China reaffirms its 'quantum supremacy' after upgrading its Zuchongzhi processor

Just a few months ago, in mid-July, China claimed 'quantum supremacy' by getting its Zuchongzhi computer to solve a problem in just 70 minutes that would have taken the best of classical computers eight years. Now, the same team of researchers has updated its quantum computer, which this time has managed to solve in just 4.2 hours a calculation that any other computer would have taken thousands of years to complete. This demonstration of what scientists themselves have called 'quantum computational advantage' was achieved using 6 more qubits (quantum bits) than the Google team used in 2019. The results of the experiment now appear on the arXiv prepublication server.

'Quantum Supremacy' is the name given to the point at which a quantum computer demonstrates its ability to complete a calculation that a classical computer could not perform in a reasonable period of time. In 2019, Google first announced that it had reached this point when its Sycamore processor managed to simulate a quantum circuit in record time and sample random numbers at its output. It was a proof of concept, with no practical applications, but it served to demonstrate that quantum computers were a reality.

The war for 'quantum supremacy'

The Google team then used 54 qubits (although only 53 worked) to perform, in 200 seconds, a calculation that would have taken a classical computer 10,000 years. Shortly after, however, IBM refuted these results by claiming that its own (classic) machines could do the same in 2.5 days, and not thousands of years.

Just a few months later, a team from the University of Science and Technology of China (USTC) managed to solve a problem much more difficult than Google's calculation. And they did it in just 70 minutes. The processor, Zuchongzhi, had 66 qubits, but for that test only 56 were used, three more than Google had used.

And now, updating the same processor, (Zuchongzhi 2.1), the same team has used 60 qubits to solve a problem that the researchers say is another three orders of magnitude more difficult than their previous experiment.

Chinese scientists in their paper said that quantum computers are emerging with the promise of solving certain computational tasks exponentially faster than current classical computers. Limited by the development of quantum hardware, quantum computing has been in the small-scale demonstration stage for several decades in the past. Recently, thanks to significant progress in superconducting and photonic platforms, the long-awaited milestone of quantum computing advantage or supremacy has been achieved using the Sycamore, Jiuzhang and Zuchongzhi quantum processors successively. These quantum processors with dozens of qubits reveal remarkable potential for quantum computing to offer new capabilities for short-term applications such as quantum simulation.

However, the authors consider that "the quantum computational advantage will be a long-term competition between classical simulation and quantum devices, rather than being a one-time experimental demonstration." In fact, "classical simulation algorithms continue to evolve," to the point where "they are almost on the verge of nullifying the quantum computational advantage that Sycamore has achieved." Therefore, "quantum hardware must be continually updated to maintain quantum computational advantage."

In their article, Qingling Zhu and his colleagues state that their updated processor has 'less noise' and is more reliable than the previous one, and that in the future they will try to improve error correction, one of the main problems to be overcome by quantum computing. 


Quantum Computational Advantage via 60-Qubit 24-Cycle Random Circuit Sampling, arXiv:2109.03494

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