# Unitary transformation

In mathematics, a unitary transformation is a transformation that preserves the inner product: the inner product of two vectors before the transformation is equal to their inner product after the transformation.

## Formal definition

More precisely, a unitary transformation is an isomorphism between two Hilbert spaces. In other words, a unitary transformation is a bijective function

$U:H_{1}\to H_{2}\,$ where $H_{1}$ and $H_{2}$ are Hilbert spaces, such that

$\langle Ux,Uy\rangle _{H_{2}}=\langle x,y\rangle _{H_{1}}$ for all $x$ and $y$ in $H_{1}$ .

## Properties

A unitary transformation is an isometry, as one can see by setting $x=y$ in this formula.

## Unitary operator

In the case when $H_{1}$ and $H_{2}$ are the same space, a unitary transformation is an automorphism of that Hilbert space, and then it is also called a unitary operator.

## Antiunitary transformation

A closely related notion is that of antiunitary transformation, which is a bijective function

$U:H_{1}\to H_{2}\,$ between two complex Hilbert spaces such that

$\langle Ux,Uy\rangle ={\overline {\langle x,y\rangle }}=\langle y,x\rangle$ for all $x$ and $y$ in $H_{1}$ , where the horizontal bar represents the complex conjugate.