Quantum oscillations in two coupled charge qubits

A practical quantum computer(1), if built, would consist of a set of coupled two-level quantum systems (qubits). Among the variety of qubits implemented(2), solid-state qubits are of particular interest because of their potential suitability for integrated devices. A variety of qubits based on Josephson junctions(3,4) have been implemented(5-8); these exploit the coherence of Cooper-pair tunnelling in the superconducting state(5-10). Despite apparent progress in the implementation of individual solid-state qubits, there have been no experimental reports of multiple qubit gates-a basic requirement for building a real quantum computer. Here we demonstrate a Josephson circuit consisting of two coupled charge qubits. Using a pulse technique, we coherently mix quantum states and observe quantum oscillations, the spectrum of which reflects interaction between the qubits. Our results demonstrate the feasibility of coupling multiple solid-state qubits, and indicate the existence of entangled two-qubit states.