Enhancement of optomechanically induced sum sideband using parametric interactions

We theoretically study radiation pressure induced generation of the frequency components at the sum sideband in an optomechanical system containing an optical parametric amplifier (OPA). It is shown that an OPA inside a cavity can considerably enhance the amplitude of sum sideband even with low power input fields. We find a new matching condition for the upper sum sideband generation. The height and width of the new peak can be adjusted by the nonlinear gain of the OPA. Furthermore, the lower sum sideband generation can be enhanced with several orders of magnitude by tuning the nonlinear gain parameter and the phase of the field pumping the OPA. The enhanced sum sideband may have potential applications to the manipulation of light in a on-chip optomechanical device and the sensitively sensing for precision measurement in the weak optomechanical coupling regime.Comment: 7 pages, 5 figure

Orbital magnetic phase and pure persistent spin current in spin-orbit coupling mesoscopic rings

By solving the Rashba model of mesoscopic rings, we give analytically the ground-state properties of the ring, including the spin polarization, the persistent charge and spin currents (PCC and PSC). These ground-state properties can be given based on four kinds of electron numbers in rings. The effect of the self-inductance of the ring leads to the self-sustained magnetic flux (SSMF) and the self-sustained PCC and PSC, which break spontaneously time-reversal symmetry to form orbital magnetic phase (OMP). To tune the spin-orbit coupling strength or electron number of the ring can induce the phase transition between the OMP and non-OMP. For exact one-dimensional rings we find the coexistence of the pure PSC and SSMF. This property of the pure PSC may provide a new scheme to measure the pure PSC.Comment: 7 pages, 2 figure

Theory of spin polarization in mesoscopic spin-orbit coupling systems

We establish a general formalism of the bulk spin polarization (BSP) and the current-based spin polarization (CSP) for mesoscopic ferromagnetic and spin-orbit interaction (SOI) semiconducting systems. Based on this formalism, we reveal the basic properties of BSP and CSP and their relationships. The BSP describes the intrinsic spin polarized properties of devices. The CSP depends on both intrinsic parameters of device and the incident current. For the non-spin-polarized incident current with the inphase spin-phase coherence, CSP equals to BSP. We give analytically the BSP and CSP of several typical nanodevice models, ferromagnetic nanowire, Rashba nanowire and rings. These results provide basic physical behaviors of BSP and CSP and their relationships.Comment: 18 page

VLBI Monitoring of the Sub-parsec-scale Jet in the Radio Galaxy 3C 66B at 22 GHz

We present measurements of proper motion of the sub-parsec scale jet at 22 GHz in the nearby FR I galaxy 3C 66B. Observations were made using the VLBA at six epochs over four years. A phase-referencing technique was used to improve the image quality of the weak and diffuse jet components. We find that the inner knots are almost stationary, though one of them was expected to be detected with the apparent speed of 0.2 mas/yr according to 8 GHz monitoring at the same observation epochs. Clear flux variations are not observed in the core at 22 GHz, in contrast, clear flux enhancement is observed in the core at 8 GHz. We discussed that this can be explained, if the jet has helical structure, that the viewing angle of the jet between 8 and 22 GHz differs by a few degree in case the jet direction is almost along our line of sight. Although these results may imply the existence of a two-zone jet, which has been suggested in certain radio galaxies, it cannot explain the fact that the jet at the higher frequency jet is slower than that at the lower frequency.Comment: 24 pages, 6 figures; accepted for publication in Ap

Fidelity Approach in Topological Superconductors with Disorders

We apply the fidelity approach to study the topological superconductivity in a spin-orbit coupled nanowire system. The wire is modeled as a one layer lattice chain with Zeeman energy and spin-orbit coupling, which is in proximity to a multi layer superconductor. In particular, we study the effects of disorders and find that the fidelity susceptibility show multiple peaks. It is revealed that the major peak indicates the topological quantum phase transition, while other peaks signal the pining of the Majorana bound states by disorders

Multiple phase estimation in quantum cloning machines

Since the initial discovery of the Wootters-Zurek no-cloning theorem, a wide variety of quantum cloning machines have been proposed aiming at imperfect but optimal cloning of quantum states within its own context. Remarkably, most previous studies have employed the Bures fidelity or the Hilbert-Schmidt norm as the figure of merit to characterize the quality of the corresponding cloning scenarios. However, in many situations, what we truly care about is the relevant information about certain parameters encoded in quantum states. In this work, we investigate the multiple phase estimation problem in the framework of quantum cloning machines, from the perspective of quantum Fisher information matrix (QFIM). Focusing on the generalized d-dimensional equatorial states, we obtain the analytical formulas of QFIM for both universal quantum cloning machine (UQCM) and phase-covariant quantum cloning machine (PQCM), and prove that PQCM indeed performs better than UQCM in terms of QFIM. We highlight that our method can be generalized to arbitrary cloning schemes where the fidelity between the single-copy input and output states is input-state independent. Furthermore, the attainability of the quantum Cramer-Rao bound is also explicitly discussed.Comment: 10 pages, 3 figures. Any suggestions and comments are welcome

Stable polarization-encoded quantum key distribution in fiber

Polarizations of single-photon pulses have been controlled with long-term stability of more than 10 hours by using an active feedback technique for auto-compensation of unpredictable polarization scrambling in long-distance fiber. Experimental tests of long-term operations in 50, 75 and 100 km fibers demonstrated that such a single-photon polarization control supported stable polarization encoding in long-distance fibers to facilitate stable one-way fiber system for polarization-encoded quantum key distribution, providing quantum bit error rates below the absolute security threshold.Comment: 14 pages, 4 figure

Quantum fisher information in noninertial frames

We investigate the performance of quantum fisher information under the Unruh-Hawking effect, where one of the observers (eg, Rob) is uniformly accelerated with respect to other partners. In the context of relativistic quantum information theory, we demonstrate that quantum fisher information, as an important measure of the information content of quantum states, has a rich and subtle physical structure comparing with entanglement or Bell nonlocality. In this work, we mainly focus on the parameterized (and arbitrary) pure two-qubit states, where the weight parameter $\theta$ and phase parameter $\phi$ are naturally introduced. Intriguingly, we prove that $\mathcal{F}_\theta$ keeps unchanged for both scalar and Dirac fields. Meanwhile, we observe that $\mathcal{F}_\phi$ decreases with the increase of acceleration $r$ but remains finite in the limit of infinite acceleration. More importantly, our results show that the symmetry of $\mathcal{F}_\phi$ (with respect to $\theta=\pi/4$) has been broken by the influence of Unruh effect for both cases.Comment: 7 pages, 3 figures. Comments are welcome

Prevent eavesdropping with bright reference pulses for practical quantum key distribution

We analyze the application of bright reference pulses to prevent the photon-number-splitting attack in weak-pulse quantum key distribution. Under the optimal eavesdropping strategy as far as we know, the optimal parameters of bright reference and signal pulses can ensure a secure transmission distance up to 146 km. To realize the quantum key distribution scenario with up-present techniques, we present an experimentally feasible scheme to create a large splitting ratio between bright reference and signal pulses, and to switch the bright reference pulses away from signal pulses to avoid the after-pulse disturbance.Comment: 18 pages, 3 figure

Left-handed polarized spin waves in ferromagnets induced by spin-transfer torque

Polarization is a fundamental property of waves that refers to the orientation of the oscillations. It has been widely used to encode information in photonics and phononics. However, the polarization of spin waves is rarely used yet in magnonics. The reason for this is that only the right-handed polarized spin waves can be accommodated in ferromagnets. Here, we report that stable left-handed polarized spin waves can be introduced into ferromagnets if a spin-polarized electrical current is presented. The right-handed and left-handed polarized spin waves coexist when the current density is larger than a critical value while the system keeps stable. The results are confirmed by micromagnetic simulations. This work provides new playgrounds to study spin waves and points to new findings for future experimental studies.Comment: 17 papes, 5 figure