The Quantum Quest for a Revolutionary Computer

Quantum computing uses strange subatomic behavior to exponentially speed up processing. It could be a revolution, or it could be wishful thinking

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Photograph by Gregg Segal for TIME

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One of the closest observers of the controversy has been Scott Aaronson, an associate professor at MIT and the author of a highly influential quantum-computing blog. He remains, at best, cautious. "I'm convinced ... that interesting quantum effects are probably present in D-Wave's devices," he wrote in an email. "But I'm not convinced that those effects, right now, are playing any causal role in solving any problems faster than we could solve them with a classical computer. Nor do I think there's any good argument that D-Wave's current approach, scaled up, will lead to such a speedup in the future. It might, but there's currently no good reason to think so."

Not only is it hard for laymen to understand the arguments in play, it's hard to understand why there even is an argument. Either D-Wave has unlocked fathomless oceans of computing power or it hasn't--right? But it's not that simple. D-Wave's hardware isn't powerful enough or well enough understood to show serious quantum speedup yet, and you can't just open the hood and watch the qubits do whatever they're doing. There isn't even an agreed-upon method for benchmarking a quantum computer. Last May a professor at Amherst College published the results of a bake-off she ran between a D-Wave and a conventional computer, and she concluded that the D-Wave had performed 3,600 times faster. This figure was instantly and widely quoted as evidence of D-Wave's triumph and equally instantly and widely denounced as meaningless.

Last month a team including Matthias Troyer, an internationally respected professor of computational physics at ETH Zurich, attempted to clarify things with a report based on an extensive series of tests pitting Google's D-Wave Two against classical computers solving randomly chosen problems. Verdict? To quote from the study: "We find no evidence of quantum speedup when the entire data set is considered and obtain inconclusive results when comparing subsets of instances on an instance-by-instance basis." This has, not surprisingly, generally been interpreted as a conspicuous failure for D-Wave.

But where quantum computing is concerned, there always seems to be room for disagreement. Hartmut Neven, the director of engineering who runs Google's quantum-computing project, argues that the tests weren't a failure at all--that in one class of problem, the D-Wave Two outperformed the classical computers in a way that suggests quantum effects were in play. "There you see essentially what we were after," he says. "There you see an exponentially widening gap between simulated annealing and quantum annealing ... That's great news, but so far nobody has paid attention to it." Meanwhile, two other papers published in January make the case that a) D-Wave's chip does demonstrate entanglement and b) the test used the wrong kind of problem and was therefore meaningless anyway. For now pretty much everybody at least agrees that it's impressive that a chip as radically new as D-Wave's could even achieve parity with conventional hardware.

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