Generating and detecting multi-qubit GHz states in circuit QED

This is one of several projects which have as a common theme the measurement of qubits and resonators states for the purpose of developing new building blocks and protocols for quantum information processing.

The methodology involves taking a non-equilibrium open system approach to modeling and employing large scale computer simulations. Here we proposed a method for the preparation and entanglement detection of multi-qubit GHZ states in circuit quantum electrodynamics. Using quantum trajectory simulations appropriate for the situation of a weak continuous measurement, we showed that the joint dispersive readout of several qubits can be utilized for the probabilistic production of high-fidelity GHZ states. When employing a nonlinear filter on the recorded homodyne signal, the selected states are found to exhibit values of the Bell-Mermin operator exceeding 2 under realistic conditions. In order to motivate an experimental use of this method, we showed that with the use of dispersive readout it is in principle possible to demonstrate a violation of the Mermin bound.The figure shows the time evolution of several individual simulated quantum trajectories during the joint dispersive measurement.

L. S. Bishop, L. Tornberg, D. Price, E. Ginossar, A. Nunnenkamp, A. A. Houck, J. M. Gambetta, J. Koch, G. Johansson, S. M. Girvin, and R. J. Schoelkopf, New J. Phys. 11, 073040 (2009)

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