# Choi matrix in QETLAB

I am using QETLAB, a package for working with quantum information theory in Matlab and I have some doubts. I am trying to calculate diamond norms using such for some quantum channels. However, when doing so, it is said that a superoperator must be the input to the function. I know the Kraus representation of the channels I am testing, so I am not sure how to get the superoperator form of such. In the example they put in their wiki for their diamond norm they use a function called ChoiMatrix that calculates the Choi matrix of a supeorperator, but I am not sure if inputing the Kraus operators there will give me what I need.

An insight about the usage of the functions would be helpful, so as the relationship between superoperators, Kraus operators and Choi matrices.

QETLAB usually deals with channels as Choi operators. You can convert your Kraus operators to the Choi matrix by providing the Kraus operators as a cell array. Example with the amplitude damping channel below.

>> damp = 0.3;
>> K = { diag([1,sqrt(1-damp)]); [0,sqrt(damp);0,0] };
>> ChoiMatrix(K)

ans =

1.0000         0         0    0.8367
0         0         0         0
0         0    0.3000         0
0.8367         0         0    0.7000