Physicists Observe the Majorana Fermion, Which Is Its Own Antiparticle

Charliemopps writes: "For the first time Princeton University scientists have observed a Majorana fermion, a long-predicted but never observed exotic particle that acts as both matter and anti-matter (abstract). "The setup they created starts with an ultrapure crystal of lead, whose atoms naturally line up in alternating rows that leave atomically thin ridges on the crystal's surface. The researchers then deposited pure iron into one of these ridges to create a wire that is just one atom wide and about three atoms thick. ...[Next, they] placed the lead and the embedded iron wire under the scanning-tunneling microscope and cooled the system to -272 degrees Celsius, just a degree above absolute zero. After about two years of painstaking work, they confirmed that superconductivity in the iron wire matched the conditions required for Majorana fermion to be created in their material." The particle is surprisingly stable. Being in both states at once seems to make it interact very weakly with its surrounding material. This could also be a major step towards quantum computing.

Comments

What is there to say?

[i kan reed]

This article hasn't gotten any meaningful comments yet. I'm not sure there's a lot to say about it. It's sure a particle alright. And it can only exist in superconductors, apparently, because it would annihilate with other instances of itself, if not contained.

And it validates an 80 year theory?

I don't know what else there is.

Re:Well that's random

[i kan reed]

Despite combining qualities usually thought to annihilate each other—matter and antimatter—the Majorana fermion is surprisingly stable; rather than being destructive, the conflicting properties render the particle neutral so that it interacts very weakly with its environment. This aloofness has spurred scientists to search for ways to engineer the Majorana into materials, which could provide a much more stable way of encoding quantum information, and thus a new basis for quantum computing.

That's what the article says. I don't quite get it, but maybe the math is more elegant than the English representation?

Re:Well that's random

[Jaime2]

From the article: "Importantly, Kitaev also outlined how such a particle could be harnessed as a qubit, the basis of a quantum computer, which added significant impetus to the search."

Fermion that is its own antiparticle

[blueg3]

The summary (and the article!) imply that it is rare and strange for a particle to be its own antiparticle. This is not the case. Plenty of boson and mesons are their own antiparticles: photons, gluons, pions, etc. This isn't a particularly weird situation.

However, fermions are another story. Fermions and bosons are the two kinds of fundamental particles. They behave very differently. While there are bosons that are their own antiparticle, there are no known fermions that have this property. All the fermions we know of are Dirac-type. It's been long postulated that there could be Majorana-type fermions, which, among other things, are their own antiparticles.

It's interesting, but not quite as crazy as implied.

Re:Fermion that is its own antiparticle

[blueg3]

That's hard to answer for a few reasons. I'm not a particle physicist, the subject is kind of complicated, and most people start off ill-informed (sorry!).

Antiparticles are not particularly weird and particle-antiparticle interactions are, in particular, not some kind of physical witchcraft. I always have disliked that it's called annihilation. At the subatomic level, particle interactions are common and they generally involve the "creation" and "destruction" of particles. For example, maybe a neutron decays into a proton, an electron, and an electron antineutrino (by way of one of its down quarks changing into an up quark). Particle interactions are all sort of a shuffling of energy between the different flavors of bundles of energy we call particles. Lots of different physical quantities, like charge, are conserved, limiting what interactions can happen.

In the interest of simplicity, a lot of what I'll say next is slightly wrong.

Antiparticles aren't particularly weird. Particles all have a set of physical properties. It turns out that for each particle, there is another particle that is basically exactly the same, except all these physical properties are opposite. So an electron has charge -1 and an antielectron (positron) has charge +1. In fact, if you look at a legal particle interaction and replace all of the particles with their antiparticles, it's still a legal particle interaction.

An implication of this is that if a particle and its antiparticle interact (not a particle and *any* antiparticle, but *its* antiparticle), the net total for any of their conserved quantities (like charge) is zero. That means the major legal interaction is that the two particles are destroy and produce photons. While photons are particles, we tend to think of them as just energy, so the particle-antiparticle interaction is an "annihilation": two particles go in, energy and zero particles come out.

The "its antiparticle" bit is important. You don't see a lot of antielectrons because a free antielectron would easily encounter an electron and annihilate. But there are plenty of antineutrinos because they interact weakly with the rest of the world. An antineutrino interacting with, say, a proton does not cause annihilation. Even an antielectron interacting with, say, a proton doesn't do anything special.

Oh, also, it turns out that, at least for the "normal matter" particles like electrons and protons, the universe seems to contain pretty much only the normal-matter particles and (relatively) no antiparticles. There doesn't seem to be any reason, in physics, for one to be preferred over the other. (It's just that in one region of space, you couldn't have a mixture and also have stable matter.) So that's weird.

This is all a long-winded way of getting to the answer that particles that are their own antiparticles aren't particularly exciting. They all have the property that conserved quantities (at least, those that are negated in antiparticles) are zero. So they all naturally have annihilation interactions: when two collide, they can annihilate and form protons. But the annihilation interaction isn't particularly dramatic or weird, it just sounds interesting. The particles all probably also have interactions with all sorts of other types of particles, too, and it really comes down to what particle it happens to collide with first. Maybe a photon and an antineutrino interact with a proton and form a neutron.

Most of the particles that are their own antiparticles are relatively neutral to normal matter (and consequently, also to normal antimatter). But they're all a very different kind of particle from normal matter. They're things like force-carriers (photons) and muons, and they interact with electrons and protons differently from how electrons and protons interact with each other.

For some real fun, look up Feynman diagrams, a neat way of writing down different legal particle interactions. One axis is space (in one dimension) and one axis is time. Now, any 90-degree rotation of a legal interaction is still a legal interaction.

Quick explanation

[Okian Warrior]

A Majorana particle is it's own antiparticle; such as, for example, a photon.

Most fermions have different antiparticles from themselves: Protons are notably different from anti-protons, electrons are different from positrons, and so on. The one exception is the neutrino, for which the question is not yet settled.

If the neutrino is its own antiparticle, we should see double-beta-decay events. A beta decay emits a neutrino, so if two happen simultaneously the neutrinos should annihilate if they are their own antiparticle. (Wikipedia link)

As yet no experiment has seen double-beta-decay, so it's likely that the neutrino has a distinct anti-neutrino - an intriguing prospect.

The article referenced in the post does not identify the fermion involved, so one can only assume that it's a "quasi particle", which is a type of vibration. Essentially a phonon (sound wave) with fermion-like properties.

Re:Well that's random

[countach44]

I apologize that I don't have time to construct a proper reply, but this article gives a nice explanation of how majorana fermions can be used to make qubits (hopefully it's not paywalled, but I'm on a university network so it's hard for me to tell): http://www.nature.com/nphys/jo...

Re: Well that's random

[smaddox]

First, this really is one of the most promising qubits, although possibly not using this material system. Their resilience to first-order scattering processes has the potential to make them far more to robust to decoherence. Second, true quantum computing (as apposed to quantum cryptography) is unlikely to ever work at room temperature. That does not, however, imply it won't be an important tool for research. Super computers already require large buildings to house them. A quantum super computer would be far more efficient as certain tasks.

As far as the summary/article goes, this journalist is clearly confused. This isn't a fundamental particle, its a "quasi-particle", which is a fancy word for an ensemble property. Phonons, for example, are a quasi-particle used to model quantization of crystal lattice vibrations.

Re:Well that's random

[Hartree]

Not so random. Maybe a long ways distant. (i.e. It's vaporware, but give us more money.)

One of the problems of current quantum computing qubits is they are easily upset by thermal and other noise from their surroundings.

There are certain systems that involve Majorana fermions that have been theorized to be what are called topologically protected states. These would be largely immune to noise in a way similar to how electron pairs in a superconductor are immune to the normal energy losses that cause resistance in a wire.

A problem with this, is, we hadn't really shown that Majorana fermions actually existed.

This is a hot area of solid state physics research. Note that these are not particles in the usual sense, but things that behave like particles (like electron pairs in a superconductor behave kinda sorta like single particles). They fall under the general term of quasiparticles.

Re:Well that's random

[Anonymous Coward]

The reason you don't get it is because that sentence is very poorly constructed. there are two properties they mention - first is the Majorana fermion being its own antiparticle, second is the fact that it is neutral, that is, it has no electric charge.
These properties are related in that a charged particle cannot be its own antiparticle, so a Majorana must be neutral, and so it doesn't feel electric fields and doesn't interact as much with the electrons and protons around it. being neutral makes it stable because it isn't disturbed by all the noise in the environment, and that's important for quantum computing 'cause you want to be able to set a qubit and have it stay there until you need it, and interference from the environment tends to ruin it.
As to calling its properties "conflicting", that's just nonsense, there's nothing wrong about being your own antiparticle - photons do it all the time! of course photons are bosons and not fermions, and while the math works just as well for fermions, for some reason we don't know of any fermion like that (well, maybe neutrinos are Majoranas, we're not sure about them).
Finally, calling what they found "a particle" is a bit confusing, the correct term is quasi-particle ,meaning it's not one of nature's fundamental particles like photons and electrons, but some stable state that lives in their carefully engineered environment and behaves as a Majorana fermion, not that it matters really.