The spin of a particle is a fundamental property in quantum physics. We shall inspect below matrix representations of such spin operators.

Suppose you have integer or half-integer spin of value s. The matrices Sx, Sy and Sz representing it have the following properties:

• Si (with i={x,y,z}) are traceless Hermitian matrices;
• Commutation relations (a): [ Si,Sj ] = i εijk Sk, where [·,·] is the commutator and εijk is the Levi-Civita symbol.
• Commutation relations (b): [ Si,S² ] = 0, where S² = Sx²+Sy²+Sz²;
• Eigenvalues: S² = j(j+1)·I and Sz = diag( -j/2, -j/2+1, … ,j/2-1, j ), where I is the identity matrix.

See also this article for more reference.

Examples

 [Sx,Sy,Sz] = spin_matrices(1/2)

 Sx = 
     0      0.5
     0.5    0

 Sy = 
     0     -0.5i
     0.5i   0

 Sz = 
     0.5    0
     0      0.5

Note:

The usual cheats are not allowed!