  The decay of spin-valley states is studied in a suspended carbon nanotube
double quantum dot via leakage current in Pauli blockade and via dephasing and
decoherence of a qubit. From the magnetic field dependence of the leakage
current, hyperfine and spin-orbit contributions to relaxation from blocked to
unblocked states are identified and explained quantitatively by means of a
simple model. The observed qubit dephasing rate is consistent with the
hyperfine coupling strength extracted from this model and inconsistent with
dephasing from charge noise. However, the qubit coherence time, although longer
than previously achieved, is probably still limited by charge noise in the
device.
