  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.
