In mathematics

Mathematics

Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...

, a unitary matrix is a (square) complex

Complex number

A complex number is a number consisting of a real part and an imaginary part. Complex numbers extend the idea of the one-dimensional number line to the two-dimensional complex plane by using the number line for the real part and adding a vertical axis to plot the imaginary part...

 matrix

Matrix (mathematics)

In mathematics, a matrix is a rectangular array of numbers, symbols, or expressions. The individual items in a matrix are called its elements or entries. An example of a matrix with six elements isMatrices of the same size can be added or subtracted element by element...

  satisfying the condition


where is the identity matrix

Identity matrix

In linear algebra, the identity matrix or unit matrix of size n is the n×n square matrix with ones on the main diagonal and zeros elsewhere. It is denoted by In, or simply by I if the size is immaterial or can be trivially determined by the context...

 in n dimensions and is the conjugate transpose

Conjugate transpose

In mathematics, the conjugate transpose, Hermitian transpose, Hermitian conjugate, or adjoint matrix of an m-by-n matrix A with complex entries is the n-by-m matrix A* obtained from A by taking the transpose and then taking the complex conjugate of each entry...

 (also called the Hermitian adjoint

Hermitian adjoint

In mathematics, specifically in functional analysis, each linear operator on a Hilbert space has a corresponding adjoint operator.Adjoints of operators generalize conjugate transposes of square matrices to infinite-dimensional situations...

) of . Note this condition implies that a matrix is unitary if and only if it has an inverse which is equal to its conjugate transpose


A unitary matrix in which all entries are real is an orthogonal matrix

Orthogonal matrix

In linear algebra, an orthogonal matrix , is a square matrix with real entries whose columns and rows are orthogonal unit vectors ....

. Just as an orthogonal matrix preserves the (real

Real number

In mathematics, a real number is a value that represents a quantity along a continuum, such as -5 , 4/3 , 8.6 , √2 and π...

) inner product of two real
vectors,
so also a unitary matrix satisfies
for all complex vectors x and y, where stands now for the standard inner product on .

If is an matrix then the following are all equivalent conditions:

1. is unitary
2. is unitary
3. the columns of form an orthonormal basis
   Orthonormal basis
   In mathematics, particularly linear algebra, an orthonormal basis for inner product space V with finite dimension is a basis for V whose vectors are orthonormal. For example, the standard basis for a Euclidean space Rn is an orthonormal basis, where the relevant inner product is the dot product of...
   
    of with respect to this inner product
4. the rows of form an orthonormal basis of with respect to this inner product
5. is an isometry
   Isometry
   In mathematics, an isometry is a distance-preserving map between metric spaces. Geometric figures which can be related by an isometry are called congruent.Isometries are often used in constructions where one space is embedded in another space...
   
    with respect to the norm from this inner product
6. is a normal matrix
   Normal matrix
   A complex square matrix A is a normal matrix ifA^*A=AA^* \ where A* is the conjugate transpose of A. That is, a matrix is normal if it commutes with its conjugate transpose.If A is a real matrix, then A*=AT...
   
    with eigenvalues lying on the unit circle
   Unit circle
   In mathematics, a unit circle is a circle with a radius of one. Frequently, especially in trigonometry, "the" unit circle is the circle of radius one centered at the origin in the Cartesian coordinate system in the Euclidean plane...
   
   .

Properties

• All unitary matrices are normal
  Normal matrix
  A complex square matrix A is a normal matrix ifA^*A=AA^* \ where A* is the conjugate transpose of A. That is, a matrix is normal if it commutes with its conjugate transpose.If A is a real matrix, then A*=AT...
  
  , and the spectral theorem
  Spectral theorem
  In mathematics, particularly linear algebra and functional analysis, the spectral theorem is any of a number of results about linear operators or about matrices. In broad terms the spectral theorem provides conditions under which an operator or a matrix can be diagonalized...
  
   therefore applies to them. Thus every unitary matrix has a decomposition of the form

where is unitary, and is diagonal and unitary. That is, a unitary matrix is diagonalizable

Diagonalizable matrix

In linear algebra, a square matrix A is called diagonalizable if it is similar to a diagonal matrix, i.e., if there exists an invertible matrix P such that P −1AP is a diagonal matrix...

 by a unitary matrix.

For any unitary matrix , the following hold:

• .
• is invertible, with .
• is also unitary.
• preserves length ("isometry
  Isometry
  In mathematics, an isometry is a distance-preserving map between metric spaces. Geometric figures which can be related by an isometry are called congruent.Isometries are often used in constructions where one space is embedded in another space...
  
  "): .
• if has complex eigenvalues, they are of modulus 1.
• Eigenspaces are Orthogonal: If matrix is normal then its eigenvectors corresponding to different eigenvalues are orthogonal.
• For any n, the set of all n by n unitary matrices with matrix multiplication forms a group
  Group (mathematics)
  In mathematics, a group is an algebraic structure consisting of a set together with an operation that combines any two of its elements to form a third element. To qualify as a group, the set and the operation must satisfy a few conditions called group axioms, namely closure, associativity, identity...
  
  , called U(n)
  Unitary group
  In mathematics, the unitary group of degree n, denoted U, is the group of n×n unitary matrices, with the group operation that of matrix multiplication. The unitary group is a subgroup of the general linear group GL...
  
  .
• Any unit-norm matrix is the average of two unitary matrices. As a consequence, every matrix is a linear combination of two unitary matrices.

See also

• Orthogonal matrix
  Orthogonal matrix
  In linear algebra, an orthogonal matrix , is a square matrix with real entries whose columns and rows are orthogonal unit vectors ....
  
  
• Hermitian matrix
• Symplectic matrix
• Unitary group
  Unitary group
  In mathematics, the unitary group of degree n, denoted U, is the group of n×n unitary matrices, with the group operation that of matrix multiplication. The unitary group is a subgroup of the general linear group GL...
  
  
• Special unitary group
  Special unitary group
  The special unitary group of degree n, denoted SU, is the group of n×n unitary matrices with determinant 1. The group operation is that of matrix multiplication...
  
  
• Unitary operator
  Unitary operator
  In functional analysis, a branch of mathematics, a unitary operator is a bounded linear operator U : H → H on a Hilbert space H satisfyingU^*U=UU^*=I...
  
  
• Matrix decomposition
  Matrix decomposition
  In the mathematical discipline of linear algebra, a matrix decomposition is a factorization of a matrix into some canonical form. There are many different matrix decompositions; each finds use among a particular class of problems.- Example :...
  
  
• Identity matrix
  Identity matrix
  In linear algebra, the identity matrix or unit matrix of size n is the n×n square matrix with ones on the main diagonal and zeros elsewhere. It is denoted by In, or simply by I if the size is immaterial or can be trivially determined by the context...