# C4XGate

*class *`qiskit.circuit.library.C4XGate(*args, _force_mutable=False, **kwargs)`

Bases: `SingletonControlledGate`

The 4-qubit controlled X gate.

This implementation is based on Page 21, Lemma 7.5, of [1], with the use of the relative phase version of c3x, the rc3x [2].

**References**

[1] Barenco et al., 1995. https://arxiv.org/pdf/quant-ph/9503016.pdf(opens in a new tab) [2] Maslov, 2015. https://arxiv.org/abs/1508.03273(opens in a new tab)

Create a new 4-qubit controlled X gate.

## Attributes

### base_class

Get the base class of this instruction. This is guaranteed to be in the inheritance tree of `self`

.

The “base class” of an instruction is the lowest class in its inheritance tree that the object should be considered entirely compatible with for _all_ circuit applications. This typically means that the subclass is defined purely to offer some sort of programmer convenience over the base class, and the base class is the “true” class for a behavioural perspective. In particular, you should *not* override `base_class`

if you are defining a custom version of an instruction that will be implemented differently by hardware, such as an alternative measurement strategy, or a version of a parametrised gate with a particular set of parameters for the purposes of distinguishing it in a `Target`

from the full parametrised gate.

This is often exactly equivalent to `type(obj)`

, except in the case of singleton instances of standard-library instructions. These singleton instances are special subclasses of their base class, and this property will return that base. For example:

```
>>> isinstance(XGate(), XGate)
True
>>> type(XGate()) is XGate
False
>>> XGate().base_class is XGate
True
```

In general, you should not rely on the precise class of an instruction; within a given circuit, it is expected that `Instruction.name`

should be a more suitable discriminator in most situations.

### condition

The classical condition on the instruction.

### condition_bits

Get Clbits in condition.

### ctrl_state

Return the control state of the gate as a decimal integer.

### decompositions

Get the decompositions of the instruction from the SessionEquivalenceLibrary.

### definition

Return definition in terms of other basic gates. If the gate has open controls, as determined from `ctrl_state`

, the returned definition is conjugated with X without changing the internal `_definition`

.

### duration

Get the duration.

### label

Return instruction label

### mutable

Is this instance is a mutable unique instance or not.

If this attribute is `False`

the gate instance is a shared singleton and is not mutable.

### name

Get name of gate. If the gate has open controls the gate name will become:

<original_name_o<ctrl_state>

where <original_name> is the gate name for the default case of closed control qubits and <ctrl_state> is the integer value of the control state for the gate.

### num_clbits

Return the number of clbits.

### num_ctrl_qubits

Get number of control qubits.

**Returns**

The number of control qubits for the gate.

**Return type**

### num_qubits

Return the number of qubits.

### params

Get parameters from base_gate.

**Returns**

List of gate parameters.

**Return type**

**Raises**

**CircuitError** – Controlled gate does not define a base gate

### unit

Get the time unit of duration.

## Methods

### control

`control(num_ctrl_qubits=1, label=None, ctrl_state=None, annotated=False)`

Controlled version of this gate.

**Parameters**

**num_ctrl_qubits**(*int*(opens in a new tab)) – number of control qubits.**label**(*str*(opens in a new tab)*| None*) – An optional label for the gate [Default:`None`

]**ctrl_state**(*int*(opens in a new tab)*|**str*(opens in a new tab)*| None*) – control state expressed as integer, string (e.g.`'110'`

), or`None`

. If`None`

, use all 1s.**annotated**(*bool*(opens in a new tab)) – indicates whether the controlled gate can be implemented as an annotated gate.

**Returns**

controlled version of this gate.

**Return type**

### inverse

`inverse(annotated=False)`

Invert this gate. The C4X is its own inverse.

**Parameters**

**annotated** (*bool*(opens in a new tab)) – when set to `True`

, this is typically used to return an `AnnotatedOperation`

with an inverse modifier set instead of a concrete `Gate`

. However, for this class this argument is ignored as this gate is self-inverse.

**Returns**

inverse gate (self-inverse).

**Return type**