From cavity to circuit quantum electrodynamics

Neereja Sundaresan, Devin Underwood, Will Shanks +2 more

Dombi, András

Patrice Bertet, Florian R. Ong, M. Boissonneault +6 more

One of the most studied model systems in quantum optics is a two-level atom strongly coupled to a single mode of the electromagnetic field stored in a cavity, a research field named cavity quantum electrodynamics or CQED. CQED has recently received renewed attention due to its implementation with superconducting artificial atoms and coplanar resonators in the so-called circuit quantum electrodynamics (cQED) architecture. In cQED, the couplings can be much stronger than in CQED due to the design flexibility of superconducting circuits and to the enhanced field confinement in one-dimensional cavities. This enabled the realization of fundamental quantum physics and quantum information processing experiments with a degree of control comparable to that obtained in CQED. The purpose of this chapter is to investigate the situation where the resonator to which the atom is coupled is made nonlinear with a Kerr-type nonlinearity, causing its energy levels to be nonequidistant. The system is then described by a nonlinear Jaynes-Cummings Hamiltonian. This considerably enriches the physics since a pumped nonlinear resonator displays bistability, parametric amplification, and squeezing. The interplay of strong coupling and these nonlinear effects constitutes a novel model system for quantum optics that can be implemented experimentally with superconducting circuits. This chapter is organized as follows. In a first section we present the system consisting of a superconducting Kerr nonlinear resonator strongly coupled to a transmon qubit. In the second section, we describe the response of the sole nonlinear resonator to an external drive. In the third section, we show how the resonator bistability can be used to perform a high-fidelity readout of the transmon qubit. In the last section, we investigate the quantum backaction exerted by the intracavity field on the qubit.

Philip Jones

Superconducting quantum bits are one of the leading frontrunners in the race to build a solid-state quantum computer. In circuit quantum electrodynamics (cQED), electronic quantum circuits, playing the role of quantum bits, qubits, are placed into superconducting cavities with incredibly weak dissipation. The resulting system exhibits both photon and qubit characteristics, and it is possible to perform complex operations by driving the system with microwaves. Progress has been rapid, with vast improvements in cavities, qubits, and in controlling the interaction between the two. As a result, the first simple quantum algorithms to utilize this architecture have recently been implemented. \nAt the same time, there have been significant developments in improving our understanding of the heat flow in nanoelectronics. In particular the development of tunnel junctions has reached such a level, that it is now possible to control and measure the temperature of small metal islands with high precision. Such techniques have enabled the first observations of quantum-limited heat conduction through the photonic modes of a circuit. \nThis thesis introduces some of the benefits that come with the unification of these two fields. Theoretical models for experiments in a superconducting cavity are presented, with heat conduction due to individual cavity photons discussed in detail, along with a novel method to vary the lifetime of a qubit over many orders of magnitude. The realisation of these experiments would be the first steps on the road to integrating normal-metal components with cQED.

Pierre Meystre, Murray Sargent

Yasunobu Nakamura

Circuit quantum electrodynamics is a radical extension of cavity quantum electrodynamics into superconducting electrical circuits, where macroscopic artiÞ cial atoms with anharmonicity, based on the nonlinearity of Josephson junctions, are strongly coupled to harmonic oscillators.We review the basic concept and recent progress.