Quantum supremacy: How does it actually work and what is the Sycamore computer that's led to huge new breakthrough?

Scientists have finally claimed to reach quantum supremacy, a landmark in an industry that could change the world.

But the announcement has been wrapped in confusion and controversy. Though quantum computing could bring profound new processing power to the world, it is also incredibly complicated both to understand and to test.

The milestone is revealed in a new paper that has been long-rumoured but is now finally published in the prestigious journal Nature. And the world is finally scrutinising what exactly Google means by quantum supremacy, whether its claim stand up – and what happens now.

Here is everything you need to know about the breakthrough.

What is quantum supremacy?

The phrase, first coined in 2012, refers to something very specific but a little complicated to define. It is the moment at which a quantum computer is able to do something that a classical computer could not.

Reaching that point would be significant because it would seem to prove the promise that scientists have been making for decades: that an entirely new kind of computer could be created that would allow for wholly fresh – and vastly improved – ways of dealing with data. That, in turn, could revolutionise the world.

But it is important to note that it is also only a milestone: the real journey is much longer, and there is an awful lot left to do. Scientists are excited about the possibility of achieving quantum supremacy because of what it means about the process of creating really useful quantum computers, not necessarily as an end in itself.

The phrase is also disputed for many reasons. Researchers including John Preskill, who first coined the phrase, have noted that it can give unnecessary hype around our understanding of quantum computers, and that it has an unfortunate echo of white supremacy.

What's more, the very importance of the test is disputed, too. That's in part because it is difficult to say exactly when quantum supremacy has been reached, because researchers could discover that any given process was actually possible to complete on a classical computer all along.

How does quantum computing work?

To understand how a quantum computer works, it's important first to understand how a traditional one works. Everything from the phone in your pocket to the computer in your desk – and all the other places that computers appear – do so using the paradigm of classic computing, which has stayed strong for years but which quantum computers could compete with, if not displace.

In a traditional computer, everything in the memory is represented as a series of bits, which work as a binary system: they are either 1 or 0, on or off. Those bits can be assembled into the vast and complex information processing systems that we use every day, by stringing them together and analysing the data they together represent.

A quantum computer works instead with qubits, not bits – they are no longer binary, but instead can be in a number of different states. By exploiting the unusual behaviour described in quantum mechanics, scientists are able to build quantum computers, which operate at that more complex and therefore more powerful level.

Just as in a traditional computer, those qubits can be assembled into a system and used to carry out operations. In a quantum computer, that is made up of a series of quantum gates, which can in turn be assembled into a quantum circuit.

That – or a vastly longer, almost impossibly more complex version of that – is how we get to a quantum computer, which is able to exploit those various strange phenomena and use them to run operations.

Theoretically, those quantum computers and their entirely new paradigm should allow scientists to conduct those operations more quickly and in new ways. It's that which leads us to quantum superiority – and to the variety of other possibilities that lie after it.

What is Google's 'Sycamore'?

Google's great achievement – and it is certainly not the only company to try – is to boil down that complexity into a functioning quantum computer. The successful chip wasn't the first of Google's experiments, and there are more likely to come or probably already being used already, but it is the one that will go down in history as perhaps achieving the first great landmark in quantum computing.

The Sycamore chip is a 54-qubit processor. That is relatively limited, and is one of the many reasons that the discovery is not practically useful – researchers want a 100-qubit or even 200-qubit system before they are really able to put it to the test, and see whether the dreams of quantum computing are realised.

But the big breakthrough is just how high-fidelity and fast the gates that make up the computer are. Scientists have lauded the precision with which the system works, and it is that which allowed Google to make its announcement.

The actual process that it did to prove its quantum supremacy was relatively simple in output: it just gave out random numbers. But it was the workings behind it that were such a breakthrough, since it used an algorithm that would take 10,000 years to give a similar output on a classical computer, but only 200 seconds on Google's Sycamore.