Orbital hyperfine interaction and qubit dephasing in carbon nanotube quantum dots

TL;DR

This paper develops a comprehensive theory of orbital hyperfine interactions in carbon nanotube quantum dots, revealing a new decoherence mechanism for valley and spin-valley qubits and estimating associated dephasing times.

Contribution

It introduces a theoretical description of orbital hyperfine interaction in carbon nanotubes, highlighting its local, Ising-like nature and its significance for qubit coherence.

Findings

Orbital hyperfine interaction couples nuclear spins to electron valley states.

The interaction is approximately of Ising type and locally defined.

Estimated inhomogeneous dephasing time T2* for nanotube valley qubits.

Abstract

Hyperfine interaction (HF) is of key importance for the functionality of solid-state quantum information processing, as it affects qubit coherence and enables nuclear-spin quantum memories. In this work, we complete the theory of the basic hyperfine interaction mechanisms (Fermi contact, dipolar, orbital) in carbon nanotube quantum dots by providing a theoretical description of the orbital HF. We find that orbital HF induces an interaction between the nuclear spins of the nanotube lattice and the valley degree of freedom of the electrons confined in the quantum dot. We show that the resulting nuclear-spin--electron-valley interaction (i) is approximately of Ising type, (ii) is essentially local, in the sense that a radius- and dot-length-independent atomic interaction strength can be defined, and (iii) has an atomic interaction strength that is comparable to the combined strength of Fermi contact and dipolar interactions. We argue that orbital HF provides a new decoherence mechanism for single-electron valley qubits and spin-valley qubits in a range of multi-valley materials. We explicitly evaluate the corresponding inhomogeneous dephasing time $T_2^*$ for a nanotube-based valley qubit.