---
title: library (latest version)
description: API reference for qiskit_bosons.mappers.library in the latest version of qiskit-bosons
source: https://quantum.cloud.ibm.com/docs/en/api/qiskit-bosons/mappers-library
---

# Mapper Library

This module provides the boson-to-qubit mappers.

[`occupation_encoding()`](/docs/api/qiskit-bosons/mappers-library-occupation-encoding#qiskit_bosons.mappers.library.occupation_encoding "qiskit_bosons.mappers.library.occupation_encoding") maps a [`BosonOperator`](/docs/api/qiskit-bosons/operators-boson-operator#qiskit_bosons.operators.BosonOperator "qiskit_bosons.operators.BosonOperator") in the occupation (Fock) basis, truncating each mode and laying its levels out on qubits as a [`LevelSpec`](/docs/api/qiskit-bosons/mappers-level-spec#qiskit_bosons.mappers.LevelSpec "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()`](/docs/api/qiskit-bosons/mappers-library-occupation-encoding#qiskit_bosons.mappers.library.occupation_encoding "qiskit_bosons.mappers.library.occupation_encoding") for the exact statement and its consequences.

|                                                                                                                                                                                                                      |                                                                                                                                                                                                                                           |
| -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| [`occupation_encoding`](/docs/api/qiskit-bosons/mappers-library-occupation-encoding#qiskit_bosons.mappers.library.occupation_encoding "qiskit_bosons.mappers.library.occupation_encoding")(op, spec\[, num\_qubits]) | Map a [`BosonOperator`](/docs/api/qiskit-bosons/operators-boson-operator#qiskit_bosons.operators.BosonOperator "qiskit_bosons.operators.BosonOperator") to a [`SparseObservable`](/docs/api/qiskit/qiskit.quantum_info.SparseObservable). |