CAREER: Realizing the ultrastrong coupling regime of quantum electrodynamics using high-impedance Josephson superconducting circuits

非技术。量子电动力学（QED）是描述光和物质如何相互作用的理论。在自然系统中，这种相互作用的强度是由精细结构常数确定的，这是一个基本的自然常数。精细结构常数远小于1，因此光和物质的相互作用很弱。近年来，利用人工原子，如量子点，制备了由材料性质控制有效精细结构常数的系统。这开启了一种令人兴奋的可能性，即使精细结构常数与自然界中发生的不同——开启了物理学探索的新领域。这个项目试图从超导结中制造人造原子，在超导结中精细结构常数可以大于1。这将阐明光与物质相互作用的基本问题。同时，这些电路可以用来形成量子计算的容错量子比特。该项目将为研究生和本科生提供最先进的实验技术培训，如纳米制造、低温测量和超导量子比特的量子控制。PI还将开发一门基于电路类比的量子力学新课程，并参与面向公众的推广活动。技术。本项目的目的是在超强耦合状态下实验实现量子电动力学。超强QED是指单个原子与有效精细结构常数超过一个单位的真空量子场耦合的情况。我们的方法是将超导量子比特（人造原子）耦合到非常高阻抗的微波（场），其阻抗接近电阻量子值。如此大的阻抗可以通过在约瑟夫森隧道结阵列或高度无序的超导膜内激发微波来实现。谐振器的有效精细结构常数是一种材料特性，因为射频（RF）场的“磁”能主要是由于移动库珀对的惯性而产生的，而不是由于用磁场对真空施加压力。利用强大的超导量子比特技术，将探索与超强光-物质相互作用机制相关的新效应，如真空的自发极化、超辐射量子相变和自旋玻色子物理中的临界行为。

Non-technical. Quantum electrodynamics (QED) is the theory that describes how light and matter interact. In natural systems the strength of this interaction is fixed by the fine structure constant which is a fundamental natural constant. The fine structure constant is much less than one so that light and matter interact only weakly. Recently, using artificial atoms, such as quantum dots, systems have been fabricated where the effective fine structure constant is controlled by the material properties. This opens the exciting possibility of making the fine structure constant different than occurs in nature - opening up new regimes of physics to explore. This project seeks to create artificial atoms out of superconducting junctions where the fine structure constant can be greater than one. This will shed light on fundamental questions of light-matter interaction. At the same time these circuits can be used to form fault tolerant qubits for quantum computing. This project will provide training to graduate and undergraduate students in state-of-the-art experimental techniques such as nanofabrication, low-temperature measurements, and quantum control of superconducting qubits. The PI will also develop a novel new course on quantum mechanics based on an analogy with electrical circuits as well as participating in outreach activities to the general public. Technical. This project aims at an experimental implementation of quantum electrodynamics (QED) in the ultrastrong coupling regime. Ultra-strong QED is a situation where a single atom is coupled to a vacuum quantum field with an effective fine structure constant exceeding a unity. Our approach is to couple superconducting qubits (artificial atoms) to very high-impedance microwaves (fields), with the impedance approaching the value of resistance quantum. Such large impedances can be achieved by exciting microwaves inside either an array of Josephson tunnel junctions or a highly disordered superconducting film. The effective fine structure constant of resonators is a material property, because the "magnetic" energy of the radio-frequency (RF) field is created predominantly due to the inertia of the moving Cooper pairs rather than due to stressing the vacuum with a magnetic field. Novel effects, associated with the ultrastrong light-matter interaction regime, such as spontaneous polarization of vacuum, superradiance quantum phase transitions, and critical behavior in the spin-boson physics, will be explored using the powerful arsenal of superconducting qubit techniques.