2,577 research outputs found
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 and phase parameter
are naturally introduced. Intriguingly, we prove that
keeps unchanged for both scalar and Dirac fields.
Meanwhile, we observe that decreases with the increase of
acceleration but remains finite in the limit of infinite acceleration. More
importantly, our results show that the symmetry of (with
respect to ) 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
Finite Time Analysis of Vector Autoregressive Models under Linear Restrictions
This paper develops a unified finite-time theory for the ordinary least
squares estimation of possibly unstable and even slightly explosive vector
autoregressive models under linear restrictions, with the applicable region
, where is the spectral radius of the transition
matrix in the \VAR(1) representation, is the time horizon and is
a universal constant. The linear restriction framework encompasses various
existing models such as banded/network vector autoregressive models. We show
that the restrictions reduce the error bounds via not only the reduced
dimensionality but also a scale factor resembling the asymptotic covariance
matrix of the estimator in the fixed-dimensional setup: as long as the model is
correctly specified, this scale factor is decreasing in the number of
restrictions. It is revealed that the phase transition from slow to fast error
rate regimes is determined by the smallest singular value of , a measure of
the least excitable mode of the system. The minimax lower bounds are derived
across different regimes. The developed non-asymptotic theory not only bridges
the theoretical gap between stable and unstable regimes but precisely
characterizes the effect of restrictions and its interplay with model
parameters. Simulations support our theoretical results.Comment: To Appear in Biometrik
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