Jan 18, 2021 
Onedimensional quantum nanowires fertile ground for Majorana zero modes
(Nanowerk News) Why is studying spin properties of onedimensional quantum nanowires important?

Quantum nanowires–which have length but no width or height–provide a unique environment for the formation and detection of a quasiparticle known as a Majorana zero mode.

A new UNSWled study (Nature Communications, "New signatures of the spin gap in quantum point contacts") overcomes previous difficulty detecting the Majorana zero mode, and produces a significant improvement in device reproducibility.


Quantum point contact structure (left) in which an applied voltage constricts electron movement to one dimension, with conductance (right) showing effect of applied magnetic field (red).

Potential applications for Majorana zero modes include faultresistant topological quantum computers, and topological superconductivity.

Majorana fermions in 1D wires

A Majorana fermion is a composite particle that is its own antiparticle.

Such unusual particle’s interest academically and commercially comes from their potential use in a topological quantum computer, predicted to be immune to the decoherence that randomises the precious quantum information.

Majorana zero modes can be created in quantum wires made from special materials in which there is a strong coupling between their electrical and magnetic properties.

In particular, Majorana zero modes can be created in onedimensional semiconductors (such as semiconductor nanowires) when coupled with a superconductor.

In a onedimensional nanowire, whose dimensions perpendicular to length are small enough not to allow any movement of subatomic particles, quantum effects predominate.

New method for detecting necessary spinorbit gap

Onedimensional semiconductor systems with strong spinorbit interaction are attracting great attention due to potential applications in topological quantum computing.

The magnetic ‘spin’ of an electron is like a little bar magnet, whose orientation can be set with an applied magnetic field.

In materials with a ‘spinorbit interaction’ the spin of an electron is determined by the direction of motion, even at zero magnetic field. This allows for all electrical manipulation of magnetic quantum properties.

Applying a magnetic field to such a system can open an energy gap such that forward moving electrons all have the same spin polarisation, and backwardmoving electrons have the opposite polarisation. This ‘spingap’ is a prerequisite for the formation of Majorana zero modes.

Despite intense experimental work, it has proven extremely difficult to unambiguously detect this spingap in semiconductor nanowires, since the spingap’s characteristic signature (a dip in its conductance plateau when a magnetic field is applied) is very hard to distinguish from unavoidable the background disorder in nanowires.

The new study finds a new, unambiguous signature for the spinorbit gap that is impervious to the disorder effects plaguing previous studies.

“This signature will become the defacto standard for detecting spingaps in the future,” says lead author Dr Karina Hudson.

Reproducibility

The use of Majorana zero modes in a scalable quantum computer faces an additional challenge due to the random disorder and imperfections in the selfassembled nanowires that host the MZM.

It has previously been almost impossible to fabricate reproducible devices, with only about 10% of devices functioning within desired parameters.

The latest UNSW results show a significant improvement, with reproducible results across six devices based on three different starting wafers.

“This work opens a new route to making completely reproducible devices,” says corresponding author Prof Alex Hamilton UNSW).
