A quantum retrograde canon: Complete population transfer in $n^{2}$-state systems
We present a novel approach for analytically reducing a family of time-dependent multi-state quantum control problems to two-state systems. The presented method translates between $SU(2)XSU(2)$ controlled $n^{2}$-state systems and two-state systems, such that the former undergo complete population transfer (CPT) if and only if the latter reach specific states. For even n, the method translates any two-state CPT scheme to CPT schemes in $n^{2}$-state systems. In particular, facilitating CPT in a four-state system via real time-dependent nearest-neighbors couplings is reduced to facilitating CPT in a two-level system. Furthermore, we show that the method can be used for operator control, and provide conditions for producing several universal gates for quantum computation as an example. In addition, we indicate a basis for utilizing the method in optimal control problems.