Quantum superposition of charge states on capacitively coupled superconducting islands

We investigate the ground state properties of a system containing two superconducting islands coupled capacitively by a wire. The ground state is a macroscopic superposition of charge states, even though the islands cannot exchange charge carriers. The ground state of the system is probed by measuring the switching current of a Bloch transistor containing one of the islands. Calculations based on superpositions of charge states on both islands show good agreement with the experiments. The ability to couple quantum mechanical charge fluctuations in two neighboring devices using a wire is relevant for realizing quantum computation with this kind of circuit.