Fully quantum-mechanical model of a SQUID ring coupled to an electromagnetic field

A quantum system comprising of a monochromatic electromagnetic field coupled to a superconducting quantum interference device (SQUID) ring with sinusoidal nonlinearity is studied. A magnetostatic flux ${\Phi }_x$ is also threading the SQUID ring, and is used to control the coupling between the two systems. It is shown that for special values of ${\Phi }_x$ the system is strongly coupled. The time evolution of the system is studied. It is shown that exchange of energy takes place between the two modes and that the system becomes entangled. A second quasiclassical model that treats the electromagnetic field classically is also studied. A comparison between the fully quantum-mechanical model with the electromagnetic field initially in a coherent state and the quasiclassical model is made.