Quantum computers can be simulated by classical computers up to a certain point. Note that the 'certain point' here is a practical limit of time/efficiency. I

Quantum computers can be simulated by classical computers up to a certain point. Note that the 'certain point' here is a practical limit of time/efficiency. In terms of computability, anything that can be computed on a quantum computer can be computed on a classical computer with exponential slowdown. It's the exponential slowdown that results in the practical limit.