  The presence of valley states is a significant obstacle to realizing quantum
information technologies in Silicon quantum dots, as leakage into alternate
valley states can introduce errors into the computation. We use a perturbative
analytical approach to study the dynamics of exchange-coupled quantum dots with
valley degrees of freedom. We show that if the valley splitting is large and
electrons are not properly initialized to valley eigenstates, then time
evolution of the system will lead to spin-valley entanglement. Spin-valley
entanglement will also occur if the valley splitting is small and electrons are
not initialized to the same valley state. Additionally, we show that for small
valley splitting, spin-valley entanglement does not affect measurement
probabilities of two-qubit systems; however, systems with more qubits will be
affected. This means that two-qubit gate fidelities measured in two-qubit
systems may miss the effects of valley degrees of freedom. Our work shows how
the existence of valleys may adversely affect multiqubit fidelities even when
the system temperature is very low.
