# OpenQASM

**OpenQASM** (Open Quantum Assembly Language) is a low-level instruction format for quantum circuits. It's a text language that describes qubit operations, gates, measurements, and control flow. Qiskit can convert circuits to OpenQASM and back.

OpenQASM is useful for circuit exchange, archival, and low-level circuit inspection.

## OpenQASM Syntax

```qasm
// Bell state circuit in OpenQASM
OPENQASM 2.0;
include "qelib1.inc";

qreg q[2];
creg c[2];

h q[0];
cx q[0], q[1];
measure q[0] -> c[0];
measure q[1] -> c[1];
```

Each line is an operation:
- `h q[0]` — Hadamard on qubit 0
- `cx q[0], q[1]` — CNOT from 0 to 1
- `measure q[i] -> c[i]` — measure qubit i into classical bit i

## Qiskit Integration

Convert Qiskit circuits to OpenQASM:

```python
from qiskit import QuantumCircuit

qc = QuantumCircuit(2, 2)
qc.h(0)
qc.cx(0, 1)
qc.measure([0, 1], [0, 1])

# Export to OpenQASM
qasm_str = qc.qasm()
print(qasm_str)

# Import from OpenQASM
qc2 = QuantumCircuit.from_qasm_str(qasm_str)
```

OpenQASM is a standard format across quantum platforms (IBM, Rigetti, IonQ). Qiskit 0.43+ supports OpenQASM 3.0, which adds more features (loops, conditionals, functions).

OpenQASM is mainly useful for interoperability and archival; most Qiskit users work with Python.