# Transpiler Pass Synthesis (`B2Q`) The [`B2QSynthesis`](../stubs/qiskit_bosons.transpiler.passes.B2QSynthesis.html.md#qiskit_bosons.transpiler.passes.B2QSynthesis) transpiler pass exposes a plugin interface (see [`B2QSynthesisPlugin`](../stubs/qiskit_bosons.transpiler.passes.synthesis.B2QSynthesisPlugin.html.md#qiskit_bosons.transpiler.passes.synthesis.B2QSynthesisPlugin)) through which implementations for mapping boson-based instructions to qubit-based ones can be registered. It is the counterpart of the boson-to-boson synthesis that the [`B2BSynthesis`](../stubs/qiskit_bosons.transpiler.passes.B2BSynthesis.html.md#qiskit_bosons.transpiler.passes.B2BSynthesis) pass performs one stage earlier (see [`synthesis`](qiskit_bosons.circuit.library.synthesis.html.md#module-qiskit_bosons.circuit.library.synthesis)). The available plugins are managed by the [`B2QSynthesisPluginManager`](../stubs/qiskit_bosons.transpiler.passes.synthesis.B2QSynthesisPluginManager.html.md#qiskit_bosons.transpiler.passes.synthesis.B2QSynthesisPluginManager). Choosing the right combination of synthesis plugins is crucial and the [`B2QSynthesis`](../stubs/qiskit_bosons.transpiler.passes.B2QSynthesis.html.md#qiskit_bosons.transpiler.passes.B2QSynthesis) transpilation pass does **not** have any default plugins defined! For this, refer to the transpiler [`presets`](qiskit_bosons.transpiler.presets.html.md#module-qiskit_bosons.transpiler.presets). ## Builtin plugins The plugins to use at runtime have to be registered in the [`B2QSynthesis.methods`](../stubs/qiskit_bosons.transpiler.passes.B2QSynthesis.html.md#qiskit_bosons.transpiler.passes.B2QSynthesis.methods) attribute. This can be done easily through the `config` argument of [`B2QSynthesis`](../stubs/qiskit_bosons.transpiler.passes.B2QSynthesis.html.md#qiskit_bosons.transpiler.passes.B2QSynthesis) which specifies the plugins to use from the [`B2QSynthesisPluginManager`](../stubs/qiskit_bosons.transpiler.passes.synthesis.B2QSynthesisPluginManager.html.md#qiskit_bosons.transpiler.passes.synthesis.B2QSynthesisPluginManager) or it can be done manually by writing into [`B2QSynthesis.methods`](../stubs/qiskit_bosons.transpiler.passes.B2QSynthesis.html.md#qiskit_bosons.transpiler.passes.B2QSynthesis.methods) directly. There is one plugin per [`BosonicGate`](../stubs/qiskit_bosons.circuit.BosonicGate.html.md#qiskit_bosons.circuit.BosonicGate) type. Each is handed the gate and the global [`B2QLayout`](../stubs/qiskit_bosons.transpiler.B2QLayout.html.md#qiskit_bosons.transpiler.B2QLayout), and inserts the equivalent qubit-based instructions. The builtin plugins are where the boson-to-qubit **encoding** and the **truncation** enter the pipeline: the plugins taking a `mapper_fn` do so through the mapper they wrap, and the `InitializeModes` plugin through the [`LevelSpec`](../stubs/qiskit_bosons.mappers.LevelSpec.html.md#qiskit_bosons.mappers.LevelSpec) it is given. This is the point of deferring the encoding to transpile time: the bosonic circuit upstream of this stage is independent of both choices. The table below lists the builtin plugins, organized by the [`BosonicGate`](../stubs/qiskit_bosons.circuit.BosonicGate.html.md#qiskit_bosons.circuit.BosonicGate) that they act upon. These are the entry points of the `qiskit_bosons.transpiler.synthesis` group, named `.`: #### An overview of the builtin `qiskit_bosons.transpiler.synthesis` entry-points. | Circuit instruction name | Plugin method name | Reference | |----------------------------------------------------------------------------------------------------------------|----------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------| | [`Evolution`](../stubs/qiskit_bosons.circuit.library.Evolution.html.md#qiskit_bosons.circuit.library.Evolution) | `MapperFn` | [`MapperFnEvolutionSynthesis`](../stubs/qiskit_bosons.transpiler.passes.synthesis.MapperFnEvolutionSynthesis.html.md#qiskit_bosons.transpiler.passes.synthesis.MapperFnEvolutionSynthesis) | | [`InitializeModes`](../stubs/qiskit_bosons.circuit.library.InitializeModes.html.md#qiskit_bosons.circuit.library.InitializeModes) | `Occupation` | [`OccupationInitializeModesSynthesis`](../stubs/qiskit_bosons.transpiler.passes.synthesis.OccupationInitializeModesSynthesis.html.md#qiskit_bosons.transpiler.passes.synthesis.OccupationInitializeModesSynthesis) | ## Additional helper functions The Pauli term order that a mapper function produces is preserved through synthesis, since a product formula synthesizes the terms in the order it receives them. These adapters wrap a mapper function to choose that order: