Probing the single-particle valley relaxation time in bilayer graphene quantum dots

Graphene and bilayer graphene (BLG) have emerged as interesting and promising host materials for qubits. Thanks to its tunable band gap, bilayer graphene allows for electrostatic confinement of charge carriers in quantum dots (QDs), similar as done in conventional III-V semiconductors. Additional to the spin degree of freedom, BLG exhibits a valley pseudospin, which distinguishes between its two energetically degenerate but inequivalent band structure minima (valleys) K and K'. The finite Berry curvature at these high symmetry points gives rise to a valley-dependent magnetic moment allowing for manipulation and control of the valley degree of freedom. Complementary to spintronics, the control over the valley enables valleytronics, where the valley degree of freedom is used as platform for quantum information processing. To assess the potential of valley qubits in BLG, it is necessary to investigate the relaxation time T₁ of an excited valley state, as it limits the lifetime of the encoded quantum information.

Here, we report single-particle valley relaxation times of up to 7 μs in a BLG quantum dot, sufficiently long to utilize a valley state for coherent manipulation. The observed dependency of T₁ on the perpendicular magnetic field can be described qualitatively and quantitatively by a model considering T₁ to be limited by electron-phonon coupling. We identify coupling to acoustic phonons via bond length change and via the deformation potential as the limiting mechanisms.