| Qiskit 2.0 migration notes |
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| Scope: circuit construction, transpilation, and backend interfaces only. |
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| ## Removal notices |
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| ### Circuit construction |
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| - Legacy instruction-level `c_if` conditions were removed. Use `QuantumCircuit.if_test(...)` with structured control flow. |
| - `Instruction.condition` is no longer present on generic `Instruction` objects. |
| - `Instruction.duration` and `Instruction.unit` were removed from most instructions. Query timing data from `Target` / `InstructionProperties` instead. |
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| ### Transpilation |
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| - `backend_properties` is no longer accepted by `transpile(...)` or `generate_preset_pass_manager(...)`. Use `target` or `backend`. |
| - `instruction_durations` and `timing_constraints` are no longer accepted by `transpile(...)` or `generate_preset_pass_manager(...)`. Use `target` or `backend`. |
| - `inst_map` was removed from `transpile(...)`, `generate_preset_pass_manager(...)`, and `Target.from_configuration(...)`. |
| - Custom basis gates are no longer accepted through `basis_gates`; define them in a `Target`. |
| - Passing `coupling_map` and/or `basis_gates` together with `backend` is discouraged and raises a warning in v2.0 because `backend` should be the single source of truth. |
| - A custom `coupling_map` can no longer be combined with `backend` or `basis_gates` when 3+ qubit gates are involved; use a custom `Target`. |
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| ### Backend interfaces and pulse-related removals |
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| - `qiskit.pulse` was removed without replacement. |
| - `BackendProperties` was removed. |
| - Pulse-related backend methods were removed from `BackendV2`: `instruction_schedule_map`, `drive_channel`, `measure_channel`, `acquire_channel`, `control_channel`. |
| - Pulse-related target methods were removed: `has_calibration`, `get_calibration`, `instruction_schedule_map`, `update_from_instruction_schedule_map`. |
| - `InstructionProperties.calibration` was removed. |
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| ## API change tables |
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| ### Circuit construction and control flow |
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| | Removed / changed API | Alternative | |
| | --- | --- | |
| | `qc.x(0).c_if(cr, 3)` | `with qc.if_test((cr, 3)): qc.x(0)` | |
| | `Instruction.condition` | Use `isinstance(op, IfElseOp)` / `isinstance(op, WhileLoopOp)` or `getattr(op, "condition", None)` for compatibility checks | |
| | `Instruction.duration` / `Instruction.unit` | Query `target[op_name][qubits]` and read `InstructionProperties.duration` | |
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| ### Transpilation interface changes |
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| | Old input / pattern | v2 alternative | |
| | --- | --- | |
| | `backend_properties=...` | Build or pass `target=...` or `backend=...` | |
| | `instruction_durations=...` | Encode durations in `Target` / `Target.from_configuration(...)` | |
| | `timing_constraints=...` | Encode timing constraints in `Target` / `Target.from_configuration(...)` | |
| | `inst_map=...` | No replacement | |
| | `basis_gates=["my_x", "cx"]` with custom gates | Define a `Target` with `custom_name_mapping` | |
| | `backend=...` together with `coupling_map=...` or `basis_gates=...` | Build one consistent `Target` instead | |
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| ### Backend interface migration: `BackendV1` to `BackendV2` |
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| | `BackendV1` access pattern | `BackendV2` replacement | Notes | |
| | --- | --- | --- | |
| | `backend.configuration().n_qubits` | `backend.num_qubits` | | |
| | `backend.configuration().coupling_map` | `backend.coupling_map` | Returns a `CouplingMap`, and `backend.target` remains the more complete source of connectivity info | |
| | `backend.configuration().backend_name` | `backend.name` | | |
| | `backend.configuration().backend_version` | `backend.backend_version` | Distinct from `BackendV2.version`, which is the interface version | |
| | `backend.configuration().basis_gates` | `backend.operation_names` | `backend.target` may contain richer information than a flat name list | |
| | `backend.configuration().dt` | `backend.dt` | | |
| | `backend.configuration().dtm` | `backend.dtm` | | |
| | `backend.configuration().max_experiments` | `backend.max_circuits` | | |
| | `backend.configuration().online_date` | `backend.online_date` | | |
| | `InstructionDurations.from_backend(backend)` | `backend.instruction_durations` | | |
| | `backend.defaults().instruction_schedule_map` | Removed | Part of pulse removal | |
| | `backend.properties().t1(0)` | `backend.qubit_properties(0).t1` | | |
| | `backend.properties().t2(0)` | `backend.qubit_properties(0).t2` | | |
| | `backend.properties().frequency(0)` | `backend.qubit_properties(0).frequency` | | |
| | `backend.properties().readout_error(0)` | `backend.target["measure"][(0,)].error` | | |
| | `backend.properties().readout_length(0)` | `backend.target["measure"][(0,)].duration` | | |
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