Extracts from the Internet

Pionic helium

Charged pi-mesons (pions), which are known today to consist of a quark and an antiquark coupled by strong interaction, were discovered in 1947. The same year, E. Fermi and E. Teller supposed that exotic atoms may be formed in which one or more orbital electrons are replaced by mesons [1]. In most cases, the atomic nucleus must quickly absorb the meson and decay. The calculations however showed that in pionic helium π4He+ a rare situation is realized where its lifetime is 1000 times longer than that of other pion atoms. The π4He+ atom consists of a heium-4 nucleus, an electron and a π--meson. Only indirect evidence of π4He+ formation in collisions was obtained earlier. M. Hori (Max Planck Institute of Quantum Optics, Germany) and co-authors were the first to obtain experimentally [2] a sufficient amount of π4He+ atoms and to perform their laser spectroscopy. A π- beam obtained on the cyclotron at Paul Scherrer Institute (Switzerland) was directed to superfluid 4He. Approximately 3.2 % of all π- delayed in helium formed π4He+ atoms with lifetime of several ns. The same target was illuminated by nanosecond laser pulses. They generated cascade processes in π4He+ that led ultimately to pion absorption and nucleus decay. The rate of neutron, proton and deuteron production due to decay had a maximum for a certain laser pulse frequency. This resonance that had statistical significance over 7 σ corresponded to the intranuclear transitions (n,l)=(17,16)→(17,15). The resonance frequency proved to be somewhat higher than the calculated one. This was possibly due to atomic collisions that perturb the energy levels as had already been noticed in anti-p4He+ spectroscopy. Thus, the experiment demonstrated realizability of laser spectroscopy of exotic meson-containing atoms. For exotic atoms see the review [3]. [1] Fermi E & Teller E, Phys. Rev. 72 399 (1947), [2] Hori M et al., Nature 581 37 (2020), [3] Men’shikov L I, Eseev M K, Phys. Usp. 44 135 (2001) [UFN 171 149 (2001)]

Strong quantum coupling at a distance of 1 m

Coupling between systems realized with quantum accuracy without loss of coherence is of importance for quantum technologies. Such a coupling is readily realized straight between objects in close proximity or through the field in the cavity. However, for distant systems the creation of quantum coupling presents considerable difficulties because the signals and scatterings of quantum information are weakened. P. Treutlein (University of Basel, Switzerland) with colleagues realized in their experiment [4] long-distance coupling through a laser beam in loop configuration with several passes in two directions. The coupling loop allows compensating some typesof losses at the expense of destructive interference of quantum noise. This method was applied for coupling of the collective spin state of 107 87Rb atoms in the magnetic field in an optical dipole trap and a mechanical oscillator which is a silicon-nitride square membrane in the optical cavity 1 m apart. The spins were perturbed using a small solenoid, and perturbation of the collective spin led to light polarization rotation which was converted to light wave amplitude perturbation resulting in the end in a force effect on the oscillator. And vice versa, membrane displacements in the reverse order affected the spin. Room-temperature quantum coupling was demonstrated in different regimes with a positive and negative effective mass of an ensemble of spins. [4] Karg T M et al., Science, online publication of May 7, 2020

Initial stage of molecule photoexcitation

The electron density redistribution in a molecule at the initial stage of its excitation by laser pulses was examined in the SLAC accelerator laboratory. P.M. Weber (Brown University, USA) with colleagues investigated relatively small organic molecules of 1,3-cyclohexadiene C6H8 in a room-temperature rarefied gas [5]. Immediately after the action of optical laser pulses,the molecules were illuminated by ultrarshort X-ray pulses from the free electron LCLS laser. From their scattering, the difference of electron densities before and after the beginning of laser pulse action was found. These direct measurements showed that at a small distance <3 Å from the molecule center the electron cloud density decreases, whereas at a large distance of (4-9) Å it increases. Such a character of photoexcitation is also reproduced in theoretical calculations as a transition to a diffuse electron 3p-state. Such studies may provide insight into the mechanisms of different photochemical processes. [5] Yong H et al., Nature Communications 11 2157 (2020)

Laser radiation with a large orbital angular momentum

H. Sroor (Wits University, South Africa) with co-authors designed a laser [6] generating radiation with a high value of orbital angular momentum - with quantum numbers up to l = 100. By only an order of magnitude smaller l were obtained in preceding works using liquid crystals. The new device contained a metasurface integrated in an IR-excited nonlinear-crystal laser generating medium with on a nonlinear crystal in an optical resonator. The metasurface consisted of an array of rectangular rods manufactured of amorphous titanium oxide TiO2 on a quartz substrate. The rods were oriented with their long sides perpendicular to the surface (in the direction of laser beam). They were of different lengths and were located on the surface in a special manner so that the wave passing through the metasurface gains a certain phase shift in each rod, and in the end acquires a large resulting angular momentum. This method of creating a metasurface allows obtaining structured light with different characteristics in a rather compact device with a small number of optical elements. The light with a large orbital angular momentum may find useful applications in various areas, including quantum communication and metrology. [6] Sroor H et al., Nature Photonics, online publication of April 27, 2020

Microwave quantum radar

Probing small objects using low-power electromagnetic pulses finds a number of important practical applications. The most interesting was the case investigating low-reflectivity objects in a medium with high thermal noises. In an optical region, pairs of photons in an entangled quantum state [7] have already been used for this purpose. This method was called quantum illumination. Quantum entanglement allows considerable increasing the signal-to-noise ratio for a reflected signal. The use of this method in the terahertz and microwave regions encounters some difficulties because here quantum technologies are worse developed and cooling to cryogen temperatures is needed. But at the same time, this frequency range is very important, for example, for noninvasive biomedical diagnostics [8]. S. Barzanjeh (the Institute of Science and Technology, Austria) with colleagues performed an experiment [9] demonstrating quantum probing of a room temperature object by microwave radiation photons. Pairs of microwave photons in quantum entangled state were generated with the help of Josephson parametric converter. One of the photons from the pair was reflected from the studied object at a distance of 1 meter and then, in the detector, was compared in phase with the first photon. An important element of these measurements compared to the previous experiments was an analog-to-digital signal conversion already at the early stages of measurements, which allowed heightening the output signal quality owing to a more convenient method of information processing. The measurements showed that the use of photons in entangled states improves substantially the results of probing compared to classical (not quantum) methods, allowing isolation of a signal from the noise level. [7] Zheltikov A M, Scully M O, Phys. Usp. 63 (2020) [ÓÔÍ 190 749 (2020)], [8] Doronina-Amitonova L V et al., Phys. Usp. 58 345 (2015) [UFN 185 371 (2015)], [9] Barzanjeh S et al., Science Advances 6 eabb0451 (2020)

Intermediate quantum statistics for anyons

In a three-dimensional system, elementary excitations - quasi-particles can only be bosons or fermions depending on the change in the phase of the common wave function upon permutation of two particles (π=0 or φ=π). However, intermediate type statistics with other φ was predicted theoretically to be realizable in two-dimensional systems. Quazi-particles satisfying this statistics were called anyons . Up to now, indirect experimental evidences only have been obtained of the fact that quasi-particles can have intermediate statistics. H. Bartolomei (the Higher Normal School, Paris, France) and co-authors showed for the first time in the direct collisional experiment [10] that colliding anyons do satisfy intermediate statistics. To this end, in the GaAs/AlGaAs heterostructure retaining a two-dimensional electron gas, two anyon sources– quantum point contacts - were created. A third analogous contact served as a splitter with interacting anyons emitted by the first two contacts. The correlations of electric currents generated by anyons that had passed through the splitter were measured. Using these correlations, statistical properties of anyons could be determined. Intermediate statistics with φ=π/3 was shown to be realized for anyons in this system. For two-dimensional systems see [11]. [10] Bartolomei H et al., Science 368 173 (2020), [11] Störmer H, UFN 170 304 (2020)

Searching for solar axions

A new experiment [12] on the search for solar axions was performed at Max Planck Institute for Physics (Germany), which continued the series of experiments carried out at B.P. Konstantinov Petersburg Nuclear Physics Institute (PNPI). The hypothetical particles - axions were proposed theoretically to explain CPsymmetry conservation in strong interactions. Axions and axion-like particles were considered as one of the key candidates for the role of dark matter (hidden mass) in the Universe. Axions were predicted to be produced effectively on the Sun invarious processes, and experiments were performed for searching for a solar axion fluxon the Earth. In the new cryogen experiment, resonant absorption of axions by 169Tm nuclei was searched using a modified low-background bolometer based on an 8-g Tm3Al5O12 crystal. A distinctive feature of the experiment was the use of a new phononsensor. It was a tungsten film evaporated on the crystal between two aluminum films that served as phonon collectors. This construction allowed the energy threshold necessary for axion detection to be overcome. Also, a heater was evaporated on the crystal for maintaining a necessary working temperature and calibration. The new detector was much more sensitive than the previous detectors that made use of 169Tm nuclei. The measurements lasted several days. Although no resonant absorption of axions has yet been registered, new restrictions on coupling constants of axions with photons and electrons were obtained. Russian scientists from Kurchatov institute, PNPI and A.M. Prokhorov Institute of General Physics of the Russian Academy of Sciences (IOFAN) took part in the experiment. [12] Abdelhameed A H et al., arXiv:2004.08121 [hep-ex]

Precession in the star orbit around a supermassive black hole

GRAVITY collaboration registered for the first time a Schwarzschild precession in the orbit of the star S2 rotating around the Galactic center supermassive black hole (BH) [13]. The explanation of an analogous Mercury orbit precession – an additional displacement of perihelion was in the early XXth century one of the most important verifications of the theory of General Relativity. S2 is a star close to BH with an extended orbit with a period of 15.6 years. This star has been monitored by a number of telescopes for 27 years. To fix the reference system, relative to which the position of the star and BH is measured, flares near BH were in particular used. A relativistic Doppler effect due to the motion of the star S2 and the gravitational red shift due to the BH field have already been recorded. The obtained set of data also testifies with a (5-6) σ confidence to the orbit precession exactly as predicted by GR. Thus, the General Relativity Theory has passed another successful test in new conditions. The star motion could in principle be affected by the gravitational field of a continuously distributed invisible matter or of compact objects. The new data impose restriction on these sources of orbit perturbations. In particular, the mass of an additional compact object (the second BH) inside a region one angular second in size around the galactic center cannot exceed 103 M. For the experimental verification of General Relativity see [14, 15]. [13] Abuter R et al., Astron. & Astrophys. 636 L5 (2020), [14] Rudenko V N, Sov. Phys. Usp. 21 893 (1978) [UFN 126 361 (1978)], [15] Turyshev S G, Phys. Usp. 52 1 (2009) [UFN 179 3 (2009)]

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