April 30th, 2019

лошадь, диаграмма, Фейнман

Игорь Иванов и Валерий Георгиевич Сербо о паре фотонов с орбитальным моментом

Fate of the Landau-Yang theorem for twisted photons: https://arxiv.org/abs/1904.12110

Landau-Yang theorem is sometimes formulated as a selection rule forbidding two real photons with zero total momentum to be in the state of the total angular momentum J=1. In this paper we discuss whether the theorem itself and this particular formulation can be extended to a pair of two {\em twisted} photons, which carry orbital angular momentum with respect to their propagation direction. We point out possible sources of confusion, which may arise both from the unusual features of twisted photons and from the fact that usual proofs of the Landau-Yang theorem operate in the center of motion reference frame, which, strictly speaking, exists only for plane waves. We show with an explicit calculation that a pair of twisted photons does have a non-zero overlap with the J=1 state. What is actually forbidden is production of a spin-1 particle by such a photon pair, and in this formulation the Landau-Yang theorem is rock-solid. Although both the twisted photon pair and the spin-1 particle can exist in the J=1 state, these two systems just cannot be coupled in a gauge-invariant and Lorentz invariant manner respecting Bose symmetry.
лошадь, диаграмма, Фейнман

Хороший, понятный пример того, как считаются сечения простых ядерных реакций

Direct capture cross section of 9Be(n,γ)10Be: https://arxiv.org/abs/1904.12115

Peter Mohr

The cross section of the 9Be(n,γ)10Be reaction was calculated in the direct capture model. All parameters of the calculations were adjusted to properties of the 9Be + n system at thermal energies. The calculated cross section at thermonuclear energies shows the expected 1/v behavior of s-wave capture at low energies, but increases towards higher energies as typical p-wave capture. Excellent agreement between new experimental data in the astrophysically relevant energy region and the present calculation is found.

7 pages, 3 figures, Phys. Rev. C, accepted for publication
лошадь, диаграмма, Фейнман

Новое в диффузионной МРТ

Достаточно подробно дана тегория.

Localization regime in diffusion NMR: theory and experiments: https://arxiv.org/abs/1904.10877

In this work we investigate the emergence of the localization regime for diffusion in various geometries: inside slabs, inside cylinders and around rods arranged on a square array. At high gradients, the transverse magnetization is strongly attenuated in the bulk, whereas the macroscopic signal is formed by the remaining magnetization localized near boundaries of the sample. As a consequence, the signal is particularly sensitive to the microstructure.
Our theoretical analysis relies on recent mathematical advances on the study of the Bloch-Torrey equation. Experiments were conducted with hyperpolarized xenon-129 gas in 3D-printed phantoms and show an excellent agreement with numerical simulations and theoretical predictions. Our mathematical arguments and experimental evidence indicate that the localization regime with a stretched-exponential decay of the macroscopic signal is a generic feature of diffusion NMR that can be observed at moderately high gradients in most NMR scanners.
лошадь, диаграмма, Фейнман

Игра в раздачу квантовых ключей для чилийских школьников

Не вникал, так что сказать, есть ли в этом что-то, или фигня, не могу.

An Instrument-Free Demonstration of Quantum Key Distribution for High-School Students: https://arxiv.org/abs/1904.10537

It has become increasingly common for high-school students to see media reports on the importance of quantum mechanics in the development of next-generation industries such as drug development and secure communication, but few of them have been exposed to fundamental quantum mechanical concepts in a meaningful classroom activity. In order to bridge this gap, we design and test a low-cost 20-minute demonstration of the Bell test, which is used in several entanglement-based quantum key distribution protocols. The demonstration introduces ideas such as the quantum state, quantum measurement, spin quantization, cryptography, and entanglement; all without using concepts beyond the 9th grade of the Chilean high-school curriculum. The demonstration can serve to promote early exposure of the future adopters and developers of quantum technology with its conceptual building blocks, and also to educate the general public about the importance of quantum mechanics in modern industry

Comments: 10 pages, 3 figures. Comments welcome