= Conditional spin measurement of identical fermions
A <measurement> addressing a spatial mode can prepare an opposite-spin product in the mode description without violating fermionic antisymmetry. For <orthogonal> orbitals $\phi_a,\phi_b$, the conditional mode state with <spin> up in $a$ and <spin> down in $b$ corresponds to the antisymmetric particle-coordinate state $[\phi_a(1)\phi_b(2)|\uparrow\downarrow\rangle-\phi_b(1)\phi_a(2)|\downarrow\uparrow\rangle]/\sqrt2$. Exchange of mode <spin> factors is different from full exchange of the <identical particles>. Only the latter must remain antisymmetric.
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