In 2019, word filtered out that a quantum computer built by Google had performed calculations that the company claimed would be effectively impossible to replicate on supercomputing hardware. That turned out to not be entirely correct, since Google had neglected to consider the storage available to supercomputers; if that were included, the quantum computer’s lead shrank to just a matter of days.
Adding just a handful of additional qubits, however, would re-establish the quantum computer’s vast lead. Recently, however, a draft manuscript was placed on the arXiv that points out a critical fact: Google’s claims relied on comparisons to a very specific approach to performing the calculation on standard computing hardware. There are other ways to perform the calculation, and the paper suggests one of those would allow a supercomputer to actually pull ahead of its quantum competitor.
More than one road to random
The calculation Google performed was specifically designed to be difficult to simulate on a normal computer. It set the 54 qubits of its Sycamore processor in a random state, then let quantum interference among neighboring qubits influence how the system evolves over time. After a short interval, the hardware started repeatedly measuring the state of the qubits. Each individual measurement produced a string of random bits, making Sycamore into a very expensive random-number generator. But if enough measurements are made, certain patterns generated by the quantum interference become apparent.