Special issue on quantum computing with superconducting qubits

Ten years ago the first superconducting qubit was demonstrated experimentally [1]. By now quantum computing with superconducting qubits has become a subject of intensive experimental and theoretical research in dozens of groups around the world. The idea of this Special Issue of the journal is to show the status of experimental research in this area after the first decade of work. Most of the best experimental groups working with superconducting qubits (with a few regrettable exceptions) are represented in this Special Issue. We hope that it gives a useful snapshot in time, demonstrating the main experimental achievements and directions of research in superconducting quantum computing. There are many possible physical realizations of qubits [2,3]. Among the candidate systems, the obvious advantages of quantum computing with Josephson junctions are the efficient control of a quantum circuit with voltage/current/microwave pulses and use of a well-developed technology suitable for large scale integration. The fast experimental progress in experiments with superconducting qubits in the last decade confirms the importance of these advantages. Superconducting qubits come in a variety of types, which are often separated into three categories: charge, flux, and phase qubits (though not all groups use this terminology). Single Cooper pair charge of an island carries the quantum information in the charge qubit (e.g., [1,4–16]), while the superconducting phase is the relevant degree of freedom for flux and phase qubits, which differ by the logic state encoding: two quantum levels in different wells of a potential profile are used in the flux qubit (e.g., [17–32]), and two levels in the same well are used in the phase qubit (e.g., [33–42]).