added script to perform quantum entanglement (#3270)

* added code to perform quantum entanglement

* Update quantum_entanglement.py

quantum/quantum_entanglement.py

+#!/usr/bin/env python3
+"""
+Build a quantum circuit with pair or group of qubits to perform
+quantum entanglement.
+Quantum entanglement is a phenomenon observed at the quantum scale
+where entangled particles stay connected (in some sense) so that
+the actions performed on one of the particles affects the other,
+no matter the distance between two particles.
+"""
+
+import qiskit
+
+
+def quantum_entanglement(qubits: int = 2) -> qiskit.result.counts.Counts:
+    """
+    # >>> quantum_entanglement(2)
+    # {'00': 500, '11': 500}
+    #      ┌───┐     ┌─┐
+    # q_0: ┤ H ├──■──┤M├───
+    #      └───┘┌─┴─┐└╥┘┌─┐
+    # q_1: ─────┤ X ├─╫─┤M├
+    #           └───┘ ║ └╥┘
+    # c: 2/═══════════╩══╩═
+    #                 0  1
+    Args:
+        qubits (int): number of quibits to use. Defaults to 2
+    Returns:
+        qiskit.result.counts.Counts: mapping of states to its counts
+    """
+    classical_bits = qubits
+
+    # Using Aer's qasm_simulator
+    simulator = qiskit.Aer.get_backend("qasm_simulator")
+
+    # Creating a Quantum Circuit acting on the q register
+    circuit = qiskit.QuantumCircuit(qubits, classical_bits)
+
+    # Adding a H gate on qubit 0 (now q0 in superposition)
+    circuit.h(0)
+
+    for i in range(1, qubits):
+        # Adding CX (CNOT) gate
+        circuit.cx(i - 1, i)
+
+    # Mapping the quantum measurement to the classical bits
+    circuit.measure(list(range(qubits)), list(range(classical_bits)))
+
+    # Now measuring any one qubit would affect other qubits to collapse
+    # their super position and have same state as the measured one.
+
+    # Executing the circuit on the qasm simulator
+    job = qiskit.execute(circuit, simulator, shots=1000)
+
+    return job.result().get_counts(circuit)
+
+
+if __name__ == "__main__":
+    print(f"Total count for various states are: {quantum_entanglement(3)}")