The creation and manipulation of multipartite entangled states is importantfor advancements in quantum computation and communication, and for testing ourfundamental understanding of quantum mechanics and precision measurements.Multipartite entanglement has been achieved by use of various forms of quantumbits (qubits), such as trapped ions, photons, and atoms passing throughmicrowave cavities. Quantum systems based on superconducting circuits have beenused to control pair-wise interactions of qubits, either directly, through aquantum bus, or via controllable coupling. Here, we describe the firstdemonstration of coherent interactions of three directly coupledsuperconducting quantum systems, two phase qubits and a resonant cavity. Weintroduce a simple Bloch-sphere-like representation to help one visualize theunitary evolution of this tripartite system as it shares a single microwavephoton. With careful control and timing of the initial conditions, this leadsto a protocol for creating a rich variety of entangled states. Experimentally,we provide evidence for the deterministic evolution from a simple productstate, through a tripartite W-state, into a bipartite Bell-state. Theseexperiments are another step towards deterministically generating multipartiteentanglement in superconducting systems with more than two qubits.
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