Tripartite interactions between two phase qubits and a resonant cavity

Multipartite entanglement is essential for quantum computation(1) and communication(2-4), and for fundamental tests of quantum mechanics(5) and precision measurements(6). It has been achieved with various forms of quantum bits (qubits), such as trapped ions(7,8), photons(9) and atoms passing through microwave cavities(10). Quantum systems based on superconducting circuits, which are potentially more scalable, have been used to control pair-wise interactions of qubits(11-16) and spectroscopic evidence for three-particle entanglement was observed(17,18). Here, we report the demonstration of coherent interactions in the time domain for three directly coupled superconducting quantum systems, two phase qubits and one resonant cavity. We provide evidence for the deterministic evolution from a simple product state, through a tripartite W state, into a (bipartite) Bell state. The cavity can be thought of as a multiphoton register or an entanglement bus, and arbitrary preparation of multiphoton states in this cavity using one of the qubits(19) and subsequent interactions for entanglement distribution should allow for the deterministic creation of another class of entanglement, a Greenberger-Horne-Zeilinger state.