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Does a quantum computer try every answer at once?

Myth · Most news explainers, and any film with a glowing chip

Stretched. The mechanism is real, and the scene pushes it past what it does.

What the scene or the claim says

Superposition lets a quantum computer test every possibility in parallel, so hard problems fall in an instant.

What is real

n qubits can hold a superposition over all 2ⁿ bit strings at once. A quantum algorithm then arranges interference so that wrong strings cancel and the right ones reinforce. That steering is the working part, and it is real.

What stops it working that way

• Measurement returns one string, chosen at random by its amplitude. Without the interference step, you read out a random guess.
• Only some problem structures allow that steering. For unstructured search, Grover's algorithm gives a square-root speedup, and Bennett, Bernstein, Brassard and Vazirani proved in 1997 that in the black-box (oracle) model no quantum algorithm can do better.
• So the exponential speedups on offer are for particular problems: factoring, simulating quantum systems, and a short list of others.

If you wanted it to work

The list of problems with the right structure is an open research question, and each new entry changes what these machines are for. The claim is easy to test on any advertised advantage: ask what structure the algorithm uses to make wrong answers cancel. If there is no answer, there is no speedup.

Learn the physics

• Why do these algorithms work?
• Is quantum computing just quantum mechanics?

Sources

• Grover 1996, quantum search
• Bennett et al. 1997, strengths and weaknesses

See it in the swipe deck, with the other cards.