We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field Bc, an approximately temperature-independent point in the measured longitudinal resistivity, which is ascribed to the direct insulator-qua
ntum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility of our device. It is found that at the direct I-QH transition, the product of the quantum mobility and is about 0.37 which is considerably smaller than 1. In contrast, at Bc, the longitudinal resistivity is close to the Hall resistivity, i.e., the product of the classical mobility and the crossing field is about 1. Therefore our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D.
We present a proposal for storing and retrieving a continuous-variable quadripartite polarization-entangled cluster state, using macroscopic atomic ensembles in a magnetic field. The Larmor precession of the atomic spins leads to a symmetry between t
he atomic canonical operators. In this scheme, each of the four spatially separated pulses passes twice through the respective ensemble in order to map the polarization-entangled cluster state onto the long-lived atomic ensembles. The stored state can then be retrieved by another four read-out pulses, each crossing the respective ensemble twice. By calculating the variances, we analyzed the fidelities of the storage and retrieval, and our scheme is feasible under realistic experimental conditions.
We propose genuine ($k$, $m$)-threshold controlling schemes for controlled teleportation via multi-particle entangled states, where the teleportation of a quantum state from a sender (Alice) to a receiver (Bob) is under the control of $m$ supervisors
such that $k$ ($kleq m$) or more of these supervisors can help Bob recover the transferred state. By construction, anyone of our quantum channels is a genuine multipartite entangled state of which any two parts are inseparable. Their properties are compared and contrasted with those of the well-known Greenberger-Horne-Zeilinger, W, and linear cluster states, and also several other genuine multipartite entangled states recently introduced in literature. We show that our schemes are secure against both Bobs dishonesty and supervisors treacheries. For the latter case, the game theory is utilized to prove that supervisors cheats can be well prevented. In addition to their practical importance, our schemes are also useful in seeking and exploring genuine multipartite entangled states and opening another perspective for the applications of the game theory in quantum information science.
In this paper, we study the thermal entanglement in a two-qubit Heisenberg XYZ system with different Dzyaloshinskii-Moriya (DM) couplings. We show that different DM coupling parameters have different influences on the entanglement and the critical te
mperature. In addition, we find that when $J_{i}$ ($i$-component spin coupling interaction) is the largest spin coupling coefficient, $D_{i}$ ($i$-component DM interaction) is the most efficient DM control parameter, which can be obtained by adjusting the direction of DM interaction.
We investigate the entanglement in a two-qubit Heisenberg XYZ system with different Dzyaloshinskii-Moriya(DM) interaction and inhomogeneous magnetic field. It is found that the control parameters ($D_{x}$, $B_{x}$ and $b_{x}$) are remarkably differen
t with the common control parameters ($D_{z}$,$B_{z}$ and $b_{z}$) in the entanglement and the critical temperature, and these x-component parameters can increase the entanglement and the critical temperature more efficiently. Furthermore, we show the properties of these x-component parameters for the control of entanglement. In the ground state, increasing $D_{x}$ (spin-orbit coupling parameter) can decrease the critical value $b_{xc}$ and increase the entanglement in the revival region, and adjusting some parameters (increasing $b_{x}$ and $J$, decreasing $B_{x}$ and $Delta$) can decrease the critical value $D_{xc}$ to enlarge the revival region. In the thermal state, increasing $D_{x}$ can increase the revival region and the entanglement in the revival region (for $T$ or $b_{x}$), and enhance the critical value $B_{xc}$ to make the region of high entanglement larger. Also, the entanglement and the revival region will increase with the decrease of $B_{x}$ (uniform magnetic field). In addition, small $b_{x}$ (nonuniform magnetic field) has some similar properties to $D_{x}$, and with the increase of $b_{x}$ the entanglement also has a revival phenomenon, so that the entanglement can exist at higher temperature for larger $b_{x}$.
Two noninteracting atoms, initially entangled in Bell states, are coupled to a one-mode cavity. Based on the reduced non-perturbative quantum master equation, the entanglement evolution of the two atoms with decay is investigated beyond rotating-wave
approximation. It is shown that the counter-rotating wave terms have great influence on the disentanglement behavior. The phenomenon of entanglement sudden death and entanglement sudden birth will occur.
The thermal entanglement is investigated in a two-qubit Heisenberg XXZ system with Dzyaloshinskii-Moriya (DM) interaction. It is shown that the entanglement can be efficiently controlled by the DM interaction parameter and coupling coefficient $J_{z}
$. $D_{x}$(the x-component parameter of the DM interaction) has a more remarkable influence on the entanglement and the critical temperature than $D_{z}$(the z-component parameter of the DM interaction). Thus, by the change of DM interaction direction, we can get a more efficient control parameter to increase the entanglement and the critical temperature.
