---
title: fermion_operator (latest version)
description: API reference for qiskit_fermions.mappers.library.fermion_operator in the latest version of qiskit-fermions
source: https://eu-de.quantum.cloud.ibm.com/docs/en/api/qiskit-fermions/mappers-library-fermion-operator
---

# fermion\_operator

`fermion_operator(operator)`

Converts an operator into a [`FermionOperator`](/docs/api/qiskit-fermions/operators-fermion-operator "qiskit_fermions.operators.FermionOperator").

This is a thin, type-agnostic wrapper around the [`SupportsFermionOperator`](/docs/api/qiskit-fermions/protocols-supports-fermion-operator "qiskit_fermions.protocols.SupportsFermionOperator") protocol method, mirroring the free-function style of [`ffsim.fermion_operator()`](https://qiskit-community.github.io/ffsim/api/stubs/ffsim.fermion_operator.html#ffsim.fermion_operator "(in ffsim)").

**Parameters**

**operator** ([*SupportsFermionOperator*](/docs/api/qiskit-fermions/protocols-supports-fermion-operator "qiskit_fermions.protocols.fermion_operator.SupportsFermionOperator")) – the operator to convert, implementing [`SupportsFermionOperator`](/docs/api/qiskit-fermions/protocols-supports-fermion-operator "qiskit_fermions.protocols.SupportsFermionOperator").

**Returns**

The operator converted into a [`FermionOperator`](/docs/api/qiskit-fermions/operators-fermion-operator "qiskit_fermions.operators.FermionOperator").

**Return type**

[*FermionOperator*](/docs/api/qiskit-fermions/operators-fermion-operator "qiskit_fermions.operators.fermion_operator.FermionOperator")

```pycon
>>> from qiskit_fermions.mappers.library import fermion_operator
>>> from qiskit_fermions.operators import MajoranaOperator
>>> maj_op = MajoranaOperator.from_dict({(0, 1): 1})
>>> fer_op = fermion_operator(maj_op)
>>> print(format(fer_op.normal_ordered().simplify()))
  0.000000e0 +1.000000e0j * ()
  0.000000e0 -2.000000e0j * (+0 -0)
```