"To the untrained eye, a circuit built with IBM’s online Quantum Experience tool looks like something out of an introductory computer-science course. Logic gates, the building blocks of computation, are arrayed on a digital canvas, transforming inputs into outputs.
But this is a quantum circuit, and the gates modify not the usual binary 1 or 0 bits, but qubits, the fundamental unit of quantum computing. Unlike binary bits, qubits can exist as a ‘superposition’ of both 1 and 0, resolving one way or the other only when measured. Quantum computing also exploits properties such as entanglement, in which changing the state of one qubit also changes the state of another, even at a distance.
Those properties empower quantum computers to solve certain classes of problem more quickly than classical computers. Chemists could, for instance, use quantum computers to speed up the identification of new catalysts through modelling.
The digital logic underlying classical computers is well known: 1 AND 0 = 0, for instance. But quantum computers are much more fluid, and researchers must come to grips with how qubit states are expressed mathematically to understand how they behave. “Quantum computing is essentially matrix vector multiplication — it’s linear algebra underneath the hood,” says Krysta Svore, principal manager of the quantum-computing group at Microsoft Research in Redmond, Washington." [1]
1. Nature 591, 166-167 (2021)
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