[en] In the framework of open quantum systems, we study the dynamics of a static polarizable two-level atom interacting with a bath of fluctuating vacuum electromagnetic field and explore under which conditions the coherence of the open quantum system is unaffected by the environment. For both a single-qubit and two-qubit systems, we find that the quantum coherence cannot be protected from noise when the atom interacts with a non-boundary electromagnetic field. However, with the presence of a boundary, the dynamical conditions for the insusceptible of quantum coherence are fulfilled only when the atom is close to the boundary and is transversely polarizable. Otherwise, the quantum coherence can only be protected in some degree in other polarizable direction. -- Highlights: •We study the dynamics of a two-level atom interacting with a bath of fluctuating vacuum electromagnetic field. •For both a single and two-qubit systems, the quantum coherence cannot be protected from noise without a boundary. •The insusceptible of the quantum coherence can be fulfilled only when the atom is close to the boundary and is transversely polarizable. •Otherwise, the quantum coherence can only be protected in some degree in other polarizable direction.

[en] The dynamics of ball bearings are important to fatigue breakage, dynamic performance and motion precision of mechanisms connected by ball bearings joints with multi-clearances. In this study, a new method is proposed for multibody dynamics analysis on mechanisms under the effects of radial internal clearances and impact of balls/cage pockets interactions of ball bearings. Including balls/rings interactions and balls/cage pockets interactions, the three dimensional dynamics models of the crank slider mechanism are established and calculated by generalized-α algorithms on the basis of Hertzian contact theory and penalty function method. The rules of eccentric trajectories of inner and outer ring center for one ball bearing joint are verified with the results calculated by XU's references. The results of dynamic errors, motion trajectories, dynamic forces are achieved under different speeds, radial clearances and number of ball bearings. The speeds and radial clearances are critical to the dynamic performances and motion precision of mechanisms, especially the number of ball bearings. The number of ball bearings is important to impact force and motion stability of the mechanism.

[en] We study how to improve the precision of the quantum estimation of phase for an uniformly accelerated atom in fluctuating electromagnetic field by reflecting boundaries. We find that the precision decreases with increases of the acceleration without the boundary. With the presence of a reflecting boundary, the precision depends on the atomic polarization, position and acceleration, which can be effectively enhanced compared to the case without boundary if we choose the appropriate conditions. In particular, with the presence of two parallel reflecting boundaries, we obtain the optimal precision for atomic parallel polarization and the special distance between two boundaries, as if the atom were shielded from the fluctuation.

[en] We probe the κ-deformation of spacetime using a two-level atom as a detector coupled to a κ-deformed massless scalar field which is invariant under a κ-Poincare algebra and written in commutative spacetime. To address the quantum bound to the estimability of the deformation parameter κ, we perform measurements on the two-level detector and maximize the value of quantum Fisher information over all possible detector preparations. We prove that the population measurement is the optimal measurement in the estimation of the deformation parameter κ. In particular, we show that the relativistic motion of the detector affects the precision in the estimation of the parameter κ, which can effectively improve this precision comparing to that of the static detector case by many orders of magnitude. (orig.)

[en] We study the Hawking effect in terms of the geometric phase for a two-level atom coupled to vacuum fluctuations in the background of Kerr black hole in a gedanken experiment. Since the atom interacts with a bath of fluctuating massless scalar field, its geometric phase for the nonunitary evolution turns out to be affected by the spacetime curvature that backscatters the vacuum field modes. We examine the geometric phase for two kinds of vacua, i.e. the Unruh vacuum and the Candelas–Chrzanowski–Howard vacuum. We find that for both of these two cases, the geometric phase of the static atom locating outside the infinite redshift surface (or the stationary atom locating inside the ergosphere) exposes that the atom behaves as if it were immersed in a thermal radiation at the Hawking temperature from the Kerr black hole. Therefore, our results show that one in principle can reveal the Hawking effect in the Kerr black hole by analyzing the correction of the geometric phase as opposed to that in a flat spacetime. (paper)

[en] We demonstrate experimentally an intrinsically stable polarization coding and decoding system composed of optical-fiber Sagnac interferometers with integrated phase modulators for quantum key distribution. An interference visibility of 98.35% can be kept longtime during the experiment without any efforts of active compensation for coding all four desired polarization states

[en] We explore the behaviors of the holographic entanglement entropy (HEE) in holographic superconductor models with logarithmic nonlinear electrodynamics (LNE) both in AdS soliton and in AdS black hole backgrounds. We observe that the slope of the HEE at the phase transition point behaves discontinuously for different LNE parameters b and geometry parameters $\mathrm{\ell <!--\; ???\; -->}$, which may be a quite general feature for the second order phase transition. Moreover, at the critical point, the stronger nonlinearity of the LNE gives rise to the smaller HEE in metal/superconductor while leaves the HEE in insulator/superconductor model as is. Interestingly, the behavior of the HEE also implies a "confinement/deconfinement" phase transition in the insulator/superconductor model, and the critical width of the phase transition depends on the chemical potential and the strength of the LNE.

[en] We address the optimal estimation of quantum parameters, in the framework of local quantum estimation theory, for a massive scalar quantum field in the expanding Robertson–Walker universe exhibiting Lorentz invariance violation (LIV). We find that, in the estimation of cosmological parameters, the ultimate bounds to the precision of the Lorentz-invariant massive scalar field can be improved due to the effects of LIV under some appropriate conditions. We also show that, in the Lorentz-invariant massive scalar field and massless scalar field due to LIV backgrounds, the optimal precision can be achieved by choosing the particles with some suitable LIV, cosmological and field parameters. Moreover, in the estimation of LIV parameter during the spacetime expansion, we prove that the appropriate momentum mode of field particles and larger cosmological parameters can provide us a better precision.

[en] With localized two-mode Gaussian quantum states in a relativistic setting, we study how to use the Gaussian interferometric power, which is given by the minimum quantum Fisher information, to guarantee the precision of the estimation of the Unruh temperature. We note that the interferometric power will be reduced with increase in the Unruh temperature because the Unruh radiation acts as a thermal bath on the probe state and it will destroy available quantum resources. By comparing the interferometric power with the entanglement, we also find that the larger squeezing parameter, the more similar the variation of entanglement and interferometric power. The maximum value of entanglement and interferometric power appears in the same condition. That is, for the states with larger entanglement, we can get higher precision of estimation. Moreover, we find that the interferometric power remains nonzero even for high Unruh temperature. This reflects the robust behavior of the Gaussian interferometric power against thermal noises.

[en] We present implementation of quantum cryptography with polarization code by wave-guide type phase modulator. At four different low input voltages of the phase modulator, coder encodes pulses into four different polarization states, 45^o, 135^o linearly polarized or right, left circle polarized, while the decoder serves as the complementary polarizers