Radiation Damping Effects in Two Level Maser Oscillators

Several experiments [1,2] have noted recently that when an inverted two-level spin system was permitted to radiate spontaneously, the resulting oscillation was characterized by an appreciable amplitude modulation. The phenomenon was first believed to be the result of interference of different spin packets in an inhomogeneously broadened spectrum [1]. A theoretical analysis (which will be reported separately) shows that this is not the case. The spins are not independent but are coupled together by means of their radiation field. This explanation has since been by its original authors

Irrational Expectations: Can a Regulator Credibly Commit to Removing an Unbundling Obligation?

There is a large empirical literature that investigates the effects of unbundling requirements on broadband operators' incentives to invest in infrastructure. To date, that literature has generally relied on industry-wide data as an indicator of how the representative operator reacts to the imposition of mandatory unbundling. In this paper, we present original findings on how specific firms reacted to the removal of an unbundling obligation that is, an act of "regulatory forbearance"either for an existing access technology or for a new access technology. We rely on three case studies to evaluate the impact of regulatory forbearance on specific incumbents and entrants that were directly affected by the regulator's decision. Our findings from the first case study appear to undermine the so-called "stepping stone" justification for unbundling an existing access technology (for example, the copper loop). In particular, there is a large discontinuity in the investment by entrants around the date of forbearance, in contrast to the steady movement up the ladder of investment predicted by the stepping stone hypothesis. Such a discontinuity suggests that either (1) the regulator failed to signal its deregulatory intentions to entrants, or (2) that the signal was clear but the entrant did not react according to the theory. We also find that incumbent investment increases significantly in response to forbearance from regulating a new access technology (for example, fiber loops). When forbearing from regulating an existing access technology, regulators can signal their future intentions to entrants by slowly increasing the regulated wholesale rate. In the case of forbearing from regulating a new technology, however, there is no equivalent mechanism by which regulators can signal their deregulatory intentions to incumbents. Because a regulator cannot credibly signal its commitment to industry participants, and because such a commitment is critical to the practical success of the stepping stone theory, the best policy for maximizing investment is to accelerate the date of forbearance for existing and new access technologies.Technology and Industry

The interaction energy of well-separated Skyrme solitons

We prove that the asymptotic field of a Skyrme soliton of any degree has a non-trivial multipole expansion. It follows that every Skyrme soliton has a well-defined leading multipole moment. We derive an expression for the linear interaction energy of well-separated Skyrme solitons in terms of their leading multipole moments. This expression can always be made negative by suitable rotations of one of the Skyrme solitons in space and iso-space.We show that the linear interaction energy dominates for large separation if the orders of the Skyrme solitons' multipole moments differ by at most two. In that case there are therefore always attractive forces between the Skyrme solitons.Comment: 27 pages amslate

GOTCHA Password Hackers!

We introduce GOTCHAs (Generating panOptic Turing Tests to Tell Computers and Humans Apart) as a way of preventing automated offline dictionary attacks against user selected passwords. A GOTCHA is a randomized puzzle generation protocol, which involves interaction between a computer and a human. Informally, a GOTCHA should satisfy two key properties: (1) The puzzles are easy for the human to solve. (2) The puzzles are hard for a computer to solve even if it has the random bits used by the computer to generate the final puzzle --- unlike a CAPTCHA. Our main theorem demonstrates that GOTCHAs can be used to mitigate the threat of offline dictionary attacks against passwords by ensuring that a password cracker must receive constant feedback from a human being while mounting an attack. Finally, we provide a candidate construction of GOTCHAs based on Inkblot images. Our construction relies on the usability assumption that users can recognize the phrases that they originally used to describe each Inkblot image --- a much weaker usability assumption than previous password systems based on Inkblots which required users to recall their phrase exactly. We conduct a user study to evaluate the usability of our GOTCHA construction. We also generate a GOTCHA challenge where we encourage artificial intelligence and security researchers to try to crack several passwords protected with our scheme.Comment: 2013 ACM Workshop on Artificial Intelligence and Security (AISec

A statistical study of the global structure of the ring current

[1] In this paper we derive the average configuration of the ring current as a function of the state of the magnetosphere as indicated by the Dst index. We sort magnetic field data from the Combined Release and Radiation Effects Satellite (CRRES) by spatial location and by the Dst index in order to produce magnetic field maps. From these maps we calculate local current systems by taking the curl of the magnetic field. We find both the westward (outer) and the eastward (inner) components of the ring current. We find that the ring current intensity varies linearly with Dst as expected and that the ring current is asymmetric for all Dst values. The azimuthal peak of the ring current is located in the afternoon sector for quiet conditions and near midnight for disturbed conditions. The ring current also moves closer to the Earth during disturbed conditions. We attempt to recreate the Dst index by integrating the magnetic perturbations caused by the ring current. We find that we need to multiply our computed disturbance by a factor of 1.88 ± 0.27 and add an offset of 3.84 ± 4.33 nT in order to get optimal agreement with Dst. When taking into account a tail current contribution of roughly 25%, this agrees well with our expectation of a factor of 1.3 to 1.5 based on a partially conducting Earth. The offset that we have to add does not agree well with an expected offset of approximately 20 nT based on solar wind pressure