21,237 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
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
Extracting Energy from Accretion into Kerr Black Hole
The highest efficiency of converting rest mass into energy by accreting
matter into a Kerr black hole is ~ 31% (Thorne 1974). We propose a new process
in which periods of accretion from a thin disk, and the associated spin-up of
the black hole, alternate with the periods of no accretion and magnetic
transfer of energy from the black hole to the disk. These cycles can repeat
indefinitely, at least in principle, with the black hole mass increasing by ~
66% per cycle, and up to ~ 43% of accreted rest mass radiated away by the disk.Comment: 4 pages, 1 figur
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