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The Von Neumann equation integrates to $\rho (t)=U(t)\rho (0)U(t{)}^{†}$, so the spectrum and purity of a density operator are invariant under any allowed open-loop control. These are the kinematic restrictions.

There is no additional orbit restriction here: the generators $i{\sigma }_z$ and $i{\sigma }_x$, together with their matrix commutator, span the special unitary Lie algebra:For freely chosen durations and piecewise continuous real control values, this gives density operator controllability. The connected reachable group is $SU(2)$; its action by conjugation is the same as that of $U(2)$, because a global phase cancels. Thus exactly the pairs ${\rho }_0\leftrightarrow {\rho }_2$ and ${\rho }_1\leftrightarrow {\rho }_3$ can be interconverted. No coherent control can turn either mixed state into either pure state, even though every orientation within each spectral orbit is reachable.

Write the unitary conjugation as $\rho (t)=U(t)\rho (0)U(t{)}^{†}$. The identity component is unchanged, and the generalized Bloch vector transforms byThese coefficients are real, by the same trace argument as in part (a). Unitary conjugation preserves the Hilbert-Schmidt inner product, so the conjugated traceless generators form another orthonormal basis. It follows that $O(U{)}^{T}O(U)=I$. The unitary group is connected, $O(I)=I$, and the determinant of an orthogonal matrix is $\pm 1$; hence $\det O(U)=1$. Thus Hamiltonian evolution rotates the generalized Bloch vector by an element of $SO(N^2-1)$.

Equivalently, in units $\hslash =1$, the Von Neumann equation givesCyclicity of the trace shows $A_{k\ell }=-A_{\ell k}$, so $A$ is a real skew-symmetric matrix. This is the generator form of Hamiltonian rotations of Bloch vectors, valid also for a time-dependent Quantum Hamiltonian. For $N>2$, Hamiltonians generally realize only a proper subgroup of all rotations of this real space; they preserve the whole density-operator spectrum, not merely its purity of a density operator.

Every density operator can be written asTherefore $⟨\varphi |\rho |\varphi ⟩={\sum }_j{p}_j|⟨{\psi }_j|\varphi ⟩{|}^2\ge 0$, so $\rho$ is a positive semidefinite operator, and $\mathrm{Tr}\rho ={\sum }_j{p}_j=1$. A pure quantum state has a rank-one density operator $|\psi ⟩⟨\psi |$, equivalently ${\rho }^2=\rho$ and $\mathrm{Tr}({\rho }^2)=1$. A mixed quantum state is not pure and has $\mathrm{Tr}({\rho }^2)<1$ in finite dimension.

The Von Neumann equation and the Pauli matrix commutator identity giveso the Bloch vector obeysWriting $\omega =|\mathbf{\omega }|$ and $\hat{\mathbf{\omega }}=\mathbf{\omega }/\omega$, the assumption $\mathbf{a}\cdot \mathbf{\omega }=0$ givesand henceUnitary evolution rotates the Bloch vector and preserves its length. Since $|\mathbf{a}|<1$, the state remains mixed, whereas $|{\uparrow }_x⟩$ has the unit Bloch vector $(1,0,0)$. The system is therefore never definitely in $|{\uparrow }_x⟩$.