21,034 research outputs found
Security assessment of audience response systems using software defined radios
Audience response systems, also known as clickers, are used at many academic institutions to offer active learning environments. Since these systems are used to administer graded assignments, and sometimes even exams, it is crucial to assess their security. Our work seeks to exploit and document potential vulnerabilities of clickers. For this purpose, we use software defined radios to perform eavesdropping attacks on an audience response system in production. The results of our study demon- strate that clickers are easily exploitable. We build a prototype and show that it is practically possible to covertly steal answers from a peer or even the entire classroom, with high levels of confidence. As a result of this study, we discourage using clickers for high-stake assessments, unless manufacturers provide proper security protection.http://people.bu.edu/staro/MIT_Conference_Khai.pdfAccepted manuscrip
Measuring the degree of unitarity for any quantum process
Quantum processes can be divided into two categories: unitary and non-unitary
ones. For a given quantum process, we can define a \textit{degree of the
unitarity (DU)} of this process to be the fidelity between it and its closest
unitary one. The DU, as an intrinsic property of a given quantum process, is
able to quantify the distance between the process and the group of unitary
ones, and is closely related to the noise of this quantum process. We derive
analytical results of DU for qubit unital channels, and obtain the lower and
upper bounds in general. The lower bound is tight for most of quantum
processes, and is particularly tight when the corresponding DU is sufficiently
large. The upper bound is found to be an indicator for the tightness of the
lower bound. Moreover, we study the distribution of DU in random quantum
processes with different environments. In particular, The relationship between
the DU of any quantum process and the non-markovian behavior of it is also
addressed.Comment: 7 pages, 2 figure
Making Clean Energy with a Kerr Black Hole: a Tokamak Model for Gamma-Ray Bursts
In this paper we present a model for making clean energy with a Kerr black
hole. Consider a Kerr black hole with a dense plasma torus spinning around it.
A toroidal electric current flows on the surface of the torus, which generates
a poloidal magnetic field outside the torus. On the surface of the tours the
magnetic field is parallel to the surface. The closed magnetic field lines
winding around the torus compress and confine the plasma in the torus, as in
the case of tokamaks. Though it is unclear if such a model is stable, we look
into the consequences if the model is stable. If the magnetic field is strong
enough, the baryonic contamination from the plasma in the torus is greatly
suppressed by the magnetic confinement and a clean magnetosphere of
electron-positron pairs is built up around the black hole. Since there are no
open magnetic field lines threading the torus and no accretion, the power of
the torus is zero. If some magnetic field lines threading the black hole are
open and connect with loads, clean energy can be extracted from the Kerr black
hole by the Blandford-Znajek mechanism.
The model may be relevant to gamma-ray bursts. The energy in the Poynting
flux produced by the Blandford-Znajek mechanism is converted into the kinetic
energy of the electron-positron pairs in the magnetosphere around the black
hole, which generates two oppositely directed jets of electron-positron pairs
with super-high bulk Lorentz factors. The jets collide and interact with the
interstellar medium, which may produce gamma-ray bursts and the afterglows.Comment: 14 pages, 1 figure, accepted by Ap
Extracting Energy from a Black Hole through Its Disk
When some magnetic field lines connect a Kerr black hole with a disk rotating
around it, energy and angular momentum are transferred between them. If the
black hole rotates faster than the disk, for a thin Keplerian
disk, then energy and angular momentum are extracted from the black hole and
transferred to the disk ( is the mass and is the angular momentum
of the black hole). This way the energy originating in the black hole may be
radiated away by the disk.
The total amount of energy that can be extracted from the black hole spun
down from to by a thin Keplerian disk is
. This is larger than which can be
extracted by the Blandford-Znajek mechanism.Comment: 8 pages, 2 figure
Phenomenology of Goldstino Couplings
A general coupling of the Goldstino to the matter field and the weak
gravitational field is constructed based on the standard and the nonlinear
Volkov-Akulov realization of SUSY. The resulting Lagrangian, which is invariant
under SUSY transformations, can give rise to explicit interactions which couple
the helicity +-1/2 states of the gravitino with the gravitational field as well
as the matter field.Comment: 7 pages; final version to appear in Modern Physics Letters
Long-Range Coulomb Effect on the Antiferromagnetism in Electron-doped Cuprates
Using mean-field theory, we illustrate the long-range Coulomb effect on the
antiferromagnetism in the electron-doped cuprates. Because of the Coulomb
exchange effect, the magnitude of the effective next nearest neighbor hopping
parameter increases appreciably with increasing the electron doping
concentration, raising the frustration to the antiferromagnetic ordering. The
Fermi surface evolution in the electron-doped cuprate NdCeCuO
and the doping dependence of the onset temperature of the antiferromagnetic
pseudogap can be reasonably explained by the present consideration.Comment: 4 pages, 4 figure
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