Defying the fine structure constant: single Cooper pair circuit free of charge offsets

Abstract

The promise of single Cooper pair quantum circuits based on tunnel junctions for metrology and quantum information applications is severely limited by the influence of "offset" charges -- random, slowly drifting microscopic charges inherent to many solid-state systems. A remedy is to shunt the junction with a sufficiently large inductance. However, the small value of the fine structure constant imposes a fundamental incompatibility between shunting with a wire-wound inductor and providing correct electromagnetic environment for single Cooper pair effects. By employing the Josephson kinetic inductance of a series array of large capacitance tunnel junctions, we have solved this conundrum and realized a new superconducting artificial atom. Its energy spectrum manifests the anharmonicity associated with single Cooper pair effects combined with total insensitivity to offset charges.