Wichterich, H.C.; (2010) Entanglement between noncomplementary parts of many-body systems. Doctoral thesis , UCL (University College London).

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Abstract

This thesis investigates the properties of entanglement in strongly correlated quantum systems, more specifically that between regions of a many-body system which may be separated spatially giving rise to a part of the system which is disregarded. The focus of the first part of this thesis is the response of a collection of spins, arranged on a one dimensional lattice, to a global quench, i.e. a rapid change in the interaction characteristics. Such a quench is seen to produce a significant amount of entanglement between distant spins. The robustness of the scheme towards random disorder is detailed and it is shown that the entanglement is sufficiently high to be distilled into almost pure Bell pairs. In a similar model system, it is explored how a von Neumann measurement with post-selection (i.e., discarding certain measurements based on the outcome) performed locally on two possibly well separated regions of spins, may give rise to a pure and entangled state of these regions, assuming the system is in its ground state. Later chapters are concerned with entanglement between noncomplementary groups of spins at quantum critical points, a situation where at zero temperature quantum fluctuations become pronounced. For spin chain models it is observed that this entanglement (as measured by negativity) assumes a finite value depending only on the ratio of the size of the regions to their separation and is further seen to be universal, i.e. independent of the microscopic details of the interaction. Universality of this form of entanglement is finally explored in a collective spin model. By casting the problem into the language of a few bosonic modes a closed form expression for the negativity in the thermodynamic limit for the entire phase diagram of the model is derived. At the quantum critical point this measure is explicitly universal in the aforementioned sense.

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