This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Quantum computers hold great promise for applications from drug discovery to cybersecurity. Yet figuring out what would give quantum computers their edge over everyday "classical" computers is a subtle problem.
A new theoretical study led by researchers at the Cavendish Laboratory shows that quantum computers are harder to make powerful than previously assumed while offering the clearest picture yet of what actually makes them work. The research, published in Physical Review Letters, helps identify the precise quantum states that are genuinely useful for quantum computation. It also increases the set of quantum calculations known to be easy for classical computers, meaning quantum devices face a higher bar to demonstrate an advantage.
At the heart of quantum computing is a kind of "magic." Quantum computers typically work with quantum bits, or qubits: particles such as electrons or atoms that can exist in two states at once. To run algorithms that outperform any classical computer, those qubits must be prepared in special starting configurations known as "magic states." These states act as the computational fuel: Without them, a quantum computer is no better than a conventional machine. But the new research expands scientists' understanding that not all magic states are equal.
Many states that appear "magic" and were previously assumed to be useful turn out to offer no quantum advantage. By identifying this class of useless magic states, the team redraws the boundary between calculations that need a quantum computer and those that can still be handled classically. "We're showing that magic is necessary but not sufficient to unlock quantum computers' full power," said Dr.
David Arvidsson-Shukur, from the Hitachi Laboratory at the Cavendish Laboratory. "If a quantum state is not magic, you can't get a quantum advantage. But having magic alone doesn't guarantee you have one either.
The picture is more nuanced and much more interesting than that." To identify which quantum states have what the team classifies as "useful" magic and which have the "useless" kind, the researchers turned to a mathematical framework developed in Cambridge in 1945 by Paul Dirac, the physicist behind the relativistic quantum equation that predicted antimatter. Working at St John's College, Dirac independently established a distribution similar to one introduced a decade earlier by MIT's John Kirkwood and extended the idea by building the mathematical framework in which to use it. This became known as the Kirkwood–Dirac distribution.
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