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Issue 12, 2024
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Extracts from the Internet


The Breit-Wheeler process

In 1934, soon after the occurrence of quantum electrodynamics, G. Breit and J.A. Wheeler analyzed theoretically the process of electron-positron pair (e+e-) production upon collision of two real (not virtual) photons. They noticed that the condition for e+e- production might be attained in collision of highly charged relativistic ions. In this case, the ion field, which is Coulomb in the rest system, can be imagined consisting of real photons. The Breit-Wheeler process could not have been registered because of the difficult ion focusing. The Breit-Wheeler process upon Au+Au collisions with a CMS energy of 200 GeV was observed for the first time in the STAR experiment, carried out at the relativistic heavy ion collider RHIC at Brookhaven National Laboratory (USA) [1]. Almost tangent (ultrarelativistic) collisions of nuclei were singled out [2] in which strong interaction is not involved in scattering. In the experiment, 6085 produced e+e- pairs were detected, the process cross section γγ → e+e- was measured, and the characteristic angular modulation predicted for the Breit-Wheeler process was observed. It confirms that it is real photons with transverse linear polarization that collide. One can hope that in a similar experiment vacuum birefringence will also be observed for photons in a magnetic field. [1] Adam J et al. Phys. Rev. Lett. 127 052302 (2021) [2] Dremin I M Phys. Usp. 63 758 (2020); UFN 190 811 (2020)

High-frequency signals in gravitational wave antenna

Gravitational waves from collision of two black holes were first detected by LIGO detector in 2015. It should not be excluded that gravitational waves are also generated in other processes in the Universe at other frequencies. A new high frequency gravitational wave detector started operating in the Australian town of Perth [3]. Its base is a quartz plate 1 mm thick and 30 mm in diameter; different modes of its volume acoustic oscillations are registered by a superconducting quantum interferometer (SQUID). The detector is thoroughly isolated from sources of acoustic and other environmental noises - its sensitivity is limited to internal thermal noises and SQUID readout noise. Two amplifiers, tuned to overtones of different oscillation modes can monitor simultaneously at two frequencies. During the first 153 days of observation, M. Goryachev (University of Western Australia) and his co-authors registered two statistically significant events. The first event occurred on May 12, 2019 at a frequency of 5.506 MHz, whereas no event was observed at the other examined frequency of 8.392 MHz. The second event was noticed on November 27, 2019 at frequencies of 5.506 MHz and 4.993 MHz. Oscillations lasted 1 - 2 seconds; in view of the known plate quality factor, this is consistent with the time of damping from a short interaction. According to estimations, an energy of the order of hundredth fractions of eV was released in the detector. It is still unclear what may have been responsible for these events. During the events, no lightnings or earthquakes occurred, and LIGO/Virgo detected no gravitational-wave bursts. No meteors or fast radio bursts have been registered either. This may have been caused by stress relaxations in the quartz plate, the action of radioactivity or cosmic rays. This may also have been high-frequency gravitational-wave signal of unknown origin with a characteristic amplitude h ≈ 2.5×10−16. Other possible explanations concern domain walls or interaction of dark matter particles with the crystal lattice. The nature of the registered signals will perhaps clear up with increasing detector sensitivity and acquisition of statistics. [3] Goryachev M et al. Phys. Rev. Lett. 127 071102 (2021)

Dissipative time crystals

Time crystals, predicted theoretically by F. Wilczek in 2012, are characterized by the fact that their properties repeat in time like atoms settled periodically in solid crystals. Time crystals have already been observed in experiments. H. Kessler (University of Hamburg, Germany) and his co-authors were the first to realize the time crystal stabilized by dissipation [4]. The experiment employed Bose-Einstein condensate of 87Rb atoms in an optical cavity, to which laser pumping radiation was directed perpendicularly to the optical axis. As the pumping intensity increased above some threshold value, the density wave phase, described by the Dicke model, set up in the cavity. Time crystal oscillations occurred between even and odd states of density waves. Crystals were stabilized by balance of periodical driving force, cavity-mediated interactions, and controlled cavity dissipation. [4] Keβler H et al. Phys. Rev. Lett. 127 043602 (2021)

Directly Measuring a Multiparticle Wave Function

As distinct from an indirect measuring of quantum wave function, realized in quantum tomography, direct measurements using a single observable make it possible to find a real or imaginary part of a wave function. The direct method has already been employed to measure the wave function of only one particle. M.-C. Chen (University of Science and Technology of China) and their co-authors proposed theoretically a new method of direct measurement of a multiparticle quantum wave function and realized it experimentally for the first time [5]. It is based on quantum teleportation of an individual multiparticle density matrix element to a unit logical qubit, where the real or imaginary part of the element is measured through quantum readout depending on the chosen measuring basis. The experiment with photons entangled in the polarization states confirmed efficiency of this method in the case of a two-photon wave function. In many cases, the new method may provide considerable advantage over the usual quantum tomography. For quantum effects, see [6-10]. [5] Chen M-C Phys. Rev. Lett. 127 030402 (2021) [6] Arbekov I M, Molotkov S N Phys. Usp. 64 617 (2021); UFN 191 651 (2021) [7] Zheltikov A M, Scully M O Phys. Usp. 63 698 (2020); UFN 190 749 (2020) [8] Belinsky A V Phys. Usp. 62 1268 (2019); UFN 189 1352 (2019) [9] Belinsky A V Phys. Usp. 63 1256 (2020); UFN 190 1335 (2020) [10] Chukbar K V Phys. Usp. 61 389 (2018); UFN 188 446 (2018)

Periodicity in fast radio burst profiles

The CHIME/FRB program for observation of fast radio bursts (FRB) at frequencies of 400-800 MHz is implemented with the Canadian radio telescope - interferometer. Some FRB profiles observed in CHIME/FRB show several peaks. The peaks of three FRBs are separated by about equal time intervals, which is indicative of FRB generation periodicity. Periodicity with a period of 216.8 ms and a significance of 6.5 σ was found in the nine-peak burst FRB 20191221A. Some indications (1.3 σ and 2.4 σ) of periodicity with periods of 2.8 ms and 10.7 ms are shown by two more FRBs [11]. These observations favor neutron-star FRB origination: on magnetars and interacting neutron stars in binary systems. In view of the discovered millisecond periodicity, the emission region must lie in a neutron star magnetosphere rather than at a distance from it, as presupposed in some models. For FRBs, see [12]. [11] Bridget C et al., arXiv:2107.08463 [astro-ph.HE] [12] Popov S B, Postnov K A, Pshirkov M S Phys. Usp. 61 965 (2018); UFN 188 1063 (2018)

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The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.

Permanent editor is Yu.N. Eroshenko.

It is compiled from a multitude of Internet sources.
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