Valley-spin blockade and spin resonance in carbon nanotubes

TL;DR

This paper demonstrates valley-spin blockade and spin resonance in low-disorder carbon nanotube quantum dots, leveraging large level spacing for robust blockade and enabling spin qubit manipulation.

Contribution

It reports the first observation of valley-spin blockade in clean carbon nanotubes using their bandgap for large level spacing.

Findings

Valley-spin blockade observed in low-disorder nanotubes.

Large level spacing enhances blockade robustness.

Single-electron spin resonance detected via blockade.

Abstract

Manipulation and readout of spin qubits in quantum dots made in III-V materials successfully rely on Pauli blockade that forbids transitions between spin-triplet and spin-singlet states. Quantum dots in group IV materials have the advantage of avoiding decoherence from the hyperfine interaction by purifying them with only zero-spin nuclei. Complications of group IV materials arise from the valley degeneracies in the electronic bandstructure. These lead to complicated multiplet states even for two-electron quantum dots thereby significantly weakening the selection rules for Pauli blockade. Only recently have spin qubits been realized in silicon devices where the valley degeneracy is lifted by strain and spatial confinement. In carbon nanotubes Pauli blockade can be observed by lifting valley degeneracy through disorder. In clean nanotubes, quantum dots have to be made ultra-small to obtain a large energy difference between the relevant multiplet states. Here we report on low-disorder nanotubes and demonstrate Pauli blockade based on both valley and spin selection rules. We exploit the bandgap of the nanotube to obtain a large level spacing and thereby a robust blockade. Single-electron spin resonance is detected using the blockade.