Frontiers of quantum physics

Abstract:

We present a method for detecting the entanglement of a state using nonequilibrium processes. A comparison of relative entropies allows us to construct an entanglement witness. The relative entropy can further be related to the quantum Jarzynski equality, allowing nonequilibrium work to be used in entanglement detection. To exemplify our results, we consider two different spin chains. © 2010 The American Physical Society.

Abstract:

We present a closest separable state to cluster states, which in turn allows us to calculate the entanglement scaling using relative entropy of entanglement. We reproduce known results for pure cluster states and show how our method can be used in quantifying entanglement in noisy cluster states. Operational meaning is given to our method, which clearly demonstrates how these closest separable states can be constructed from two-qubit clusters in the case of pure states. We also discuss the issue of finding the critical temperature at which the cluster state becomes only classically correlated and the importance of this temperature to our method. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Abstract:

The entanglement properties of phase transition in a twodimensional harmonic lattice, similar to the one observed in recent ion trap experiments, are discussed for both finite number of particles and thermodynamical limit. We show that for the ground state at the critical value of the trapping potential, two entanglement measures, the negativity between two neighbouring sites and the block entropy for blocks of size 1, 2 and 3, change abruptly. Entanglement thus indicates quantum phase transitions in general, not only in the finite-dimensional case considered inWu et al (2004 Phys. Rev. Lett. 93 250404). Finally, we consider the thermal state and compare its exact entanglement with a temperature entanglement witness introduced in Anders (2008 Phys. Rev. A 77 062102). © IOP Publishing Ltd. and Deutsche Physikalische Gesellschaft.