Make a matrix representation of a fermion ladder operator.

Operators are mapped as follows: a_j^\dagger -> Z0 .. Z{j-1} (X_j - iY_j) / 2 a_j -> Z0 .. Z{j-1} (X_j + iY_j) / 2

index This is a nonzero integer. The integer indicates the tensor factor and the sign indicates raising or lowering.
n_qubits int

Number qubits in the system Hilbert space.

The corresponding Scipy sparse matrix.