In mathematics, an antiunitary transformation, is a bijective antilinear map between two complex Hilbert spaces such that for all and in , where the horizontal bar represents the complex conjugate. If additionally one has then is called an antiunitary operator. Antiunitary operators are important in quantum theory because they are used to represent certain symmetries, such as time reversal. 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 leave the absolute value of scalar product invariant: for all and in . Due to Wigner's theorem these transformations can either be unitary or antiunitary. Geometric Interpretation Congruences of the plane form two distinct 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 correspond (up to translation) to unitaries and antiunitaries, respectively. Properties holds for all elements of the Hilbert space and an antiunitary . When is antiunitary then is unitary. This follows from For unitary operator the operator , where is complex conjugate operator, is antiunitary. The reverse is also true, for antiunitary the operator is unitary. For antiunitary the definition of the adjoint operator is changed to compensate the complex conjugation, becoming The adjoint of an antiunitary is also antiunitary and (This is not to be confused with the definition of unitary operators, as the antiunitary operator is not complex linear.) Examples The complex conjugate operator is an antiunitary operator on the complex plane. The operator where is the second Pauli matrix and is the complex conjugate operator, is antiunitary. It satisfies . 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 , . The operator is just simple complex conjugation on For , the operator acts on two-dimensional complex Hilbert space. It is defined by Note that for so such may not be further decomposed into 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),