Transformation of Quantum States in Quantum Computation
Quantum computation is based on transformation of quantum states. Quantum bits are two-level quantum systems, and as the simplest elementary building blocks for a quantum computer, they provide a convenient labeling for pairs of states and their physical realizations. Closed quantum systems evolve unitarily as determined by their Hamiltonians, but to perform quantum computation one must be able to control the Hamiltonian to effect an arbitrary selection from a universal family of unitary transformations.
Linli Xu, Wenya Tian and Elwin Mao
J. Lu "Transformation of Quantum States in Quantum Computation", Applied Mechanics and Materials, Vols. 80-81, pp. 276-278, 2011