  Pauli blockade mechanisms -- whereby carrier transport through quantum dots
is blocked due to selection rules even when energetically allowed -- are a
direct manifestation of the Pauli exclusion principle, as well as a key
mechanism for manipulating and reading out spin qubits. Pauli spin blockade is
well established for systems such as GaAs QDs, but is to be further explored
for systems with additional degrees of freedom, such as the valley quantum
numbers in carbon-based materials or silicon. Here we report experiments on
coupled bilayer graphene double quantum dots, in which the spin and valley
states are precisely controlled, enabling the observation of the two-electron
combined blockade physics. We demonstrate that the doubly occupied single dot
switches between two different ground states with gate and magnetic-field
tuning, allowing for the switching of selection rules: with a
spin-triplet--valley-singlet ground state, valley-blockade is observed; and
with the spin-singlet--valley-triplet ground state, robust spin blockade is
shown.
