Zero-point fluctuations in the ground state of a mesoscopic normal ring

We investigate the persistent current of a ring with an in-line quantum dot capacitively coupled to an external circuit. Of special interest is the magnitude of the persistent current as a function of the external impedance in the zero temperature limit when the only fluctuations in the external circuit are zero-point fluctuations. These are time-dependent fluctuations which polarize the ring-dot structure and we discuss in detail the contribution of displacement currents to the persistent current. We have earlier discussed an exact solution for the persistent current and its fluctuations based on a Bethe ansatz. In this work, we emphasize a physically more intuitive approach using a Langevin description of the external circuit. This approach is limited to weak coupling between the ring and the external circuit. We show that the zero temperature persistent current obtained in this approach is consistent with the persistent current calculated from a Bethe ansatz solution. In the absence of coupling our system is a two level system consisting of the ground state and the first excited state. In the presence of coupling we investigate the projection of the actual state on the ground state and the first exited state of the decoupled ring. With each of these projections we can associate a phase diffusion time. In the zero temperature limit we find that the phase diffusion time of the excited state projection saturates, whereas the phase diffusion time of the ground state projection diverges.Comment: 12 pages, 5 figure

Internal flows and energy circulation in light beams

We review optical phenomena associated with the internal energy redistribution which accompany propagation and transformations of monochromatic light fields in homogeneous media. The total energy flow (linear-momentum density, Poynting vector) can be divided into spin part associated with the polarization and orbital part associated with the spatial inhomogeneity. We give general description of the internal flows in the coordinate and momentum (angular spectrum) representations for both nonparaxial and paraxial fields. This enables one to determine local densities and integral values of the spin and orbital angular momenta of the field. We analyse patterns of the internal flows in standard beam models (Gaussian, Laguerre-Gaussian, flat-top beam, etc.), which provide an insightful picture of the energy transport. The emphasize is made to the singular points of the flow fields. We describe the spin-orbit and orbit-orbit interactions in the processes of beam focusing and symmetry breakdown. Finally, we consider how the energy flows manifest themselves in the mechanical action on probing particles and in the transformations of a propagating beam subjected to a transverse perturbation.Comment: 50 pages, 21 figures, 173 references. This is the final version of the manuscript (v1) modified in accord to the referee's remarks and with allowance for the recent development. The main changes are: additional discussion of the energy flows in Bessel beams (section 4.1), a lot of new references are added and the Conclusion is shortened and made more accurat

Social Network-Based Digital Stroke Prevention: Opportunities, Results and Prospects

Aim. To study the possibilities and limitations of the social network as a digital medical tool, which is aimed at improving programs for primary and secondary stroke prevention in young people.Material and methods. The study was carried out in the format of online training for volunteers. At the first stage of the work, the online school “Stroke in Young People” was announced in 8 medical blogs. As part of the school, a special account was created for readers (n=1354). At the second stage, 49 respondents (4% of men, whose average age was 24.4±5.2 years) were selected from 1354 listeners, who were surveyed on “Awareness of risk factors and stroke symptoms among users of social networks” before and after the online school.Results. The online school audience is predominantly female (91%), and 43% of readers were in the 25-34 age group. The total number of people who listened to and read the online school material is 8712 people. 17% worked in the healthcare system, and 22% of respondents had a history of stroke. 38 (78%) people of the 2nd stage among the respondents independently searched for information about stroke earlier, and 30 (61%) received this information passively from medical workers in 2020. Before the online school start, the majority of respondents (over 60%) were aware of 2 out of 7 stroke risk factors (dyslipidemia and arterial hypertension) and 3 out of 6 stroke signs (drooping of the face half, weakness in the limbs and difficulty speaking). Less than 40% of the participants considered the stroke risk factors for diabetes mellitus, other cardiovascular diseases (CVD), obesity, and alcohol use; less than 20% were aware of stroke symptoms such as impaired vision and coordination and very severe headache. After completing online learning, the greatest increase in knowledge was found among the following risk factors – smoking and other CVDs (p<0.05); stroke symptoms - headache and drooping of the face half (p<0.05).Conclusion. The online school aroused interest among healthcare workers and people without medical education, including those with stroke. Most of the respondents believed that they knew how to prevent a stroke (over 80%) and would be able to provide first aid to a person with a stroke (over 90%). At the same time, the awareness of risk factors and stroke symptoms was low prior to the start of learning, even though the study included healthcare workers and stroke survivors. Online learning has led to increased awareness of some risk factors and stroke symptoms. Social media can be one of the tools for medical prevention of stroke in young people, but program planning should take into account the way the material is presented and its readability