7,749 research outputs found
Ionospheric effects in active retrodirective array and mitigating system design
The operation of an active retrodirective array (ARA) in an ionospheric environment (that is either stationary or slowly-varying) was examined. The restrictions imposed on the pilot signal structure as a result of such operation were analyzed. A 3 tone pilot beam system was defined which first estimates the total electron content along paths of interest and then utilizes this information to aid the phase conjugator so that correct beam pointing can be achieved
Quantum information with Gaussian states
Quantum optical Gaussian states are a type of important robust quantum states
which are manipulatable by the existing technologies. So far, most of the
important quantum information experiments are done with such states, including
bright Gaussian light and weak Gaussian light. Extending the existing results
of quantum information with discrete quantum states to the case of continuous
variable quantum states is an interesting theoretical job. The quantum Gaussian
states play a central role in such a case. We review the properties and
applications of Gaussian states in quantum information with emphasis on the
fundamental concepts, the calculation techniques and the effects of
imperfections of the real-life experimental setups.
Topics here include the elementary properties of Gaussian states and relevant
quantum information device, entanglement-based quantum tasks such as quantum
teleportation, quantum cryptography with weak and strong Gaussian states and
the quantum channel capacity, mathematical theory of quantum entanglement and
state estimation for Gaussian states.Comment: 170 pages. Minors of the published version are corrected and listed
in the Acknowledgement part of this versio
Influence of pure-dephasing by phonons on exciton-photon interfaces: Quantum microscopic theory
We have developed a full quantum microscopic theory to analyze the time
evolution of transversal and longitudinal components of an exciton-single
photon system coupled to bulk acoustic phonons. These components are subjected
to two decay processes. One is radiative relaxation and the other is
pure-dephasing due to exciton-phonon interaction. The former results in a decay
with an exponent linear to time, while the latter causes a faster initial decay
than the radiative decay. We analyzed the dependence of the components on the
duration of the input one-photon pulse, temperature, and radiative relaxation
rates. Such a quantitative analysis is important for the developments of
atom-photon interfaces which enable coherent transfer of quantum information
between photons and atomic systems. We found that, for a GaAs spherical quantum
dot in which the exciton interacts with bulk phonons, the maximal probability
of the excited state can be increased up to 75 %. This probability can be
considered as the efficiency for quantum information transfer from photon to
exciton.Comment: 9pages, 5figure
Spin melting and refreezing driven by uniaxial compression on a dipolar hexagonal plate
We investigate freezing characteristics of a finite dipolar hexagonal plate
by the Monte Carlo simulation. The hexagonal plate is cut out from a piled
triangular lattice of three layers with FCC-like (ABCABC) stacking structure.
In the present study an annealing simulation is performed for the dipolar plate
uniaxially compressed in the direction of layer-piling. We find spin melting
and refreezing driven by the uniaxial compression. Each of the melting and
refreezing corresponds one-to-one with a change of the ground states induced by
compression. The freezing temperatures of the ground-state orders differ
significantly from each other, which gives rise to the spin melting and
refreezing of the present interest. We argue that these phenomena are
originated by a finite size effect combined with peculiar anisotropic nature of
the dipole-dipole interaction.Comment: Proceedings of the Highly Frustrated Magnetism (HFM2006) conference.
To appear in a special issue of J. Phys. Condens. Matte
Long wavelength iteration of Einstein's equations near a spacetime singularity
We clarify the links between a recently developped long wavelength iteration
scheme of Einstein's equations, the Belinski Khalatnikov Lifchitz (BKL) general
solution near a singularity and the antinewtonian scheme of Tomita's. We
determine the regimes when the long wavelength or antinewtonian scheme is
directly applicable and show how it can otherwise be implemented to yield the
BKL oscillatory approach to a spacetime singularity. When directly applicable
we obtain the generic solution of the scheme at first iteration (third order in
the gradients) for matter a perfect fluid. Specializing to spherical symmetry
for simplicity and to clarify gauge issues, we then show how the metric behaves
near a singularity when gradient effects are taken into account.Comment: 35 pages, revtex, no figure
Measured Quantum Fourier Transform of 1024 Qubits on Fiber Optics
Quantum Fourier transform (QFT) is a key function to realize quantum
computers. A QFT followed by measurement was demonstrated on a simple circuit
based on fiber-optics. The QFT was shown to be robust against imperfections in
the rotation gate. Error probability was estimated to be 0.01 per qubit, which
corresponded to error-free operation on 100 qubits. The error probability can
be further reduced by taking the majority of the accumulated results. The
reduction of error probability resulted in a successful QFT demonstration on
1024 qubits.Comment: 15 pages, 6 figures, submitted to EQIS 2003 Special issue, Int. J.
Quantum Informatio
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