Faculty of Physics
M.V. Lomonosov Moscow State University

# Issue 2, 2021

Optics and spectroscopy. Laser physics

## Two-photon interference of spectrally entangled pairs generated in a photonic crystal fiber

### A. V. Belinsky, N. S. Starshinov, A. B. Fedotov

Moscow University Physics Bulletin 2021. N 2.

It is shown that the spectrally entangled two-photon states formed in optical parametric processes exhibit nonlocal nonclassical second-order interference effects that depend on the joint spectral amplitude of correlated photon pairs. This makes it possible to experimentally relate the recorded interference patterns to the degree of spectral anticorrelation of a two-photon wave packet, which can act as an alternative parameter to the Schmidt and Fedorov parameters for characterizing spectral entanglement. Based on the results of two-photon interference in the Mach-Zehnder interferometer, the parameter of spectral anticorrelation of the state formed during the four-wave mixing from the photonic crystal fiber was determined.

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Theoretical and mathematical physics

## Piecewise constant signal estimation by the registration result in a linear scheme

### N. G. Mikheev, A. I. Chulichkov, V. A. Antonyuk

Moscow University Physics Bulletin 2021. N 2.

This paper proposes a method and an algorithm for piecewise constant signal reconstruction by the registration result obtained by device, the effect of which can be described by a linear operator (blur) and an additive noise. The form of the linear operator and statistical properties of the noise are known. The range of the piecewise constant signal is also assumed to be given. The investigation of the piecewise constant signal reconstruction accuracy dependence on the number of possible signal values was carried out.

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## The main characteristics of photodetectors of the photo-resistive type based on a composite of poly 3-hexylthiophene and silicon nanoparticles

### K. A. Savin$^{1,2}$, E. S. Ibragimov$^1$, A. S. Vorontsov$^1$, M. N. Martyshov, E. A. Forsh$^3$, P. A. Forsh$^1$, I. V. Novikov$^1$

Moscow University Physics Bulletin 2021. N 2.

The spectral dependences of current sensitivity and kinetics of rise and fall of photoconductivity of photo-resistances based on a composite of polymer poly 3-hexylthiophene and silicon nanoparticles were investigated. It was found that the introduction of silicon nanoparticles allows to vary in a wide range both the spectral range of operation and current sensitivity of photoresistances based on polymer polymer 3-hexylthiophene composite. The time characteristics of the photo-resistances studied in this work do not yield to the common photodetectors based on P3HT/fullerene.

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## Comparison of the fluence effect of ion irradiation on the structure of multi-walled carbon nanotubes of various diameters

### A. P. Evseev$^{1,2}$, Yu. V. Balakshin$^{2,3}$, E. A. Vorobyeva$^{2,5}$, A. V. Stepanov$^4$, K. D. Kushkina$^1$, A. A. Tatarintsev$^1$, A. A. Sheukhin$^{2,3}$

Moscow University Physics Bulletin 2021. N 2.

Multiwalled carbon nanotubes (MWCNTs) with different initial diameters were irradiated with accelerated He+ ions in the fluence range from 2.5×1015 to 3×1016 ion/cm2. Using the method of Raman light scattering, it was shown that with increasing fluence, the number of defects and the degree of amorphization of MWCNTs rapidly increase. The change in the average diameter of MWCNTs as a function of the irradiation fluence is shown, and the change in the nature of this dependence for initial nanotubes of various diameters is discussed. Simulation of the sputtering of nanotubes is carried out; it is shown that the estimate of the sputtering yield is in agreement with the experimentally measured dynamics of a change in the average diameter of nanotubes.

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## High power W-band extended interaction klystron with hollow electron beam

### V. Y. Rodyakin$^1$, V. M. Pikunov$^1$, V. N. Aksenov$^{1,2}$

Moscow University Physics Bulletin 2021. N 2.

The results of theoretical analysis of electron beam optics system and interaction region of the high power W-band extended interaction klystron with hollow electron beam are prezented. The computer code PARS is used for numerical simulation. As result of optimization, the design of extended interaction klystron with 6 maltigap cavities has been developed. The klystron provides output power about 8.6 kW with 56 dB gain, 32% electronic efficiency and 40% total efficiency with depressed collector. Developed double-stage depressed collector shows beam energy recovery efficiency 54% in dynamic regime with taking into account of secondary electron emission from collector’s walls.

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Optics and spectroscopy. Laser physics

## Simulation of quantum tomography process of biphoton polarization states on a quantum computer

### S. D. Manko$^1$, D. N. Frolovtsev$^1$, S. A. Magnitskiy$^{1,2}$

Moscow University Physics Bulletin 2021. N 2.

The process of quantum tomography of polarization-entangled photon pairs was simulated on quantum computers of different generations of the IBM Q Experience platform. Simulation was carried out for all 4 two-qubit Bell states. It was turned out that the value of the contribution of the basis states absent in the Bell states, even on quantum computers of the first generations, does not exceed 10%, and it is decreasing to a few percent on computers of the latest generation. The quality of modeling was estimated using Fidelity. The values of Fidelity are in the range (68.6-94.9)%, depending on the generation of computers. It was shown that for Bell states, Fidelity can be calculated from 3 tomographic measurements, instead of 9 required to determine the density matrix using quantum state tomography.

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Condensed matter physics

## Thermal-Magnetic properties of terbium before and after severe plastic deformation

### Yu. V. Korneeva, V. B. Chzhan, I. S. Tereshina, T. P. Kaminskaya

Moscow University Physics Bulletin 2021. N 2.

The structure, thermal and magnetothermal properties of the rare-earth metal terbium, which has been purified and then subjected to the procedure of intensive plastic deformation (SPD), are studied. The original Tb sample and the deformed one contain nanoscale structural elements, which is confirmed by AFM studies. In high-purity Tb, the magnitude of the magnetocaloric effect (MCE) in the region of transition to the paramagnetic state exceeds the (MCE) values shown by Tb samples that have not passed the purification procedure. It was found that the FEM practically disappears in the deformed sample. The magnetic properties of samples in the paramagnetic region are analyzed. It is established that the method of intensive plastic deformation allows to obtain a material with new thermal and magnetothermal properties, due to the transformation of the structure, exchange and magnetocrystalline interactions.

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Physics of Earth, atmosphere, and hydrosphere

## Deformation of the vertical velocity profile during deceleration of a flow under the influence of adverse pressure gradient

### O. N. Melnikova, H. Yang$^2$

Moscow University Physics Bulletin 2021. N 2.

It is shown experimentally that the reverse pressure gradient leads to periodic deceleration of the boundary layer of a plane-parallel flow in an open channel. In the process of braking, due to the action of the inverse gradient of pressure and friction force, an instantaneous vertical velocity profile is formed with a local minimum located above the viscous layer between two points of inflection. It is shown that in a thin layer containing the upper inflection point, the plane-parallel flow is unstable, which is confirmed experimentally - the formation of a cylindrical vortex is recorded in this layer, at the critical value of the inverse pressure gradient at the end of the deceleration cycle

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