Google’s latest quantum-computing chip, called Willow and announced in December, has made headlines for being able to perform in five minutes a calculation that would take the best of conventional supercomputers an estimated 10 trillion trillion years, far longer than the age of the universe. But that’s not actually the big deal about it.
Rather, Willow has passed a performance threshold showing that quantum computers can be made ever bigger without being overwhelmed by technical difficulties that would scupper a calculation. Many researchers in the field seem to accept such bold claims. Computer scientist Scott Aaronson of the University of Texas at Austin, a reliably hard-headed commentator on quantum computing, has called it “a real milestone for the field”.
Quantum computers store information in components (quantum bits or qubits) governed by the rules of quantum physics. This means each qubit can encode data not just as binary digits 1 and 0, as in the bits of ordinary computers, but as any “mixture” of the two. That massively expands the possibilities for computation, so that a quantum computer can carry out some calculations much faster than a conventional one. There are potential benefits of this “quantum advantage” in some areas of finance, the simulations of complex materials, searching large data banks, and a host of other tasks.
But the big catch is the problem of errors. All computers incur errors: random flipping of the value of their bits, a 1 to a 0 say. Conventional computers are protected against such errors by storing each bit in multiple copies. But quantum rules prohibit that strategy. It’s essential in quantum computation that the states of the qubits remain unknowable during the computation – but it is impossible to make a copy of an unknown quantum state and to safeguard against errors in that way. Unchecked, errors would quickly overwhelm a calculation of any real complexity.
Ways to get around this problem – to enable quantum error correction – have been a major focus since even before the first true prototype devices were created in the 2010s. Typically they involve spreading the information – the 1 or 0, you might say – across several of the qubit devices (the “physical qubits”) so that they act as a single “logical qubit” that is resistant to errors.
The idea works in theory. But since each physical qubit is error-prone, grouping ever more of them together into a logical qubit could just add more sources of error: the error rate could grow faster than the error suppression you get from spreading out the information.
Here, then, is what’s special about Willow: it passes the threshold at which the error rate gets smaller, rather than larger, as more physical qubits are used to make a logical qubit. In other words, Willow surpasses the break-even point: in theory the error rate for a logical qubit could be as small as you like, provided that you make it from enough physical qubits. In principle one could make immense quantum circuits capable of carrying out much more complicated quantum computations than have been feasible to date, with the kind of accuracy thought to be required for widespread commercial applications. The Willow chip hosts just 105 qubits, but the Google team says their current goal is to achieve a machine with a million qubits, all of them fully error-corrected.
This latest advance doesn’t come out of the blue. Researchers at Google, IBM and elsewhere have been achieving a steady reduction in error rates – and corresponding improvements in accuracy – in quantum computers over recent years. Neither does the latest advance clear the path to making quantum computers off-the-shelf products for all takers. There’s still painstaking engineering work to be done to make the basic hardware better – in particular, to give physical qubits lower intrinsic error rates and to allow them to operate coherently together for longer than the fraction of a second currently possible. There’s much to be done, too, on the software side, developing algorithms that can take advantage of quantum speed-up for a wider range of problems.
Frankly, it’s mostly a matter of “boring” engineering – as most of the work to convert a bright idea to a useful technology usually is. That shouldn’t detract, however, from the fact that the theory of quantum mechanics, which celebrates its centenary this year, is no longer just a famously esoteric and counterintuitive description of the universe at its finest grain, but is now the basis of powerful and potentially transformative technologies. It’s a classic illustration that you never know where ideas will lead.
