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.

## Reference:

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

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