Quantum computing is now ready to go – or is it? Google appears to have reached an impressive milestone known as quantum supremacy, where a quantum computer is able to perform a calculation that is practically impossible for a classical one. But there are plenty of hurdles left to jump over before the technology hits the big time.
For a start, the processors need to be more powerful. Unlike classical computers, which store data as either a 0 or a 1, quantum computers use qubits that store data as a mixture of these two states.
Google’s quantum computer, called Sycamore, consisted of only 54 qubits – one of which didn’t work. For quantum computers to really come into their own, they will probably need thousands.
But scaling up the number of qubits won’t be easy. Qubits must be isolated from vibrations as they can be easily disturbed, and there are many competing ideas on how best to do this. As well as Google, IBM, Microsoft, Intel and others are all looking at how to advance the technology.
Also, on the quantum computer to-do list is error correcting codes. Classical computers have mechanisms to make sure that when little mistakes happen they are automatically rectified.
The same will be needed for quantum computers, especially considering the delicate nature of qubits. In 2016, a team from Yale University showed that error correction is possible with at least one type of qubit – although not the type used by Google. The challenge now is to build a quantum computer that has quantum supremacy, as well as error correcting codes.
The final, and perhaps biggest, next step is to actually do something useful. Google’s quantum computer tackled a task called a random circuit sampling problem. In such a problem, after a series of calculations each qubit outputs a 1 or 0. The aim is to calculate the probability of each possible outcome occurring.
Google says Sycamore was able to find the answer in just a few minutes – a task it estimates would take 10,000 years on the most powerful supercomputer. Although that is impressive, there is no practical use for it.
“We shouldn’t get too carried away with this,” says Ciarán Gilligan-Lee at University College London. This is an important step in the era of quantum computing, but there’s still a long way to go, he says.
The hope is that quantum computers could eventually help revolutionise our understanding of chemistry and material science by performing simulations that are too complicated for classical computers. “There are certain quantities that you’d like to know that you can’t easily learn from experiment and can’t calculate with supercomputers today. This is where quantum computers can help,” says Scott Aaronson at the University of Texas at Austin.
Quantum computers could also be used to crack some forms of encryption that keep the internet secure. However, people are already working on alternatives that wouldn’t be so easily broken.