We experimentally confirm the functionality of a coupling element for flux-based superconducting qubits, with a coupling strength J whose sign and magnitude can be tuned {\it in situ}. To measure the effective J, the groundstate of a coupled two-qubit system has been mapped as a function of the local magnetic fields applied to each qubit. The state of the system is determined by directly reading out the individual qubits while tunneling is suppressed. These measurements demonstrate that J can be tuned from antiferromagnetic through zero to ferromagnetic.
@article{arxiv.cond-mat/0608253,
title = {Sign- and magnitude-tunable coupler for superconducting flux qubits},
author = {R. Harris and A. J. Berkley and M. W. Johnson and P. Bunyk and S. Govorkov and M. C. Thom and S. Uchaikin and A. B. Wilson and J. Chung and E. Holtham and J. D. Biamonte and A. Yu. Smirnov and M. H. S. Amin and Alec Maassen van den Brink},
journal= {arXiv preprint arXiv:cond-mat/0608253},
year = {2011}
}