12.7 Qiskit plus IBM Quantum

Source: Ideas from Computational Physics (Landau, Páez, Bordeianu), §12.7. Original sketches — package names (Aer, IBMProvider, …) evolve; check current Qiskit docs. Continues 12.6. Labs: 12.7b.

Qiskit is an open-source SDK for circuits, simulators, and IBM hardware. Running the same circuit on a noiseless simulator and on a real backend shows how decoherence and gate error tilt histograms away from ideal $|\beta_{00}\rangle$ peaks.

Local install (sketch)

conda create --name qiskit python=3.11
conda activate qiskit
pip install "qiskit[visualization]"
# IBM runtime / provider packages: follow current IBM Quantum docs

Save an API token once (local machine only):

# token_sketch.py — do not commit real tokens
# from qiskit_ibm_runtime import QiskitRuntimeService
# QiskitRuntimeService.save_account(channel="ibm_quantum", token="YOUR_TOKEN")

Bell circuit in Qiskit

# beta00_sketch.py — structure
from qiskit import QuantumCircuit

circuit = QuantumCircuit(2)
circuit.h(0)
circuit.cx(0, 1)
# circuit.draw("mpl")  # local matplotlib

Same gate order as Composer / Cirq: H on qubit 0, CNOT control→target.

Statevector and density city plot

On a statevector simulator, retrieve the amplitudes and visualize $\mathrm{Re}/\mathrm{Im}$ of the density matrix $\rho=|\psi\rangle\langle\psi|$ (city plot). For ideal $|\beta_{00}\rangle$ the imaginary part is zero and the real part shows the $|00\rangle$ / $|11\rangle$ block structure (Figure 12.6).

Graded labs lock the ideal amplitudes $1/\sqrt2$ without importing Qiskit.

Hardware path (conceptual)

1. Authenticate with your saved account.
2. Pick an operational backend with enough qubits (historically “least busy”; APIs change).
3. measure_all() (or explicit measures), transpile to the device’s native gates / coupling map.
4. Submit the job, monitor, download counts.

Expect $|00\rangle$ and $|11\rangle$ to dominate, with small leakage into $|01\rangle$ / $|10\rangle$ from noise — unlike the perfect simulator.

Composer vs Qiskit vs Cirq

	Composer	Cirq	Qiskit
UI	drag-drop	Python	Python
Default cloud	IBM	Google ecosystem	IBM
This course grades	concepts	amplitude / bit locks	amplitude / bit locks

Takeaway

Build once, compare ideal statevector vs noisy counts. Token hygiene matters. Next chapter topics (QFT, algorithms) reuse the same circuit vocabulary.

Quiz

Qiskit / IBM

1 / 3

Minimal Bell circuit gate sequence is…

Pick an answer to see if you are right.

Next: 12.7b Qiskit labs.