Strong coupling QED of mesoscopic point contacts

量子电动力学(QED)描述了我们周围所有的低能物理，捕捉了物质(电子)和电磁场(光子)之间的相互作用。在真空中，由于精细结构常数很小，电子和光子之间的耦合很弱，然而腔量子电动力学(CQED)以及近年来的电路量子电动力学(CQED)已经导致了对量子态的前所未有的控制。在裸露的电子-光子相互作用仍然很弱的情况下，通过实现高精细腔来实现相干耦合，从而使“物质”和“场”混合。目前，原子和凝聚态物理面临的两个主要挑战是：一方面在材料中定制强物质-光耦合，另一方面探索强关联多体系统。介观固态电路由于其独特的性质允许同时攻击强光物质相互作用和多体关联：电荷-电荷关联是自然存在的，并且可以通过调节通过相干导体的传输来调节，而光-电荷相互作用可以通过电路制造达到有效的精细结构常数的数量级。本项目的主要目的是研究量子电动力学的一个新领域，即同时存在强电子-电子和电子-光子相互作用的非平衡量子电路。为了达到这个具有挑战性的目标，我们将把重点放在结合多体复杂性和概念简单性的电路上，即量子点接触(QPC)和超导点接触(SPC)。这两个系统都提供了在单通道水平上对电子散射进行精细的实验控制，使得从众所周知的隧穿机制到理想传输通道的强关联极限的散射幅度能够被调谐，从而实现了费米液体关联和SPC超导QPC。我们遵循两个方向：第一，点接触耦合到优化的射频(RF)电路，从而能够有效地检测电子散射时发出的光子；第二，使用高阻抗RF电路来实现强QED耦合，从而引起多光子发射，从而产生强烈的反作用效应。理论建模和定量描述是这些实现的组成部分。这一法德合作项目结合了两个实验小组和两个理论小组的互补专业知识。它在这一两国合作中提供了一个独特的研究环境，以解决一个非常及时和极具挑战性的相关研究课题，也超出了介观固态社区的范畴。

Quantum electrodynamics (QED), describing all the low energy physics around us, captures the interaction of matter (electrons) and the electromagnetic field (photons). In vacuum the coupling between electrons and photons is weak due to the smallness of the fine-structure constant, yet cavity Quantum electrodynamics (CQED), and in recent years also circuit quantum electrodynamics (cQED), have led to an unprecedented level of control over quantum states. While the bare electron-photon interaction remains weak, coherent coupling has been achieved by implementing high finesse cavities such that the ‘matter’ and the ‘field’ hybridize. Currently, two main challenges in atomic and condensed matter physics are addressed: Tailoring strong matter-light coupling ‘in the material’ on the one hand and exploring strongly correlated many-body systems on the other hand. Mesoscopic solid state circuits allow to attack simultaneously both strong light-matter interactions and many-body correlations due to their unique properties: Charge-charge correlations naturally exist and can be tuned, e.g. by tuning the transmission through coherent conductors, and light-charge interactions can, by circuit fabrication, reach effective fine structure constants on the order of 1. The main goal of this project is to investigate a novel domain of QED, namely out-of-equilibrium quantum circuits in presence of both strong electron-electron and electron-photon interactions. To attack this challenging goal, we will focus on circuits that combine many-body complexity with conceptual simplicity, namely, quantum point contacts (QPCs) and superconducting point contacts (SPCs). Both systems offer an exquisite experimental control over the electronic scattering at the single channel level which enables the tuning of the scattering amplitudes from the well-understood tunneling regime to the strongly correlated limit of a perfectly transmitting channel, thus QPCs implementing Fermi liquid correlations and SPC superconducting ones. We follow two directions: In one, the point contacts are coupled to optimized radiofrequency (RF) circuits enabling an efficient detection of photons emitted upon electron scattering; in two, high impedance RF circuits are employed to achieve strong QED coupling giving rise to multi-photon emission and thus strong back-action effects. Theoretical modelling and quantitative descriptions accompany are integral part of these realizations. This French-German project combines the complementary expertise of two experimental and two theory groups. It offers a unique research environment within this binational cooperation to attack a very timely and highly challenging research topic of relevance also beyond the mesoscopic solid state community.