Matrix Representation
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Matrix Representation

Matrix Rep. Same basics as introduced already. Convenient method of working with vectors.Superposition Complete set of vectors can be used to express any other vector.Complete set of N vectors can form other complete sets of N vectors.Can find set of vectors for Hermitian operator satisfying Eigenvectors and eigenvaluesMatrix method Find superposition of basis states that are eigenstates of particular operator. Get eigenvalues.

Copyright – Michael D. Fayer, 2007


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Any vector can be written as

with

To get this, project out

Orthonormal basis set in N dimensional vector space

basis vectors

Copyright – Michael D. Fayer, 2007


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Left mult. by

The N2 scalarproducts

N values of j ; N for each yi

are completely determined by

Operator equation

Substituting the series in terms of bases vectors.

Copyright – Michael D. Fayer, 2007


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In terms of the vector representatives

vector

vector representative,

must know basis

(Set of numbers, gives you vector when basis is known.)

The set of N linear algebraic equations can be written as

double underline means matrix

Writing

Matrix elements of A in the basis

gives for the linear transformation

Know the aij because we know A and

Copyright – Michael D. Fayer, 2007


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The product of matrix and vector representative xis a new vector representative y with components

array of coefficients - matrix

The aij are the elements of the matrix .

Copyright – Michael D. Fayer, 2007


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The unit matrix

The zero matrix

ones down principal diagonal

Gives identity transformation

Corresponds to

Matrix Properties, Definitions, and Rules

Two matrices, and are equal

if aij = bij.

Copyright – Michael D. Fayer, 2007


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Using the same basis for both transformations

has elements

Example

Law of matrix multiplication

Matrix multiplication

Consider

operator equations

Copyright – Michael D. Fayer, 2007


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Multiplication NOT Commutative

Matrix addition and multiplication by complex number

Inverse of a matrix

identity matrix

inverse of

transpose of cofactor matrix (matrix of signed minors)

determinant

Multiplication Associative

Copyright – Michael D. Fayer, 2007


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For matrix defined as

Transpose

interchange rows and columns

Complex Conjugate

complex conjugate of each element

Hermitian Conjugate

complex conjugate transpose

Reciprocal of Product

Copyright – Michael D. Fayer, 2007


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determinant of transpose is determinant

complex conjugate of product is product of complex conjugates

determinant of complex conjugate is complex conjugate of determinant

Hermitian conjugate of product is product of Hermitian conjugates in reverse order

determinant of Hermitian conjugate is complex conjugateof determinant

Rules

transpose of product is product of transposes in reverse order

Copyright – Michael D. Fayer, 2007


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Powers of a matrix

Definitions

Symmetric

Hermitian

Real

Imaginary

Unitary

Diagonal

Copyright – Michael D. Fayer, 2007


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vector representatives in particular basis

then

becomes

row vector transpose of column vector

transpose

Hermitian conjugate

Column vector representative one column matrix

Copyright – Michael D. Fayer, 2007


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Superposition of can form N new vectors linearly independenta new basis

complex numbers

Change of Basis

orthonormal basis

then

Copyright – Michael D. Fayer, 2007


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Important result. The new basis will be orthonormal if , the transformation matrix, is unitary (see book and Errata and Addenda ).

New Basis is Orthonormal

if the matrix

coefficients in superposition

meets the condition

is unitary

Copyright – Michael D. Fayer, 2007


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Vector orthonormal

vector – line in space (may be high dimensionality abstract space)written in terms of two basis sets

Same vector – different basis.

The unitary transformation can be used to change a vector representativeof in one orthonormal basis set to its vector representative in another orthonormal basis set.

x – vector rep. in unprimed basisx' – vector rep. in primed basis

change from unprimed to primed basis

change from primed to unprimed basis

Unitary transformation substitutes orthonormal basis for orthonormal basis .

Copyright – Michael D. Fayer, 2007


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Example orthonormal

Consider basis

Vector - line in real space.

In terms of basis

Vector representative in basis

Copyright – Michael D. Fayer, 2007


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Change basis by rotating axis system 45 orthonormal° around .

Can find the new representative of , s'

is rotation matrix

For 45° rotation around z

Copyright – Michael D. Fayer, 2007


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Example – length of vector orthonormal

Then

vector representative of in basis

Same vector but in new basis.Properties unchanged.

Copyright – Michael D. Fayer, 2007


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Can go back and forth between representatives of a vector by

change from primed to unprimed basis

change from unprimed to primed basis

components of in different basis

Copyright – Michael D. Fayer, 2007


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In the basis can write by

or

Change to new orthonormal basis using

or

with the matrix given by

Because is unitary

Consider the linear transformation

operator equation

Copyright – Michael D. Fayer, 2007


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Extremely Important by

Can change the matrix representing an operator in one orthonormal basisinto the equivalent matrix in a different orthonormal basis.

Called

Similarity Transformation

Copyright – Michael D. Fayer, 2007


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Example by

Can insert between because

Therefore

In basis

Go into basis

Relations unchanged by change of basis.

Copyright – Michael D. Fayer, 2007


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Isomorphism between operators in abstract vector space byand matrix representatives.

Because of isomorphism not necessary to distinguishabstract vectors and operatorsfrom their matrix representatives.

The matrices (for operators) and the representatives (for vectors)can be used in place of the real things.

Copyright – Michael D. Fayer, 2007


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Hermitian Operators and Matrices by

Hermitian operator

Hermitian operator Hermitian Matrix

+ - complex conjugate transpose - Hermitian conjugate

Copyright – Michael D. Fayer, 2007


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Theorem by (Proof: Powell and Craseman, P. 303 – 307, or linear algebra book)

For a Hermitian operator A in a linear vector space of N dimensions,there exists an orthonormal basis,relative to which A is represented by a diagonal matrix .The vectors, , and the corresponding real numbers, ai, are thesolutions of the Eigenvalue Equationand there are no others.

Copyright – Michael D. Fayer, 2007


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There exists some new basis eigenvectors by

in which represents operator and is diagonal eigenvalues.

To get from to unitary transformation.

Similarity transformation takes matrix in arbitrary basisinto diagonal matrix with eigenvalues on the diagonal.

Application of Theorem

Operator A represented by matrixin some basis . The basis is any convenient basis.

In general, the matrix will not be diagonal.

Copyright – Michael D. Fayer, 2007


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Matrices and Q.M. by

Previously represented state of system by vector in abstract vector space.Dynamical variables represented by linear operators.Operators produce linear transformations.Real dynamical variables (observables) are represented by Hermitian operators.Observables are eigenvalues of Hermitian operators.Solution of eigenvalue problem gives eigenvalues and eigenvectors.

Copyright – Michael D. Fayer, 2007


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Matrix Representation by

Hermitian operators replaced by Hermitian matrix representations.In proper basis, is the diagonalized Hermitian matrix and the diagonal matrix elements are the eigenvalues (observables).A suitable transformation takes (arbitrary basis) into (diagonal – eigenvector basis)Diagonalization of matrix gives eigenvalues and eigenvectors.Matrix formulation is another way of dealing with operators and solving eigenvalue problems.

Copyright – Michael D. Fayer, 2007


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All ideas about matrices also true for infinite dimensional matrices.

All rules about kets, operators, etc. still apply.

ExampleTwo Hermitian matricescan be simultaneously diagonalized by the same unitarytransformation if and only if they commute.

Copyright – Michael D. Fayer, 2007


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matrix elements of matrices.a

Example – Harmonic Oscillator

Have already solved – use occupation number representation kets and bras (already diagonal).

Copyright – Michael D. Fayer, 2007




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Adding the matrices and and multiplying by ½ gives

The matrix is diagonal with eigenvalues on diagonal. In normal unitsthe matrix would be multiplied by .

This example shows idea, but not how to diagonalize matrix when youdon’t already know the eigenvectors.

Copyright – Michael D. Fayer, 2007


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In terms of the components ½ gives

This represents a system of equations

Diagonalization

Eigenvalue equation

eigenvalue

matrix representingoperator

representative of eigenvector

We know the aij.We don't knowa - the eigenvaluesui - the vector representatives, one for each eigenvalue.

Copyright – Michael D. Fayer, 2007


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Then substituting one eigenvalue at a time into system of equations,the ui (eigenvector representatives) are found.N equations for u's gives only N - 1 conditions.Use normalization.

Besides the trivial solution

A solution only exists if the determinant of the coefficients of the ui vanishes.

know aij, don't know a's

Expanding the determinant gives Nth degree equation for thethe unknown a's (eigenvalues).

Copyright – Michael D. Fayer, 2007


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a equations, and b not eigenkets.Coupling g.

These equations define H.

The matrix elements are

And the Hamiltonian matrix is

Example - Degenerate Two State Problem

Basis - time independent kets orthonormal.

Copyright – Michael D. Fayer, 2007


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These only have a solution if the determinant of the coefficients vanish.

Dimer Splitting

Expanding

E0

2g

Excited State

Energy Eigenvalues

Ground StateE = 0

The corresponding system of equations is

Copyright – Michael D. Fayer, 2007


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To obtain Eigenvectors coefficients vanish. Use system of equations for each eigenvalue.

Eigenvectors associated with l+ andl-.

and are the vector representatives of andin the

We want to find these.

Copyright – Michael D. Fayer, 2007


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Matrix elements of coefficients vanish.

The matrix elements are

The result is

First, for the eigenvaluewrite system of equations.

Copyright – Michael D. Fayer, 2007


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Substitute coefficients vanish.

Multiplying the matrix by the column vector representative gives equations.

An equivalent way to get the equations is to use a matrix form.

Copyright – Michael D. Fayer, 2007


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Always get coefficients vanish.N – 1 conditions for the N unknown components.Normalization condition gives necessary additional equation.

Then

and

Eigenvector in terms of thebasis set.

The two equations are identical.

Copyright – Michael D. Fayer, 2007


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Substituting coefficients vanish.

These equations give

Using normalization

Therefore

For the eigenvalue

using the matrix form to write out the equations

Copyright – Michael D. Fayer, 2007


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Can diagonalize by transformation coefficients vanish.

diagonal not diagonal

Transformation matrix consists of representatives of eigenvectorsin original basis.

complex conjugate transpose

Copyright – Michael D. Fayer, 2007


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Factoring out coefficients vanish.

after matrix multiplication

more matrix multiplication

diagonal with eigenvalues on diagonal

Then

Copyright – Michael D. Fayer, 2007


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