Introduction to Superconducting Quantum Circuits

Standard textbooks on quantum mechanics typically illustrate the theory using examples from the microscopic world, such as atoms, electrons or molecules. At this scale, quantum effects are striking and easily noticeable. At the macroscopic level, quantum mechanics seems however often counter-intuitive. Features like state superposition and entanglement lead to well-known logical paradoxes, challenging our understanding of what we call ‘reality’. Controlling quantum features in a macroscopic physical object could open the way for building a new generation of quantum machines with tremendous computational power. Superconducting electrical circuits are an example of such a macroscopic quantum system. As of today, the cutting-edge level of control exhibited by these circuits has led them to be considered as one of the foremost technologies for physically implementing quantum computers. Moreover, it is possible to make hybrid systems in which the quantum variables of an electrical circuit are coupled to various microscopic degrees of freedom, thereby demonstrating that these circuits constitute a general interface to the quantum world. The purpose of this chapter is to provide an introduction to superconducting quantum circuits, elucidating how such systems can exhibit quantum behavior and how they can be controlled to serve as a building block of quantum processors.