Big Blue says Google’s claim to have solved a problem that classical computers can’t doesn’t “meet the threshold” of proof.
Google created quite a stir a few weeks ago when a NASA scientist accidentally posted--and then quickly took down--a research paper on the NASA Ames Research Center website that claimed Google had achieved “quantum supremacy, defined as the point where quantum computers can do things that classical computers can’t. I wrote about the leaked paper here and here .
Last week, the Google paper and claim reappeared in the prestigious scientific journal Nature . As before, it argued that its device reached “quantum supremacy” and that “a state-of-the-art supercomputer would require approximately ‘10,000 years’ to perform the equivalent task.
In a blog response , IBM scientists Edwin Pednault, John Gunnels, Dmitri Maslov, and Jay Gambetta cry foul and suggest that Google re-cork the champagne. They write:
Based on the original meaning of the term “quantum supremacy,” as proposed by John Preskill (the Richard P. Feynman Professor of Theoretical Physics at CalTech) in 2012…this threshold has not been met.
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. This is in fact a conservative, worst-case estimate, and we expect that with additional refinements the classical cost of the simulation can be further reduced.
Then, of course, the IBM paper heads for the high weeds, but hang in there:
This particular notion of “quantum supremacy” is based on executing a random quantum circuit of a size infeasible for simulation with any available classical computer. Specifically, the paper shows a computational experiment over a 53-qubit quantum processor that implements an impressively large two-qubit gate quantum circuit of depth 20, with 430 two-qubit and 1,113 single-qubit gates, and with predicted total fidelity of 0.2%. Their classical simulation estimate of 10,000 years is based on the observation that the RAM memory requirement to store the full state vector in a Schrödinger-type simulation would be prohibitive, and thus one needs to resort to a Schrödinger-Feynman simulation that trades off space for time.
For those of us who are not theoretical physicists and are probably more concerned about Mr. Schrödinger’s poor cat than anything else, a little review might be handy.
Unlike traditional computers which use a stream of electrical or optical pulses called bits, representing 1s or 0s, quantum computers use qubits, which are typically subatomic particles such as electrons or photons. At the quantum level, qubits can represent thousands of possible combinations of 1 and 0—a quality called superposition of states that allows them to solve problems by simultaneously considering numerous possibilities. A connected group of qubits can provide far more processing power than the same number of binary bits.
There are currently 53-qubit processors in existence: one from IBM and one at Google. IBM has made its effort an open-source communal effort inviting virtually any qualified entities or individuals to access its machine through the cloud.
Google’s main failing in its simulation, according to IBM researchers, was to underestimate some of the strengths of high-performance classsical computers:
The concept of “quantum supremacy” showcases the resources unique to quantum computers, such as direct access to entanglement and superposition. However, classical computers have resources of their own such as a hierarchy of memories and high-precision computations in hardware, various software assets, and a vast knowledge base of algorithms, and it is important to leverage all such capabilities when comparing quantum to classical.
The IBM reseachers do pay grudging respect to their Google peers:
Building quantum systems is a feat of science and engineering and benchmarking them is a formidable challenge. Google’s experiment is an excellent demonstration of the progress in superconducting-based quantum computing, showing state-of-the-art gate fidelities on a 53-qubit device, but it should not be viewed as proof that quantum computers are “supreme” over classical computers.
A headline that includes some variation of “Quantum Supremacy Achieved” is almost irresistible to print, but it will inevitably mislead the general public. First because, as we argue above, by its strictest definition the goal has not been met. But more fundamentally, because quantum computers will never reign “supreme” over classical computers, but will rather work in concert with them, since each have their unique strengths.
This is shaping up as a great food fight, and I can't wait to see what happens next.