In mathematics, a Hermitian matrix (or self-adjoint matrix) is a complex square matrix that is equal to its own conjugate transpose—that is, the element in the i-th row and j-th column is equal to the complex conjugate of the element in the j-th row and i-th column, for all indices i and j:
or in matrix form:
Hermitian matrices can be understood as the complex extension of real symmetric matrices.
If the conjugate transpose of a matrix is denoted by then the Hermitian property can be written concisely as
Hermitian matrices are named after Charles Hermite, who demonstrated in 1855 that matrices of this form share a property with real symmetric matrices of always having real eigenvalues. Other, equivalent notations in common use are although in quantum mechanics, typically means the complex conjugate only, and not the conjugate transpose.
Hermitian matrices can be characterized in a number of equivalent ways, some of which are listed below:
A square matrix is Hermitian if and only if it is equal to its adjoint, that is, it satisfies
This is also the way that the more general concept of self-adjoint operator is defined.
An matrix is Hermitian if and only if
A square matrix is Hermitian if and only if it is unitarily diagonalizable with real eigenvalues.
Hermitian matrices are fundamental to quantum mechanics because they describe operators with necessarily real eigenvalues. An eigenvalue of an operator on some quantum state is one of the possible measurement outcomes of the operator, which necessitates the need for operators with real eigenvalues.
In this section, the conjugate transpose of matrix is denoted as the transpose of matrix is denoted as and conjugate of matrix is denoted as
See the following example:
The diagonal elements must be real, as they must be their own complex conjugate.
Well-known families of Hermitian matrices include the Pauli matrices, the Gell-Mann matrices and their generalizations. In theoretical physics such Hermitian matrices are often multiplied by imaginary coefficients, which results in skew-Hermitian matrices.
Here, we offer another useful Hermitian matrix using an abstract example. If a square matrix equals the product of a matrix with its conjugate transpose, that is, then is a Hermitian positive semi-definite matrix. Furthermore, if is row full-rank, then is positive definite.
The entries on the main diagonal (top left to bottom right) of any Hermitian matrix are real.
By definition of the Hermitian matrix
Only the main diagonal entries are necessarily real; Hermitian matrices can have arbitrary complex-valued entries in their off-diagonal elements, as long as diagonally-opposite entries are complex conjugates.
A matrix that has only real entries is symmetric if and only if it is Hermitian matrix. A real and symmetric matrix is simply a special case of a Hermitian matrix.
by definition. Thus (matrix symmetry) if and only if ( is real).
So, if a real anti-symmetric matrix is multiplied by a real multiple of the imaginary unit then it becomes Hermitian.
Every Hermitian matrix is a normal matrix. That is to say,
The finite-dimensional spectral theorem says that any Hermitian matrix can be diagonalized by a unitary matrix, and that the resulting diagonal matrix has only real entries. This implies that all eigenvalues of a Hermitian matrix A with dimension n are real, and that A has n linearly independent eigenvectors. Moreover, a Hermitian matrix has orthogonal eigenvectors for distinct eigenvalues. Even if there are degenerate eigenvalues, it is always possible to find an orthogonal basis of Cn consisting of n eigenvectors of A.
The sum of any two Hermitian matrices is Hermitian.
The inverse of an invertible Hermitian matrix is Hermitian as well.
If then so as claimed.
The product of two Hermitian matrices A and B is Hermitian if and only if AB = BA.
Thus An is Hermitian if A is Hermitian and n is an integer.
If A and B are Hermitian, then ABA is also Hermitian.
For an arbitrary complex valued vector v the product is real because of This is especially important in quantum physics where Hermitian matrices are operators that measure properties of a system, e.g. total spin, which have to be real.
The Hermitian complex n-by-n matrices do not form a vector space over the complex numbers, C, since the identity matrix In is Hermitian, but i In is not. However the complex Hermitian matrices do form a vector space over the real numbers R. In the 2n2-dimensional vector space of complex n × n matrices over R, the complex Hermitian matrices form a subspace of dimension n2. If Ejk denotes the n-by-n matrix with a 1 in the j,k position and zeros elsewhere, a basis (orthonormal with respect to the Frobenius inner product) can be described as follows:
together with the set of matrices of the form
and the matrices
where denotes the imaginary unit,
An example is that the four Pauli matrices form a complete basis for the vector space of all complex 2-by-2 Hermitian matrices over R.
If n orthonormal eigenvectors of a Hermitian matrix are chosen and written as the columns of the matrix U, then one eigendecomposition of A is where and therefore
The singular values of are the absolute values of its eigenvalues:
Since has an eigendecomposition , where is a unitary matrix (its columns are orthonormal vectors; see above), a singular value decomposition of is , where and are diagonal matrices containing the absolute values and signs of 's eigenvalues, respectively. is unitary, since the columns of are only getting multiplied by . contains the singular values of , namely, the absolute values of its eigenvalues.
The determinant of a Hermitian matrix is real:
(Alternatively, the determinant is the product of the matrix's eigenvalues, and as mentioned before, the eigenvalues of a Hermitian matrix are real.)
Additional facts related to Hermitian matrices include:
Main article: Rayleigh quotient
In mathematics, for a given complex Hermitian matrix M and nonzero vector x, the Rayleigh quotient is defined as:: p. 234 
For real matrices and vectors, the condition of being Hermitian reduces to that of being symmetric, and the conjugate transpose to the usual transpose for any non-zero real scalar Also, recall that a Hermitian (or real symmetric) matrix has real eigenvalues.
It can be shown that, for a given matrix, the Rayleigh quotient reaches its minimum value (the smallest eigenvalue of M) when is (the corresponding eigenvector). Similarly, and
The Rayleigh quotient is used in the min-max theorem to get exact values of all eigenvalues. It is also used in eigenvalue algorithms to obtain an eigenvalue approximation from an eigenvector approximation. Specifically, this is the basis for Rayleigh quotient iteration.
The range of the Rayleigh quotient (for matrix that is not necessarily Hermitian) is called a numerical range (or spectrum in functional analysis). When the matrix is Hermitian, the numerical range is equal to the spectral norm. Still in functional analysis, is known as the spectral radius. In the context of C*-algebras or algebraic quantum mechanics, the function that to M associates the Rayleigh quotient R(M, x) for a fixed x and M varying through the algebra would be referred to as "vector state" of the algebra.