  We study the dephasing of two-electron states in a single quantum dot in both
GaAs and Si. We investigate dephasing induced by electron-phonon coupling and
by charge noise analytically for pure orbital excitations in GaAs and Si, as
well as for pure valley excitations in Si. In GaAs, polar optical phonons give
rise to the most important contribution, leading to a typical dephasing rate of
~5.9 GHz. For Si, intervalley optical phonons lead to a typical dephasing rate
of ~140 kHz for orbital excitations and ~1.1 MHz for valley excitations. For
harmonic, disorder-free quantum dots, charge noise is highly suppressed for
both orbital and valley excitations, since neither has an appreciable dipole
moment to couple to electric field variations from charge fluctuators. However,
both anharmonicity and disorder break the symmetry of the system, which can
lead to increased dipole moments and therefore faster dephasing rates.
