Optimal generation of single-qubit operation from an always-on   interaction by algebraic decoupling
  We present a direct algebraic decoupling approach to generate arbitrary single-qubit operations in the presence of a constant interaction by applying local control signals. To overcome the difficulty of undesirable entanglement generated by the untunable interaction, we derive local control fields that are designed to both drive the qubit systems back to unentangled states at the end of the time interval over which the desired single-qubit operation is completed. This approach is seen to be particularly relevant for the physical implementation of solid-state quantum computation and for the design of low-power pulses in NMR. 
Jun Zhang and K. Birgitta Whaley