5 Dec 2018, By David Schneider
Experts in the field give a hard-headed assessment of just how hard the technology is
In November, IEEE Spectrum published an expert-written feature titled, “The Case Against Quantum Computing.” In it, the author, Mikhail Dyakonov, defended his view that practical general-purpose quantum computers would not be built anytime in the foreseeable future. As you might expect at this time of great enthusiasm for quantum computing, the article ruffled more than a few feathers.
As it turns out, while that article was being prepared, a committee of experts assembled by the U.S. National Academies of Science, Engineering and Medicine had been grappling with the very same question: When will quantum computers mature into something of practical commercial value? More fully, the task before the committee was to provide “an independent assessment of the feasibility and implications of creating a functional quantum computer capable of addressing real-world problems. . . .” In particular, the committee was to estimate “the time and resources required, and how to assess the probability of success.”
That committee released its 205-page report to the public yesterday, and experts from the committee conducted a webinar about it for reporters earlier in the day. Having edited “The Case Against Quantum Computing,” I was very much interested to find out what the group had concluded.
The committee consisted of 13 experts in quantum computing, including: John Martinis of the University of California, Santa Barbara, who heads Google’s quantum-hardware efforts; David Awschalom of the University of Chicago, who formerly directed the Center for Spintronics and Quantum Computation at UCSB; and Umesh Vazirani of the University of California, Berkeley, who co-directs the Berkeley Quantum Information and Computation Center. So this task force was made up of people who you might expect to give an optimistic answer the question put to them.
Yet to their credit, the committee members did not sugar coat their assessment of the difficulties that they and other researchers face in their efforts to design and build practical, general-purpose quantum computers. Quite the opposite. Indeed, as I read through various parts of the report, I repeatedly found justification for Dyakonov’s skepticism about the prospects for quantum computing.
This stands in contrast to the rosy picture of the field you’ll find in much of the popular press—one in which quantum computers that can do all sorts of wonderful things are said to be maybe 5 or 10 years away. If “The Case Against Quantum Computing” didn’t disabuse you of that notion, the new NAS report should. In it, the committee highlights as a “key finding” that:
Given the current state of quantum computing and recent rates of progress, it is highly unexpected that a quantum computer that can compromise RSA 2048 or comparable discrete logarithm- based public key cryptosystems will be built within the next decade.
Let me translate. Compromising this kind of encryption is just one example of the kinds of computations that quantum computers are, in principle, suited for. So here the committee is saying in no uncertain terms that there’s slim prospects for anyone being able to build a quantum computer with such practical capabilities in the next decade.
Okay, if not a decade, then how long might it be before we see practical machines? The committee members were not prepared to commit themselves to any estimate. The most they felt they could offer was some general guidance to the quantum computing community for how to report on progress in a way that would make such an estimate possible.
Bravo to the committee members for not pretending that they had an answer. Indeed, the report summary makes clear that the challenges are enormous and that “there is no guarantee that all of these challenges will be overcome.” Such statements echo what Dyakonov wrote in “The Case Against Quantum Computing.” The much longer committee report elaborates on many of the reasons for Dyakonov’s pessimism, and it adds a few more I wasn’t aware of.
In particular, the committee describes the challenge in creating a self-reinforcing cycle of technical progress that leads to commercial applications that lead to private-sector investment that leads to further technical progress. Such a cycle has powered the enormous advances in conventional computing over the decades. So you can understand why enthusiasts of this approach would want things to play out similarly for quantum computing. That would require that the kinds of quantum computers that can be built in the near term serve have least some commercial applications. And that might not be in the cards.
The kind of machine that might soon be built, something the committee calls a “noisy intermediate-scale quantum computer” or NISQ computer, probably isn’t going to be of much practical use. “There are at present no known algorithms/applications that could make effective use of this class of machine,” says the committee. That might change. Or it might not. And if it doesn’t, it seems unlikely that industry will keep investing in quantum computing for long enough for the technology to pay dividends.
In the end, the committee concludes that even if a practical general-purpose quantum computer ultimately proves impossible to construct, people will learn a lot in their failed efforts to design and build one. “Key Finding” number 6 of the report reads:
Quantum computing is valuable for driving foundational research that will help advance humanity’s understanding of the universe. As with all foundational scientific research, discoveries in this field could lead to transformative new knowledge and applications.
Yes, sure. Even skeptic Dyakonov would agree with that. In “The Case Against Quantum Computing,” he acknowledges that “experimental research [on quantum computing] is beneficial and may lead to a better understanding of complicated quantum systems.” But as the committee members themselves point out, fundamental research of any kind is worthy, because it might lead to something else of use or benefit to society. The trick for governments and other patrons of science and engineering is figuring out which kinds of fundamental research are more worthy than others. And for that, this new NAS report appears most helpful, even if it only serves as a wake-up call to those who had assumed that the quantum-computing enterprise was guaranteed to be destined for success.