  In analogy to the Coulomb and the Pauli spin blockade, based on the
electrostatic repulsion and the Pauli exclusion principle respectively, the
concept of valley blockade in Silicon nanostructures is explored. The valley
parity operator is defined. Valley blockade is determined by the parity
conservation of valley composition eigenvectors in quantum transport. A Silicon
quantum changeover switch based on a triple of donor quantum dots capable to
separate electrons having opposite valley parity by virtue of the valley parity
conservation is proposed. The quantum changeover switch represents a novel kind
of hybrid quantum based classical logic device.
