Quantum Down Conversion and Multipartite Entanglement via a Mesoscopic   SQUID Ring
  In this paper we study, by analogy with quantum optics, the superconducting quantum interference device (SQUID) ring mediated quantum mechanical interaction of an input electromagnetic field oscillator mode with two or more output oscillator modes at subintegers of the input frequency. We show that through the nonlinearity of the SQUID ring multiphoton downconversion can take place between the input and output modes with the resultant output photons being created in an entangled state. We also demonstrate that the degree of this entanglement can be adjusted by means of a static magnetic flux which controls the strength of the interaction between these modes via the SQUID ring. 
P. B. Stiffell, M. J. Everitt, T. D. Clark, C. J. Harland, J. F. Ralph