Unitary matrix

Complex matrix whose conjugate transpose equals its inverse From Wikipedia, the free encyclopedia

In linear algebra, an invertible complex square matrix U is unitary if its matrix inverse U−1 equals its conjugate transpose U*, that is, if

where I is the identity matrix.

In physics, especially in quantum mechanics, the conjugate transpose is referred to as the Hermitian adjoint of a matrix and is denoted by a dagger (), so the equation above is written

A complex matrix U is special unitary if it is unitary and its matrix determinant equals 1.

For real numbers, the 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

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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, Ux, Uy⟩ = ⟨x, y.
  • U is normal ().
  • U is diagonalizable; that is, U is unitarily similar to a diagonal matrix, as a consequence of the spectral theorem. Thus, U has a decomposition of the form where V is unitary, and D is diagonal and unitary.
  • The eigenvalues of lie on the unit circle, as does .
  • The eigenspaces of are orthogonal.
  • U can be written as U = eiH, where e indicates the matrix exponential, i is the imaginary unit, and H is a Hermitian matrix.

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

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

Equivalent conditions

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If U is a square, complex matrix, then the following conditions are equivalent:[2]

  1. is unitary.
  2. is unitary.
  3. is invertible with .
  4. The columns of form an orthonormal basis of with respect to the usual inner product. In other words, .
  5. The rows of form an orthonormal basis of with respect to the usual inner product. In other words, .
  6. is an isometry with respect to the usual norm. That is, for all , where .
  7. is a normal matrix (equivalently, there is an orthonormal basis formed by eigenvectors of ) with eigenvalues lying on the unit circle.

Elementary constructions

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2 × 2 unitary matrix

One general expression of a 2 × 2 unitary matrix is

which depends on 4 real parameters (the phase of a, the phase of b, the relative magnitude between a and b, and the angle φ). The form is configured so the determinant of such a matrix is

The sub-group of those elements with is called the special unitary group SU(2).

Among several alternative forms, the matrix U can be written in this form:

where and above, and the angles can take any values.

By introducing and has the following factorization:

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

Another factorization is[3]

Many other factorizations of a unitary matrix in basic matrices are possible.[4][5][6][7][8][9]

See also

References

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