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

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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.