Extracts from the Internet

Limits on spin-mass interactions

Some theories include interactions between mass and intrinsic angular momentum (spin) of objects transported by axions or axion-like particles. A hypothetic axion field was initially proposed to explain the absence of CP violation in strong interactions, while its quanta, i.e., axions can be dark matter particles. The researchers from Princeton University (USA) J. Lee, A. Almasi, and M. Romalis conducted a search for interactions between mass, which consisted of 250 kg of lead bricks, and an ensemble of spin-polarized neutrons and electrons in a nearby co-magnetometer. The neutron spins in 3He nuclei and the electron spins in potassium nuclei gained certain polarization through collisions with 87Rb atoms which had been polarized by laser light. The load was set into vibration, and the corresponding periodic signal was sought in rotation of the polarization plane of light transmitted through the co-magnetometer. The spin-mass interactions might change the spin states, which would have affected the signal. No such interaction was noticed with the available precision, and new limits on the coupling constants were obtained in the range of the axion-like particle masses of <10−6 eV.In the case of neutrons, these limits are an order of magnitude better than those shown in the previous laboratory experiments. Source: Phys. Rev. Lett. 120 161801 (2018)

Nuclear fission reactions in a nanowire array

A. Curtis (the University of Colorado, USA) et al. investigated reactions of nuclear fusion in hot plasma produced in the action of laser pulses on a nanowire array. A titanium-sapphire laser generated pulses 60 fs long with energy up to 1.65 J. These pulses irradiated a beam of deuterated polyethylene (CD2) nanowires 5 µm long and 0.2 to 0.4 µm in diameter. The mean density of this medium was 16-19 % of the density of matter of the nanowires themselves. Owing to the use of such an array, the laser radiation could penetrate the intervals between the nanowires, and their heating was more effective than would be possible upon pulse action on a solid surface. A quick heating induced a nanowire explosion. In the plasma produced, deuterons were accelerated up to energies of several MeV and underwent thermonuclear reactions. Neutron detectors registered neutrons with energies of 2.45 MeV typical of these reactions. The neutron yield efficiency (2×106 per J) was record for lasers with pulse energy of ≈ 1 J. It is 500 times as high as that upon irradiation of a flat CD2 surface and an order of magnitude higher than in experiments with deuterium clusters. In experiments with inertial target confinement, whose aim is thermonuclear energetics, the neutron yield per J is much larger, but it needs a laser pulse energy of ≈ MJ. And nanowires can make the basis of compact neutron sources for the study of materials. Source: Nature Communications 9 1077 (2018)

Einstein – Podolsky – Rosen steering in a Bose – Einstein Condensate

In 1935, E. Schrodinger introduced the concept of Einstein – Podolsky – Rosen (EPR) steering in which the results of measuring of one part of a quantum entangled system alter the state of the other distant part and affect the results of its measurements. This steering has already been demonstrated for only pairs of particles, whereas for systems with a larger number of particlesit has not hitherto been observed. This was attained for the first time by the group of researchers from the University of Basel (Switzerland) headed by P. Treutlein. A cloud of ≈ 600 87Rb atoms was confined in a magnetic trap and was transferred to the state of Bose – Einstein condensate. After the trap potential was switched off the cloud of atoms fell freely and expanded, and the collective spin states of different spatial parts of the condensate were registered from the resonance light absorption. The quantum inequalities were verified which confirmed both quantum entanglement of the condensate parts and the Einstein – Podolsky – Rosen steering. The experimental methods and the effect observed may appear to be useful for the design of new quantum sensors. Source: Science 360 409 (2018)

Triple quantum correlations

It can be assumed thatin quantum measurements with three and more possible outcomes, the quantum process of measuring is always so organized inherently that an alternative is only chosen between some two outcomes and thus all the outcomes are turned over. So it is hypothesized that only binary quantum correlations exist. X.-M. Hu (the University of Science and Technology of China) with colleagues reported the first direct experimental proof of the existence of correlations stronger than binary. Pairs of quantum entangled photons were produced each of which could propagate in two different ways. In one way the photon could have one and in the other way two polarization states, so that each of the photons was in superposition of three quantum states and was a qutrit (by analogy with qubit). The photons were sent to two laboratories where randomly chosen measurements of photon states were carried out with three possible results. The measuring technique allowed verification of the binary correlation hypothesis formulated as inequalities analogous to Bell inequalities. These inequalities were violated at the level of 9.3σ, which excludes the presence of only binary correlations and testifies to triple correlations in the course of measurements. Source: Phys. Rev. Lett. 120 180402 (2018)

Single photons emitted by a pair of quantum entangled atoms

R. Blatt (the University of Innsbruck and the Institute for Quantum optics and Quantum Information, Austria) with colleagues demonstrated for the first time a single-photon emission by simultaneously two quantum entangled atoms. The atoms were spatially separated, but jointly emitted single photons in free space. This was possible because the atoms were connected with a common optical mode created by the interferometer arms. Two 138Ba+ ions were trapped into a linear Paul trap and with the help of laser pulses were transferred to a certain superposition of excited states so that the atoms appeared to be quantum entangled. Their emission upon transition to lower states was registered. Single-photon emission was registered from the characteristic interference pattern, the indistinguishability of the emitters reaching the level of 0.99 ± 0.06. The interference pattern is fairly sensitive to the degree of entanglement and to conditions near the atoms. Its distortions allowed an experimental measurement of the magnetic field gradient. Thus, this effect may turn out to be useful for the design of sensitive magnetometers. Source: Phys. Rev. Lett. 120 193603 (2018)

Rhenium superconductivity in multilayer structures

D.P. Pappas (the National Institute of Standards and Technology, USA) et al. examined superconductivity of the transition metal rhenium Re in multilayer metal structures. Electrodeposition was used to place the Re films inside multilayer structures of Cu, Au, and Pd films. The electrical resistance and magnetic susceptibility point to the critical temperature near Tc=6° K when the Re layer was bilaterally covered by Cu and Au layers. Analogous structures with Pd layers showed a somewhat lower Tc. For comparison, a three-dimensional Re crystal demonstrated the maximal Tc=3° K. Also revealed was a low loss level at radio frequencies for multilayers with Re deposited on the resonators. Superconducting films with Re and Cu or Au can be integrated into standard electron components and find practical application. Source: Appl. Phys. Lett. 112 182601 (2018)

Quantum spin ice in Pr2Hf2O7

In interacting spin systems, macroscopic correlated states are possible referred to as quantum spin liquids. Some evidence of their appearance has already been obtained experimentally, but no stable quantum liquids have yet been observed in three-dimensional systems. R. Sibille (Paul Scherrer Institute, Switzerland) with colleagues used the inelastic neutron scattering method to study Pr2Hf2O7 crystals and found that at a temperature below 0.05° K they contain a quantum spin liquid in the form of so-called spin ice. Small dipole moments in Pr2Hf2O7 exclude the possibility that this state is a classical dipolar spin ice. An interesting feature of quantum spin ice predicted theoretically is the appearance in it of emergent electrodynamics in which the role of potentials and fields satisfying the Maxwell equations is played by spin correlations and quasiparticles. In particular, magnetic monopoles are possible in this “electrodynamics”. The experiment of R. Sibille et al. has shown the presence of a half-integral excitation continuum in PrPr2Hf2O7, which testifies to the existence of emergent electrodynamics. Source: Nature Physics, online publication of April 30, 2018

Radio wave propagation in ice

The IceCube detector located in the Antarctic ice on the South Poledetected cosmic neutrinos with energies up to PeV. They were registered from optical Vavilov – Cherenkov radiation of the products of neutrino interactions. It is predicted that at higher energies the most sensitive method of neutrino registering will be the Askaryan effect, i.e., the coherent Vavilov-Cherenkov radiation in the radio-frequency range. In this connection, the radio-wave propagation in the Antarctic ice is being studied. The models of variation of ice permittivity n with depth were constructed with allowance for the ice compression and density variation. The n variability must lead to the fact that the radio ray, which initially propagated at a shallow depth along the ice surface, bends downward to form a zone of shadow or a “forbidden” zone that the ray fails to reach. S.W. Barwick (the University of California, Irvine, USA) with colleagues measured radio wave propagation in the Antarctic ice to obtain an unexpected result: the signal was also registered in “forbidden” zones. The mechanism of this effect remains unclear. It may be due to unaccounted microscopic radio wave scattering. This effect is of importance because the position of signal receivers at a rather shallow depth in the shadowed region can noticeably increase the efficiency of registration of radio waves from neutrinos. For Vavilov – Cherenkov radiation and the Askaryan effect see the paper by B.M. Bolotovsky in Phys. Usp. 52 1099 (2009). Source: arXiv:1804.10430 [astro-ph.IM]

Gravitational waves and the equation of state of the neutron star matter

On October 16, 2017 LIGO/Virgo detectors registered the gravitational wave burst GW170817 that was due to the neutron star merge. This gave new limits on the tidal deformability of neutron stars involved in the collision.The tidal deformability influences the approaching of objects at the last stages before they merge. As a result, in the case of neutron star merging, the form of gravitational-wave signal somewhat differs from that observed upon the merge of two black holes. Investigating the tidal deformability, one can obtain data on the equation of state of the neutron star matter. E. Annala (the University of Helsinki, Finland) with colleagues analyzed the GW170817 signal and derived the family of equations of state that agree with the form of GW170817signal and with other known data. The maximal radius R of a 1.4M neutron star was found not to exceed 13.6 km. Another group of scientists from Indiana University and the University of Florida (USA) carried out a similar analysis to obtain the constraint R(1.4M)<13.76 km, which is consistent with the results of the first group. The properties of nuclear matter inside a neutron star have not yet been exactly calculated theoretically, and therefore the data on neutron star merging are important for the clarification. In particular, the researchers try to find out whether the so-called quark matter is present inside neutron stars. Source: Phys. Rev. Lett. 120 172702 (2018), Phys. Rev. Lett. 120 172703 (2018)

Mercury perihelion shift

In 1915, A. Einstein appealed to the General Relativity Theory to explain why the elliptic Mercury’s orbit rotates quicker by 43 arcseconds per century than was predicted by the Newton theory. The new theoretical calculations by C. Will (the University of Florida, USA, and Pierre and Marie Curie University, France) showed the existence of two earlier unaccounted corrections equal to several millionth fractions of the main contribution of 43” per century. The first correction comes from the relativistic influence of other planets on the Mercury acceleration, and the second is associated with the interaction of the Mercury velocity and the gravimagnetic potential due to distant planets. The found additional contributions will possibly be discovered by the European-Japanese Mission BepiColombo, in the framework of which two Mercury satellites are planned to be launched at the end of 2018. BepiColombo will be able to reveal for the first time the relativistic effect associated with the influence on the Mercury motion of not only the Sun, but also of other planets, which is of importance for verification of the General Relativity and other gravitational theories. For the calculation of small corrections in the General Relativity see the book by C. Will Theory and Experiment in Gravitational Physics (Physics Today, 1982; Cambridge University Press, 1993) [in Russian: Teoriyaieksperiment v gravitatsionnoy fizike. M.: Energoatomizdat, 1985] and also C. Will’s paper in Phys. Usp. 37 697 (1994). Source: Phys. Rev. Lett. 120 191101 (2018)

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