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I want to create a matrix of the form:

$X_\alpha = 1\otimes 1\otimes 1 ...\otimes X \otimes ... \otimes 1$

where $X$ is the Pauli $x$ matrix and is the $\alpha$th factor, and $1$ is the $2\times 2$ identity matrix. I have found answer on how to create a block diagonal matrices of this form for a specific placement of $X$ but I am trying to generalize it for an arbitrary $\alpha$.

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1 Answer 1

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See this book on page 53 (section 3.4.1):

SpinQ[S_] := IntegerQ[2S] && S>=0
op[S_?SpinQ, n_Integer, k_Integer, a_?MatrixQ] /;
               1<=k<=n && Dimensions[a] == {2S+1,2S+1} :=
  KroneckerProduct[IdentityMatrix[(2S+1)^(k-1), SparseArray],
                   a,
                   IdentityMatrix[(2S+1)^(n-k), SparseArray]]

You'll be using S=1/2 and can simplify the above to

op[n_Integer, k_Integer, a_?MatrixQ] /;
             1<=k<=n && Dimensions[a] == {2,2} :=
  KroneckerProduct[IdentityMatrix[2^(k-1), SparseArray],
                   a,
                   IdentityMatrix[2^(n-k), SparseArray]]
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