Pauli Blockade of Tunable Two-Electron Spin and Valley States in Graphene Quantum Dots

Abstract

Pauli blockade mechanisms-whereby carrier transport through quantum dots (QD) is blocked due to selection rules even when energetically allowed-are a direct manifestation of the Pauli exclusion principle, as well as a key mechanism for manipulating and reading out spin qubits. The Pauli spin blockade is well established for systems such as GaAs QDs, but is to be further explored for systems with additional degrees of freedom, such as the valley quantum numbers in carbon-based materials or silicon. Here we report experiments on coupled bilayer graphene double quantum dots, in which the spin and valley states are precisely controlled, enabling the observation of the two-electron combined blockade physics. We demonstrate that the doubly occupied single dot switches between two different ground states with gate and magnetic-field tuning, allowing for the switching of selection rules: with a spin-triplet-valley-singlet ground state, valley blockade is observed; and with the spin-singlet-valley-triplet ground state, robust spin blockade is shown.

Nanomaterials for Quantum Information Science and Engineering

“…Spin and valley blockade was recently shown in a graphene DQD system, as seen in the charge stability diagram in Fig. 2c [169] . Dispersive charge readout was also recently shown for graphene charge states [170] .…”

“…Spin-valley qubits in graphene quantum dots could theoretically be viable, but this will require significant enhancement of the spin-orbit coupling. Recent experiments have demonstrated the prerequisites for the use of graphene quantum dots as qubits [166,169,170] , but coherent control of spins and the measurement of coherence times remains the next task. While recent experiments have focused on bulk bilayer graphene, graphene nanoribbons grown by bottom-up methods are also worth exploring for spin/valley qubits due to the inherent lateral confinement, lifting of the valley degeneracy, and bandgap [160] .…”

“…It's been determined that in two-carrier GQDs, the ground state is best expressed as a spintriplet valley-singlet state at low magnetic fields [168] . In the high field regime, however, it was recently reported that the ground state switches to the spin-singlet valley-triplet state [169] . Finally, a crucial prerequisite is the ability to read-out and manipulate spins.…”

Gate‐Controlled Quantum Dots Based on 2D Materials

“…Besides mapping the spin-valley states, Pauli blockade is demonstrated recently in gate-defined graphene quantum dot [191] . Tunneling through particular spin (valley) states can be blocked upon gate and magnetic field tuning, due to Pauli exclusion principle of spin (valley) degree of freedom.…”

“…The recent demonstration of spin and valley blockade [ 191 ] provides a crucial tool towards qubit manipulation and readout. With such highly tunable and high‐quality gate‐defined quantum dots, demonstrating a graphene‐based qubit is believed to be within reach.…”