## How to build a quantum computer

• There are calculations in areas such as materials science, chemistry, medicine, and optimisation that even today’s supercomputers cannot solve.
• In many cases quantum computers will change this soon.
• Calculations that would take a thousand years on our current most powerful computers may soon take just minutes or even seconds on a quantum computer.
• But what is a quantum computer, and could you build one at home?

The quantum computer, first proposed in 1981 by the great physicist Richard Feynman, is yet to become a mainstream commercial product because it is proving extremely difficult to design and build.

As you probably know, the quantum world is a weird one, exhibiting some very strange characteristics. It is precisely these bizarre behaviours that can be leveraged to produce something remarkable.

Conventional computers – the ones we use every day – utilise bits to store and process information. So values of 0 and 1 are represented as voltages across astonishingly small transistors, and they get processed to do what we ask them via software.

### Quantum computers

A quantum computer is somewhat fancier. It uses qubits to represent not just 0 and 1, but also any other value in between. This ‘in between’ feature is down to a strangeness in quantum physics called superposition. You may have heard things like ‘quantum particles can be in many places at once.’ That’s what superposition is referring to; in simple terms, a quantum bit can represent 0 and 1, and any linear combination thereof, all at the same time.

The quantum world is a weird one, exhibiting some very strange characteristics.

The mathematics of quantum physics can be used to manipulate these superposed states all at once and allow the quantum computer to perform calculations on an unimaginable scale. After a calculation, we can observe the quantum system and allow the states to collapse to single determined results – so, back to the conventional 0s and 1s for our answer.

The key here is that the power comes from the information space. With N bits in a conventional computer, we can only represent one state at any one time. But with the superposition advantage in quantum computing, we have the equivalent of 2 to the power of N, or 2N, bits of information all at the same time. So one hundred qubits or more represents a ridiculously powerful computing potential. Importantly, the enormous information space means fewer operations are required to perform a calculation.

Take the classic ‘the travelling salesman problem.’ The aim is to compute the shortest possible route to visit a set of cities once and return to the starting point. This is computationally extremely difficult to solve for an arbitrary set of cities; every possible option must be computed.

A quantum computer could represent each possible outcome in its array of superposed states and deliver the answer with ease at great speed. Or a problem for the future – imagine a city full of self-driving cars. A quantum computer one day will be the perfect machine to immediately determine how to adjust services like traffic lights and in-car controls, to ensure optimal traffic flow.

So how can you build a quantum computer at home? The good news is you could certainly give it a go. The bad news is it’s somewhat challenging and rather costly.

First of all you’ll need to create some qubits. One popular approach is to electromagnetically secure a few individual ions (electrically charged atoms) in a confined physical space via the construction of an ion trap. Calcium ions are a good choice.

With your ion trap built, you’ll be the proud owner of a few qubits. But these precious things mustn’t interact with anything – that would interfere with their quantum properties, which you need to be in control of. So you must isolate your qubits inside an ultra-high vacuum system.

Next, you need to control the motions and the states of your qubits, for which you’ll need some lasers and electromagnetic equipment. By controlling the qubits’ motions and states you will be creating gates, which – just like in a conventional computer – allow calculations to be performed. Finally, you will use a specialist camera to observe your qubits to determine the results of your calculations.

Sounds simple. Some have tried this as a hobby. They’ve learned that a garage is essential, as is 12 months of spare time and an understanding family. Upwards of £50K would also come in handy. So this may become more than a hobby. For now, you may choose to look at some of the online simulators and quantum computer platforms available for your experimentation.

Dr James Henderson is a freelance writer based in the UK