The First Dance of the Majoranas
Majoranas are particles theorized to appear in exotic new materials. They have been observed a few times so far, but managing to do stuff with them has been more tricky. I took one of the first steps towards changing this in an experiment last year. I allowed the Majoranas to move, and even to dance a little. In return, they showed me some of their secrets.
Universes, real and imaginary
For every type of particle, there is an antiparticle. Electrons have positrons. Protons have antiprotons. For photons, all other photons are a potential nemesis. In all cases, combining a particle and antiparticle leads to annihilation. They’ll never combine to form something new. Instead, they just disappear in a puff of energy.
We can also imagine universes where the rules would be a bit different. There, a particle and antiparticle might only sometimes annihilate. Other times, they might combine to form a new particle instead. In this world, knowing what types of particles we have would no longer tell us what they woud do if we combine them. We would have to know their life histories as well.
The easiest example is a pair that emerges together from nothingness. Combining the two will send the particles straight back to the void. Or if one particle breaks into two, combining the parts will just rebuild the original. But two particles that have never met wouldn´t have a predetermined destiny. They can do whatever they like. They get to choose randomly.
The destinies of particles also depend on the way they dance. Swapping the positions of particles will change the results of combinations. But even dances that return particles to their original positions can have powerful effects. Behavior like this doesn’t happen with the particles of our universe. So observing it would be fascinating.
Back to the real world
I have taken you on a fanciful tour of imaginary universes. Now it is time to return to more practical matters. Are these universes actually useful for anything?
It turns out they are! Defects and excitations in some materials can act like particles. For exotic enough materials, they can act like the particles of our imaginary universes. Their strange properties then become something we can observe in the lab. And use as part of quantum technology.
Quantum error correcting codes are examples of these exotic materials. The surface code, for example, is a bunch of quantum things called qubits on a two-dimensional lattice. We continuously measure the qubits in fun quantum ways to keep track of how they correlate with their neighbors. The measurements are designed to detect the effects of errors and noise. With the results, we can keep things clean enough to do cool quantum stuff. Like quantum computation.
Making small changes to the measurements will make small changes to the correlations. By doing this, we can get some qubits that are less correlated with their neighbors than most.
Let´s call them Majoranas
We can move these patches of weirdness around just like particles. So let’s give them a name that befits their status. We’ll call them Majoranas.
Since we can move Majoranas, we can also combine pairs of them which will bring us back to a normal, undeformed code. The pair of Majoranas are gone, but have they annihilated?
Sometimes they don’t disappear completely, but leave a trace in the results of the measurements. This mark can’t be swept under the carpet. Future measurements in the same place will keep telling the same story.
The only way to deal with this is by messing around with the qubits nearby. This can move the place where the weird measurement result occurs. So now these are behaving like particles too. We call these fermions (because of reasons). Fortunately, these fermions are pretty simple creatures: combine two, and they’ll always annihilate.
The five-qubit experiment
To experience the full Majorana magic, we need our code to be built on a lattice of qubits that stretches to the horizon.
Until we get that, five qubits will be enough. With these, we can make the measurements that will create two pairs of Majoranas. These pairs as created from nothing, and so would surely annihilate if recombined.
But that would be boring. So instead we can do measurements that have the effect of moving Majoranas. We can swap two of them from different pairs, creating the simplest dance of all. Afterward, we’ll no longer have two pairs that will annihilate. Instead, we’ll have two pairs of total strangers.
Combining one of these pairs will now give a random outcome. Either they’ll annihilate, or they’ll form a fermion. We use a measurement to make the Majorana combine. The result of that measurement will tell us what happened.
Combining the other pair will not be so random. Since everything came from nothing to nothing, it must return. So if one pair becomes a fermion, the other must become its antiparticle: another fermion. The results for the two pairs will always be perfectly correlated.
This result is the signature we’re looking for. The randomness shows that destinies can be changed. The pairs that were once fated to annihilate can do something different after the dance.
Particles can’t just come from nowhere
The correlation shows that the particles don’t simply pop up out of nowhere whenever they get a chance. Instead, they´re properly responsible particles that care about returning like ashes to ashes and dust to dust.
With both the randomness and the correlations we have something impossible for the dull particles of our universe, like teapots and Higgs Bosons. We have proof of Majoranas.
Finding this signature of these new particles would be a great result. As well as it being extremely interesting to see particles with such exotic behavior, we also like to put them to work. Specifically, we’d like to use them to make quantum computers, turning programs into the choreography of these impossible particles.
5Q quantum processor
But I’m getting ahead of myself. We have to do the experiment first. So where to get the five qubits required? It’s hardly something we can get on the internet for cheap.
Or is it? IBM made a device called the 5Q quantum processor, and they let anyone play with it. For free! You can go online, log in and mess around with the cutting edge of quantum technology. So naturally that’s what I did.
When I ran the experiment, I got exactly the results expected. It was a bit messier than I’d thought, but the results were clear. I had indeed coaxed the five qubits into becoming a mini-universe and played around with the impossible particles therein. And you can have a go as well if you’d like.
See my paper or my video for more on this experiment.
For another couple of fun experiments with Majoranas, check out the following.
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