Direct measurement of electron intervalley relaxation in a Si/SiGe quantum dot

The presence of nondegenerate valley states in silicon can drastically affect electron dynamics in silicon-based heterostructures, leading to electron spin relaxation and spin-valley coupling. In the context of solid-state spin qubits, it is important to understand the interplay between spin and valley degrees of freedom to avoid or alleviate these decoherence mechanisms. Here we report the observation of relaxation from the excited valley state to the ground state in a Si/Si-Ge quantum dot at zero magnetic field. Valley-state readout is aided by a valley-dependent tunneling effect, which we attribute to valley-orbit coupling. We find a long intervalley relaxation time of 12.0 +/- 0.3 ms, a value that is unmodified when a magnetic field is applied. Furthermore, we compare our findings with the spin relaxation time and find that the spin-valley "hot spot" relaxation is roughly 4 times slower than intervalley relaxation, consistent with established theoretical predictions. The precision of this technique, adapted from electron spin readout via energy-dependent tunneling, is an improvement over indirect valley relaxation measurements and could be a useful probe of valley physics in spin-qubit and valley-qubit implementations.