Bad Qubits — Learn Quantum by Doing

Multiple choice

In Shor's period-finding circuit for factoring an n-bit number N, why does the input register use 2n qubits rather than just n?

So the QFT window Q = 2^(2n) satisfies Q >= N^2, giving the continued-fraction step enough resolution
To hold two independent copies of the exponent for error checking
Because each value a^x mod N requires two qubits to store
Because the Hadamard layer needs twice as many gates as qubits