Tripartite interactions between two phase qubits and a resonant cavity | NIST (original) (raw)

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. With the development of quantum information science 1 , entanglement of multiparticle systems has become a resource for a new information technology. In particular, three-particle or tripartite entanglement allows for teleportation 2 , secret sharing 4 and dense coding 20 , with connections to cosmology 21. Over the past decade, the development of exquisite control over quantum systems has led to various demonstrations of tripartite entanglement 8-10. Genuine tripartite entanglement is delineated by two inequivalent classes of states 22 : Greenberger-Horne-Zeilinger and W, where the W state involves only a single photon shared amongst three systems. Using multipartite entanglement in a solid-state-qubit system has only recently received theoretical attention 23-25. Thus far in superconducting systems, bipartite entanglement has been verified by two-qubit quantum state tomography 13 and used to carry out a quantum algorithm 15. Spectroscopic evidence for three-particle entanglement was observed for two current-biased phase qubits coupled to a lumped element consisting of an inductor-capacitor circuit and a cavity as well as for transmon qubits 17,18. In the experiments described below, we first verified the spectroscopic signature of three coupled systems. Next, we demonstrated coherent interactions. Frequency detuning of the third system was used to verify the proper change in the time evolution of two versus three coupled systems. Finally, we describe a free-evolution process as a means of deterministically preparing arbitrary single-photon tripartite entangled states and a corresponding visualization LETTERS NATURE PHYSICS