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| Issue 12, 2024 |
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Extracts from the Internet | ||
Complex numbers in quantum mechanics1 March 2022The mathematical formalism in quantum mechanics, based on the state vector in a complex Hilbert space, successfully describes experimental data. However, attempts were made to reformulate quantum mechanics in a real Hilbert space using real numbers only. In the recent paper by M.-O. Renou and their co-authors it has been shown that the theory thus formulated is not fully equivalent to the theory in a complex Hilbert space, and the discrepancy can be revealed in experiment. The difference in the results can be written as relations resembling the Bell inequalities. The idea of the experiment suggests the use of a “quantum net”, where observers A and C each sends one particle from a quantum entangled pair to observer B, who performs their common quantum measurement, thus entangling in a certain way the states remaining with A and C. Then a correlation analysis in the spirit of Bell test is carried out. This scheme was realized in two experiments. In [1], quantum entangled photon pairs were used, while in [2] these were superconducting qubits controlled by microwave pulses. In both cases, it has been shown with high fidelity that the formalism of quantum mechanics in a complex Hilbert space is valid, whereas the real-number formulation is excluded at a high level of statistical confidence. It was 43 σ in [2] and 4.5 σ in [1]. [1] Zheng-Da Li Z.-D. et al. Phys. Rev. Lett. 128 040402 (2022) [2] Chen M.-C. et al. Phys. Rev. Lett. 128 040403 (2022) Dissipation-time uncertainty relation1 March 2022In 2020, G. Falasco and M. Esposito (University of Luxemburg) used mathematical methods of statistical mechanics to show that the dissipation-dependent entropy production rate bounds the evolution pace of physical processes and to derive the corresponding “dissipation-time” relation. Its limiting case is an infinite time of evolution in the absence of dissipation, i.e., the process is reversible. L.-L. Yan (Zhengzhou University, China) and their co-authors performed an experiment [3], in which this relation was verified for a non-equilibrium system on the basis of laser-induced electronic transitions in a unit # ion. Investigated was energy transfer between two dedicated levels, which were two heat baths, and the role of dissipative processes was played by transitions onto other levels. Each individual transition in an ion is random, but these transitions are on the average determined by the character of dissipation.The existence of restriction on the energy transfer rate corresponding to the entropy production (dissipation) and described by the dissipation-time relation was shown for the first time. These results are of importance for investigation of relation between quantum mechanics and thermodynamics and may appear to be useful for quantum information facilities because quantum operations can be speed up through dissipation control. For the limiting speed of quantum dynamics, see [4]. [3] Yan L L et al. Phys. Rev. Lett. 128 050603 (2022) [4] Zheltikov A M Phys. Usp. 64 370 (2021); UFN 191 386 (2021) Quantum boomerang1 March 2022The Anderson localization effect was predicted as far back as 1958, but its interesting feature, the counterintuitive quantum effect called “quantum boomerang”, has been theoretically discovered only recently, in 2019. The effect is as follows: the Anderson localization in a disordered medium in combination with time reversal symmetry results in the fact that the quantum wave packet of a particle starts propagating in the reverse direction and returns to its starting point. R. Sajjad (University of California, Santa Barbara, USA) and their co-authors were the first to experimentally observe the predicted quantum boomerang effect [5], but instead of return of the particles themselves, they considered the return of their momenta to the initial values. A Bose-Einstein condensate of 107 7Li atoms was placed into an optical lattice, and the interatomic interaction was switched off via Feshbach resonance.The second pulsating phase-shifted lattice had a pulsed action on the atoms. Measurements have shown that the pulses received by the atoms returned to zero values with time, which is an analog of wave packet return in the quantum boomerang effect. [5] Sajjad R et al. Phys. Rev. X (2022) (in press) SiC-based qubit1 March 2022Solid-state quantum logic cells based on spin defects are promising for the use in quantum information and communication devices owing to a long coherence time and the possibility of creating optical interfaces. Of particular interest are cells in the form of neutral divacancies in silicon carbide SiC, since they are well compatible with other semiconducting devices. However, the defect spin state readout, typically realized by observation of spin-dependent fluorescence after laser excitation, is difficult. In this method, the average number of photons registered per readout cycle is <<1. In the new experimental approach, developed by C.P. Anderson (University of Chicago, USA) and their co-authors [6], both read out within a cycle and a long-lived coherent state – nearly 5 swere achieved. This time is about two orders of magnitude longer than was reached previously for SiC-based qubits. The method of spin-charge conversion in combination with spin-selective ionization by laser pulses was used. The readout process only comes down to recording the presence or absence of an electron at the electron level after ionization. The photon signal in this case is 104 times stronger than in ordinary fluorescence, and the readout is much more efficient. For quantum nets, see [7]. [6] Anderson C P et al. Science Advances 8 eabm5912 (2022) [7] Sukachev D D Phys. Usp. 64 1021 (2021); UFN 191 1077 (2021) An unusual radio transient1 March 2022N. Hurley-Walker (International Center for Radio Astronomy Research, Australia) and her colleagues revealed an unusual variable radio source with periodicity of 18.2 min [8], which has never been observed before. According to the archive data of low-frequency observations on Murchison Widefield Array (MWA) radio telescopes, 71 pulses from this source were registered from January to March 2018. The pulses persisted from 30 to 60 s and contained shorter (<0.5 s) peaks as well. The emission dispersion measure in combination with models of electron density in the Galaxy testifies to the source localization within our Galaxy at a distance of 1.3 ± 0.5 kpc. It has a nonthermal spectrum with index α=-1.16, and the brightness temperatureof 1016 K is indicative of a coherent emission mechanism. The emission is linearly polarized at a level of 88 ± 1 %, which may show the presence of a strong magnetic field in the source. The origin of this unusual radio transient remains unknown. This may be a very long-period magnetar (a strongly magnetized neutron star), a white dwarf, or even a new type of cosmic object. [8] Hurley-Walker N et al. Nature 601 526 (2022) |
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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