  Non-adiabatic quantum effects, often experimentally observed in
semiconductors nano-devices such as single-electron pumps operating at high
frequencies, can result in undesirable and uncontrollable behaviour. However,
when combined with the valley degree of freedom inherent to silicon, these
unfavourable effects may be leveraged for quantum information processing
schemes. By using an explicit time evolution of the Schrodinger equation, we
study numerically non-adiabatic transitions between the two lowest valley
states of an electron in a quantum dot formed in a SiGe/Si heterostructure. The
presence of a single atomic layer step at the top SiGe/Si interface opens an
anti-crossing in the electronic spectrum as the centre of the quantum dot is
varied. We show that an electric field applied perpendicularly to the interface
allows tuning of the anti-crossing energy gap. As a result, by moving the
electron through this anti-crossing, and by electrically varying the energy
gap, it is possible to electrically control the probabilities of the two lowest
valley states.
