Single electron occupation in a bilayer graphene double quantum dot

Graphene quantum dots (QDs) are an attractive platform for hosting spin qubits since the low nuclear spin densities and weak spin-orbit interaction in graphene promise long spin coherence times. Physically etched graphene QDs have been studied for about a decade. However, the influence of disorder, in particular the edge disorder prevented a precise control of the number of confined charge carriers.
We study an electrostatically gated bilayer graphene double QD (DQD) embedded in hexagonal boron nitride. Finger gates modulate the potential landscape along a one-dimensional channel confined by split gates and enable the formation of a DQD.
We control the number of charge carriers on each of the QDs from the few-electron regime down to the last. Tunnel coupling and capacitive interdot coupling increase with the QD occupation, leading eventually to the formation of a single QD. Furthermore, we can form a DQD in the low electron/hole occupation. Finite bias spectroscopy reveals the excited state spectrum of the first electrons in the DQD.
The precise control of the electron occupation is a key requirement for making such a device a suitable building block for spin qubit devices. The measured interdot tunnel coupling on the order of 2 GHz is in a regime compatible with spin qubits.