Tunable Valley Splitting and Bipolar Operation in Graphene Quantum Dots

Quantum states in graphene are 2-fold degenerate in spins, and 2-fold in valleys. Both degrees of freedom can be utilized for qubit preparations. In our bilayer graphene quantum dots, we demonstrate that the valley g-factor g(v), defined analogously to the spin g-factor g(s) for valley splitting in a perpendicular magnetic field, is tunable by over a factor of 4 from 20 to 90, by gate voltage adjustments only. Larger g(v) results from larger electronic dot sizes, determined from the charging energy. On our versatile device, bipolar operation, charging our quantum dot with charge carriers of the same or the opposite polarity as the leads, can be performed. Dots of both polarities are tunable to the first charge carrier, such that the transition from an electron to a hole dot by the action of the plunger gate can be observed. Addition of gates easily extends the system to host tunable double dots.

Automated summary

Quantum states in graphene have two-fold degeneracy in spins and valleys, which can be utilized for qubit preparations. In bilayer graphene quantum dots, the valley g-factor can be tuned by gate voltage adjustments, resulting in larger g-factor with larger electronic dot sizes. Bipolar operation on the versatile device allows for the observation of transitions from electron dots to hole dots. Addition of gates can extend the system to host tunable double dots.