# SGate

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

Bases: `SingletonGate`

Single qubit S gate (Z**0.5).

It induces a $\pi/2$ phase, and is sometimes called the P gate (phase).

This is a Clifford gate and a square-root of Pauli-Z.

Can be applied to a `QuantumCircuit`

with the `s()`

method.

**Matrix Representation:**

**Circuit symbol:**

```
┌───┐
q_0: ┤ S ├
└───┘
```

Equivalent to a $\pi/2$ radian rotation about the Z axis.

Create new S 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.

### decompositions

Get the decompositions of the instruction from the SessionEquivalenceLibrary.

### definition

Return definition in terms of other basic gates.

### 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

Return the name.

### num_clbits

Return the number of clbits.

### num_qubits

Return the number of qubits.

### params

The parameters of this `Instruction`

. Ideally these will be gate angles.

### unit

Get the time unit of duration.

## Methods

### control

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

Return a (multi-)controlled-S gate.

One control qubit returns a `CSGate`

.

**Parameters**

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

]**ctrl_state**(*int**|**str**| None*) – control state expressed as integer, string (e.g.``’110’`), or ``None`

. If`None`

, use all 1s.**annotated**(*bool**| None*) – indicates whether the controlled gate should be implemented as an annotated gate. If`None`

, this is handled as`False`

.

**Returns**

controlled version of this gate.

**Return type**

### inverse

`inverse(annotated=False)`

Return inverse of S (SdgGate).

**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 the inverse of this gate is always a `SdgGate`

.

**Returns**

inverse of `SGate`

**Return type**

### power

`power(exponent, annotated=False)`

Raise this gate to the power of `exponent`

.

Implemented either as a unitary gate (ref. `UnitaryGate`

) or as an annotated operation (ref. `AnnotatedOperation`

). In the case of several standard gates, such as `RXGate`

, when the power of a gate can be expressed in terms of another standard gate that is returned directly.

**Parameters**

**exponent**(*float*) – the power to raise the gate to**annotated**(*bool*) – indicates whether the power gate can be implemented as an annotated operation. In the case of several standard gates, such as`RXGate`

, this argument is ignored when the power of a gate can be expressed in terms of another standard gate.

**Returns**

An operation implementing `gate^exponent`

**Raises**

**CircuitError** – If gate is not unitary