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

Particle-hole symmetry plays an important role in the characterization of topological phases in solid-state systems(1). 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(2). In the low-energy limit, graphene is a prime example of a gapless particle-hole symmetric system described by an effective Dirac equation(3,4) in which topological phases can be understood by studying ways to open a gap by preserving (or breaking) symmetries(5,6). 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(7) while preserving particle-hole symmetry. Here we show that bilayer graphene allows the realization of electron-hole double quantum dots that exhibit near-perfect particle-hole symmetry, in which transport occurs via the creation and annihilation of single electron-hole pairs with opposite quantum numbers. Moreover, we show that particle-hole symmetric spin and valley textures lead to a protected single-particle spin-valley blockade. The latter will allow robust spin-to-charge and valley-to-charge conversion, which are essential for the operation of spin and valley qubits.

Automated summary

Particle-hole symmetry plays an important role in characterizing topological phases in solid-state systems. This study reveals that bilayer graphene exhibits nearly perfect particle-hole symmetry and has a protected single-particle spin-valley blockade, which is essential for the operation of spin and valley qubits.