Superconducting Qubits

Abstract

Superconducting qubits were initially developed with the goal of realizing a superposition of macroscopically distinct quantum states by exploiting superconducting circuits. This basic idea resulted from the quantum mechanical description of the Josephson junction, the key element for producing superconducting qubits. Because the phase across a Josephson junction and its charge are canonical conjugates, there are two alternative realizations of superconducting qubits. The first one is based on the charge degree of freedom, termed charge qubit. The second utilizes the phase (or flux) degree of freedom and correspondingly are called phase (flux) qubits. Nowadays, the most robust superconducting qubit is the transmon. In practical applications, quantum state initialization and manipulations are heavily restricted by the quantum coherence of the qubit itself and of the qubit‐based systems. The main source of decoherence is interactions with the environment. Their relatively large values result from the macroscopic size of the quantum bits. Still, their circuit architecture enables the implementation of different types of coupling schemes between superconducting qubits and qubit‐resonator systems. The handling of superconducting quantum structures requires special experimental methods, including qubit fabrication, cooling to milliKelvin temperatures, experimental characterization, and readout. Concerning applications, superconducting qubits are promising candidates for both quantum simulators and universal quantum computing. This article covers a description of basic types of superconducting qubits and gives a general description of their use that includes dissipation and decoherence, coupling schemes, experimental realization, and basic measurement techniques. Finally, their use as building blocks for the realization of quantum computation is discussed.