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Roger Highfield presents your guide to Quantum Computing.

Quantum computers represent a new way to process information: they offer a way to crack what are currently thought to be unbreakable codes; model complex chemical processes, such as the way that drugs work in the body; simulate particle collisions; and answer many questions that lie beyond the capability of current ‘classical’ computers.

They should also be able to achieve all this with a business end that would only represent a tiny fraction of the hardware seen in an ordinary laptop.

Beginner’s Guide to Quantum Computing, with Seth Lloyd

Seth Lloyd is a professor of mechanical engineering and physics at the Massachusetts Institute of Technology, who proposed the first technologically feasible design for a quantum computer. Here’s his bluffer’s guide to quantum computing.

Who invented the quantum computer?

The basic idea can be glimpsed in a corner of fundamental physics born more than a century ago. The name reflects how they build on the principles of quantum mechanics, which was developed at the start of the 20th century to describe nature at the smallest scales of energy levels of atoms and subatomic particles and chunks, called quanta.

In the 1980s, it was realised that quantum theory had implications for computing, extending theoretical work done by the English mathematician Alan Turing in the 1930s, who came up with a mathematical portrait of a universal computer that would be capable of comporting itself like any other computer.

In 1985, the British physicist David Deutsch pointed out that, because Turing was working with classical physics, his universal computer was not so universal after all. Turing’s theory needed to be extended to quantum mechanics and Deutsch proposed on 8th July 1985 a universal computer based on quantum physics, which would have calculating powers that Turing’s classical computer (even in theory) could not simulate. Others who played a key role were American physicists Paul Benioff and Richard Feynman, and Soviet physicist Yuri Manin.

Quantum theory, which is deeply counterintuitive, states that when we look at particles we unavoidably alter them and that particles can be in two places at once, a quality called superposition. Moreover, two particles can be related, or ‘entangled,’ so that their properties are linked, regardless of their relative distance in space and time. Albert Einstein termed this ‘spooky action at a distance’ and this strange property means quantum computers can do simultaneous calculations, rather than one at a time.

However, David Deutsch points out: ‘There was a controversy a while ago about whether interference or entanglement was the basic phenomenon that gives quantum computing its power. I had always said interference and I still would. One can argue it either way and it’s kind of a philosophical question.’

History and Future of Quantum Computers, with Seth Lloyd

Seth Lloyd is a professor of mechanical engineering and physics at the Massachusetts Institute of Technology, who proposed the first technologically feasible design for a quantum computer. He refers to himself as a ‘quantum mechanic’ and in 2018 was interviewed by Roger Highfield of the Science Museum to discuss the rise of quantum theory and the potential of quantum computers.

What can quantum computers do that traditional ones cannot?

In 1992, the MIT mathematician Peter Shor showed that calculations that would take a normal computer longer than the history of the universe could be done in a reasonable time by a quantum computer, and the next year Seth Lloyd, now a professor at MIT, came up with the first technologically feasible design for a working quantum computer and started working on the problem.

Classical computers encode information in bits, which can take the value of 1 or 0. You can think of these 1s and 0s  as the currency of on/off switches that ultimately control how a computer works. Quantum computers, on the other hand, explained Prof Lloyd, are based on ‘qubits’, which can represent both a 1 and a 0 at the same time. Entanglement means that qubits in a superposition can be correlated with each other. Harnessing these two principles, qubits can be thought of as much more sophisticated currency for information processing, enabling quantum computers to tackle difficult problems that are intractable using today’s computers.

What is remarkable is that if you add extra qubits, the process scales geometrically: a quantum computer with two qubits could run four calculations at the same time. But a 1,000-qubit device could process more simultaneous calculations than there are particles in the known universe. However, one big issue is a kind of drop-out called decoherence, where qubits lose their useful ambiguity and become humdrum 1s and 0s.

Who is trying to build a quantum computer?

Glimpsing the power of quantum computing, recent years have seen a trickle of interest by researchers turn into a torrent. Google, IBM, Intel, venture capitalists and start-ups – and not just those from Silicon Valley – are racing to create the next generation of quantum computers to help us solve problems, like modelling complex chemical processes, and complicated quantum processes that underpin chemical reactions. ‘To be more precise, the great hope is that quantum computers will solve the electronic structure problem, meaning figuring out accurately how electrons behave in atoms and molecules, which is “intractable” on classical computers for any molecules of a realistic size and hence complexity (because the computational complexity scales exponentially),’ said Prof Peter Coveney of University College London, who leads the CompBioMed Centre of Excellence.

By the start of this century, there were one qubit machines but now we have 20 qubit chips with an expectation that will rise to around 100 by the end of this year, according to his UCL colleague, Prof John Morton, a quantum technologist.

Thousands of people are now involved in a global quest for quantum supremacy (some call it quantum advantage) which, roughly speaking, refers to the point where a quantum computer can crunch sums that a conventional computer couldn’t hope to simulate. Prof Morton expects that point to be reached this year.

The Race to Build a Quantum Computer, with John Morton

Prof John Morton, a quantum technologist at University College London and was interviewed by Roger Highfield of the Science Museum in 2018 to discuss the commercial interest in quantum computers, ‘quantum supremacy’ and more.

What is in a quantum computer?

There are several different kinds, depending on the types of qubits, how you manipulate the qubits, and how you get them to talk to one another.

One relies on a loop of superconductors – materials that have zero resistance to the flow of electricity at low temperatures – that incorporate a ‘Josephson junction’ that allows current flow clockwise and anticlockwise simultaneously, a superposition of states – in other words, precisely what you need for a qubit. In a further complication, qubits in these superconducting computers can come in different types (transmons or Xmons).

In another kind of quantum computer, qubits can be created when the spin of a single atom of phosphorus inside a silicon chip can be put into a superposition, manipulated and then read by applying a microwave pulse. Other kinds of quantum computer harness charged atoms, or ions (one, developed at the University of Sussex, can be seen in the Science Museum’s Tomorrow’s World gallery), the spin of protons, or ‘quantum dots’ in semiconductors.

Ultimately it is down to ‘empowering atoms’, according to Prof Lloyd.

Recently the University of Sussex led an international collaboration consisting of Google, Denmark’s Aarhus University, the Riken research institute in Japan and the University of Siegen in Germany to draw up a blueprint to build a large-scale quantum computer.

What uses will quantum computers be put to?

Because they operate on a fundamentally different principles to existing computers, they are suited to solving mathematical problems, like finding very large prime numbers, which is important in cryptography, so it’s likely that quantum computers could be used to crack many of the systems that keep our online information secure.

The good news is that quantum-based cryptographic systems would be much more secure than their conventional analogues, thanks to the ‘no-cloning theorem’ which forbids eavesdroppers from creating copies of a transmitted quantum cryptographic key.

Researchers are also excited about the prospect of using quantum computers to model complicated quantum processes that underpin chemical reactions, a task that taxes even the biggest supercomputers, according to Prof Peter Coveney, who does such work with a big team in University College London.

Eventually, researchers hope they’ll be able to use quantum simulations to design entirely new molecules for use in medicine. Others are studying the implications for fields such as machine learning, to uncover new patterns in nature, and in artificial intelligence.

What gives Quantum Computers their Power?

Prof John Morton, a quantum technologist at University College London and was interviewed by Roger Highfield of the Science Museum in 2018 to discuss the extraordinary power of quantum computers.

How Quantum Computers will change our lives

When will we all get a quantum computer?

Assuming that we will one day all get access to quantum computing via the cloud, Prof John Morton estimates that we will be able to tap this power in a useful way, for instance from our smartphones, within the next decade or so.