Detecting Dual Superconductivity in the Ground State of Gauge Theories -   II
  A monopole creation operator is constructed: its vacuum expectation value is an order parameter for dual superconductivity in that, if different from zero it signals spontaneous breaking of the U(1) symmetry corresponding to monopole charge conservation. The operator is tested on compact U(1) gauge theory on lattice. For SU(2) gauge theory it clearly demonstrates that confinement is produced by dual superconductivity. 
L. Del Debbio A. Di Giacomo G. Paffuti P. Pieri