The Next Big Thing in Quantum Computing

What are qubits and quDits, and how could they affect our lives in everything from security to medicine? We break it all down.

Imagine if the brain power of the device you’re reading this on was so advanced that it could store information as multiple things simultaneously?

If you think of regular qubits as being black and white, quDits would be their cousins that can exist in any colour of the rainbow. The results could have profound implications for quantum computing, given the advance in processing power and unprecedented scale of calculations.

This is all typical quantum gobbledygook, indeed. Let’s unpack it.

What are qubits?

Quantum computers, on the other hand, delve into the realm of subatomic particles and their quantum states to store information.

So if you place one inside a magnetic field, it will align with that field no different to how a compass would with the magnetic field of Earth.

This is the lowest energy state for an electron, and is aptly called ‘spin down’. Its value is theoretically zero, or else the tiniest bit above that according to interference from energy.

Forcing it into its theoretically highest energy state, an ‘up spin’, takes energy. Think of it like twisting the dial on your compass to point north. Either way, its state is defined as one or the other.

The quantum superposition

So far, this is the same as the two separate states of one and zero on a standard computer. But as we know, in the quantum world, things can exist simultaneously, so the spin could be up or down before we measure it.

Abstractly speaking, it might seem that a qubit contains an infinite amount of information because its value can encode an infinite sequence of digits.

With zero and one as the parameters, there are infinite possibilities when decimal points come on board. An in-between number could be as simple as 0.9, or as crazy as 0.64926509473949693962.

Quantum mechanics has an important rule, however. Even though the value of a qubit can float in this limbo of possible values, a measurement, once extracted, must be an ordinary bit — a classical one or zero.

The implications for computing are enormous. In a game of chess, a quantum computer would be able to analyze all possibilities at once and simply follow through with the best option. A standard computer can only analyze situations step by step.

Quantum amigos

When qubits make friends with other qubits, they become entangled, and this is where things start to get exponential and crazy.

Now here’s a mind-blowing fact: If you had 300 qubits, i.e. 2300, you could conduct more parallel computations than there are atoms in the known universe.

This is all far too complex and abstract, so let’s discuss the applications.

Enter the mighty quDit

Now researchers have created a microchip with two entangled quDits, both in possession of 10 states, totalling 100 dimensions. QuDits are the measuring unit of quantum information in a d-level system; that is, a system where the number of states is greater than two (expressed on paper as D>2).

An entangled, high-dimensional photon could be blue, yellow, green, or red, all at the same time. The amount of information that can be stored on a single photon is therefore exponentially greater.

While qubit superpositions are incredibly fragile, the team managed to send entangled photons through a 24-km-long optical fibre telecommunications system without the photons losing their coherence. Good luck trying to hack quantum communications applications on that scale.

To open this web page, your computer used classical bits while communicating with a server potentially thousands of miles away. When a telecommunications system capable of maintaining the coherence of quDits is developed, this will have enormous implications for computing, both regular and mind-bending.

“While this will take some additional years, it is the final step required to achieve systems that can outperform classical computers and communications,” says Kues.

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