2,281 research outputs found
Attack Detection in Sensor Network Target Localization Systems with Quantized Data
We consider a sensor network focused on target localization, where sensors
measure the signal strength emitted from the target. Each measurement is
quantized to one bit and sent to the fusion center. A general attack is
considered at some sensors that attempts to cause the fusion center to produce
an inaccurate estimation of the target location with a large mean-square-error.
The attack is a combination of man-in-the-middle, hacking, and spoofing attacks
that can effectively change both signals going into and coming out of the
sensor nodes in a realistic manner. We show that the essential effect of
attacks is to alter the estimated distance between the target and each attacked
sensor to a different extent, giving rise to a geometric inconsistency among
the attacked and unattacked sensors. Hence, with the help of two secure
sensors, a class of detectors are proposed to detect the attacked sensors by
scrutinizing the existence of the geometric inconsistency. We show that the
false alarm and miss probabilities of the proposed detectors decrease
exponentially as the number of measurement samples increases, which implies
that for sufficiently large number of samples, the proposed detectors can
identify the attacked and unattacked sensors with any required accuracy
Nonlocal effective medium analysis in symmetric metal-dielectric multilayer metamaterials
The optical nonlocality in symmetric metal-dielectric multilayer
metamaterials is theoretically and experimentally investigated with respect to
transverse-magnetic-polarized incident light. A nonlocal effective medium
theory is derived from the transfer-matrix method to determine the nonlocal
effective permittivity depending on both the frequency and wave vector in a
symmetric metal-dielectric multilayer stack. In contrast to the local effective
medium theory, our proposed nonlocal effective medium theory can accurately
predict measured incident angle-dependent reflection spectra from a fabricated
multilayer stack and provide nonlocal dispersion relations. Moreover, the bulk
plasmon polaritons with large wave vectors supported in the multilayer stack
are also investigated with the nonlocal effective medium theory through the
analysis of the dispersion relation and eigenmode.Comment: 21 pages, 7 figure
Dispersive response of atoms trapped near the surface of an optical nanofiber with applications to quantum nondemolition measurement and spin squeezing
We study the strong coupling between photons and atoms that can be achieved
in an optical nanofiber geometry when the interaction is dispersive. While the
Purcell enhancement factor for spontaneous emission into the guided mode does
not reach the strong-coupling regime for individual atoms, one can obtain high
cooperativity for ensembles of a few thousand atoms due to the tight
confinement of the guided modes and constructive interference over the entire
chain of trapped atoms. We calculate the dyadic Green's function, which
determines the scattering of light by atoms in the presence of the fiber, and
thus the phase shift and polarization rotation induced on the guided light by
the trapped atoms. The Green's function is related to a full
Heisenberg-Langevin treatment of the dispersive response of the quantized field
to tensor polarizable atoms. We apply our formalism to quantum nondemolition
(QND) measurement of the atoms via polarimetry. We study shot-noise-limited
detection of atom number for atoms in a completely mixed spin state and the
squeezing of projection noise for atoms in clock states. Compared with
squeezing of atomic ensembles in free space, we capitalize on unique features
that arise in the nanofiber geometry including anisotropy of both the intensity
and polarization of the guided modes. We use a first principles stochastic
master equation to model the squeezing as function of time in the presence of
decoherence due to optical pumping. We find a peak metrological squeezing of ~5
dB is achievable with current technology for ~2500 atoms trapped 180 nm from
the surface of a nanofiber with radius a=225 nm.Comment: To be appeared on PR
Computing Equilibrium in Matching Markets
Market equilibria of matching markets offer an intuitive and fair solution
for matching problems without money with agents who have preferences over the
items. Such a matching market can be viewed as a variation of Fisher market,
albeit with rather peculiar preferences of agents. These preferences can be
described by piece-wise linear concave (PLC) functions, which however, are not
separable (due to each agent only asking for one item), are not monotone, and
do not satisfy the gross substitute property-- increase in price of an item can
result in increased demand for the item. Devanur and Kannan in FOCS 08 showed
that market clearing prices can be found in polynomial time in markets with
fixed number of items and general PLC preferences. They also consider Fischer
markets with fixed number of agents (instead of fixed number of items), and
give a polynomial time algorithm for this case if preferences are separable
functions of the items, in addition to being PLC functions.
Our main result is a polynomial time algorithm for finding market clearing
prices in matching markets with fixed number of different agent preferences,
despite that the utility corresponding to matching markets is not separable. We
also give a simpler algorithm for the case of matching markets with fixed
number of different items
Fast initialization of the spin state of an electron in a quantum dot in the Voigt configuration
We consider the initialization of the spin-state of a single electron trapped
in a self-assembled quantum dot via optical pumping of a trion level. We show
that with a magnetic field applied perpendicular to the growth direction of the
dot, a near-unity fidelity can be obtained in a time equal to a few times the
inverse of the spin-conserving trion relaxation rate. This method is several
orders-of-magnitude faster than with the field aligned parallel, since this
configuration must rely on a slow hole spin-flip mechanism. This increase in
speed does result in a limit on the maximum obtainable fidelity, but we show
that for InAs dots, the error is very small.Comment: 4 pages, 4 figure
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