  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.
