Researchers may have made a major step towards solving one of the central problems with quantum computers.
The technology, which could lead to entirely new kinds of much more powerful computers, has been hailed as potentially bringing a whole range of benefits, including allowing us to build new kinds of AI and research that could bring better batteries and other materials. It works by harnessing the strange rules that govern atoms and other extremely small particles to make computers that are not simply made up of ones and zeros, like our existing machines, but states in between.
That promise has however been held back in part by the fact that the systems have inherently chaotic behaviour. Over time, the systems can develop errors, which mean that their results are unreliable.
The new research however details an experiment in which a quantum computer was able to check and reset parts of itself as it was running. It is among the largest demonstrations of this kind of control, which is believed to be essential to make quantum computers that we can actually rely on for their transformative possibilities.
The idea works by allowing a quantum computer to spot and correct errors as they work. That could finally allow for the building of a fault-tolerant quantum computer – which works not by getting rid of errors but by correcting them as they happen – but nobody has yet managed it.
“To build a fault-tolerant quantum computer, we have to make these checks and corrections not just once or twice, but a huge number of times during a calculation,” said Jebediah Pixley, from Rutgers University, who is one of the senior authors of the study. “This experiment is a landmark on that road.”
In the experiment, researchers were also able to increase how often the quantum computer checked and reset its own systems, and saw it quickly shift from chaos into a controllable state. That confirmed a prediction about quantum behaviour and the way that order can emerge from chaos, the researchers said.
Part of the trouble with quantum computers is related specifically to how they work. A classical computer is made up of bits that can be on or off, but a quantum computer uses qubits that can have multiple possibilities between the two until they are measured.
That makes them powerful. But it also means that information is very sensitive: heat, tiny problems with the hardware or stray signals from elsewhere can alter the qubits’ state and then lead to errors that add up to make the system unreliable.
“The challenge was not simply to measure a qubit, but to do it repeatedly while the rest of the processor continued to operate,” said Maika Takita, principal research scientist for quantum error correction and dynamic circuits experiments at IBM, and another author on the study. “This experiment shows that present-day hardware can coordinate quantum operations, measurements and resets thousands of times across a large system.”
In the study, researchers used one of the most powerful quantum processors in the world, made by IBM, and set it to switch between two competing tasks. One mixed information up, spreading chaos, and another then checked those scrambled qubits and reset any that had needed it.
In all, the team did almost 5,000 operations and 5,000 checks and resets. The researchers say that is the biggest successful demonstration of that approach ever done.
The researchers could also change the mix of scrambling and controlling. When the scrambling happened more than half the time, the system stayed chaotic, and when there was more checking, the system became ordered in the way it would need to be to work.
“Small changes can lead to large and nearly impossible-to-predict consequences, like the classic idea that a butterfly flapping its wings in Africa could eventually contribute to a hurricane hitting Louisiana,” said Justin Wilson, an associate professor of physics at Louisiana State University and another co-author.
“But this out-of-control behaviour can be brought under control by randomly intervening, and there is a sharply defined rate at which that happens. It wasn’t obvious that this transition between chaos and control would survive in quantum physics, but our results show that it does.”
The work is reported in a new paper, ‘Order from chaos with adaptive circuits on quantum hardware’, published in the journal Nature Physics.
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