A Purdue University physicist has observed evidence of
long-sought Majorana fermions, special particles that could unleash the
potential of fault-tolerant quantum computing.

Leonid Rokhinson, an associate professor of physics, led a
team that is the first to successfully demonstrate the fractional a.c.
Josephson effect, which is a signature of the particles.

“The search for this particle is for condensed-matter
physicists what the Higgs boson search was for high-energy particle
physicists,” Rokhinson says. “It is a very peculiar object because it
is a fermion yet it is its own antiparticle with zero mass and zero
charge.”

The pursuit of Majorana fermions is driven by their
potential to encode quantum information in a way that solves a problem dogging
quantum computing. The current carriers of quantum bits, the basic unit of
information in quantum computing, are delicate and easily destroyed by small
disturbances from the local environment. Information stored through Majorana
fermions could be protected from such perturbances, resulting in a much more
resilient quantum bit and ‘fault-tolerant’ quantum computing, he says.

“Information could be stored not in the individual
particles, but in their relative configuration, so that if one particle is
pushed a little by a local force, it doesn’t matter,” Rokhinson says.
“As long as that local noise is not so strong that it alters the overall
configuration of a group of particles, the information is retained. It offers
an entirely new way of dealing with information.”

Majorana fermions also have the unique ability to retain a
history of their interactions that can be used to encode quantum information,
he says.

“Other particles are interchangeable and if two
electrons trade places, it is as if nothing had happened, but when you swap two
Majorana fermions, it leaves a mark by altering their quantum mechanical
state,” Rokhinson says. “This change in state is like a passport book
full of stamps and provides a record of exactly how the particle arrived at its
current destination.”

Rokhinson observed a variation of the Josephson effect that
is a unique signature of Majorana fermions. The effect describes the way an
electrical current traveling between two superconductors oscillates at a
frequency that depends on the applied voltage. The reverse also is true; an
oscillating current generates specific voltage, proportional to the frequency.
In the presence of Majorana fermions the frequency-voltage relationship should
change by a factor of two in what is called the fractional a.c. Josephson
effect, he says.

Rokhinson used a 1D semiconductor coupled to a
superconductor to create a hybrid nanowire in which Majorana particles are
predicted to form at the ends. When alternating current is applied through a
set of two such wires, a specific voltage is generated across the device, which
Rokhinson measured. As a magnetic field was applied and varied from weak to
strong, the resulting steps in voltage became twice as tall, a signature of the
formation of Majorana particles, he says.

Victor Yakovenko, a professor of physics at the University
of Maryland, was one of the first theorists to predict the fractional a.c.
Josephson effect.

The effect is very unusual and is specific to Majorana
particles, which makes this observation more definitive than signatures
obtained through other approaches, he says.

“Majorana particles are the only particle that can
produce this effect, and experimental observation of it is a tremendous
breakthrough,” Yakovenko says. “Of course, it will take time and
independent confirmation to firmly establish it, but this is very
exciting.”

The observation of this special state does not mean
fault-tolerant quantum computing will happen any time soon, if at all,
Yakovenko says.

“Whether or not these particles will work for quantum
computing has yet to be seen, but in the process of trying we will learn a lot
of unknown quantum physics,” he says. “This could open the door to a
whole new field of the topological effects of quantum mechanics.”

A paper detailing the work will be published in Nature
Physics.

Rokhinson next plans to perform follow-up experiments and to
modify the system to probe different properties of the observed state.

Source: Purdue University