Google Has a Useful Quantum Algorithm That Outperforms a Supercomputer

"Anonymous Coward" writes:

An approach it calls "quantum echoes" takes 13,000 times longer on a supercomputer

[...] Today, Google and a large collection of academic collaborators are publishing a paper describing a computational approach that demonstrates a quantum advantage compared to current algorithms-and may actually help us achieve something useful.

Google's latest effort centers on something it's calling "quantum echoes." The approach could be described as a series of operations on the hardware qubits that make up its machine. These qubits hold a single bit of quantum information in a superposition between two values, with probabilities of finding the qubit in one value or the other when it's measured. Each qubit is entangled with its neighbors, allowing its probability to influence those of all the qubits around it. The operations that allow computation, called gates, are ways of manipulating these probabilities. Most current hardware, including Google's, perform manipulations on one or two qubits at a time (termed one- and two-qubit gates, respectively.

[...] So how do you turn quantum echoes into an algorithm? On its own, a single "echo" can't tell you much about the system-the probabilities ensure that any two runs might show different behaviors. But if you repeat the operations multiple times, you can begin to understand the details of this quantum interference. And performing the operations on a quantum computer ensures that it's easy to simply rerun the operations with different random one-qubit gates and get many instances of the initial and final states-and thus a sense of the probability distributions involved.

This is also where Google's quantum advantage comes from. Everyone involved agrees that the precise behavior of a quantum echo of moderate complexity can be modeled using any leading supercomputer. But doing so is very time-consuming, so repeating those simulations a few times becomes unrealistic. The paper estimates that a measurement that took its quantum computer 2.1 hours to perform would take the Frontier supercomputer approximately 3.2 years. Unless someone devises a far better classical algorithm than what we have today, this represents a pretty solid quantum advantage.

But is it a useful algorithm? The repeated sampling can act a bit like the Monte Carlo sampling done to explore the behavior of a wide variety of physical systems. Typically, however, we don't view algorithms as modeling the behavior of the underlying hardware they're being run on; instead, they're meant to model some other physical system we're interested in. That's where Google's announcement stands apart from its earlier work-the company believes it has identified an interesting real-world physical system with behaviors that the quantum echoes can help us understand.

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