Equivalent circuits for small signal performance of spin ½ particles

A quantum system composed of a large number of non‐interacting spin ½ particles, biased by a large constant magnetic field along the z ‐axis and excited by a small circularly polarized electromagnetic wave in the xy ‐plane is considered. Its small signal reflectance about the equilibrium state resulting from the biasing field and a pumping activity is calculated in the complex frequency plane. It is suggested that the equilibrium state can be locally passive or active depending on the pumping sign and level. Equivalent circuits containing imaginary resistors , i.e. frequency independent reactances, are presented for both passive and active cases.