In its official announcement, Google said that its 54-qubit Sycamore processor was able to perform a calculation, which involved generated random numbers, in 200 seconds that would have taken the world’s most powerful supercomputer 10,000 years. In other words, says the company, its processor performed a calculation would be essentially impossible on a traditional computer, meeting the definition of " quantum supremacy ."
Not so fast, says IBM. In a blog post responding to Google's announcement, IBM researchers disputed Google's claim.
"In [Google's] paper," say the researchers, "it is argued that their device reached 'quantum supremacy' and that 'a state-of-the-art supercomputer would require approximately 10,000 years to perform the equivalent task.' We argue that an ideal simulation of the same task can be performed on a classical system in 2.5 days and with far greater fidelity."
In fact, say the researchers, this is a conservative, worst-case estimate, and they expect that with additional refinements the classical cost of the simulation can be further reduced. As a result, they say, if the term "quantum supremacy" describes the point where quantum computers can do things that classical computers can't, this threshold has not been met.
"When [Google's] comparison to classical was made, they relied on an advanced simulation that leverages parallelism, fast and error-free computation, and large aggregate RAM, but failed to fully account for plentiful disk storage," say the IBM researchers.
In contrast, IBM's classical simulation approach uses both RAM and hard drive space to store and manipulate the state vector. In addition, say the researchers, their simulation approach features a number of "nice" properties that do not directly transfer from the classical to quantum worlds, and that a number of factors could substantially accelerate their simulation, including new and better classical hardware and code optimizations to more efficiently utilize the classical hardware, as well as the potential of leveraging GPU-direct communications to run the kind of supremacy simulations of interest.