Why people believe this
The phrase both 0 and 1 simultaneously appears in almost every quantum computing explainer. It is a convenient shorthand that feels intuitive because it borrows from classical logic.
The correction
A qubit in superposition is not simultaneously 0 and 1 in any classical sense. It is in a quantum state described by a complex linear combination: alpha|0> + beta|1>, where |alpha|^2 + |beta|^2 = 1. These amplitudes are not probabilities — they are complex numbers that can interfere. When you measure, you get 0 with probability |alpha|^2 or 1 with probability |beta|^2. Before measurement, the qubit does not have a definite value. Saying it is both at once imports classical intuition into a domain where it does not belong.
Try it in the simulator
What to do
Place a single H gate on q0. Switch to the Shot histogram tab and run with 1024 shots. You get roughly 50% zeros and 50% ones — random each time. Now add a second H gate after the first. Run again. You get 100% zeros. Two H gates cancel. That is interference, not two simultaneous values.
Research notes
Tags