Noiseless manipulation of helical edge state transport by a quantum magnet

TL;DR

This paper demonstrates that in a quantum-spin-Hall insulator, a magnetically gapped region can allow noiseless, low-energy electron transport with suppressed thermal noise, enabling a robust noiseless current splitter.

Contribution

It shows that current in such systems is carried by electrons below the magnet-induced gap, leading to noiseless transport and potential device applications.

Findings

Absence of shot noise in magnetically gapped helical edge states

Exponential suppression of thermal noise

Device acts as a noiseless current splitter

Abstract

The current through a helical edge state of a quantum-spin-Hall insulator may be fully transmitted through a magnetically gapped region due to a combination of spin-transfer torque and spin pumping [Meng {\em et al.}, Phys. Rev. B {\bf 90}, 205403 (2014)]. Using a scattering approach, we here argue that in such a system the current is effectively carried by electrons with energies below the magnet-induced gap and well below the Fermi energy. This has striking consequences, such as the absence of shot noise, an exponential suppression of thermal noise, and an obstruction of thermal transport. For two helical edges covered by the same quantum magnet, the device can act as robust noiseless current splitter.