B1: Copy Oracle

Editorial

The oracle takes single qubit computational basis states $x$ and $y$ and outputs $y' = x \oplus y$. Its truth table is as follows:

From the truth table, the bit $y$ flips only when $x = 1$. Such an operation can be achieved by using a controlled-$X$ gate ¹, with $x$ as the control qubit and $y$ as the target qubit.

Thus, the problem can be solved using the controlled-$X$ gate.

Sample Code

Below is a sample program:

from qiskit import QuantumCircuit, QuantumRegister
 
 
def solve() -> QuantumCircuit:
    x, y = QuantumRegister(1), QuantumRegister(1)
    qc = QuantumCircuit(x, y)
 
    qc.cx(x, y)
 
    return qc