SPICE Workshop on Quantum Matter for Quantum Technologies, May 21st - May 23rd 2024
Artem Denisov
Low nuclear spin density is an essential property of a material to host highly coherent spin qubits. Another crucial criterion is the ability to
fabricate high-quality quantum dots (QDs) with a known and tunable number of carriers. Bilayer graphene (BLG), which benefits from the low
nuclear spin density of carbon, stands out as one of the few platforms that meet both requirements. Additionally, intrinsic to the hexagonal
graphene lattice, the two-fold valley degeneracy provides an opportunity to define a qubit in the two-dimensional subspace spanned by the valley
and the spin degrees of freedom.
We will start with a broad overview of the band structure and gap opening mechanism in bilayer graphene followed by a short introduction
into physics of quantum dots. We will cover device fabrication techniques and experimental challenges in working with van der Waals
heterostructures compared to conventional semiconductors. In the second part of the talk, we will focus on the single-carrier quantum dot
physics in BLG, where the ground state exhibits a two-fold degeneracy, with the two states having opposite spin and valley quantum numbers. By
breaking the time-reversal symmetry of this ground state with an out-of-plane magnetic field, we will demonstrate how a novel type of
qubit (Kramers qubit), encoded in the two-dimensional spin-valley subspace, becomes accessible. The Kramers qubit is robust against known
spin- and valley-mixing mechanisms, as it requires a simultaneous change of both quantum numbers, resulting in long relaxation and coherence
times. We demonstrate ultra-long spin-valley relaxation times of the Kramers qubit exceeding 30 seconds, which is about two orders of
magnitude longer than the spin relaxation time of 400 ms.