Extracts from the Internet


Bose – Einstein condensation of photons in a double potential

M. Weitz (the University of Bonn, Germany) and his colleagues performed an experiment in which photons in an irreversible process transited to a split state of Bose – Einstein condensate. A microcavity was used that consisted of mirrors of special form, so that a confining potential with two minima spaced by 13µm was formed for photons in the cavity center. Between the mirrors was a dye solution with molecules reemitting laser photons, which enabled the photon gas to be cooled to room temperature. The photons were accumulated at the potential minima and could tunnel between them, and the photon loss that made the process irreversible was compensated by a photon flux from outside. In the double potential, photons were divided according to the form of their wave function that could be symmetric or antisymmetric. The observation of the emission showed that photons occupied several lower energy levels in the cavity, and nearly 1200 photons underwent transition to the state of Bose-Einstein condensate. The Bose – Einstein condensate of photons was first obtained by M. Weitz and his colleagues in 2010. Source: Science 366 894 (2019)

Evolution of a topological knot

Different types of topological defects are being investigated both theoretically and experimentally in different fields of physics. In Bose – Einstein condensate of nonzero-spin particles, topological defects must be quite diversified owing to a great many possible ways of symmetry violation. The observation of a knot type topological defect in Bose-Einstein condensate of spin 1 particles was already reported in 2016 in the paper by D.S. Hall (the Amherst College, USA) et al. T. Ollikainen (Aalto University, Finland) with colleagues performed a new experiment investigating knot evolution in Bose – Einstein condensate of spin 1 87Rb atoms in an optical dipole trap. A topological knot (interlocked magnetization rings) was created using a quadrupole magnetic field. After the knot formation, the magnetic field in the trap was switched over to a uniform one, and the knot began to decay. After its decay, a configuration was formed with the fraction of ferromagnetic phase increasing from the non-magnetic center to the periphery, and then a spin vortex with a polar core appeared and remained stable during several seconds. The transformation of topological defects to vortices is possibly a fairly universal process. Source: Phys. Rev. Lett. 123 163003 (2019)

Atomic gravimeter with trapped atoms

Atomic interferometers found wide application in gravimetry and in fundamental studies, but their precision is limited to a small time of free fall of atoms and the noise effect. Researchers from the University of California, Berkley (USA) demonstrated a new type of atomic gravimeter. Its main distinction is atom confinement in an optical lattice at the upper point of trajectory. Cesium atoms cooled to ≈ 300 nK were transported by laser pulses to a quantum superposition of different trajectories of motion. The optical lattice created by laser beams were switched on when the atoms were at the top of the trajectories, and the phase differences were measured after the lattice potential adiabatic shutdown. The atoms held in the lattice during ≈ 20 s allowed the gravimeter to be quite compact since the whole path of the atoms in it was only 2 mm. This also allowed overcoming one of the main obstacles due to vibrations through reducing its effect by three to four orders of magnitude. The new gravimeters can be used in exploration of minerals and in investigation of gravitation theories. Source: Science 366 745 (2019)

Observation of Fano antiresonances by electron spectroscopy

Fano resonance is interference of different wave processes causing spectral line asymmetry. Fano resonance and antiresonance were observed earlier in a number of optical and some other experiments (see, e.g., Phys. Usp. 22 236 (1979) and Phys. Usp. 62 823 (2019)). K.C. Smith (the University of Washington, USA) and co-authors were the first to perform successful observations of Fano antiresonance using a scanning transmission electron microscope with aberration correction. Dimers consisting of a gold disc hundreds of nm in diameter and a gold rod 5µm long located at 50 nm from the disc edge were examined. The disc plasmon spectrum was ≥ 10 times wider than the rod spectrum, and their electromagnetic coupling was weak. These conditions are precisely what is necessary for the occurrence of Fano antiresonance. The measured dimer electron spectra show the presence of Fano antiresonance and are well consistent with the theoretical model elaborated by the authors of the experiment. As compared with the usual description of Fano resonance, this model allows also for the dissipative effects. One of the factors making this experimentsuccessful was the application of new generation of monochromators upgrading electron spectroscopy. Source: Phys. Rev. Lett. 123 177401 (2019)

Data consistency in cosmology

In recent years, cosmology has generally come to a self-consistent picture when all theobservational data well correspond to each other. This has led to the confirmation of the standard flat ΛCDM cosmological model. However, discordances at the level of 6 % in the Hubble constant values obtained from different data have recently been noticed. These discordances possibly result from some unaccounted measurement errors. One more disagreement was revealed in the Planck observations of relic radiation (Planck Legacy 2018 data set). The lensing amplitude was found to be enhanced compared to that predicted in the ΛCDM model. The Planck Legacy 2018 data give some preference to the closed ΛCDM+ΩK model with a positive spatial curvature. However, the assumption concerning a closed Universe leads to greater discordances in other data sets. This was reported in the paper by E.Di Valentino, A. Melchiorri and J. Silk. They have shown that although the positive curvature accounts for the anomalous lensing amplitude, the agreement of other data sets (parameters of baryonic acoustic oscillations etc.) referring mostly to the relatively close Universe region z < 3 is violated. To clarify the situation, further studies are needed. If the indicated discrepancies, most of which are of order 3 σ, are not due to statistical fluctuation or measurement errors, they may testify to new interesting physical effects in cosmology. Source: Nature Astronomy, Online publication of November 4, 2019

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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.

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