Qiskit: Add Quantum Half Adder (#3405)

* Qiskit: Add Quantum Half Adder

* fixup! Format Python code with psf/black push

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

quantum/half_adder.py

@@ -0,0 +1,59 @@
+#!/usr/bin/env python3
+"""
+Build a half-adder quantum circuit that takes two bits as input,
+encodes them into qubits, then runs the half-adder circuit calculating
+the sum and carry qubits, observed over 1000 runs of the experiment
+.
+
+References:
+- https://en.wikipedia.org/wiki/Adder_(electronics)
+- https://qiskit.org/textbook/ch-states/atoms-computation.html#4.2-Remembering-how-to-add-
+"""
+
+import qiskit as q
+
+
+def half_adder(bit0: int, bit1: int) -> q.result.counts.Counts:
+    """
+    >>> half_adder(0, 0)
+    {'00': 1000}
+    >>> half_adder(0, 1)
+    {'01': 1000}
+    >>> half_adder(1, 0)
+    {'01': 1000}
+    >>> half_adder(1, 1)
+    {'10': 1000}
+    """
+    # Use Aer's qasm_simulator
+    simulator = q.Aer.get_backend("qasm_simulator")
+
+    qc_ha = q.QuantumCircuit(4, 2)
+    # encode inputs in qubits 0 and 1
+    if bit0 == 1:
+        qc_ha.x(0)
+    if bit1 == 1:
+        qc_ha.x(1)
+    qc_ha.barrier()
+
+    # use cnots to write XOR of the inputs on qubit2
+    qc_ha.cx(0, 2)
+    qc_ha.cx(1, 2)
+
+    # use ccx / toffoli gate to write AND of the inputs on qubit3
+    qc_ha.ccx(0, 1, 3)
+    qc_ha.barrier()
+
+    # extract outputs
+    qc_ha.measure(2, 0)  # extract XOR value
+    qc_ha.measure(3, 1)  # extract AND value
+
+    # Execute the circuit on the qasm simulator
+    job = q.execute(qc_ha, simulator, shots=1000)
+
+    # Return the histogram data of the results of the experiment.
+    return job.result().get_counts(qc_ha)
+
+
+if __name__ == "__main__":
+    counts = half_adder(1, 1)
+    print(f"Half Adder Output Qubit Counts: {counts}")