Prof Vlatko Vedral FInstP

Abstract:

For N hard-core bosons on an arbitrary lattice with d sites and independent of additional interaction terms we prove that the hard-core constraint itself already enforces a universal upper bound on the Bose-Einstein condensate given by Nmax=(N/d)(d-N+1). This bound can only be attained for one-particle states |φ) with equal amplitudes with respect to the hard-core basis (sites) and when the corresponding N-particle state |Ψ) is maximally delocalized. This result is generalized to the maximum condensate possible within a given sublattice. We observe that such maximal local condensation is only possible if the mode entanglement between the sublattice and its complement is minimal. We also show that the maximizing state |Ψ) is related to the ground state of a bosonic "Hubbard star" showing Bose-Einstein condensation.

Abstract:

A paramount topic in quantum foundations, rooted in the study of the Einstein-Podolsky-Rosen (EPR) paradox and Bell inequalities, is that of characterizing quantum theory in terms of the spacelike correlations it allows. Here, we show that to focus only on spacelike correlations is not enough: we explicitly construct a toy model theory that, while not contradicting classical and quantum theories at the level of spacelike correlations, still displays an anomalous behavior in its timelike correlations. We call this anomaly, quantified in terms of a specific communication game, the "hypersignaling" phenomena. We hence conclude that the "principle of quantumness," if it exists, cannot be found in spacelike correlations alone: nontrivial constraints need to be imposed also on timelike correlations, in order to exclude hypersignaling theories.