Quasi-phase-matching of high-order-harmonic generation using multimode polarization beating

The generalization of quasi-phase-matching using polarization beating and of multimode quasi-phase-matching (MMQPM) for the generation of high-order harmonics is explored, and a method for achieving polarization beating is proposed. If two (and in principle more) modes of a waveguide are excited, modulation of the intensity, phase, and/or polarization of the guided radiation will be achieved. By appropriately matching the period of this modulation to the coherence length, quasi-phase-matching of high-order-harmonic radiation generated by the guided wave can occur. We show that it is possible to achieve efficiencies with multimode quasi-phase-matching greater than the ideal square wave modulation. We present a Fourier treatment of QPM and use this to show that phase modulation, rather than amplitude modulation, plays the dominant role in the case of MMQPM. The experimental parameters and optimal conditions for this scheme are explored

Developing and evaluating a hybrid wind instrument

A hybrid wind instrument generates self-sustained sounds via a real-time interaction between a computed excitation model (such as the physical model of human lips interacting with a mouthpiece) and a real acoustic resonator. Attempts to produce a hybrid instrument have so far fallen short, in terms of both the accuracy and the variation in the sound produced. The principal reason for the failings of previous hybrid instruments is the actuator which, controlled by the excitation model, introduces a fluctuating component into the air flow injected into the resonator. In the present paper, the possibility of using a loudspeaker to supply the calculated excitation signal is evaluated. A theoretical study has facilitated the modeling of the loudspeaker-resonator system and the design of a feedback and feedforward filter to successfully compensate for the presence of the loudspeaker. The resulting self-sustained sounds are evaluated by a mapping of their sound descriptors to the input parameters of the physical model of the embouchure, both for sustained and attack sounds. Results are compared with simulations. The largely coherent functioning confirms the usefulness of the device in both musical and research contexts

AC Josephson effect in finite-length nanowire junctions with Majorana modes

It has been predicted that superconducting junctions made with topological nanowires hosting Majorana bound states (MBS) exhibit an anomalous 4\pi-periodic Josephson effect. Finding an experimental setup with these unconventional properties poses, however, a serious challenge: for finite-length wires, the equilibrium supercurrents are always 2\pi-periodic as anticrossings of states with the same fermionic parity are possible. We show, however, that the anomaly survives in the transient regime of the ac Josephson effect. Transients are moreover protected against decay by quasiparticle poisoning as a consequence of the quantum Zeno effect, which fixes the parity of Majorana qubits. The resulting long-lived ac Josephson transients may be effectively used to detect MBS.Comment: 9 pages, 4 figures, published version (with supplementary material

Moral Status and Agent-Centred Options

If we were required to sacrifice our own interests whenever doing so was best overall, or prohibited from doing so unless it was optimal, then we would be mere sites for the realisation of value. Our interests, not ourselves, would wholly determine what we ought to do. We are not mere sites for the realisation of value — instead we, ourselves, matter unconditionally. So we have options to act suboptimally. These options have limits, grounded in the very same considerations. Though not merely such sites, you and I are also sites for the realisation of value, and our interests (and ourselves) must therefore sometimes determine what others ought to do, in particular requiring them to bear reasonable costs for our sake. Likewise, just as my moral status grounds a requirement that others show me appropriate respect, so must I do to myself

The phase diagram of the U(2)×U(2)U(2) \times U(2) Sigma Model and its Implications for Chiral Hierarchies

Motivated by the issue of whether it is possible to construct phenomenologically viable models where the electroweak symmetry breaking is triggered by new physics at a scale $\Lambda \gg 4\pi v$, where $v$ is the order parameter of the transition ($v\sim 250$ GeV) and $\Lambda$ is the scale of new physics, we have studied the phase diagram of the $U(2) \times U(2)$ model. This is the relevant low energy effective theory for a class of models which will be discussed below. We find that the phase transition in these models is first order in most of parameter space. The order parameter can not be made much smaller than the cut-off and, consequently a large hierarchy does not appear sustainable. In the relatively small region in the space of parameters where the phase transition is very weakly first order or second order the model effectively reduces to the O(8) theory for which the triviality considerations should apply.Comment: LaTeX file. 32 pages, 10 appended PostScript files, uses epsfig.st