Powerful than previously realized

Quantum computers promise huge speedups on some computational problems because they harness a strange physical property called entanglement, in which the physical state of one tiny particle depends on measurements made of another. In quantum computers, entanglement is a computational resource, roughly like a chip’s clock cycles — kilohertz, megahertz, gigahertz — and memory in a conventional computer.

In a recent paper in the journal Proceedings of the National Academy of Sciences, researchers at MIT and IBM’s Thomas J. Watson Research Center show that simple systems of quantum particles exhibit exponentially more entanglement than was previously believed. That means that quantum computers — or other quantum information devices — powerful enough to be of practical use could be closer than we thought.

Where ordinary computers deal in bits of information, quantum computers deal in quantum bits, or qubits. Previously, researchers believed that in a certain class of simple quantum systems, the degree of entanglement was, at best, proportional to the logarithm of the number of qubits.

“For models that satisfy certain physical-reasonability criteria — i.e., they’re not too contrived; they’re something that you could in principle realize in the lab — people thought that a factor of the log of the system size was the best you can do,” says Ramis Movassagh, a researcher at Watson and one of the paper’s two co-authors. “What we proved is that the entanglement scales as the square root of the system size. Which is really exponentially more.”

That means that a 10,000-qubit quantum computer could exhibit about 10 times as much entanglement as previously thought. And that difference increases exponentially as more qubits are added.