A scheme is proposed for generating Greenberger-Horne-Zeilinger (GHZ) entangled states of multiple superconducting quantum-interference device (SQUID) qubits by the use of a microwave cavity. The scheme operates essentially by creating a single photon through an auxiliary SQUID built in the cavity and performing a joint multiqubit phase shift with assistance of the cavity photon. It is shown that entanglement can be generated using this method, deterministic and independent of the number of SQUID qubits. In addition, we show that the present method can be applied to preparing many atoms in a GHZ entangled state, with tolerance to energy relaxation during the operation.
We report on the observation of quantum interference of a collective single spin excitation with a spin ensemble of ${N}_{\mathrm{a}}\ensuremath{\approx}{10}^{5}$ atoms. Detection of a single photon scattered from the atoms creates the single spin excitation, a Fock state embedded in the collective spin of the ensemble. The state of the atomic ensemble is then detected by a quantum nondemolition measurement of the collective spin. A macroscopic difference of the order of Na in the marginal distribution of the collective spin state arises from the interference between the single excited spin and Na atoms. These hybrid discrete-continuous manipulation and measurement procedures of collective spin states in an atomic ensemble pave the road towards generation of even more exotic ensemble states for quantum information processing, precision measurements, and communication.