[en] Quantum effects like the superposition principle contradict our experience of daily life. Decoherence can be viewed as a possible explanation why we do not observe quantum superposition states in the macroscopic world. In this article, we use the qubit ansatz to discuss decoherence in the simplest possible model system and propose a visualization for the microscopic origin of decoherence, and the emergence of a so-called pointer basis. Finally, we discuss the possibility of ‘macroscopic’ quantum effects. (paper)

[en] The resource theories of quantum coherence attract a lot of attention in recent years. Especially, the monotonicity property plays a crucial role here. In this paper we investigate the monotonicity property for the coherence measures induced by the Rényi α-relative entropy, which present in [Phys. Rev. A 94 (2016) 052336]. We show that the Rényi α-relative entropy of coherence does not in general satisfy the monotonicity requirement under the subselection of measurements condition and it also does not satisfy the extension of monotonicity requirement, which presents in [Phys. Rev. A 93 (2016) 032136]. Due to the Rényi α-relative entropy of coherence can act as a coherence monotone quantifier, we examine the trade-off relations between coherence and mixedness. Finally, some properties for the single qubit of Rényi 2-relative entropy of coherence are derived . (paper)

[en] A feasible scheme for protecting the Greenberger–Horne–Zeilinger (GHZ) entanglement state in non-Markovian environments is proposed. It consists of prior weak measurement on each qubit before the interaction with decoherence environments followed by post quantum measurement reversals. It is shown that both the fidelity and concurrence of the GHZ state can be effectively improved. Meanwhile, we also verified that our scenario can enhance tripartite nonlocality remarkably. In addition, the result indicates that the larger the weak measurement strength, the better the effectiveness of the scheme with the lower success probability.

[en] Optical lattices can be used to simulate quantum baths and hence they can be of fundamental help to study, in a controlled way, the emergence of decoherence in quantum systems. Here we show how to implement a pure dephasing model for a two-level system coupled to an interacting spin bath. In this scheme it is possible to implement a large variety of spin environments embracing Ising, XY and Heisenberg universality classes. After having introduced the model, we calculate exactly the decoherence for the Ising and the XY spin bath model. We find universal features depending on the critical behaviour of the spin bath, both in the short- and long-time limits. The rich scenario that emerges can be tested experimentally and can be of importance for the understanding of the coherence loss in open quantum systems

[en] We consider the geometric global quantum discord (GGQD) of two-qubit systems. By analyzing the symmetry of geometric global quantum discord we give an approach for deriving analytical formulae of the extremum problem which lies at the core of computing the GGQD for arbitrary two-qubit states. Furthermore, formulae of GGQD of arbitrary two-qubit states and some concrete examples are presented. (paper)

[en] We present a family of coherence quantifiers based on the generalized $\mathrm{\alpha <!--\; ??\; -->}$–z-relative R$\stackrel{´<!--\; ??\; -->}{e}$nyi entropy. These quantifiers satisfy all the standard criteria for well-defined measures of coherence and include some existing coherence measures as special cases.

[en] It is shown that the conditions for classical behavior, in the sense of zero dispersion of all macroscopic variables, for systems of qubits with different dynamics, depend on whether the interaction is local or global. In the case of local interaction, macroscopic coarse-graining is enough. On the other hand, coarse-grained macroscopic variables of globally interacting large systems of qubits have negligible dispersion only if the system is exposed to some form of the environmental decoherence.

[en] We investigate how the Unruh effect affects the transition between classical and quantum decoherences for a general class of initial states and find that: (i) The quantum decoherence exists while λt⩽λt-tilde (the transition time) and the classical one can also affect the system's evolution while λt⩾λt-tilde for both the bit and phase-bit flips, which are different from the cases in inertial frame; (ii) The classical decoherence will not occur, while the quantum decoherence still dominates the evolution of system as λt⩾λt-tilde for the phase flip; And (iii) as the Unruh temperature increases, the λt-tilde, compared with that in inertial frame, will be bigger for phase flip but smaller for bit flip. However, the λt-tilde does not change no matter what the Unruh temperature is for phase-bit flip.

[en] Polyaniline microfibers were synthesized by using a self-assembly method. The electron hopping transport in this partially dedoped polyaniline single fiber showed typical bias-dependent characteristics. The magnetoresistance properties of the fiber were systematically investigated. In this microfiber, both positive and negative magnetoresistances were observed. An apparent positive magnetoresistance (> 1%) was found at 50 K. This temperature is much higher than previous reports of single or few nanofiber/tubes bundles (< 10 K). With the increase in temperature, the positive magnetoresistance was converted to the negative magnetoresistance where the transition happened at ca. 90 K. Moreover, the magnetoresistance was also sensitive to the applied bias. These observations could be explained by using the combination or competition of electron wave function shrinkage effect, quantum decoherence effect, as well as localized states coupling effect.

[en] We investigate the phenomenon of sudden transitions in geometric quantum correlation of two qubits in spin chain environments at finite temperature. It is shown that when only one qubit is coupled to the spin environment, the geometric discord exhibits a double sudden transition behavior, which is closely related to the quantum criticality of the spin chain environment. When two qubits are uniformly coupled to a common spin chain environment, the geometric discord is found to display a sudden transition behavior whereby the system transits from pure classical decoherence to pure quantum decoherence. Moreover, an interesting scaling behavior is revealed for the frozen time, and we also present a scheme to prolong the time during which the discord remains constant by applying bang–bang pulses. (paper)