# Mapper Library This module provides the boson-to-qubit mappers. [`occupation_encoding()`](../stubs/qiskit_bosons.mappers.library.occupation_encoding.html.md#qiskit_bosons.mappers.library.occupation_encoding) maps a [`BosonOperator`](../stubs/qiskit_bosons.operators.BosonOperator.html.md#qiskit_bosons.operators.BosonOperator) in the occupation (Fock) basis, truncating each mode and laying its levels out on qubits as a [`LevelSpec`](../stubs/qiskit_bosons.mappers.LevelSpec.html.md#qiskit_bosons.mappers.LevelSpec) describes. A bosonic encoding is **not** an algebra isomorphism: a bosonic mode’s Fock space is infinite-dimensional and must be truncated first, which breaks the canonical commutation relation on the highest retained level. See [`occupation_encoding()`](../stubs/qiskit_bosons.mappers.library.occupation_encoding.html.md#qiskit_bosons.mappers.library.occupation_encoding) for the exact statement and its consequences.