Chenhao Wang
In quantum information theory, the state space of a multipartite quantum system is modeled by a tensor product. In the tensor-product space $\mathbb C^{d_1}\otimes\cdots\otimes\mathbb C^{d_p}$, a nonzero vector is a \emph{product state} if it can be written as $\lvert \varphi_1\rangle\otimes\cdots\otimes\lvert \varphi_p\rangle$ with $\lvert \varphi_j\rangle\in\mathbb C^{d_j}\setminus\{0\}$. An \emph{unextendible product basis} (UPB) is a finite family of pairwise orthogonal product states such that no nonzero product state is orthogonal to all of them. UPBs play a key role in investigating quantum entanglement and nonlocal phenomena. Finding a smallest UPB is a natural extremal problem: it asks how few pairwise orthogonal product states suffice to prevent any further product state from being added. The general minimum-size problem for UPBs has been studied for over two decades since the seminal work of Alon and Lovász. For local dimensions $d_1,\ldots,d_p\ge2$, let $f_m(d_1,\ldots,d_p)$ be the minimum cardinality of a UPB and let $f_{LB}(d_1,\ldots,d_p)=1+\sum_{j=1}^{p}(d_j-1)$ be the natural lower bound. Alon and Lovász determined exactly when $f_m$ attains the lower bound $f_{LB}$, but the obstructed multipartite cases remained open in general. We prove a stabilization theorem: for every non-all-qubit system with $p\ge3$, whenever parity prevents the natural lower bound $f_{LB}$ from being attained, the true minimum is exactly $f_{LB}+1$. Equivalently, if the number of even local dimensions is positive and even, and at least one local dimension is greater than two, then $f_m(d_1,\ldots,d_p)=f_{LB}(d_1,\ldots,d_p)+1$. The proof is built on a unified graph-theoretic framework. Our result, together with earlier work, settles the minimum-cardinality problem for UPBs in all finite quantum systems.