Effects of applied fields on quantum coupled double-well systems

Effects of time-dependent applied fields on quantum coupled double-well (DW) systems with Razavy's hyperbolic potential have been studied. By solving the Schr\"{o}dinger equation for the DW system, we have obtained time-dependent occupation probabilities of the eigenstates, from which expectation values of positions $x_1$ and $x_2$ of particles ($<x_1+x_2 >$), the correlation ($\Gamma(t)$) and the concurrence ($C(t)$) expressing a degree of the entanglement of the coupled DW system, are obtained. Analytical expressions for $<x_1+x_2 >$, $\Gamma(t)$ and $C(t)$ are derived with the use of the rotating-wave approximation (RWA) for sinusoidal fields. Model calculations have indicated that $<x_1+x_2 >$, $\Gamma(t)$ and $C(t)$ show very complicated time dependences. Results of the RWA are in good agreement with exact ones evaluated by numerical methods for cases of weak couplings and small applied fields in the near-resonant condition. Applications of our method to step fields are also studied.