Qiskit Tutorial: Building Your First Quantum Circuit

Ready to write your first quantum program? In this tutorial, we will use Qiskit, the open-source Python SDK developed by IBM, to build a basic quantum circuit that creates a Bell State (a perfectly entangled state of two qubits).

Let's get started.

Prerequisites

Ensure you have Python installed (version 3.10 or higher is recommended). You can install Qiskit using pip from your terminal:

pip install qiskit qiskit-aer

qiskit is the core SDK. qiskit-aer is the high-performance local simulator package.

Step 1: Initialize the Circuit

We will create a quantum circuit with two qubits and two classical bits. The classical bits are used to store the results of measuring the qubits.

Create a new Python file named bell_state.py and add the following imports:

from qiskit import QuantumCircuit
from qiskit_aer import AerSimulator
Create a Quantum Circuit acting on 2 qubits and 2 classical bits
qc = QuantumCircuit(2, 2)

Step 2: Apply Quantum Gates

To entangle our two qubits, we need to perform two steps:

1. Put the first qubit (qubit 0) into superposition. We do this using a Hadamard (H) Gate.
2. Entangle the two qubits. We do this using a Controlled-NOT (CNOT) Gate where qubit 0 is the control and qubit 1 is the target.

# Apply a Hadamard gate to qubit 0
qc.h(0)
Apply a CNOT gate with control qubit 0 and target qubit 1
qc.cx(0, 1)

Step 3: Add Measurement

Now, we must measure our qubits. The measurement collapses the quantum state into a classical 0 or 1, which we store in the classical bits.

# Measure both qubits into the classical bits
qc.measure([0, 1], [0, 1])

Step 4: Simulate the Circuit

We will use Qiskit Aer's AerSimulator to run this circuit 1,024 times. Since the qubits are entangled, we expect to measure either 00 or 11 roughly 50% of the time each. We should almost never see 01 or 10.

# Initialize the simulator
simulator = AerSimulator()
Run the circuit on the simulator
job = simulator.run(qc, shots=1024)
Get the results
result = job.result()
counts = result.get_counts()
print("Measurement Counts:", counts)

Run the Code

Execute the script in your terminal:

python bell_state.py

You should see output similar to this:

Measurement Counts: {'00': 507, '11': 517}

As expected, the qubits collapsed into the same state almost every time, proving they were entangled. Congratulations. You have successfully programmed and simulated your first quantum circuit.