The strange particle that holds the key to ‘quantum supercomputers’

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Last Updated12/07/2023

Researchers at Microsoft announced that they had figured out a way to create a strange kind of particle that could potentially revolutionise quantum computing.
These particles are called Majorana zero modes, whose unique properties could help build quantum computers that are less fragile, and more computationally robust, than they are today.

All subatomic particles that make up matter are called fermions.
In 1928, the British physicist Paul Dirac found the Dirac equation, which described the behaviour of subatomic particles that moved at near the speed of light.
The equation predicted the existence of an antiparticle for each particle, such that if the two meet, they annihilate each other.
Based on it, scientists found the first antiparticle, the positron (the anti-electron), in 1932.
In 1937, the Italian physicist Ettore Majorana found that the Dirac equation also allowed particles that satisfied certain conditions to be their own antiparticles.
In his honour, fermions that are their own antiparticles are called Majorana fermions.

All particles have four quantum numbers associated with them.
No two particles in the same system can have the same four quantum numbers.
- The numbers are together like each particle’s ID.
The characteristic feature of fermions is that one of these numbers, called the quantum spin, has only half-integer values, like 1/2, 3/2, 5/2, etc.
This is why any particle, even two particles bound to each other in some way, can be a fermion: the total quantum spin needs to have a half-integer value.
Most of the rules that apply to single fermions also apply to these pairs, or bound states.
When these bound states are their own antiparticles i.e. if they meet, they annihilate each other, they are Majorana fermions.
Physicists call such bound states Majorana zero modes.

Majorana zero modes can be used to realise a powerful form of computing called topological quantum-computing.
A quantum computer today can use individual electrons as qubits – its fundamental units of information.
Information can be encoded in some property of each electron, like its spin.
Then, the computer manipulates that information by having the electrons interact with each other according to the quirky rules of quantum mechanics.
These quirks allow the computers to access computational techniques and pathways not available to systems that are limited to the possibilities of classical physics.
For example, a qubit can have the values 0 and 1 at the same time due to a property called quantum superposition.
- But a semiconductor in a classical computer can have only one value at a time, 0 or 1.

The information is protected thanks to topological degeneracy.
Degeneracy in quantum mechanics means that the system has multiple states at the same energy.
In topological systems, the system has multiple states at the lowest or ground state energy.
- That is, the quantum system can exist in two (or more) possible states at its lowest energy.
This is usually not possible: in its ground state – i.e. when a system has the least amount of energy – it will have a particular configuration and will exist in a particular state.
If a system can exist in two possible states, or configurations, at its ground state, then the information encoded in that energy level can be recovered from one state or the other.

Study of those properties of matter that don’t change when it undergoes continuous deformation – i.e. when it’s stretched, folded, twisted, etc. but not ruptured or glued to itself.
- For example, a rubber band that’s continuously deformed will continue to have one hole.
- A pair of shorts that’s continuously deformed will always have three holes.
- This is why a rubber band (no matter how big) can’t seamlessly transform into a pair of shorts.
It will need to undergo a discontinuous deformation.
- That is, the rubber band and the shorts are in topologically different states.
If they are also topologically degenerate, the rubber band and the shorts would be two possible states of the same system in its ground state.
So the information can be stored between different topological properties, such as in the number of holes each state contains.
In effect, Majorana zero modes can work as qubits and they won’t easily lose the information vested with them.
This is why people building quantum computers are interested in finding them.

To create Majorana zero modes in a system
- To be a Majorana zero mode, any bound state should obey the Dirac equation and should be its own antiparticle.
- A topological superconductor is built to allow particles to meet these conditions.
Confirming the presence of Majorana zero modes is tricky
- They need to be inferred indirectly, from their effects on the surrounding material.