  The valley-orbit coupling in a few-electron Si quantum dot is expected to be
a function of its occupation number N. We study the spectrum of multivalley Si
quantum dots for 2 <= N <= 4, showing that, counterintuitively,
electron-electron interaction effects on the valley-orbit coupling are
negligible. For N=2 they are suppressed by valley interference, for N=3 they
vanish due to spinor overlaps, and for N = 4 they cancel between different
pairs of electrons. To corroborate our theoretical findings, we examine the
experimental energy spectrum of a few-electron metal-oxide-semiconductor
quantum dot. The measured spin-valley state filling sequence in a magnetic
field reveals that the valley-orbit coupling is definitively unaffected by the
occupation number.
