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It’s been more than 40 years since the physicist Richard Feynman pointed out that building computing devices based on quantum principles could unlock powers far greater than those of “classical” computers. In a 1981 keynote speech often credited with launching the field of quantum computing, Feynman concluded with a now-famous quip:
“Nature isn’t classical, dammit, and if you want to make a simulation of nature, you’d better make it quantum mechanical.”
It’s been nearly 30 years since the mathematician Peter Shor came up with the first potentially transformative use for quantum computers. Much of the security of the digital world is built upon the assumption that factoring large numbers is a challenging and time-consuming task. Shor showed how to use qubits — quantum objects that can exist in mixtures of 0 and 1 — to do it in a heartbeat, at least relative to known classical methods.
Researchers feel quite confident (although not entirely certain) that Shor’s quantum algorithm beats all classical algorithms because — despite the tremendous incentives — no one has successfully broken modern encryption with a classical machine. But for tasks less glamorous than factoring, it’s hard to say for sure whether quantum methods are superior. Searching for further blockbuster applications has become something of a haphazard guessing game.
“This is a silly way to go about this,” said Crystal Noel, a physicist at Duke University.
Over the last 20 years, a loose confederation of mathematically inclined physicists and physically inclined mathematicians has endeavored to more clearly identify the power of the quantum realm. Their goal? To find a way to quantify quantumness. They dream of a number they can assign to an arrangement of qubits produced by some quantum calculation. If the number is low, then it would be easy to simulate that calculation on a laptop. If it’s high, the qubits represent the answer to a truly hard problem beyond the reach of any classical device.
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How might we measure the computational power of a quantum system? Researchers have identified an assortment of physical properties that could do the trick. Peter Greenwood for Quanta Magazine
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