Quantum Electric Transport Meets Quantum Optics: Josephson Photonics with Strong Charge-Light Coupling

光子学和量子电子学目前是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、通信和传感具有至关重要的相关性。这些场处理自然界中的两个基本量子，即光子和电荷载流子，根据量子电动力学理论绑定在一起。然而，探索量子导体的量子光学的发展直到最近才作为一个交叉学科领域出现，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部件的器件，即直流偏压约瑟夫森结(JJ)和微波谐振器。它们允许达到强电荷-光相互作用的领域，结合基本完美地将电能转化为光子能量。该项目的目标是通过设计用于量子微波的新光源和探索从弱电荷-光子耦合到强电荷-光子耦合的通道，来推动这类新设备的全部潜力。我们将使用的平台在概念上与电路量子电动力学(CQED)中开发的平台相关，CQED是腔QED的固态模拟，使Haroche和Wineland获得了2012年的诺贝尔奖。然而，在约瑟夫森光子学中，对JJ施加直流电压偏置，允许直接解决库珀对转移，并诱导远离平衡的电荷-光子耦合。这一机制对快速量子微波器件特别有用，但也形成了我们对约瑟夫森物理理解的最后一个缺口：从传统的约瑟夫森机制到库仑阻塞机制的交叉，在传统的约瑟夫森机制中，JJ两端的超导相位差几乎是一个经典变量，在库仑阻塞机制中，转移电荷几乎是一个很好的量子数，通过非相干隧道事件演化。在量子力学中，位相差和转移的库珀对的数目形成一组由海森伯测不准关系联系起来的共轭变量。在中间区域，无论是相还是电荷都不是好的量子数，我们还没有深入研究。具体地说，我们将证明库珀对流经直流偏置的JJ可以被利用来制造有用的器件，其噪声温度接近量子力学允许的极限，以用于非经典光和放大器的明亮源。在演示了在微波范围内工作的原理之后，我们将使其中一些器件适应太赫兹范围。作为补充，我们将研究JJ的量子到经典的跃迁。我们将使用这些设备的一个特殊功能来监控两者：非经典光子态和电荷电流噪声。这个项目汇集了两个实验小组和一个理论小组的深厚专业知识。

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. The goal of this project is to push forward the full potential of this new class of devices by designing new light sources for quantum microwaves and by exploring the passage from weak to strong charge-photon coupling. The platforms we will use are conceptually related to those developed in circuit Quantum Electrodynamics (cQED), the solid state analog of cavity QED that gained Haroche and Wineland the Nobel Prize in 2012. However, in Josephson photonics a dc-voltage bias is applied to the JJ which allows to address the Cooper pair transfer directly and to induce charge-photon coupling far from equilibrium. This regime is particularly useful for fast quantum microwave devices but forms the last gap in our understanding of Josephson physics: the crossover from the conventional Josephson regime, where the superconducting phase difference across the JJ is almost a classical variable, to the Coulomb blockade regime, where the transferred charge is almost a good quantum number that evolves via incoherent tunnel events. Quantum mechanically, phase difference and number of transferred Cooper pairs form a set of conjugate variables linked by a Heisenberg uncertainty relation. The intermediate regime, where neither phase nor charge are good quantum numbers, has not been investigated in depth.Specifically, we will show that the flow of Cooper pairs through dc-biased JJs can be exploited to produce useful devices in form of bright sources for non-classical light and amplifiers, whose noise temperature approaches the limits allowed by quantum mechanics. After demonstrating the principles at work in the microwave range, we will adapt some of these devices to the THz range. In a complementary effort, we will investigate the quantum to classical transition of JJs. We will use a particular feature of these devices which allows monitoring both: non-classical photon states and charge current noise. This project brings together the profound expertise of two experimental and one theory group.