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quantum computers

Tiny ‘hashtags’ could catch Majorana quasiparticles

New research describes how to coax “hashtag”-shaped nanowires into generating Majorana quasiparticles.

These quasiparticles are exotic states that if realized, can be used to encode information with very little risk of decoherence—one of quantum computing’s biggest challenges—and thus, little need for quantum error correction.

“The ‘hashtag’ structures whose quantum properties are studied in this paper have an unworldly beauty…”

“This was a really good step toward making things happen,” says Chris Palmstrøm, professor of electrical and computer engineering and of materials at the University of California, Santa Barbara.

In 2012, Dutch scientists Leo Kouwenhoven and Erik Bakkers (also authors of the paper) from the Delft and Eindhoven Universities of Technology in the Netherlands, reported the first observation of states consistent with these quasiparticles. At the time, however, they stopped short of definitive proof that they were in fact the Majoranas, and not other phenomena.

“The ‘hashtag’ structures whose quantum properties are studied in this paper have an unworldly beauty and look nearly as impossible as a tower by Escher. They are single crystals with the topology of a circle,” says Michael Freedman, mathematician and director of Microsoft Corporation’s Research Station Q, which has its headquarters on the UCSB campus.

This team of scientists, working under the aegis of Station Q, is part of a greater international effort to build the first topological quantum computer.

Braiding the Majoranas

The quasiparticles get their name from Italian physicist Ettore Majorana, who predicted their existence in 1937, around the birth of quantum mechanics. They have the unique distinction of being their own antiparticles—they can annihilate one another. They also have the quality of being non-Abelian, resulting in the ability to “remember” their relative positions over time—a property that makes them central to topological quantum computation.

“If you are to move these Majoranas physically around each other, they will remember if they were moved clockwise or anticlockwise,” says Mihir Pendharkar, a graduate student researcher in the Palmstrøm group.

This operation of moving one around the other, he continues, is what is referred to as “braiding.” Computations could in theory be performed by braiding the Majoranas and then fusing them, releasing a poof of energy—a “digital high”—or absorbing energy—a “digital low.” The information is contained and processed by the exchange of positions, and the outcome is split between the two or more Majoranas (not the quasiparticles themselves), a topological property that protects the information from the environmental perturbations (noise) that could affect the individual Majoranas.

However, before any braiding can take place, these fragile and fleeting quasiparticles must first be generated.

In this work, semiconductor wafers started their journey with patterning of gold droplets at the Delft University of Technology. With the gold droplets acting as seeds, Indium antimonide (InSb) semiconductor nanowires grew at the Eindhoven University of Technology. Next, the nanowires traveled across the globe to Santa Barbara, where Palmstrøm group researchers carefully cleaned and partially covered them with a thin shell of superconducting aluminum. The nanowires then returned to the Netherlands for low temperature electrical measurements.

Zero energy

“The Majorana has been predicted to occur between a superconductor and a semiconductor wire,” Palmstrøm explains. Some of the intersecting wires in the infinitesimal hashtag-shaped device are fused together, while others barely miss one another, leaving a very precise gap. This clever design, according to the researchers, allows for some regions of a nanowire to go without an aluminum shell coating, laying down ideal conditions for the measurement of Majoranas.

“What you should be seeing is a state at zero energy,” Pendharkar says. This “zero-bias peak” is consistent with the mathematics that results in a particle being its own antiparticle and was first observed in 2012. “In 2012, they showed a tiny zero-bias blip in a sea of background,” Pendharkar says. With the new approach, he continues, “now the sea has gone missing,” which not only clarifies the 2012 result and takes the researchers one step closer to definitive proof of Majorana states, but also lays a more robust groundwork for the production of these quasiparticles.

Clear evidence points to Majorana fermions

Majoranas, because of their particular immunity to error, can be used to construct an ideal qubit (unit of quantum information) for topological quantum computers, and, according to the researchers, can result in a more practicable quantum computer because its fault-tolerance will require fewer qubits for error correction.

“All quantum computers are going to be working at very low temperatures,” Palmstrøm says, “because ‘quantum’ is a very low energy difference.” Thus, say the researchers, cooling fewer fault-tolerant qubits in a quantum circuit would be easier, and done in a smaller footprint, than cooling more error-prone qubits plus those required to protect from error.

The final step toward conclusive proof of Majoranas will be in the braiding, an experiment the researchers hope to conduct in the near future. To that end, the scientists continue to build on this foundation with designs that may enable and measure the outcome of braiding.

The work appears in Nature.

Source: UC Santa Barbara

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