*Quantum physics is perhaps one of the most puzzling areas of modern scientist to the non-specialist. And yet, the implications of our understanding of this branch of physics are potentially vast. One of the most promising future applications of quantum physics is in the field of computing, as Dr Tom Kemp describes…*

Only quantum physicists really understand quantum physics, and even they have trouble explaining in everyday terms the strange behaviour of sub-atomic particles such as electrons and photons. An electron for example can in effect be in more than one state of spin at once, or even in more than one place at the same time, a phenomenon called “superposition”. Only when it is pinned down, by observing it using a scientific instrument is it seen to have one particular spin or to occupy one particular location. A simple analogy is with tossing a coin. While the coin is spinning in the air, it could be thought of as *both* heads and tails at the same time rather than one or the other. Only when it lands do we see it as *either* heads or tails.

The information handled by a conventional computer is based on “bits”, which are like little gates that can be either open or closed – spoken of as 1 or 0. Imagine three such bits linked together. They can represent eight different combinations (000, 010, 001, 011, 100, 110, 101, and 111), but only one at a time. Quantum computers are based on what are called “qubits”, each of which can be both 0 and 1 at the same time. Three linked qubits can represent the same eight combinations, but thanks to superposition, they can do it simultaneously. Therefore, in principle, a quantum computer can make its calculations vastly faster than a conventional one. For example, in 2019, Google’s experimental quantum computer called Sycamore used a mere 53-qubit chip to perform a task in 200 seconds that they claimed would have taken the world’s fastest conventional computer 10,000 years (others reckoned it would only have taken it two and a half days, but that is still a pretty impressive difference!).

Another very strange property of quantum behaviour is called “entanglement”, in which a pair of particles in different, even quite distant places retain a mysterious connection to one another. Any interaction with one particle will have an instantaneous effect on the state of the other particle, even though nothing, not even information, should be able to travel faster than the speed of light. In principle this could be the basis of a hugely powerful quantum version of the internet.

The kinds of problems quantum computing would be particularly valuable for are those with a huge number of independent variables. For example, breaking a very large (say 500 digit) number down to its prime factors is presently impossible in a realistic length of computing time. A quantum computer could therefore be important in both cracking and creating many encryption systems based on prime numbers. Another deceptively simple problem is discovering the optimal route for a delivery van delivering to a range of different places. A more obviously valuable potential use is to simulate the reactions of complex biological molecules such as enzymes, leading to the design of possible new drugs and other medical treatments. Machine learning and artificial intelligence would be hugely advanced by the ability to take into account so quickly many more variable factors, for example in driverless cars, remote weapons systems, and weather and climate forecasts. By the process of entanglement, many independent telescopes could be linked together to revolutionize the study of the Universe.

Not everybody is optimistic that building a practical quantum computer is imminent. At present there are still formidable engineering difficulties to overcome. Quantum states of subatomic particles are extremely unstable and short-lived, and can only be maintained at a temperature fractionally above absolute zero. Even then, the ubiquitous background radiation of the Universe tends to cause them to break down. This is called decoherence and requires continuous, subtle corrections to be made. However, given the huge scientific, social, commercial and military benefits that would come from such a large increase in computing power, it is not surprising that the big IT corporations – Google, Microsoft, IBM etc.- as well as national governments around the world are spending eye-watering sums of money on research into quantum computing. Nevertheless, we are probably still a few decades away from solving the practical problems and having the everyday company or desktop quantum computer.

But watch this space!

**Dr Tom Kemp** is Emeritus Research Fellow in Biology at St John’s College, Oxford, and Associate Editor of the Inspire Programme for Years 9, 10 & 11

## Further Resources

What is quantum computing? – IBM

How does a quantum computer work? – Scientific American

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