Quantum correlations and exchange symmetry

Marek Gazdzicki, Francesco Giacosa

Searches for post-quantum correlations depend on two distinct ingredients: how abundant such correlations are in the underlying correlation space and how efficiently the chosen representation preserves their distinguishability from quantum correlations. We quantify these effects by introducing the popularity rate and the projection acceptance, whose product defines the search performance. We apply these concepts in binary-outcome Bell scenarios with symmetry-related sites, using the CHSH and pyramid representations. Particular attention is paid to the distinction between exchange-symmetric synchronous correlations and correlations satisfying only exchange symmetry. In the synchronous quantum sector, the common-vector representation follows automatically from synchrony, whereas exchange symmetry alone defines a substantially larger correlation space. We show that this distinction can qualitatively change the effectiveness of a projected search: the pyramid representation, which retains substantial discriminatory power in the synchronous sector, has no discriminatory power when only exchange symmetry is imposed. Our results provide a simple framework for assessing and comparing searches for post-quantum correlations.

Domenico Costantini, Ubaldo Garibaldi

Martin P. van Exter, J. Woudenberg, H. Di Lorenzo Pires +1 more

We experimentally demonstrate the importance of two-photon symmetry for the propagation of spatial quantum correlations in a multiple-scattering disordered system. Two distinguishable entangled photons with tunable spatial exchange symmetry are sent through the scattering medium. The two photons are observed to have an ensemble-averaged tendency to cluster together (bosonic symmetry) or to avoid each other (fermionic symmetry). An intermediate degree of clustering is observed for anyonic exchange symmetry.

Farid Shähandeh

Hailin Huang, Zhao‐Yu Sun, B. Wang

The singularity of quantum correlation has been widely regarded as a valuable indicator for quantum phase transition (QPT) in low-dimensional quantum systems. In this paper, in a two-leg spin ladder with ring exchange, the critical points of the system cannot be detected by analyzing the singularity of the quantum discord (a kind of quantum correlation). Instead, the discord exhibits a novel odd–even effect in the symmetry-breaking (SB) regions, which can be used to detect the two critical points between the SB regions and the non-SB regions. The underlying mechanism of the odd–even effect is explained.

I.-O. Stamatescu