A Comparative Dynamical Study of a Bound Entangled State

In this article, a comparative dynamical study of an open quantum system is investigated for one of the bound entangled states proposed by Bennett et al. The study is conducted under the influence of Heisenberg, bi-linear bi-quadratic and Dzyaloshinskii-Moriya (DM) interaction. During the study, an auxiliary qutrit interacts with one of the qutrits of the selected two qutrit bound entangled state through different interactions. It is observed that, although the auxiliary qutrit plays a significant role during the interaction, the probability amplitude of the qutrit does not affect the open quantum system. In the present work, the computable cross-norm or realignment (CCNR) criterion has been used to detect the bound entanglement of the state and the negativity has been applied to measure the free entanglement. In this three-fold study, it is found that the Dzyaloshinskii-Moriya (DM) interaction performs better to free the chosen bound entangled state among all the interactions.Comment: 10 pages, 3 figure

Herring-Flicker coupling and thermal quantum correlations in bipartite system

In this letter we study thermal quantum correlations as quantum discord and entanglement in bipartite system imposed by external magnetic field with Herring-Flicker coupling ie. $J(R)=1.642 e^{-2 R} R^{5/2}+O(R^{2}e^{-2R})$. The Herring-Flicker coupling strength is the function of $R$, which is the distance between spins and systems carry XXX Heisenberg interaction. By tuning the coupling distance $R$, temperature and magnetic field quantum correlations can be scaled in the bipartite system. We find the long sustainable behaviour of quantum discord in comparison to entanglement over the coupling distance $R$. We also investigate the situations, where entanglement totally dies but quantum discord exist in the system. The present findings in the letter may be useful for designing quantum wires, data bus, solid state gates and quantum processors.Comment: 5 pages, 3 figure

Scaling, stability range, magnetic excitations and light-induced effects in Cu2OSeO3

The delicate energetic balance between the competing symmetric and asymmetric magnetic interactions found in magnetic skyrmion materials provides an interesting framework to investigate the fundamental properties of novel spin textures and the potential technological applications of topological spin vortices. The projects in this thesis take advantage of a set of complementary techniques, namely, elastic magnetic neutron scattering, inelastic photon scattering and SQUID magnetisation, that enable us to investigate the effect of changes in the atomic lattice and external stimuli upon correlated electron systems such as the multiferroic skyrmion material Cu2OSeO3. Small-Angle Neutron Scattering has been extensively used to investigate the emergence of magnetic skyrmion long-range magnetic ordering in different materials since the distance between these magnetic vortices usually lies between 10 and 200 nm, which is within the resolution of this technique. Moreover, the reciprocal space resolution and intense neutron flux of the state-of-the-art Small-Angle Neutron Scattering instruments at the Australian Centre for Neutron Scattering have proved to be comparable to the best neutron instruments around the world and sensitive to small variations in magnetic neutron scattering patterns from small single crystals. Additionally, the Raman scattering setup at the Ulrich laboratories in the School of Physics of UNSW allows us to investigate the spin dynamics of small samples under different sample environments with excellent energy resolution. Furthermore, the extensive sample growth and characterisation experience of our collaborators in the Tilo Soehnel group at the University of Auckland has enabled us to investigate the role of different thermal protocols, sample orientations, and light illumination on the stability of the long-range magnetic ordering of pristine and atomically substituted single crystals of Cu2OSeO3. The combined projects have unveiled the role of thermal fluctuations, magnetic anisotropies, thermal protocols, and internal pressure in the transition from non-topological to topological spin states of matter

Dynamics of Metastable Magnetic Skyrmions

Skyrmions, vortex-like objects composed of magnetic moments, have seen a recent surge of research interest due to their unique transport and topological properties. With an ever-increasing demand for more efficient memory and computation, skyrmionic devices have been conceived as an ultra-low power, high density data storage solution. While they are found in a range of materials, in this thesis we will primarily be concerned with skyrmions found in bulk chiral magnets. In such systems, skyrmions are typically only at equilibrium in a small range of temperature and applied magnetic field. However, they can exist in a metastable state over a much wider range of the magnetic phase diagram, formed by cooling the system under an applied magnetic field. Metastable skyrmions therefore have technological application by enabling the existence of skyrmions at room temperature and zero applied magnetic field. However, they also posses a finite, temperature-dependent lifetime, which places limitations on the stability of metastable skyrmions, and also restricts the population remaining after the cooling process. This lifetime is realised in nanoscopic mechanisms which are governed by topological defects, known as Bloch points. Due to the locality of these structures, the development of real-space imaging techniques are vital for gaining true understanding of skyrmion formation and annihilation. In this thesis, the dynamics of metastable skyrmions are thoroughly investigated through the use of magnetometry, and a range of neutron and x-ray scattering techniques. The effect of chemical substitution, or doping, on the magnetic phase transitions in Zn-doped Cu$_2$OSeO$_3$ is explored, and found to introduce pinning effects which dramatically increases the lifetime of metastable skyrmions. Furthermore, by adapting x-ray imaging methods for cryogenic sample environments, the first real-space observation of the vertical, tube-like, structure of skyrmions is demonstrated. The results open the door to a variety of experiments capable of further investigation into the dynamics of the skyrmion spin texture

Quantum-enhanced measurements without entanglement

Quantum-enhanced measurements exploit quantum mechanical effects for increasing the sensitivity of measurements of certain physical parameters and have great potential for both fundamental science and concrete applications. Most of the research has so far focused on using highly entangled states, which are, however, difficult to produce and to stabilize for a large number of constituents. In the following we review alternative mechanisms, notably the use of more general quantum correlations such as quantum discord, identical particles, or non-trivial Hamiltonians; the estimation of thermodynamical parameters or parameters characterizing non-equilibrium states; and the use of quantum phase transitions. We describe both theoretically achievable enhancements and enhanced sensitivities, not primarily based on entanglement, that have already been demonstrated experimentally, and indicate some possible future research directions