Schur--Horn bound on field-free molecular orientation at finite temperature

The maximum field-free orientation attainable from a \emph{thermal} molecular ensemble within a finite rotational subspace has not been characterised analytically. Here we derive a Schur–Horn-type upper bound on the field-free orientation $\text{avg}\cos \theta$ achievable by \emph{any} $M$-conserving unitary control acting on a Boltzmann ensemble truncated to rotational levels $J\le \text{Jmax}$. The bound is the sum, over magnetic-quantum-number sectors, of the sorted Boltzmann weights paired with the sorted spectrum of $\cos \theta$; it is purely kinematic, set by temperature, the rotational constant, and $\text{Jmax}$ alone, and interpolates exactly between the zero-temperature subspace eigenvalue and a finite-temperature ceiling fixed by the rotational partition function. Benchmarked on LiH against the analytical $N$-subpulse resonant protocol of Hong $\mathit{\text{et}}\mathit{\text{al.}}$ [Phys.\ Rev.\ Research $\mathbf{\text{7}}$, L012049 (2025)], it reveals three regimes: the protocol saturates the bound to within about $1.6\mathrm{\%}$ for $T\le \text{SI}5\text{kelvin}$, loses roughly $10\mathrm{\%}$ of its zero-temperature orientation at $T=B\mathrm{/}\text{kB}\approx \text{SI}10.8\text{kelvin}$, and leaves a $10$–$40\mathrm{\%}$ gap above $\text{SI}10\text{kelvin}$. Optimising the subpulse areas and carrier phases within the fixed layout closes only $\approx 7\mathrm{\%}$ of that gap, the optimal phase offsets vanishing, which localises the dominant loss in the rigidity of the analytic layout rather than in the choice of areas or phases. The bound is a control-independent target for coherent-control design, applies unchanged to any linear polar molecule in a ${}^1{\mathrm{\Sigma }}^{+}$ state, and is mapped across the $(\text{Jmax},T)$ plane.