Macroscopic Entanglement and Relative Phase

Benjamin P. Stein

Aurel Bulgac, Shi Jin

The relative phase of the order parameters in the collision of two condensates can influence the outcome of their collision in the case of weak coupling. With increasing interaction strength, however, the initially independent phases of the two order parameters in the colliding partners quickly become phase locked, as the strong coupling favors an overall phase rigidity of the entire condensate, and upon their separation the emerging superfluid fragments become entangled.

Thomas Krüger

Yonghong Ma, Quanzhen Ding, E Wu

As an essential quantum resource in quantum information processing, entanglement between macroscopic systems has been applied to both quantum technologies and foundational studies of the boundary between the quantum and classical worlds. Inspired by the proposal by Li et al. [Phys. Rev. Lett. 117, 015502 (2016)], we propose a scheme that is composed of two separated nitrogen-vacancy centers coupled to the vibration mode of a nanotube for creating entanglement between the two ensembles. An approximate analytical solution related to the calculation of macroscopic entanglement for this two-spin-ensemble model is derived. This kind of analytical solution will be crucial for directly testing the contribution of the experimental parameters, especially for the calculation of macroscopic entanglement in follow-up work. Including the dephasing and mechanical dissipation, we show that two spin ensembles are entangled with time evolution under realistic experimental conditions. This macroscopic entanglement is applicable for applied ideas related to quantum computation and communication, as well as the studies of the boundary between quantum and classical worlds. Moreover, we describe how to realize this practical model and demonstrate this macroscopic entanglement in experiments.

Janet Anders, Vlatko Vedral

This paper summarises the results of our research on macroscopic entanglement in spin systems and free Bosonic gases. We explain how entanglement can be observed using entanglement witnesses which are themselves constructed within the framework of thermodynamics and thus macroscopic observables. These thermodynamical entanglement witnesses result in bounds on macroscopic parameters of the system, such as the temperature, the energy or the susceptibility, below which entanglement must be present. The derived bounds indicate a relationship between the occurrence of entanglement and the establishment of order, possibly resulting in phase transition phenomena. We give a short overview over the concepts developed in condensed matter physics to capture the characteristics of phase transitions in particular in terms of order and correlation functions. Finally we want to ask and speculate whether entanglement could be a generalised order concept by itself, relevant in (quantum induced) phase transitions such as BEC, and that taking this view may help us to understand the underlying process of high-T superconductivity.

Časlav Brukner, Vlatko Vedral, Anton Zeilinger