  With silicon being the go-to material for spin qubits, and motivated by the
demand of a scalable quantum computer architecture for fast and reliable
quantum information transfer on-chip, we study coherent electron transport in a
silicon double quantum dot. We first examine the valley-orbital dynamics in a
silicon double dot, and discuss how to properly measure the tunnel couplings as
well as the valley phase difference between two quantum dots. We then focus on
possible phase and spin flip errors during spin transport across a silicon
double dot. In particular, we clarify correction on the effective $g$-factor
for the electron spin from the double dot confinement potential, and quantify
the resulting phase error. We then study spin fidelity loss due to spin-valley
mixing, which is a unique feature of silicon quantum dots. We show that a small
phase correction between valleys can cause a significant coherence loss. We
also investigate spin flip errors caused by either an external inhomogeneous
magnetic field or the intrinsic spin-orbit coupling. We show that the presence
of valleys makes it possible to have much broader (in terms of interdot
detuning) level anti-crossings compared to typical anti-crossings in, for
example, a GaAs double dot, and such broad anti-crossings lead to amplification
of spin flip errors. Lastly, we design a pulse sequence to suppress various
possible spin flip errors by taking advantage of the multiple level
anti-crossings in a silicon double dot and employing Landau-Zener transitions.
