Singlet-Triplet Physics and Shell Filling in Carbon Nanotube Double Quantum Dots

TL;DR

This paper demonstrates a gate-tunable singlet-triplet qubit in a carbon nanotube double quantum dot, revealing shell structures and inelastic cotunneling that align with theoretical predictions, advancing spin-based quantum computing research.

Contribution

It presents the first observation of a gate-tunable singlet-triplet qubit in a CNT DQD with detailed shell structure analysis and cotunneling measurements.

Findings

Existence of shell structures with four and eight electrons in CNT DQD

Observation of inelastic cotunneling via singlet and triplet states

Splitting between singlet and triplet states matches theoretical models

Abstract

An artifcial two-atomic molecule, also called a double quantum dot (DQD), is an ideal system for exploring few electron physics. Spin-entanglement between just two electrons can be explored in such systems where singlet and triplet states are accessible. These two spin-states can be regarded as the two states in a quantum two-state system, a so-called singlet-triplet qubit. A very attractive material for realizing spin based qubits is the carbon nanotube (CNT), because it is expected to have a very long spin coherence time. Here we show the existence of a gate-tunable singlet-triplet qubit in a CNT DQD. We show that the CNT DQD has clear shell structures of both four and eight electrons, with the singlet-triplet qubit present in the four-electron shells. We furthermore observe inelastic cotunneling via the singlet and triplet states, which we use to probe the splitting between singlet and triplet, in good agreement with theory.