  We expand on previous work that treats relaxation physics of low-lying
excited states in ideal, single electron, silicon quantum dots in the context
of quantum computing. These states are of three types: orbital, valley, and
spin. The relaxation times depend sensitively on system parameters such as the
dot size and the external magnetic field. Generally, however, orbital
relaxation times are short in strained silicon (from a tenth of a microsecond
to picoseconds), spin relaxation times are long (microseconds to greater than
seconds), while valley relaxation times are expected to lie in between. The
focus is on relaxation due to emission or absorption of phonons, but for spin
relaxation we also consider competing mechanisms such as charge noise. Where
appropriate, comparison is made to reference systems such as quantum dots in
III-V materials and silicon donor states. The phonon bottleneck effect is shown
to be rather small in the silicon dots of interest. We compare the theoretical
predictions to some recent spin relaxation experiments and comment on the
possible effects of non-ideal dots.
