# Transpiler Pass Synthesis (`F2Q`) The [`F2QSynthesis`](../stubs/qiskit_fermions.transpiler.passes.F2QSynthesis.html.md#qiskit_fermions.transpiler.passes.F2QSynthesis) transpiler pass exposes a plugin interface (see [`F2QSynthesisPlugin`](../stubs/qiskit_fermions.transpiler.passes.synthesis.F2QSynthesisPlugin.html.md#qiskit_fermions.transpiler.passes.synthesis.F2QSynthesisPlugin)) through which implementations for mapping fermion-based instructions to qubit-based ones can be registered. It is the counterpart of the fermion-to-fermion synthesis that the [`F2FSynthesis`](../stubs/qiskit_fermions.transpiler.passes.F2FSynthesis.html.md#qiskit_fermions.transpiler.passes.F2FSynthesis) pass performs one stage earlier (see [`synthesis`](qiskit_fermions.circuit.library.synthesis.html.md#module-qiskit_fermions.circuit.library.synthesis)). The available plugins are managed by the [`F2QSynthesisPluginManager`](../stubs/qiskit_fermions.transpiler.passes.synthesis.F2QSynthesisPluginManager.html.md#qiskit_fermions.transpiler.passes.synthesis.F2QSynthesisPluginManager). Choosing the right combination of synthesis plugins is crucial and the [`F2QSynthesis`](../stubs/qiskit_fermions.transpiler.passes.F2QSynthesis.html.md#qiskit_fermions.transpiler.passes.F2QSynthesis) transpilation pass does **not** have any default plugins defined! For this, refer to the transpiler [`presets`](qiskit_fermions.transpiler.presets.html.md#module-qiskit_fermions.transpiler.presets). ## Builtin plugins The plugins to use at runtime have to be registered in the [`F2QSynthesis.methods`](../stubs/qiskit_fermions.transpiler.passes.F2QSynthesis.html.md#qiskit_fermions.transpiler.passes.F2QSynthesis.methods) attribute. This can be done easily through the `config` argument of [`F2QSynthesis`](../stubs/qiskit_fermions.transpiler.passes.F2QSynthesis.html.md#qiskit_fermions.transpiler.passes.F2QSynthesis) which specifies the plugins to use from the [`F2QSynthesisPluginManager`](../stubs/qiskit_fermions.transpiler.passes.synthesis.F2QSynthesisPluginManager.html.md#qiskit_fermions.transpiler.passes.synthesis.F2QSynthesisPluginManager) or it can be done manually by writing into [`F2QSynthesis.methods`](../stubs/qiskit_fermions.transpiler.passes.F2QSynthesis.html.md#qiskit_fermions.transpiler.passes.F2QSynthesis.methods) directly. The table below lists the builtin plugins, organized by the [`FermionicGate`](../stubs/qiskit_fermions.circuit.FermionicGate.html.md#qiskit_fermions.circuit.FermionicGate) that they act upon. These are the entry points of the `qiskit_fermions.transpiler.synthesis` group, named `.`: #### An overview of the builtin `qiskit_fermions.transpiler.synthesis` entry-points. | Circuit instruction name | Plugin method name | Reference | Notes | |------------------------------------------------------------------------------------------------------------------------------------|-----------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------| | [`Evolution`](../stubs/qiskit_fermions.circuit.library.Evolution.html.md#qiskit_fermions.circuit.library.Evolution) | `MapperFn` | [`MapperFnEvolutionSynthesis`](../stubs/qiskit_fermions.transpiler.passes.synthesis.MapperFnEvolutionSynthesis.html.md#qiskit_fermions.transpiler.passes.synthesis.MapperFnEvolutionSynthesis) | | | [`InitializeModes`](../stubs/qiskit_fermions.circuit.library.InitializeModes.html.md#qiskit_fermions.circuit.library.InitializeModes) | `TrivialOccupation` | [`TrivialOccupationInitializeModesSynthesis`](../stubs/qiskit_fermions.transpiler.passes.synthesis.TrivialOccupationInitializeModesSynthesis.html.md#qiskit_fermions.transpiler.passes.synthesis.TrivialOccupationInitializeModesSynthesis) | | | [`FermionicMeasure`](../stubs/qiskit_fermions.circuit.library.FermionicMeasure.html.md#qiskit_fermions.circuit.library.FermionicMeasure) | `TrivialOccupation` | [`TrivialOccupationFermionicMeasureSynthesis`](../stubs/qiskit_fermions.transpiler.passes.synthesis.TrivialOccupationFermionicMeasureSynthesis.html.md#qiskit_fermions.transpiler.passes.synthesis.TrivialOccupationFermionicMeasureSynthesis) | | | [`OrbitalRotation`](../stubs/qiskit_fermions.circuit.library.OrbitalRotation.html.md#qiskit_fermions.circuit.library.OrbitalRotation) | `GivensDecomposition` | [`GivensDecompositionOrbitalRotationSynthesis`](../stubs/qiskit_fermions.transpiler.passes.synthesis.GivensDecompositionOrbitalRotationSynthesis.html.md#qiskit_fermions.transpiler.passes.synthesis.GivensDecompositionOrbitalRotationSynthesis) | **DEPRECATED** | | [`PrepareSlaterDeterminant`](../stubs/qiskit_fermions.circuit.library.PrepareSlaterDeterminant.html.md#qiskit_fermions.circuit.library.PrepareSlaterDeterminant) | `GivensDecompositionSlater` | [`GivensDecompositionSlaterDeterminantSynthesis`](../stubs/qiskit_fermions.transpiler.passes.synthesis.GivensDecompositionSlaterDeterminantSynthesis.html.md#qiskit_fermions.transpiler.passes.synthesis.GivensDecompositionSlaterDeterminantSynthesis) | **DEPRECATED** | ## Qubit-stage synthesis The plugins above map fermionic instructions to qubit ones. The following is instead a plugin of Qiskit’s [`HighLevelSynthesis`](https://quantum.cloud.ibm.com/docs/api/qiskit/qiskit.transpiler.passes.HighLevelSynthesis) for the qubit stage, which recognizes the image of a fermionic hop between adjacent modes and emits it as a single [`XXPlusYYGate`](https://quantum.cloud.ibm.com/docs/api/qiskit/qiskit.circuit.library.XXPlusYYGate). The Jordan-Wigner preset registers it. ## 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: