Tuesday, September 17, 2019

quantum mechanics - Tensor product of two different Pauli matrices sigma2otimeseta1


I'm solving problem 3.D in H. Georgi Lie Algebra etc for fun where one is to compute the matrix elements of the direct product σ2η1 where [σ2]ij and [η1]xy are two different Pauli matrices in two different two dimensional spaces.


Defining the basis in our four dimensional tensor product space |1=|i=1|x=1|2=|i=1|x=2|3=|i=2|x=1|4=|i=2|x=2


Now we know that when we multiply representations, the generators add in the sense of


[J12a(g)]jyix=[J1a]jiδyx+δji[J2a]yx,

where the Js are the generators corresponding to the different representations D1 and D2 (g stands for the group elements).



Using all of this I find that in the basis of (1) the matrix representation of the tensor product is given by


σ2η1=(01i0100ii0010i10)


(The bold 1 is just notation, see below!) I am not asking you to redo the calculations for me but does (3) make sense?


Appendix. My calculations were done in the following fashion [using equation (2)]: 1|σ2η1|1=j=1,y=1|σ2η1|i=1,x=1=[σ2]11δ11+δ11[η1]11=0.

Similarly for eg 1|σ2η1|2=j=1,y=1|σ2η1|i=1,x=2=[σ2]11δ12+δ11[η1]12=1.
This is how the bold 1 was obtained.


So are my calculations (4),(5) totally wrong?


The Pauli matrices σ1=(0110)σ2=(0ii0)σ3=(1001).




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