Periodic Reporting for period 1 - N-MuQuaS (Non-locality in Multipartite Quantum Systems)
Reporting period: 2016-04-01 to 2018-03-31
The overall aim of this project was therefore to significantly improve our understanding of the phenomenon of non-locality in multipartite quantum systems. The project was structured around three sub-projects, each defining more concrete objectives.
First, we aimed at understanding the relation between non-locality and entanglement—another key notion in quantum information—in the multipartite scenario. The second aim was to design methods of detection of non-locality in the multipartite scenario with the aid of quantities that are within reach of current experimental technology, and also to explore the phenomenon of non-locality in many-body interacting quantum systems. Finally, we aimed at addressing a more fundamental problem, that is, to propose information-theoretic principles allowing to single out the set of quantum correlations from the nonsignaling ones.
As for the first aim, we have provided a general construction of multipartite quantum states that are genuinely entangled but not genuinely non-local, thus showing that the inequivalence between entanglement and non-locality is a more generic feature of multipartite quantum systems than we thought. As for the second aim, we have introduced a general toolbox for studying non-locality in one-dimensional many-body interacting quantum systems with the aid of few-body correlations; within this approach the energy of a quantum many-body system can tell us whether it is non-local. We have also shown how the number of particles sharing genuine non-locality in a multipartite system can be certified from only two-body correlations. Finally, we have constructed a general class of Bell inequalities, involving an arbitrary number of measurements and outcomes, which are maximally violated by the maximally entangled states and provided numerical evidence that these inequalities can be used for self-testing.
In the second part of the project we have established a link between quantum many-body systems and non-locality. We have introduced a method allowing one to reveal non-locality of low-energy states of one-dimensional many-body interacting quantum systems with the aid of quantities that are measurable in current experiments. On the other hand, we have pointed out that tools frequently used to study quantum many-body systems can be harnessed to study Bell inequalities, in particular to efficiently compute the maximal classical and quantum values thereof. We have also provided a construction of Bell-like inequalities capable of revealing entanglement depth in multipartite quantum states.
Finally, in the third part of the project, among other, we have constructed and fully characterized a class of Bell inequalities involving arbitrary numbers of measurements and outcomes whose maximal quantum violation is achieved on the maximally entangled states. We have also pointed out certain device-independent applications of our Bell inequalities such as for instance self-testing. Interestingly, violation of these inequalities have been tested on entangled states generated in an experiment involving a large-scale integrated photonic quantum chip.
Each of the above accomplishments has involved extensive international collaboration with different European institutions and has lead to independent well-received scientific publications, all of them freely accessible online and many of them already published in renowned physical journals. Moreover, thanks to the fellowship the results obtained during the action have been presented on scientific events. Last but not least, I engaged in a variety of outreach and popularization activities.