quantum noise
Tiny disturbances that cause errors in a quantum computer while it runs.
Qubits are disturbed by heat, stray signals and imperfect control. Each disturbance can cause a small error. The errors build up over a long circuit, so the answer drifts away from the true one. Noise is the main reason today's quantum computers are limited. A machine's error rate is one of its most important numbers.
Think of it as
Static on a phone line: a little is fine, but on a long call it garbles more and more of what is said.
Example
Two machines have the same number of qubits. One makes an error in 1 operation of every 1,000, the other in 1 of every 100. On a long circuit, only the first may give a usable answer.
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