关键的缺点，即低电压波动引起的相扩散。这个继续项目的目标是解决这个问题，特别是在强电荷-光耦合（有效精细结构常数为1）的情况下，其中初步的实验数据显示存在多达9个光子的多光子共振。这不仅为量子微波工程提供了迷人的新工具，而且为研究新的物理学铺平了道路，即量子状态中的相位稳定（量子锁定），量子相位滑移可能发生，预计将被实验监测。作为概括，我们将考虑基本约瑟夫森光子元素的网络来揭示同步的量子特性，并展望在太赫兹区开发相同步高强度源的前景。这项工作计划的一个重要组成部分将是继续与我们在CEA/Saclay的实验伙伴进行卓有成效和成功的合作。

Photonics and quantum electronics are currently among the fastest developing fields in physics due to their paramount relevance for future information processing, communication, and sensing. These fields deal with two fundamental quanta in nature, namely, photons and charge carriers, bound together according to the theory of quantum electrodynamics. However, a development which explores the quantum optics of quantum conductors has emerged only very recently as an interdisciplinary field with new prospects in creating and controlling quantum microwave radiation via quantum electronics. Particularly powerful are devices based on the key components of superconducting quantum electrical circuits, namely, dc-voltage biased Josephson junctions (JJ) and microwave resonators. They allow reaching the domain of strong charge-light interaction in combination with a basically perfect conversion of electrical into photonic energy. In the first funding period (2017-2021) and in close collaboration with our French partners at the CEA Saclay we substantially pushed forward this new field of Josephson Photonics. Bright versatile sources for single and entangled microwave photons have been described and developed and fundamental properties in the high impedance regime beyond the quantum resistance have been explored. However, possible technological applications still suffer from one crucial drawback, namely, phase diffusion by low voltage fluctuations. The goal of this continuation project is to solve this problem, particularly in the regime of strong charge-light coupling (effective fine structure constant on the order of 1), where preliminary experimental data show the existence of multi-photon resonances of up to nine photons. This not only offers fascinating new tools for quantum microwave engineering but also paves the road to investigate new physics, namely, phase stabilization in the quantum regime (quantum locking), where quantum phase slips may occur which are expected to be monitored experimentally. As a generalization, we will consider networks of basic Josephson Photonic elements to reveal quantum properties of synchronization with the prospect to develop phase-synchronized high intensity sources in the THz regime. An essential part of this work program will be the continuation of the very fruitful and successful collaboration with our experimental partners at the CEA/Saclay.