how to convert density matrix into density operator?

[Ronghui]

If I have the density matrix for para-Hydrogen as 1/2 {{0,0,0,0},....}, how can I convert it into Bra and Ket form? Thanks.

Ronghui

[kuprov]

One way is to get your bra-ket density matrices (using kronecker products) and then compute the expansion coefficients with your density matrix, i.e.

rho=a1*A1+a2*A2+...   =>

a_k=trace(A_k'*rho)/norm(A_k);

So standard orthogonal expansion in matrix space, see Lecture 12 of the Spin Dynamics course.

[Ronghui]

Thank you very much.

[MichaelTayler]

Hi Ronghui,

There is a neat trick for this, which is used internally in SpinDynamica.  If the density matrix is represented in the Zeeman ket-bra basis, it can be flattened to a vector corresponding to the coefficients of the Zeeman ket-bra operators.  One can then take the dot product with the appropriately-ordered list of basis operators.

To solve your problem in SpinDynamica you can use the ExpressOperator routine, which works more-or-less in the way that Ilya (kuprov) has described:

Code: [Select]

op=opI[1].opI[2]

ExpressOperator[
op,ZeemanKetBraOperatorBasis[]
]
For more information you can look up the help message by executing

Code: [Select]

?ExpressOperator