783 research outputs found
Squeezing-Enhanced Phase-Shift-Keyed Binary Communication in Noisy Channels
We address the use of squeezing in binary phase-shift-keyed (PSK) channels at fixed energy. In particular, we assess homodyne receivers against the Helstrom bound in the presence of phase noise. We also take into account possible imperfections in the generation of squeezing and the effect of losses during propagation. We find that squeezing is a useful resource if its amplitude is below a given threshold depending on the energy of the signals and on the properties of the channel. Squeezing enhancement is present also when phase-noise becomes large
Dephasing due to Nuclear Spins in Large-Amplitude Electric Dipole Spin Resonance
We analyze effects of the hyperfine interaction on electric dipole spin resonance when the amplitude of the quantum-dot motion becomes comparable or larger than the quantum dot's size. Away from the well-known small-drive regime, the important role played by transverse nuclear fluctuations leads to a Gaussian decay with characteristic dependence on drive strength and detuning. A characterization of spin-flip gate fidelity, in the presence of such additional drive-dependent dephasing, shows that vanishingly small errors can still be achieved at sufficiently large amplitudes. Based on our theory, we analyze recent electric dipole spin resonance experiments relying on spin-orbit interactions or the slanting field of a micromagnet. We find that such experiments are already in a regime with significant effects of transverse nuclear fluctuations and the form of decay of the Rabi oscillations can be reproduced well by our theory
Vortex Loops and Majoranas
We investigate the role that vortex loops play in characterizing eigenstates
of interacting Majoranas. We first give some general results, and then we focus
on ladder Hamiltonian examples to test further ideas. Two methods yield exact
results: i.) We utilize the mapping of spin Hamiltonians to quartic
interactions of Majoranas and show under certain conditions the spectra of
these two examples coincide. ii) In cases with reflection-symmetric
Hamiltonians, we use reflection positivity for Majoranas to characterize
vortices. Aside from these exact results, two additional methods suggest wider
applicability of these results: iii.) Numerical evidence suggests similar
behavior for certain systems without reflection symmetry. iv.) A perturbative
analysis also suggests similar behavior without the assumption of reflection
symmetry.Comment: 28 page
Fate of the Quasi-condensed State for Bias-driven Hard-Core Bosons in one Dimension
Bosons in one dimension display a phenomenon called quasi-condensation, where
correlations decay in a powerlaw fashion. We study the fate of
quasi-condensation in the non-equilibrium steady-state of a chain of hard-core
bosons coupled to macroscopic leads which are held at different chemical
potentials. It is found that a finite bias destroys the quasi-condensed state
and the critical scaling function of the quasi-condensed fraction, near the
zero bias transition, is determined. Associated critical exponents are
determined and numerically verified. Away from equilibrium, the system exhibits
exponentially decaying correlations that are characterized by a bias-dependent
correlation length that diverges in equilibrium. In addition, power-law
corrections are found, which are characterized by an exponent that depends on
the chain-leads coupling and is non-analytic at zero bias. This
exactly-solvable nonequilibrium strongly-interacting system has the remarkable
property that, the near-equilibrium state at infinitesimal bias, cannot be
obtained within linear response. These results aid in unraveling the intricate
properties spawned by strong interactions once liberated from equilibrium
constraints.Comment: 7 pages, 4 figure
Motion Planning from Demonstrations and Polynomial Optimization for Visual Servoing Applications
Vision feedback control techniques are desirable for a wide range of robotics applications due to their robustness to image noise and modeling errors. However in the case of a robot-mounted camera, they encounter difficulties when the camera traverses large displacements. This scenario necessitates continuous visual target feedback during the robot motion, while simultaneously considering the robot's self- and external-constraints. Herein, we propose to combine workspace (Cartesian space) path-planning with robot teach-by-demonstration to address the visibility constraint, joint limits and “whole arm” collision avoidance for vision-based control of a robot manipulator. User demonstration data generates safe regions for robot motion with respect to joint limits and potential “whole arm” collisions. Our algorithm uses these safe regions to generate new feasible trajectories under a visibility constraint that achieves the desired view of the target (e.g., a pre-grasping location) in new, undemonstrated locations. Experiments with a 7-DOF articulated arm validate the proposed method.published_or_final_versio
Polaritonic stop-band transparency via exciton-biexciton coupling in CuCl
Radiation is almost completely reflected within the exciton-polariton stop band of a semiconductor, as in the typical case of CuCl. We predict, however, that a coherently driven exciton-biexciton transition allows for the propagation of a probe light beam within the stop band. The phenomenon is reminiscent of electromagnetically induced transparency effects occurring in three-level atomic systems, except that it here involves delocalized electronic excitations in a crystalline structure via a frequency and wave-vector selective polaritonic mechanism. A well-developed transparency, favored by the narrow linewidth of the biexciton, is established within the stop band where a probe pulse may propagate with significant delays. The transparency window can be controlled via the pump beam detuning and intensity
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