Entanglement gauge and the non-Abelian geometric phase with two photonic   qubits
  We introduce the entanglement gauge describing the combined effects of local operations and nonlocal unitary transformations on bipartite quantum systems. The entanglement gauge exploits the invariance of nonlocal properties for bipartite systems under local (gauge) transformations. This new formalism yields observable effects arising from the gauge geometry of the bipartite system. In particular, we propose a non-Abelian gauge theory realized via two separated spatial modes of the quantized electromagnetic field manipulated by linear optics. In this linear optical realization, a bi-partite state of two separated spatial modes can acquire a non-Abelian geometric phase. 
Karl-Peter Marzlin, Stephen D. Bartlett, and Barry C. Sanders