Bad Qubits — Learn Quantum by Doing

Multiple choice

To measure the Z-type plaquette check

Z_0 Z_1

with a fresh ancilla a prepared in

|0\rangle

, two CNOTs are applied: CNOT(0->a) then CNOT(1->a). What value does the ancilla hold afterward, for data

|q_0 q_1\rangle

The product $q_0 \cdot q_1$ (logical AND of the two data bits)
A copy of $q_0$ only, ignoring $q_1$
The parity $q_0 \oplus q_1$ , with the data register left untouched in $|q_0 q_1\rangle$
The data qubits collapsed to a single computational basis state