# Unitary matrix

In mathematics, a complex square matrix U is unitary if its conjugate transpose U is also its inverse – that is, if

${\displaystyle U^{*}U=UU^{*}=I,}$

where I is the identity matrix. In physics, especially in quantum mechanics, the Hermitian conjugate of a matrix is denoted by a dagger (†) and the equation above becomes

${\displaystyle U^{\dagger }U=UU^{\dagger }=I.}$

The real analogue of a unitary matrix is an orthogonal matrix. Unitary matrices have significant importance in quantum mechanics because they preserve norms, and thus, probability amplitudes.

## Properties

For any unitary matrix U of finite size, the following hold:

• Given two complex vectors x and y, multiplication by U preserves their inner product; that is,
${\displaystyle \langle Ux,Uy\rangle =\langle x,y\rangle }$.
${\displaystyle U=VDV^{*}\;}$
where V is unitary and D is diagonal and unitary.
• ${\displaystyle \left|\det(U)\right|=1}$.
• Its eigenspaces are orthogonal.
• U can be written as U = eiH, where e indicates matrix exponential, i is the imaginary unit and H is a Hermitian matrix.

For any nonnegative integer n, the set of all n-by-n unitary matrices with matrix multiplication forms a group, called the unitary group U(n).

Any square matrix with unit Euclidean norm is the average of two unitary matrices.[1]

## Equivalent conditions

If U is a square, complex matrix, then the following conditions are equivalent:

1. U is unitary.
2. U is unitary.
3. U is invertible with U−1 = U.
4. The columns of U form an orthonormal basis of ${\displaystyle \mathbb {C} ^{n}}$ with respect to the usual inner product.
5. The rows of U form an orthonormal basis of ${\displaystyle \mathbb {C} ^{n}}$ with respect to the usual inner product.
6. U is an isometry with respect to the usual norm.
7. U is a normal matrix with eigenvalues lying on the unit circle.

## Elementary constructions

### 2 × 2 unitary matrix

The general expression of a 2 × 2 unitary matrix is:

${\displaystyle U={\begin{bmatrix}a&b\\-e^{i\theta }b^{*}&e^{i\theta }a^{*}\\\end{bmatrix}},\qquad \left|a\right|^{2}+\left|b\right|^{2}=1,}$

which depends on 4 real parameters (the phase of ${\displaystyle a}$, the phase of ${\displaystyle b}$, the relative magnitude between ${\displaystyle a}$ and ${\displaystyle b}$, and the angle ${\displaystyle \theta }$). The determinant of such a matrix is:

${\displaystyle \det(U)=e^{i\theta }.}$

The sub-group of such elements in U where ${\displaystyle \det(U)=1}$ is called the special unitary group SU(2).

The matrix U can also be written in this alternative form:

${\displaystyle U=e^{i\varphi }{\begin{bmatrix}e^{i\varphi _{1}}\cos \theta &e^{i\varphi _{2}}\sin \theta \\-e^{-i\varphi _{2}}\sin \theta &e^{-i\varphi _{1}}\cos \theta \\\end{bmatrix}},}$

which, by introducing φ1 = ψ + Δ and φ2 = ψ − Δ, takes the following factorization:

${\displaystyle U=e^{i\varphi }{\begin{bmatrix}e^{i\psi }&0\\0&e^{-i\psi }\end{bmatrix}}{\begin{bmatrix}\cos \theta &\sin \theta \\-\sin \theta &\cos \theta \\\end{bmatrix}}{\begin{bmatrix}e^{i\Delta }&0\\0&e^{-i\Delta }\end{bmatrix}}.}$

This expression highlights the relation between 2 × 2 unitary matrices and 2 × 2 orthogonal matrices of angle θ.

Many other factorizations of a unitary matrix in basic matrices are possible.