Particle-hole symmetry protects spin-valley blockade in graphene quantum dots

Particle-hole symmetry plays an important role for the characterization of topological phases in solid-state systems. It is found, for example, in free-fermion systems at half filling, and it is closely related to the notion of antiparticles in relativistic field theories. In the low energy limit, graphene is a prime example of a gapless particle-hole symmetric system described by an effective Dirac equation, where topological phases can be understood by studying ways to open a gap by preserving (or breaking) symmetries. An important example is the intrinsic Kane-Mele spin-orbit gap of graphene, which leads to a lifting of the spin-valley degeneracy and renders graphene a topological insulator in a quantum spin Hall phase, while preserving particle-hole symmetry. Here, we show that bilayer graphene allows realizing electron-hole double quantum-dots that exhibit nearly perfect particle-hole symmetry, where transport occurs via the creation and annihilation of single electron-hole pairs with opposite quantum numbers. Moreover, we show that this particle-hole symmetry results in a protected single-particle spin-valley blockade. The latter will allow robust spin-to-charge conversion and valley-to-charge conversion, which is essential for the operation of spin and valley qubits.