Certifying beyond quantumness of locally quantum no-signalling theories through quantum input Bell test

TL;DR

This paper demonstrates that by extending Bell tests to include quantum inputs, one can certify beyond-quantum correlations in locally quantum no-signalling theories, revealing the need for additional principles to distinguish quantum from beyond-quantum correlations.

Contribution

It introduces a generalized Bell test with quantum inputs that can certify beyond-quantum states, highlighting the necessity of new principles to characterize quantum correlations.

Findings

Beyond-quantum correlations can be certified with quantum input Bell tests.

Standard Bell tests cannot distinguish beyond-quantum states from quantum states.

Additional principles sensitive to local quantum inputs are required to isolate quantum correlations.

Abstract

Physical theories constrained with local quantum structure and satisfying the no-signalling principle can allow beyond-quantum global states. In a standard Bell experiment, correlations obtained from any such beyond-quantum bipartite state can always be reproduced by quantum states and measurements, suggesting local quantum structure and no-signalling to be the axioms to isolate quantum correlations. In this letter, however, we show that if the Bell experiment is generalized to allow local quantum inputs, then beyond-quantum correlations can be generated by every beyond-quantum state. This gives us a way to certify beyond-quantumness of locally quantum no-signalling theories and in turn suggests requirement of additional information principles along with local quantum structure and no-signalling principle to isolate quantum correlations. More importantly, our work establishes that the additional principle(s) must be sensitive to the quantum signature of local inputs. We also generalize our results to multipartite locally quantum no-signalling theories and further analyze some interesting implications.