Back in 2019, Google proudly announced they had achieved what quantum computing researchers had been looking for for years: proof that esoteric technique could outperform traditional ones. But this demonstration of “quantum supremacy” is disputed by researchers claiming to have beaten Google to a relatively normal supercomputer.
To be clear, no one is saying Google lied or misrepresented its work – the painstaking, groundbreaking research that led to the announcement of Quantum Supremacy in 2019 is still hugely important. But if this new article is correct, classical competition against quantum computing is still anyone’s game.
You can read the full story of how Google took quantum from theory to reality in the original article, but here’s the very short version. Quantum computers like Sycamore are not yet better than classical computers, except perhaps for one task: simulating a quantum computer.
It sounds like a loophole, but the point of quantum supremacy is to show the viability of the method by finding even one very specific, weird task that it can do better than even the fastest supercomputer. Because it puts the quantum foot in the door to expand that library of tasks. Maybe in the end, all tasks will be faster in quantum, but for Google’s purposes in 2019, only one was, and they showed how and why in detail.
Today, a team from the Chinese Academy of Sciences led by Pan Zhang published a paper describing a new technique for simulating a quantum computer (specifically, some of the noise patterns it emits) that appears to take a tiny fraction of the estimated time for the classic calculation to do so in 2019.
Not being an expert in quantum computing or a professor of statistical physics myself, I can only give a general idea of the technique used by Zhang et al. They presented the problem as a large 3D tensor network, with the 53 sycamore qubits represented by a grid of nodes, extruded 20 times to represent the 20 cycles that the sycamore gates went through in the simulated process. The mathematical relationships between these tensors (each its own set of interrelated vectors) were then computed using a cluster of 512 GPUs.
In the original Google article, it was estimated that achieving this simulation scale on the most powerful supercomputer available at the time (Summit at Oak Ridge National Laboratory) would take around 10,000 years – although to be clear , this was their estimate for 54 qubits doing 25 cycles; 53 qubits making 20 is considerably less complex but would still take on the order of a few years according to their estimate.
Zhang’s group claims to have done it in 15 hours. And if they had access to a proper supercomputer like Summit, it could be accomplished in seconds – faster than Sycamore. Their paper will be published in the journal Physical Review Letters; you can read it here (PDF).
These results have not yet been fully verified and reproduced by those familiar with these things, but there is no reason to believe that this is some sort of error or hoax. Google has even admitted that the baton may be passed several times before supremacy is firmly established, as it is incredibly difficult to build and program quantum computers while classical computers and their software are constantly being improved. (Others in the quantum world were skeptical of their claims initially, but some are direct competitors.)
Google offered the following comment acknowledging the march of progress here:
In our 2019 article, we said that classical algorithms would improve (in fact, Google invented the method used here for simulating random circuits in 2017, and methods of trading fidelity against computational costs in 2018 and 2019) – but the key point is that quantum technology is improving exponentially faster. We therefore do not believe that this classical approach can keep pace with quantum circuits in 2022 and beyond, despite significant improvements in recent years.
As University of Maryland quantum scientist Dominik Hangleiter told Science, this is by no means a black eye for Google or a punch to quantum in general: “The Google experiment has what she was supposed to do, start this race. »
Google may well retaliate with new claims of its own – it hasn’t stood still either. But the fact that it’s even competitive is good news for everyone involved; it’s an exciting area of computing and work like that of Google and Zhang continues to raise the bar for everyone.