Scientific breakthroughs don’t always go exactly to plan. On Friday, a draft research paper erroneously uploaded to Nasa’s website accidentally tipped the world off that Google had reached a quantum computing milestone: quantum supremacy.
It’s a goal that Google – and its competitors – had in their sights for years. In 2017, the firm predicted it’d reach quantum supremacy by the end of that year, but that deadline came and went without any breakthrough. In the intervening years IBM and Intel nipped at Google’s heels, testing quantum computers with ever increasing numbers of qubits – the units of information that are the reason that quantum computers are so potentially powerful.
Now it appears that Google has reached this particular milestone ahead of its competitors. The draft paper details how Google researchers used a quantum processor called Sycamore containing 53 functioning qubits to solve a random sampling problem that would have taken the world’s best supercomputers 10,000 years to work out. It took Sycamore just three minute and 20 seconds. Google, which partnered with Nasa for this project, did not respond to requests for comment.
But this breakthrough doesn’t mean that useful quantum computers are just around the corner. Not by a long shot. Instead, Google has just kicked open the door to the next era of quantum computing. And that’s where things start to get really interesting.
Quantum supremacy isn’t quite as exciting as it sounds, says Simon Benjamin, professor of quantum technologies at the University of Oxford. It just means the moment that a quantum computer completes a task that conventional computers find impossible. In Google’s case, that meant telling the quantum computer to run a random set of instructions and then measure the results. The researchers then tried to get a supercomputer to predict what the quantum computer would produce, to make sure that the results really were only achievable by a quantum computer.
This task is – practically speaking – pointless. It’s good at sorting out quantum computers from their classical kin, but that’s it. And that means that, in some ways, quantum supremacy is pointless too. “It’s [a milestone] that must be passed to get to the really good stuff, but it is not going to have any immediate consequences at all,” says Benjamin.
Some researchers, including Benjamin, reckon that the term quantum supremacy, which was popularised by the theoretical physicist John Preskill, is a little overblown. Benjamin has a suggested alternative – quantum inimitability – which he says conveys the technical aspects of supremacy a little more accurately, without inviting any Terminator-esque comparisons.
But just because the actual task Google used to demonstrate supremacy is pointless, it doesn’t mean that this isn’t a big deal. “[The supremacy test] aims to be something that gives you very high confidence than we’ve ever had before that this machine is doing something computationally that cannot be replicated on a classical computer,” says Toby Cubitt at University College London’s Quantum Science and Technology Institute.
That’s big news, and a massive stamp of approval for quantum computing. In October 2017, IBM demonstrated that it could simulate quantum computers with 56 qubits on non-quantum supercomputers. And if you can simulate quantum computers, why bother going through the trouble of creating room-size device that needs to be cooled to within a whisker of absolute zero?
Google’s paper waves a massive flag reminding everyone that there is a whole category of calculations that can only be solved by using quantum computers, and opens the door to people who might actually want to start designing problems to be solved by those computers.
But what comes next is much more exciting. Google’s experiment puts a line in the sand between two eras of quantum computing. Welcome to the supremacy era. Ahead of us is something called noisy intermediate-scale quantum (NISQ). If the end goal for quantum computing is to be able to run calculations that no classical computer could dream of running – complex molecular interactions or cracking cryptographic codes – then NISQ is like quantum’s awkward infancy.
The problem is that even the most promising quantum computers are disastrously error prone. Imagine asking a computer to perform a calculation and once every 1,000 times or more it’d just spit out a completely random answer. And that’s per calculation – a programme could be made up of millions of individual calculations, so Google’s Sycamore processor would be hopeless at tackling any kind of useful quantum problem.
The NISQ era is all about making quantum computers that just about work, despite all this noise. The key, Benjamin says, is working out how to squeeze value out of imperfect machines. That could mean engineering programmes that are simple enough to run on quantum computers while also yielding useful results. And the other part will be improving hardware so that quantum computers produce useful results while still only using a relatively low number of qubits.
Benjamin has a name for the moment we crack the challenge of making noisy quantum computers that are actually useful: quantum advantage. “[It’s about] how we can squeeze value out of imperfect machines,” he says. “At least then we’ll be in an era when quantum computers have started to become useful tools.”
And if we get past quantum advantage, that’s when we can start thinking about the big challenges that only quantum computers can solve, such as factorising very large numbers, or modeling quantum mechanics. This will require a different kind of quantum computer altogether, one that has ironed out all of its errors and uses millions of qubits to make computations.
While Google might have the lead now, there’s no way of knowing whether its quantum computer – based on superconducting electronic circuits – will be the basis of future, useful, quantum computers. “It’s hard to predict which hardware will lead to a million qubit computer,” says Natalia Ares at the University of Oxford. In the early years of quantum computing ion traps were the most popular kind of hardware, but that was later leapfrogged by the superconducting machines favoured by Google and IBM.
For Ares, the Google news is exciting, but it’s important not to expect useful quantum computers any time soon. The largest quantum computer to date is Google’s 72-qubit processor called Bristlecone, and that hasn’t even demonstrated quantum supremacy. We’re a really long way off the millions of qubits we’ll need to crack difficult quantum problems. “We are far from doing anything very useful but it shows that we are making progress,” Ares says. Google may have inched ahead for now, but the marathon is only getting started.
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