In quantum computers the following two effects should be seen:
If an operator acts on an arbitrary qubit $Q_n$ of a quantum system $S$ consisting of several qubits than we get a new quantum system $S'$ where the amplitudes of all qubits have changed.
If an operator acts on one qubit of a quantum system $T$ consisting of two entangled QuBits than both qubits are affected.
So which one of these is the reason for the exponential speed-up expected from quantum computers?
