Plan for quantum supremacy

Things are getting real for researchers in the UC Santa Barbara John Martinis/Google group. They are making good on their intentions to declare supremacy in a tight global race to build the first quantum machine to outperform the world’s best classical supercomputers. But what is quantum supremacy in a field where horizons are being widened on a regular basis, in which teams of the brightest quantum computing minds in the world routinely up the ante on the number and type of quantum bits (“qubits”) they can build, each with their own range of qualities?
“Let’s define that, because it’s kind of vague,” said Google researcher Charles Neill. Simply put, he continued, “we would like to perform an algorithm or computation that couldn’t be done otherwise. That’s what we actually mean.”
Neill is lead author of the group’s new paper, “A blueprint for demonstrating quantum supremacy with superconducting qubits,” now published in the journal Science. Fortunately, nature offers up many such complex situations, in which the variables are so numerous and interdependent that classical computers can’t hold all the values and perform the operations. Think chemical reactions, fluid interactions, even quantum phase changes in solids and a host of other problems that have daunted researchers in the past. Something on the order of at least 49 qubits — roughly equivalent to a petabyte (one million gigabytes) of classical random access memory — could put a quantum computer on equal footing with the world’s supercomputers. Just recently, Neill’s Google/Martinis colleagues announced an effort toward quantum supremacy with a 72-qubit chip possessing a “bristlecone” architecture that has yet to be put through its paces. But according to Neill, it’s more than the number of qubits on hand.
“You have to generate some sort of evolution in the system which leads you to use every state that has a name associated with it,” he said. The power of quantum computing lies in, among other things, the superpositioning of states. In classical computers, each bit can exist in one of two states — zero or one, off or on, true or false — but qubits can exist in a third state that is a superposition of both zero and one, raising exponentially the number of possible states a quantum system can explore. Additionally, say the researchers, fidelity is important, because massive processing power is not worth much if it’s not accurate. Decoherence is a major challenge for anyone building a quantum computer — perturb the system, the information changes. Wait a few hundredths of a second too long, the information changes again. Agencies

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