Last week, the long-anticipated confirmation that the Majorana quasiparticle actually exists finally came to pass. With that confirmation also came a method for using it in a new kind of quantum computing that is far more stable than what’s currently available.
That research appears to have been a watershed moment. Two new papers released this week share some of the same origins as last week’s research, but with some significant new twists.
In the first, published in Nature Communications, researchers at the University of Sydney in Australia and Microsoft's Station Q, also in Sydney, offer further proof of the existence of Majorana fermions in semiconducting nanowires. This confirmation takes the form of proof that an electron inside these one-dimensional semiconducting nanowires will have a quantum spin opposite to its momentum in a finite magnetic field.
The other paper, which appears in the journal Science Advances, details the work of an international team of scientists led by a group at the University of Pittsburgh. That team has provided a new approach to generating Majorana fermions in nanowires that involves the use of quantum dots.
Since 2012, collaborators at TU Delft University and Eindhoven University, both in the Netherlands, found that the preferred way to produce Majorana quasiparticles was to take a semiconducting nanowire made from indium antimonide and place a superconductor next to it.
When a magnetic field is applied to this combination of materials, a new kind of material, referred to as a topological superconductor, is produced. By precisely tuning all the ingredients involved in the creation of this topological superconductor, it was strongly believed—albeit not confirmed—that Majorana particles would form on either end of the nanowire bordered by the superconductor.
The University of Pittsburgh scientists use the exact same materials; and, in fact, the materials were grown by the Dutch collaborators from last week’s research. However, this latest research addresses one of the key challenges in producing Majoranas with this material mix.
To generate Majorana fermions with this set of materials, it is necessary to make a network of nanowires, build a nanoscale device around that network, and then attach superconducting contacts that serve as electrostatic gate electrodes like in transistors. Then it is necessary to cool the device down to milliKelvin temperatures and apply Tesla-scale magnetic fields.
“Our group has a unique approach to generating Majorana fermions; the steps that follow after the nanowire has been synthesized are unique,” explained Sergey Frolov, a professor at the University of Pittsburgh and co-author of the paper, in an e-mail interview with IEEE Spectrum. “We are assembling a chain of quantum dots. The dots are all defined in a nanowire using electrostatic gates, so the dots form a chain along the nanowire. The two end sites of the chain are going to have Majorana fermions.”
So far, Frolov and his colleagues have demonstrated a building block of this chain: a two-dot molecule. The advantage of building a chain is that the site of each Majorana particle is well known and can be precisely controlled. This will be very useful once the team is ready to braid Majorana states, according to Frolov.
The act of braiding Majoranas involves them exchanging places along the nanowire at the boundary of the superconducting material. This phenomenon is referred to in quantum mechanics as exchanging statistics of the particles. These statistics describe how the quantum mechanics of the system change when two indistinguishable particles switch places.
Until last week, the problem was that gettting Majoranas to exchange places seemed to be an impossible task. If they ever came in direct contact, they would annihilate each other. What last week’s paper revealed was the Dutch researchers’ method for creating a hashtag structure in which a four-way junction of nanowires contained four Majoranas. While, strictly speaking, the structure did not physically induce the Majoranas to exchange places, the effect of applying a small current to the four-way junction effectively produced the same result as a physical exchange.
This network of hashtag nanowires could form the basis of quantum bits (qubits) that could be stored and manipulated simply by braiding (swapping) Majoranas. This would offer a far more robust approach to creating qubits because the Majoranas could maintain superposition far longer than qubits formed via previous techniques. The result: far greater time to perform calculations.
Frolov concedes that there is still a long way to go before Majoranas could form the basis of qubits used in topological quantum computers. “We are currently learning how to control two Majorana fermions,” says Frolov. “A single topological qubit requires on the order of 10 Majorana fermions, so we need to scale up our efforts by a factor of few to reach a single quantum bit. To build a quantum computer, we need to scale up by a factor of a thousand to a million.”
While using the Majorana fermions is of central interest to both lines of research published this week, both teams are interested in other implications of their work.
Maja Cassidy, a professor at the University of Sydney and co-author of the Nature Communications paper, suggested in a press release that their work with Majoranas will be useful in spintronic systems, where the quantum spin and not the charge is used for information in classical systems.
Frolov believes his group’s research could uncover something completely new: “We are also fascinated by the non-trivial braiding property [from the standpoint that it could present] strong evidence for the existence of a third class of fundamental particles, which are not fermions and not bosons.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.