# Matrix Representation of Operators and Wavefunctions

- Page ID
- 95629

## Vector Representation of an eigenstate

For a set of vectors \(\{|1 \rangle, |2 \rangle, ... | \infty \rangle \}\) that spans the space we are interested in, the arbitrary eigenstate can be decomposed

\[ | \psi \rangle = \sum_i^n c_i | i \rangle = \begin{pmatrix} c_1 \\ c_2 \\ ... \\ c_n \end{pmatrix} \label{1A} \]

The \(\{|1 \rangle, |2 \rangle, ... | \infty \rangle \}\) constitutes a basis (one of many possible) for the space.

Most of the operators we are discussed are linear so

\[ \hat{A} | \psi \rangle = \hat{A} \left( \sum_i^n c_i | i \rangle \right) = \sum_i^n c_i \hat{A} | i \rangle \label{2A}\]

## Matrix Representation of an Operator

Operators can be expressed as matrices that "operator" on the eigenvector discussed above

\[ \hat{A} | i \rangle = \sum_i^n A_{ij} | i \rangle \label{3A} \]

The number \(A_{ij}\) is the \(ij^{th}\) matrix element of \(A\) in the basis select.

Hermitian Operators

Hermitian operators are operators that satisfy the general formula

\[ \langle \phi_i | \hat{A} | \phi_j \rangle = \langle \phi_j | \hat{A} | \phi_i \rangle \label{Herm1}\]

If that condition is met, then \(\hat{A}\) is a Hermitian operator. For any operator that generates a real eigenvalue (e.g., observables), then that operator is Hermitian. The Hamiltonian \(\hat{H}\) meets the condition and a Hermitian operator. Equation \ref{Herm1} can be rewriten as

\[A_{ij} =A_{ji}\]

where

\[A_{ij} = \langle \phi_i | \hat{A} | \phi_j \rangle\]

and

\[A_{ji} = \langle \phi_j | \hat{A} | \phi_i \rangle\]

Therefore, when applied to the Hamiltonian operator

\[\boxed{H_{ij}^* =H_{ji}.}\]

## Multiplication

We can define an inner product (dot product) of two eigenstates \(| \phi_1 \rangle\) and \(| \phi_2\ \rangle\)

\[ \langle \phi_1 | \phi_2 \rangle = \sum_{i=1}^n c_i^* c_j \label{4A}\]

which looks like this in vector representations

\[ \langle \phi_1 | \phi_2 \rangle = ( c_1^* \; c_2^* \; ... c_n^* ) \begin{pmatrix} c_1 \\ c_2 \\ ... \\ c_n \end{pmatrix} \label{5A} \]

This form emphasizes the *dot product *nature of the *inner product* multiplication.

From equation \(\ref{5A}\), we can express the bra form of the eigenstate as

\[ \langle \phi | = \sum_i^n c_i^* \langle i | \label{6A}\]

## Completeness Relation

For vectors \(|i \rangle\) forming an orthonormal basis \(\langle i | j \rangle = \delta_{ij}\) for all space then

\[ \sum_i^n | i \rangle \langle j| = 1 \label{7A}\]

Those terms in this sum are *outer products *and are matrices (remember inner products are scalars).

## Diagonal Representation of an operator

\[\hat{A} = \sum_i ^n \lambda_i | i \rangle \langle i| \label{8A}\]

This is a matrix that has non-zero element everywhere except the diagonal. E.g.,

\[ \begin{pmatrix} 4 & 0 & 0 \\ 0 & -1 & 0 \\ 0 & 0 & -4 \end{pmatrix}\]