Christian Volk

Quantum dots in bilayer graphene

Graphene quantum dots (QDs) are attractive building blocks for quantum circuits with potential applications in quantum information. Its weak spin-orbit and hyperfine coupling makes graphene an interesting host material for spin and spin-valley qubits. Bilayer graphene (BLG) is of particular interest as it offers the chance to open a gate-voltage-controllable band gap allowing the confinement of electrons in QDs. The demonstration of gate-defined QDs in ultra-clean heterostructures of BLG encapsulated in hexagonal Boron Nitride (hBN) in combination with a graphite back gate [1-3] has stimulated a cascade of follow-up experiments.

In this talk, I will present recent results of our work on single and double quantum dots (DQDs) in BLG. The finite Berry curvature results in an orbital magnetic moment pointing perpendicular to the BLG plane with opposite sign for the two valleys (K, K’) and for electrons and holes. This magnetic moment couples to magnetic fields allowing control over the valley degree of freedom. We investigate the intrinsic Kane-Mele spin-orbit coupling which leads to a lifting of the spin-valley degeneracy [4]. We show that these Kramer’s doublets have different ordering for electron and hole states preserving particle-hole symmetry. We describe transport through an electron-hole DQD by creation and annihilation of single electron-hole pairs with opposite quantum numbers. We use the electron-hole symmetry to achieve a protected spin-valley blockade which will allow spin-to-charge and valley-to-charge conversion, which is essential for the operation of spin and valley qubits [5].

Furthermore, I will present spin and valley relaxation times (T1) of single-electron states in BLG QDs. Using pulsed-gate spectroscopy, we extract spin relaxation times exceeding 200 μs at finite magnetic fields [6] and valley relaxation times of around 7 µs, both sufficiently long for coherent manipulation. Finally, I will show our recent results on coherent charge oscillations in a BLG DQD [7]. Our work paves the way for the implementation of spin and valley-qubits in graphene.

[1] L. Banszerus et al., Nano Lett. 18, 4785 (2018).
[2] M. Eich et al., Physical Review X, 8, 3, 031023 (2018).
[3] E. Icking et al., Adv. Electron. Mater. 8, 2200510 (2022).
[4] L. Banszerus, S. Möller et al., Nat. Commun. 12, 5250 (2021).
[5] L. Banszerus, S. Möller et al., Nature 618, 51 (2023).
[6] L. Banszerus, K. Hecker et al., Nat. Commun. 13, 3637 (2022).
[7] K. Hecker, L. Banszerus et al., arXiv: 2303.10119 (2023).