Spin-valley locked excited states spectroscopy in a one-particle bilayer graphene quantum dot

TL;DR

This study resolves the excited states spectrum of a bilayer graphene quantum dot in small magnetic fields with high precision, confirming theoretical predictions and enabling future valley qubit relaxation time measurements.

Contribution

It provides the first high-resolution measurement of the excited states spectrum in a single-carrier bilayer graphene quantum dot within relevant magnetic fields, verifying theoretical models.

Findings

Spectrum matches theoretical predictions

Established an upper bound on inter-valley mixing

Demonstrated a technique for measuring valley qubit relaxation times

Abstract

Current semiconductor qubits rely either on the spin or on the charge degree of freedom to encode quantum information. By contrast, in bilayer graphene the valley degree of freedom, stemming from the crystal lattice symmetry, is a robust quantum number that can therefore be harnessed for this purpose. The simplest implementation of a valley qubit would rely on two states with opposite valleys as in the case of a single-carrier bilayer graphene quantum dot immersed in a small perpendicular magnetic field ($B_\perp\lesssim 100$mT). However, the single-carrier quantum dot excited states spectrum has not been resolved to date in the relevant magnetic field range. Here, we fill this gap, by measuring the parallel and perpendicular magnetic field dependence of this spectrum with an unprecedented resolution of $4\mu$eV. We use a time-resolved charge detection technique that gives us access to individual tunnel events. Our results come as a direct verification of the predicted spectrum and establish a new upper-bound on inter-valley mixing, equal to our energy resolution. Our charge detection technique opens the door to measuring the relaxation time of a valley qubit in a single-carrier bilayer graphene quantum dot.