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

# InitializeModes

*class* `InitializeModes(occupation)`

Bases: [`BosonicGate`](/docs/api/qiskit-bosons/circuit-bosonic-gate "qiskit_bosons.circuit.bosonic_gate.BosonicGate")

Prepares a bosonic Fock state.

This gate declares the occupation of the modes it is placed on, i.e. the Fock state

$$
\lvert n_0, n_1, \ldots, n_{M-1} \rangle
= \prod_m \frac{(b^\dagger_m)^{n_m}}{\sqrt{n_m!}} \lvert \text{vac} \rangle \, ,
$$

where $n_m$ is `occupation[m]`. Synthesis emits the gates that set each mode’s qubits to the pattern encoding its occupation level.

> **Note**
>
> The occupation entries are **non-negative integers**, not booleans: a single mode may hold arbitrarily many bosons.

> **Note**
>
> This gate does not itself fix a truncation. The occupation it declares must of course be representable by whatever cutoff the boson-to-qubit encoding uses (an occupation of 3 requires `max_occupation >= 3`) which the synthesis plugin checks at transpile time, since that is where the cutoff is known. [`max_occupation`](#qiskit_bosons.circuit.library.InitializeModes.max_occupation "qiskit_bosons.circuit.library.InitializeModes.max_occupation") reports the largest level this gate declares.

> **Caution**
>
> This is an early development prototype. Beware of changes to its interface without warning during the pre-release development of this package.

Initializing an instance of this gate can be done with the arguments listed below.

**Parameters**

**occupation** (*Sequence\[*[*int*](https://docs.python.org/3/builtins/functions.html#int)*]*) – the boson number of each mode in the [`BosonicRegister`](/docs/api/qiskit-bosons/circuit-bosonic-register "qiskit_bosons.circuit.BosonicRegister") being initialized by this gate.

**Raises**

[**ValueError**](https://docs.python.org/3/builtins/exceptions.html#ValueError) – if any occupation entry is negative.

## Attributes

### max\_occupation

The largest occupation level this gate declares.

A boson-to-qubit encoding must retain at least this many levels to represent the declared Fock state; see [`occupation_encoding()`](/docs/api/qiskit-bosons/mappers-library-occupation-encoding "qiskit_bosons.mappers.library.occupation_encoding").

### occupation

Type: [`tuple`](https://docs.python.org/3/builtins/stdtypes.html#tuple)\[[`int`](https://docs.python.org/3/builtins/functions.html#int), ...]

The boson number of each mode in the [`BosonicRegister`](/docs/api/qiskit-bosons/circuit-bosonic-register "qiskit_bosons.circuit.BosonicRegister") initialized by this gate.

## Methods

### vacuum

*classmethod* `vacuum(num_modes)`

Builds the gate for the vacuum state $\lvert 0, \ldots, 0 \rangle$.

**Parameters**

**num\_modes** ([*int*](https://docs.python.org/3/builtins/functions.html#int)) – the number of bosonic modes to initialize.

**Returns**

An [`InitializeModes`](#qiskit_bosons.circuit.library.InitializeModes "qiskit_bosons.circuit.library.InitializeModes") gate declaring zero bosons in every mode.

**Return type**

[*InitializeModes*](#qiskit_bosons.circuit.library.InitializeModes "qiskit_bosons.circuit.library.initialize_modes.InitializeModes")