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. Graphene is a textbook example of a gapless particle-hole symmetric system, where topological phases can be understood by studying ways to open a gap by breaking symmetries [1]. An important example is the intrinsic Kane-Mele spin-orbit gap of graphene, which renders graphene a topological insulator in a quantum-spin Hall phase [2].           

Here, we show that the Kane-Mele spin-orbit gap leads to a lifting of the spin-valley degeneracy in bilayer graphene quantum dots [3], resulting in Kramer's doublets with different ordering for electron and hole states preserving particle-hole symmetry. We observe the creation of single electron-hole pairs with opposite quantum numbers and use the electron-hole symmetry to achieve a protected spin-valley blockade in electron-hole double quantum dots. The latter will allow spin-to-charge conversion and valley-to-charge conversion, which is essential for the operation of spin and valley qubits.

 

[1]  F. Haldane, Phys. Rev. Lett. 61, 2015 (1988)

[2]  C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801 (2005)

[3]  L. Banszerus et al., Nat. Commun 12, 5250 (2021)