> For the complete documentation index, see [llms.txt](https://docs.sirraya.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.sirraya.org/examples/bell_state_end_to_end.md).

# bell\_state\_end\_to\_end

The smallest complete run through the real pipeline — the right first example to run.

```bash
cargo run --example bell_state_end_to_end
```

## What it does

Parses a two-line Bell-state OpenQASM 2.0 program, runs it through `optimize_ir`, lowers it to IBM's native gate set, and exports real IBM-basis QASM via `to_ibm_qasm`. Separately, it generates a reference measurement distribution by running the compiled circuit on the local simulator 4,096 times — each "shot" re-runs the circuit from a fresh `|00⟩` state, since `QuantumRegister`'s measurement collapses the state and there's no batched-shots primitive on the register itself.

```rust
let circuit = qasm::parse(source).expect("parse failed");
let circuit = optimize_ir(&circuit);
let backend_circuit = lower(&circuit, Backend::IbmQ);

let qasm_text = to_ibm_qasm(&backend_circuit, "bell_state").expect("IBM export failed");
fs::write("bell.qasm", &qasm_text).expect("failed to write bell.qasm");
```

## Output files

| File                         | What it is                                                                                                                       |
| ---------------------------- | -------------------------------------------------------------------------------------------------------------------------------- |
| `bell.qasm`                  | Real IBM-basis QASM (`rz`/`sx`/`x`/`cx`), ready to hand to Qiskit or IBM Quantum Platform                                        |
| `bell_reference_counts.json` | The simulator's 4,096-shot reference distribution, hand-serialized to JSON (no `serde_json` dependency needed for this one file) |

## Why it matters

This is the exact handoff point the companion script [`submit_ibm.py`](/examples.md#submit_ibmpy) expects: run this example first, then

```bash
python3 submit_ibm.py --qasm bell.qasm --real --backend <name> \
  --compare bell_reference_counts.json
```

to submit the same circuit to real IBM Quantum hardware and see how far the real result drifts from the ideal simulator's distribution (measured as total variation distance).

## Related

* [`pipeline_end_to_end`](/examples/pipeline_end_to_end.md) — the fuller version of this same pipeline shape, across all three backends, with diagrams
* [`gate_cheatsheet`](/examples/gate_cheatsheet.md) — what the `H` and `CX` gates in this circuit actually cost once decomposed


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