Qiskit 1.0 migration notes

Scope: circuit construction, transpilation, and backend interfaces only.

## Removal notices

### Execution and backend access

- `qiskit.execute` was removed. Use `transpile(circuit, backend)` followed by `backend.run(...)`.
- `qiskit.Aer` was removed from the top-level namespace. Use `qiskit_aer.Aer`.
- `qiskit.BasicAer` was removed from the top-level namespace. Use `qiskit.providers.basic_provider`.

### Circuit construction and serialization

- `QuantumCircuit.qasm` was removed. Use `qiskit.qasm2.dump` or `qiskit.qasm2.dumps`.
- `qiskit.converters.ast_to_dag` was removed along with the legacy OpenQASM 2 parser path. Parse into a `QuantumCircuit` with `qiskit.qasm2.load`, `QuantumCircuit.from_qasm_file`, or `QuantumCircuit.from_qasm_str`, then convert with `circuit_to_dag`.

## API change tables

### Circuit construction: gate helpers removed

| Removed | Alternative |
| --- | --- |
| `QuantumCircuit.cnot` | `QuantumCircuit.cx` |
| `QuantumCircuit.toffoli` | `QuantumCircuit.ccx` |
| `QuantumCircuit.fredkin` | `QuantumCircuit.cswap` |
| `QuantumCircuit.mct` | `QuantumCircuit.mcx` |
| `QuantumCircuit.i` | `QuantumCircuit.id` |
| `QuantumCircuit.squ` | `QuantumCircuit.unitary` |

### Circuit construction: methods replaced by `append(...)`

| Removed | Alternative |
| --- | --- |
| `QuantumCircuit.diagonal` | `QuantumCircuit.append(DiagonalGate(...), qubits)` |
| `QuantumCircuit.hamiltonian` | `QuantumCircuit.append(HamiltonianGate(...), qubits)` |
| `QuantumCircuit.isometry` | `QuantumCircuit.append(Isometry(...), qubits)` |
| `QuantumCircuit.iso` | `QuantumCircuit.append(Isometry(...), qubits)` |
| `QuantumCircuit.uc` | `QuantumCircuit.append(UCGate(...), qubits)` |
| `QuantumCircuit.ucrx` | `QuantumCircuit.append(UCRXGate(...), qubits)` |
| `QuantumCircuit.ucry` | `QuantumCircuit.append(UCRYGate(...), qubits)` |
| `QuantumCircuit.ucrz` | `QuantumCircuit.append(UCRZGate(...), qubits)` |

### Circuit construction: other removed methods

| Removed | Alternative |
| --- | --- |
| `QuantumCircuit.bind_parameters` | `QuantumCircuit.assign_parameters` |
| `QuantumCircuit.snapshot` | `qiskit-aer` save instructions |

### Transpilation and backend execution

| Removed / changed API | Alternative |
| --- | --- |
| `qiskit.execute(circuit, backend)` | `backend.run(transpile(circuit, backend))` |
| `from qiskit import Aer` | `from qiskit_aer import Aer` |
| `from qiskit import BasicAer` | `from qiskit.providers.basic_provider import BasicProvider` |

### OpenQASM parsing and DAG conversion

| Removed | Alternative |
| --- | --- |
| `qiskit.converters.ast_to_dag` | `circuit_to_dag(qiskit.qasm2.load(...))` |
| `qiskit.qasm.Qasm(...).parse()` workflow | `qiskit.qasm2.load(...)` or `QuantumCircuit.from_qasm_file(...)` |