title: Tomography and entanglement in coupled Josephson junction qubits.
abstract: We provide an alternative interpretation of experimental results that were represented as demonstrating entanglement between two macroscopic quantum Josephson oscillators. We model the experimental system using the well-established classical equivalent circuit of a resistively and capacitively shunted Josephson junction. Simulation results are used to generate the corresponding density matrix which shows features quite similar to the previously published matrix generated from experimental data. Since our data are generated by a classical model, we therefore submit that state tomography cannot be used to determine absolutely whether or not quantum entanglement has taken place. Analytical arguments are given for why the classical analysis provides an adequate explanation of the experimental results.
authors.affiliation: Department of Applied Science, University of California, Davis, California 95616, USA.
authors.forename: Niels
authors.initials: N
authors.lastname: Grønbech-Jensen
authors.forename: Jeffrey E
authors.initials: JE
authors.lastname: Marchese
authors.forename: Matteo
authors.initials: M
authors.lastname: Cirillo
authors.forename: James A
authors.initials: JA
authors.lastname: Blackburn