Signature of long-sought particle that could revolutionize quantum computing

Sep 26, 2012 by Elizabeth K. Gardner

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 .

"The search for this particle is for condensed-matter physicists what the search was for high-energy ," Rokhinson said. "It is a very peculiar object because it is a 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 , 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 said.

"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 said. "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 , he said.

"Other particles are interchangeable and if two 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 said. "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 said.

Rokhinson used a one-dimensional 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 said.

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 said.

"Majorana particles are the only particle that can produce this effect, and experimental observation of it is a tremendous breakthrough," Yakovenko said. "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 said.

"Whether or not these particles will work for has yet to be seen, but in the process of trying we will learn a lot of unknown quantum physics," he said. "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 the next issue of the journal Nature Physics and is currently available online. Co-authors include Xinyu Liu and Jacek Furdyna of the University of Notre Dame, who designed the material specifically for these experiments. Furdyna also has an honorary degree from Purdue. The work was partially supported by grants from the Army Research Office and the National Science Foundation.

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

Explore further: Breakthrough in light sources for new quantum technology

More information: The Fractional ac Josephson Effect in a Semiconductor-Superconductor Nanowire as a Signature of Majorana Particles, Nature Physics, 2012.

ABSTRACT
Topological superconductors that support Majorana fermions have been predicted when one-dimensional semiconducting wires are coupled to a superconductor. Such excitations are expected to exhibit non-Abelian statistics and can be used to realize quantum gates that are topologically protected from local sources of decoherence. Here we report the observation of the fractional a.c. Josephson effect in a hybrid semiconductor-superconductor InSb/Nb nanowire junction, a hallmark of topological matter. When the junction is irradiated with a radio frequency f0 in the absence of an external magnetic field, quantized voltage steps (Shapiro steps) with a height ∆V=hf0/2e are observed, as is expected for conventional superconductor junctions, where the supercurrent is carried by charge-2e Cooper pairs. At high magnetic fields the height of the first Shapiro step is doubled to hf0/e, suggesting that the supercurrent is carried by charge-e quasiparticles. This is a unique signature of the Majorana fermions, predicted almost 80 years ago.

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User comments : 13

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Mumrah
5 / 5 (1) Sep 26, 2012
Can someone clarify, are they proposing a new fundamental particle (as in the standard model needs revising) or an excitation/condensate or some such?
vega12
5 / 5 (1) Sep 26, 2012
Excitation is what they are talking about. They are doing condensed matter physics afterall. I also was initially confused by their insistence on using the word "particle" so much.
GSwift7
4 / 5 (4) Sep 26, 2012
Can someone clarify, are they proposing a new fundamental particle (as in the standard model needs revising) or an excitation/condensate or some such?


No, this is a non-fundamental quasi-particle that is predicted to form at the ends of a superconducting wire. It is an electrical effect produced at the surface boundary of the superconductor (if it is confirmed).

Note, there is more than one type of Majorana fermion. The one I just described is the type they are talking about above.

The other thing people talk about when they use the term Majorana fermion is a classification of fundamental particle that is something like the opposite of a Dirac fermion. All the fundamental particles we have classified so far appear to be Dirac fermions, though the neurtino reamains unclassified in this regard. That's high energy particle physics, this story is condensed particle physics. Entirely different fields.
Lurker2358
1 / 5 (8) Sep 26, 2012
At this rate, I seriously doubt this type of quantum computer would ever be more economical than a conventional computer.

Nobody has even shown a practical application of the few quantum algorithms that do exist, except for quantum data encryption.

While they're screwing around trying to get just a few quantum bits to work together, everybody else has a 4 core 4ghz conventional computer with 6 gigs of ram crunching away at whatever they want to do, and it only cost $500.
antialias_physorg
5 / 5 (3) Sep 26, 2012
Man...first the Higgs, now the Majorana. These are truly exciting times.
GSwift7
3.7 / 5 (3) Sep 26, 2012
At this rate, I seriously doubt this type of quantum computer would ever be more economical than a conventional computer.


I think most people in the field would agree, but the main point here is really the basic research. They don't really have enough of a foundation to do much in the way of applied research yet. Probably won't in our lifetime either.

Man...first the Higgs, now the Majorana. These are truly exciting times


"These are not the majoranas you are looking for."

I'll bet you are thinking of the high energy particle physics version of the term.
Sonhouse
not rated yet Sep 26, 2012
At this rate, I seriously doubt this type of quantum computer would ever be more economical than a conventional computer.

Nobody has even shown a practical application of the few quantum algorithms that do exist, except for quantum data encryption.

While they're screwing around trying to get just a few quantum bits to work together, everybody else has a 4 core 4ghz conventional computer with 6 gigs of ram crunching away at whatever they want to do, and it only cost $500.

Quantum computers will do what a 4000 core 4000 ghz machine will never do because the bits can be 1 and 0 at the same time, a leg up that will never be done with classical computers. It is as Einstein said, Spooky computing, to paraphrase:)
rkolter
3 / 5 (2) Sep 26, 2012
At this rate, I seriously doubt this type of quantum computer would ever be more economical than a conventional computer.


At this rate, I seriously doubt flying machines will ever be invented.

Nobody has even shown a practical application of the few quantum algorithms that do exist, except for quantum data encryption.


Nobody has even shown that practical manned flight is even possible except for a few moments.

While they're screwing around trying to get just a few quantum bits to work together, everybody else has a 4 core 4ghz conventional computer with 6 gigs of ram crunching away at whatever they want to do, and it only cost $500.


While they're screwing around trying to get man off the ground, everyone else has a steam engine powered boat capable of crossing the Atlantic in half the time of ships just twenty years ago.

Yeah. I tend not to believe people who go to either extreme about the future. This physics may lead to something more practical later.
maxb500_live_nl
1 / 5 (1) Sep 26, 2012
They copied the exact same method that Dutch researchers used to discover the majorana fermion and made world headlines with it in April this year and even became famous on the prior work.

The fact that the discovery and method is not even refered to is disturbing to me. It was realy big headline news and still is. They litteraly copied their method almost exactly.
ValeriaT
1.3 / 5 (3) Sep 26, 2012
Aren't the commonly known Cooper pairs the Majorana particles soughed - just under different name?
Argiod
1 / 5 (4) Sep 27, 2012
Imagine a computer so sensitive it can detect your thoughts and be influenced at the quantum level. Schrodenger showed us that the mind of the observer collapses the quantum state into the reality we desire, or fear, depending on our mind set.

Once the rest of the scientific community acknowledges that quantum effects are affected by our thoughts, they will find that the only stable quantum computer that is truly fault tollerant will be one operated by someone with a stable, fault-tollerant mind... perhaps a Tibetan monk with decades of mental discipline.
antialias_physorg
not rated yet Sep 27, 2012
Aren't the commonly known Cooper pairs the Majorana particles soughed - just under different name?

Cooper pairs aren't electrically neutral.
Lex Talonis
1 / 5 (3) Oct 01, 2012
Marijuhuana Particles?