# MCXRecursive

*class *`qiskit.circuit.library.MCXRecursive(num_ctrl_qubits=None, label=None, ctrl_state=None, *, duration=None, unit='dt', _base_label=None)`

Bases: `MCXGate`

Implement the multi-controlled X gate using recursion.

Using a single clean ancilla qubit, the multi-controlled X gate is split into four sub-registers, each one of them uses the V-chain method.

The method is based on Lemma 9 of [2], first shown in Lemma 7.3 of [1].

**References**

- Barenco et al., 1995. https://arxiv.org/pdf/quant-ph/9503016.pdf
- Iten et al., 2015. https://arxiv.org/abs/1501.06911

Create new MCX 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 behavioral 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 parametrized gate with a particular set of parameters for the purposes of distinguishing it in a `Target`

from the full parametrized 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_ancilla_qubits

The number of ancilla qubits.

### num_clbits

Return the number of clbits.

### num_ctrl_qubits

### 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

### get_num_ancilla_qubits

*static *`get_num_ancilla_qubits(num_ctrl_qubits, mode='recursion')`

Get the number of required ancilla qubits.

### inverse

`inverse(annotated=False)`

Invert this gate. The MCX is its own inverse.

**Parameters**

**annotated** (*bool*) – 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**