They can be simulated on classical computers, but in exponential time. In general, you need 2^n bits to represent n qubits. You can see this in the case of a b

They can be simulated on classical computers, but in exponential time. In general, you need 2^n bits to represent n qubits. You can see this in the case of a basic quantum gate, the hadamard gate: H(a) produces a qubit with equal probably of being 1 or 0 if observed; H(H(a)) will always return the original value of A. Doing this to an entangled vector of qubits, performing another transform, then hadamard again produces a quantum circuit that takes an impractical number of classical bits to simulate.