12.6 Accessing the IBM Quantum computer

Source: Ideas from Computational Physics (Landau, Páez, Bordeianu), §12.6–12.6.1. Original walkthrough — UI labels change over time; follow current IBM docs. Continues 12.5b. Next: 12.7 Qiskit.

Personal desktop QPUs are not a commodity yet. IBM offers cloud QPUs and simulators through the IBM Quantum platform (formerly quantum-computing.ibm.com). You will need an account (IBMid, or sometimes Google/GitHub SSO). Availability and login flows vary by region.

Account and token

1. Open the IBM Quantum login page and create / authenticate an IBMid.
2. From the dashboard, copy an API token if you will call backends from a laptop (used in 12.7).
3. Follow IBM’s current lab / Runtime docs for job submission — paths move; search “IBM Quantum Runtime get started.”

Never commit tokens to git. Store them in a local secrets file or environment variable.

12.6.1 Quantum Composer

Composer is a drag-and-drop circuit editor: colorful gate tiles, qubit wires, classical registers, and a run button that targets a simulator or a hardware backend.

Typical first Bell build:

1. File → New (or equivalent) — start with a few qubits and a classical register.
2. Delete unused qubits so only q[0] and q[1] remain.
3. Drop H on q[0].
4. Drop CNOT with control on q[0] and target on q[1].
5. Save (e.g. beta_00) and run.

Ideal outcome:

$$|\beta_{00}\rangle=\tfrac1{\sqrt2}(|00\rangle+|11\rangle),$$

visible as a two-bar histogram and matching statevector entries.

IBM endianness caveat

IBM often writes multi-qubit kets with q[0] on the right:

$$|q_{n-1}\ldots q_1 q_0\rangle.$$

So the bitstring 01 means $q_1=0$, $q_0=1$. When comparing to Cirq / textbook left-to-right ordering, reverse the labels carefully.

Troubleshooting (Composer / Runtime)

Symptom	What to check
Job sits in queue forever	Backend busy or fair-share limits — try a simulator, or pick a less popular backend
Counts look nothing like $\|\beta_{00}\rangle$	Did you measure? Wrong CNOT direction? Endianness when reading bitstrings?
Circuit “runs” but gates look rearranged	Transpile maps your gates to the device’s native set and coupling map — expect SWAPs on hardware
Simulator OK, hardware noisy	Shot noise + decoherence; increase shots, simplify depth, compare to ideal histogram
Token / auth errors	Regenerate API token; never paste an expired key from an old notebook

Endianness reminder: when a histogram bar is labeled 01, confirm whether that is $|q_1 q_0\rangle$ (IBM-style, $q_0$ rightmost) or textbook $|q_0 q_1\rangle$. Mismatched labeling is the most common “my Bell is wrong” false alarm.

Transpile reminder: hardware backends only natively support a gate set and a connectivity graph. The compiler inserts SWAPs so two-qubit gates land on linked qubits — depth grows; simulators usually skip that pain.

Takeaway

Tool	Role
Dashboard	account, token, backends
Composer	graphical circuits → Bell in minutes
Simulators	ideal histograms
Hardware	same circuit, noisier counts

Quiz

IBM Quantum / Composer

1 / 3

A Composer Bell pair starts with…

Pick an answer to see if you are right.

Next: 12.7 Qiskit + IBM.