Entanglement is a resource in quantum information theory when state manipulation is restricted to local operations assisted by classical communication (LOCC). It is therefore of paramount importance to decide which LOCC transformations are possible and, particularly, which states are maximally useful under this restriction. While the bipartite maximally entangled state is well known (it is the only state that cannot be obtained from any other and, at the same time, it can be transformed to any other by LOCC), no such state exists in the multipartite case. In order to cope with this fact, we introduce here the notion of the maximally entangled set (MES) of n-partite states. This is the set of states which are maximally useful under LOCC manipulation; i.e., any state outside of this set can be obtained via LOCC from one of the states within the set and no state in the set can be obtained from any other state via LOCC. We determine the MES for states of three and four qubits and provide a simple characterization for them. In both cases, infinitely many states are required. However, while the MES is of measure zero for 3-qubit states, almost all 4-qubit states are in the MES. This is because, in contrast to the 3-qubit case, deterministic LOCC transformations are almost never possible among fully entangled four-partite states. We determine the measure-zero subset of the MES of LOCC convertible states. This is the only relevant class of states for entanglement manipulation.
Local operations assisted with classical communication (LOCC) and/or its stochastic generalization induce an ordering relation among bipartite entangled states based on their inter-convertibility. This, in multipartite scenario, leads to different classes of entanglement where states within a class can be inter-converted but conversion among different classes is not possible, and thus results in incomparable multipartite entangled resources. Here we show that a bona-fide ordering relation, under the operational paradigm of LOCC, can be established between two such incomparable states based on their performance in some operational task. Entanglement assisted local state discrimination provides one such platform for resource comparison and the notion of genuinely nonlocal product states becomes quite pertinent in this regard. We provide the detailed framework along with explicit examples exhibiting how incomparable resources can be put into well defined ordered relation.
The characterization of possible transformations between quantum states and the quantification of entanglement are central for understanding quantum resources. Explicit results are, however, scarce since they require complicated optimizations over transformation protocols or state decompositions. We introduce a method to optimize functions of the entanglement spectrum of bipartite states over mixed-state decompositions with arbitrary accuracy. Our results are based on a convergent semidefinite programming hierarchy of optimizations over entanglement spectra using a natural polynomial basis and Schur-Weyl duality. Our approach can also be used to determine non-polynomial measures, such as the entanglement of formation, and to characterize the dimensionality of entanglement. Moreover, extracting the decomposition allows us to formulate criteria to determine state transformations under local operations and classical communication (LOCC). In particular, we provide a hierarchy of certificates of LOCC non-convertibility from pure to mixed states in any dimension and a heuristic approach for LOCC convertibility.
In order to better understand the class of quantum operations that preserve the positivity of partial transpose (PPT operations) and its relation to the widely used class of local operations and classical communication (LOCC), we study the problem of distinguishing orthogonal maximally entangled states (MES) by PPT operations. Firstly, we outline a rather simple proof to show that the number of
d⊗d PPT distinguishable MES is at most
d, which slightly generalizes existing results on this problem. Secondly, we construct 4 MES in
4⊗4 state space that cannot be distinguished by PPT operations. Before our work, it was unknown whether there exists
d MES in
d⊗d state space that are locally indistinguishable. This example leads us to a novel phenomenon of Entanglement Catalysis Discrimination. Moreover, we find there exists a set of locally indistinguishable states
K such that
K⊗m is locally distinguishable for some finite
m. As an interesting application, we exhibit a quantum channel with one sender and two receivers, whose one-shot zero-error local capacity is not optimal, but multi-use would enhance the capacity to achieve the full output dimension even without entangled input. Finally, we consider the entanglement cost of distinguishing three Bell states and a
2⊗2 entangled basis. In the former case a bipartite pure entangled state with the largest Schmidt coefficient at most 2/3 is necessary and sufficient, while in the latter case an additional Bell state, or one ebit, should be supplied.