Last year, Google gained international recognition when its prototype quantum computer performed a calculation in minutes that researchers estimate would make a supercomputer in 10. 000 years. That was the definition of quantum superiority – the moment a quantum machine does something impractical for a conventional computer.
On Thursday, China’s leading quantum research group made its own quantum supremacy statement in Science magazine. A system called Jiuzhang provided results in minutes that were calculated by the world’s third most powerful supercomputer, which took over 2 billion years of effort.
The two systems work differently. Google builds quantum circuits out of supercold, superconducting metal, while the team at the University of Science and Technology in Hefei recorded the result by manipulating photons and light particles.
No quantum computer is ready to do useful work yet. However, the evidence that two radically different forms of technology can outperform supercomputers will fuel the hopes – and investments – of the embryonic industry.
Google and competitors like IBM, Microsoft, Amazon, Intel and several large startups have spent a lot of money on the development of quantum computer hardware in recent years. Google and IBM are providing access to their latest prototypes over the Internet, while Microsoft and Amazon’s cloud platforms each host a Smorgasboard of quantum hardware from others, including Honeywell.
The potential performance of quantum computers is based on their basic building blocks, the so-called qubits. Like the bits in conventional computers, they can represent zeros and ones of data. However, qubits can also use quantum mechanics to achieve an unusual state, which is known as superposition and which includes the possibilities of both. With enough qubits, it is possible to use arithmetic links that conventional computers cannot – a benefit that increases as more qubits work together.
Quantum computers are not yet ruling the world because engineers have not been able to reliably assemble enough qubits. The quantum mechanical effects on which they depend are very sensitive. Google and the Chinese group were able to conduct their supremacy experiments because they managed to correct qubits in relatively large numbers.
Google’s experiment used a superconducting chip called Sycamore with 54 qubits that had cooled to fractions of a degree above absolute zero. One qubit broke, but the remaining 53 were enough to demonstrate the supremacy over traditional computers on a carefully selected statistical problem. It is unclear how many qubits a quantum computer needs to do useful work. Expert estimates range from hundreds to millions.
The Chinese team also used a statistical test to back up their claim of quantum superiority, but their quantum disks are made up of photons moving through optical circuits placed on a laboratory bench and guided by mirrors. Each photon read out at the end of the process corresponds to a qubit and shows the result of a calculation.
The researchers reported that they measured up to 76 photons from the Jiuzhang machine, but a more modest 43 on average. Members wrote code to simulate the work of the quantum system on Sunway TaihuLight, China’s most powerful supercomputer and third fastest in the world. The researchers reckon it would have taken the supercomputer more than 2 billion years to do what Jiuzhang did in just over 3 minutes.
The Chinese team was led by Jian-Wei Pan, whose extensive research team benefited from the efforts of the Chinese government to play a bigger role in quantum technology. Her accomplishments include demonstrating the use of quantum encryption over record breaking distances, including using a satellite specifically designed for quantum communications to secure a video call between China and Austria. Encryption, rooted in quantum mechanics, is theoretically unbreakable, although in practice it could still be undermined.
One difference between Jiuzhang and Google’s Sycamore is that the photonic prototype is not easily reprogrammable to perform various calculations. His settings were effectively hard-coded into his optical circuitry. Christian Weedbrook, CEO and founder of quantum computer startup Xanadu in Toronto, which is also working on the photonic quantum computer, says the result is still remarkable as a reminder that there are several viable ways to make cracking quantum numbers work bring. « It is a milestone in the photonic quantum computer, » he says, « but also good for us all. ”
Various forms of quantum hardware are being developed in science and industry. Qubits based on superconducting circuits are best known, also thanks to the heavy investments of Google and IBM. Quantum computers made from qubits based on individual atoms that float in electrical fields, so-called ion traps, are offered by the industrial giant Honeywell and startups such as IonQ and are available via the cloud services of Amazon and Microsoft.
Weedbrook, who put his first prototypes online for early customers in September, says his team can make more flexible devices than Jiuzhang, and he believes photonic quantum computers can catch up soon. They have the advantage that they use the same components that are used in many telecommunications networks.
Proponents of the photonic quantum computer and ion traps say their technologies should be easier to scale than the superconducting chips favored by IBM and Google because they don’t have to build their devices in ultra-cold refrigerators. However, no one knows exactly which form of quantum computer will come in handy first. « We all have pros and cons, » says Weedbrook.
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Quantum mechanics, quantum computers, computing, quantum superiority, Pan Jianwei, boson sampling
World news – AU – China is claiming quantum superiority
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