Gühne, Otfried
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Publication Access status: Metadata only , Certifying quantum separability with adaptive polytopesThe concept of entanglement and separability of quantum states is relevant for several fields in physics. Still, there is a lack of effective operational methods to characterise these features. We propose a method to certify quantum separability of two- and multipArticle quantum systems based on an adaptive polytope approximation. This leads to an algorithm which, for practical purposes, conclusively recognises two-pArticle separability for small and medium-size dimensions. For multipArticle systems, the approach allows to characterise full separability for up to five qubits or three qutrits; in addition, different classes of entanglement can be distinguished. Finally, our methods allow to identify systematically quantum states with interesting entanglement properties, such as maximally robust states which are separable for all bipartitions, but not fully separable.Source Type:Article3 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Metadata only , Covariance-Matrix-Based Criteria for Network Entanglement(2023); Quantum networks offer a realistic and practical scheme for generating multipArticle entanglement and implementing multipArticle quantum communication protocols. However, the correlations that can be generated in networks with quantum sources and local operations are not yet well understood. Covariance matrices, which are powerful tools in entanglement theory, have been also applied to the network scenario. We present simple proofs for the decomposition of such matrices into the sum of positive semi-definite block matrices and, based on that, develop analytical and computable necessary criteria for preparing states in quantum networks. These criteria can be applied to networks where nodes share at most one source, such as all bipartite networks.Source Type:Article3 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Metadata only , MultipArticle singlet states cannot be maximally entangled for the bipartitions(2024); One way to explore multipArticle entanglement is to ask for maximal entanglement with respect to different bipartitions, leading to the notion of absolutely maximally entangled states or perfect tensors. A different path uses unitary invariance and symmetries, resulting in the concept of multipArticle singlet states. We show that these two concepts are incompatible in the sense that the space of pure multipArticle singlet states does not contain any state for which all partitions of two pArticles vs the rest are maximally entangled. This puts restrictions on the construction of quantum codes and contributes to discussions in the context of the anti-de Sitter/conformal field theory correspondence and quantum gravity.Source Type:Article1 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Quantum LOSR networks cannot generate graph states with high fidelity(2024); ; Quantum networks lead to novel notions of locality and correlations and an important problem concerns the question of which quantum states can be experimentally prepared with a given network structure and devices and which not. We prove that all multi-qubit graph states arising from a connected graph cannot originate from any quantum network with bipartite sources, as long as feed-forward and quantum memories are not available. Moreover, the fidelity of a multi-qubit graph state and any network state cannot exceed 9/10. Similar results can also be established for a large class of multi-qudit graph states.Source Type:Article12 64
