We detect in real time interdot tunneling events in a weakly coupled two-electron double quantum dot in GaAs. At finite magnetic fields, we observe two characteristic tunneling times $T_d$ and $T_b$, belonging to, respectively, a direct and a blocked (spin-flip-assisted) tunneling. The latter corresponds to the lifting of a Pauli spin blockade, and the tunneling times ratio $\eta =T_b/T_d$ characterizes the blockade efficiency. We find pronounced changes in the behavior of $\eta$ upon increasing the magnetic field, with $\eta$ increasing, saturating, and increasing again. We explain this behavior as due to the crossover of the dominant blockade-lifting mechanism from the hyperfine to spin-orbit interactions and due to a change in the contribution of the charge decoherence.

• Received 24 February 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.206802