Why people believe this
A 1% error rate per gate sounds small. Classical computers tolerate noise too — error correcting codes in memory, checksums in storage. Surely a little quantum noise just slightly degrades results.
The correction
Quantum noise is catastrophic in a way classical noise is not. A 1% depolarizing error per gate on a 100-gate circuit gives an overall fidelity of roughly 0.99^100 = 36%. The result is nearly useless. Classical bits can be copied and checked — quantum states cannot be cloned, so error correction requires enormous overhead. On NISQ devices with no error correction, even shallow circuits of a few dozen gates produce unreliable results.
Visual demonstration
Fidelity decay by error rate per gate
At just 1% error per gate, a 100-gate circuit retains only 37% fidelity. At 5% it collapses to near zero.
Try it in the simulator
What to do
Load the Bell state. Simulate with noise at 0% — perfect 50/50. Now drag the noise slider to 5%, then 10%, then 20%. Watch the distribution degrade toward a uniform mixture. At 20% error per gate the Bell state is nearly destroyed after just 3 gates.
Research notes
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