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In mathematics, an antiunitary transformation, is a bijective antilinear map

$$U:H_1\to H_2\,$$

between two complex Hilbert spaces such that

$$\langle Ux, Uy \rangle = \overline{\langle x, y \rangle}$$

for all x and y in H_1, where the horizontal bar represents the complex conjugate. If additionally one has H_1 = H_2 then U is called an antiunitary operator.

Antiunitary operators are important in Quantum Theory because they are used to represent certain symmetries, such as time-reversal symmetry. Their fundamental importance in quantum physics is further demonstrated by Wigner's Theorem.

Invariance transformations

In Quantum mechanics, the invariance transformations of complex Hilbert space H leave the absolute value of scalar product invariant:

$$|\langle Tx, Ty \rangle| =|\langle x, y \rangle|$$

for all x and y in H. Due to Wigner's Theorem these transformations fall into two categories, they can be unitary or antiunitary.
Geometric Interpretation

Congruences of the plane form two distinguish classes. The first conserves the orientation and is generated by translations and rotations. The second does not conserve the orientation and is obtained from the first class by applying a reflection. On the complex plane these two classes corresponds (up to translation) to unitaries and antiunitaries, respectively.
Properties

$$\langle Ux, Uy \rangle = \overline{\langle x, y \rangle} = \langle y, x \rangle$$ holds for all elements x, y of the Hilbert space and an antiunitary U .
When U is antiunitary then U^2 is unitary. This follows from

$$\langle U^2x, U^2y \rangle = \overline{\langle Uy, Ux \rangle} = \langle x, y \rangle$$ .

For unitary operator V the operator VK , where K is complex conjugate operator, is antiunitary. The reverse is also true, for antiunitary U the operator VK is unitary.
For antiunitary U the adjoint operator U^* is also antiunitary and

$$U U^* = U^* U = 1.$$

Examples

The complex conjugate operator $$K, K z = \overline{z}$$, is an antiunitary operator on the complex plane.
The operator

$$U = \sigma_y K = \begin{pmatrix} 0&-i\\ i&0 \end{pmatrix} K,$$

where \sigma_y is the second Pauli matrix and K is the complex conjugate operator, is an antiunitary. It satisfies U^2 = -1 .
Decomposition of an antiunitary operator into a direct sum of elementary Wigner antiunitaries

An antiunitary operator on a finite-dimensional space may be decomposed as a direct sum of elementary Wigner antiunitaries $$W_\theta, 0\le\theta\le\pi$$. The operator $$W_0:C\rightarrow C$$ is just simple complex conjugation on C

$$W_0(z)=\overline{z}\,$$

For 0<\theta\le\pi, the operation W_\theta acts on two-dimensional complex Hilbert space. It is defined by

$$W_\theta((z_1,z_2)) = (e^{i\theta/2} \overline{z_2}, e^{-i\theta/2}\overline{z_1}). \,$$

Note that for 0<\theta\le\pi

$$W_\theta(W_\theta((z_1,z_2)))=(e^{i\theta}z_1,e^{-i\theta}z_2),\,$$

so such W_\theta may not be further decomposed into $$W_0'$$s, which square to the identity map.

Note that the above decomposition of antiunitary operators contrasts with the spectral decomposition of unitary operators. In particular, a unitary operator on a complex Hilbert space may be decomposed into a direct sum of unitaries acting on 1-dimensional complex spaces (eigenspaces), but an antiunitary operator may only be decomposed into a direct sum of elementary operators on 1 and 2 dimensional complex spaces.
References

Wigner, E. "Normal Form of Antiunitary Operators", Journal of Mathematical Physics Vol 1, no 5, 1960, pp. 409–412
Wigner, E. "Phenomenological Distinction between Unitary and Antiunitary Symmetry Operators", Journal of Mathematical Physics Vol1, no5, 1960, pp.414–416