Extracts from the Internet

Overcoming the standard quantum limit in measurements

In 1967, V.B. Braginskii derived the limit on the sensitivity of optomechanical measuring devices that is of quantum origin (JETP 26 831 (1968), see also the review by V.B. Braginskii and Yu.I. Vorontsov in Sov. Phys. Usp. 18 644 (1975)) [Phys.-Usp.] and is called a “standard quantum limit”. The limit is associated with the noises and with the backreaction effect of the device on the system. The laser interferometers LIGO/Virgo registering the gravitational waves from merging of black holes and neutron stars have closely approached this limit. (see Phys.-Usp. 43 691 (2000) and Phys.-Usp. 59 968 (2016). It was shown theoretically that the standard quantum limit can be overcome using quantum nondemolition measurements, but the earlier attempts have not been successful in optical mechanics. D. Mason (University of Copenhagen, Denmark) with colleagues was the first to overcome the standard quantum limit. In their experiment, the backreaction and noises were correlated and partially compensated each other under destructive interference. A lattice of holes – a photon crystal was made in a 3.6 mm × 3.6 mm × 20 nm membrane. The reflected laser light contained information about noises that (information) influenced the membrane position measurement by a synchronous detector. The sensitivity reached in the experiment was by 30 % higher than the standard quantum limit. Source: Nature Physics online publication of 27 May, 2019.

Quantum C-NOT gate teleportation between ion qubits

In quantum information devices, one of the methods of spatially separated qubit interaction is quantum teleportation of quantum gates implementing logical operations. There exists an efficient protocol of quantum “controlled NOT gate” (C-NOT) teleportation that had already been demonstrated for photon and superconducting qubits. Y. Wan (National Institute of Standards and Technology and University of Colorado, USA) with co-authors applied for the first time the protocol for the interaction of ion-based qubits. The protocol only contains local operations, interaction through the classical channel, and one pair of quantum entangled particles. The qubits were realized on the states of hyperthin splitting of 9Be+ ions in a Paul trap. Quantum tomography showed the precision of C-NOT gate teleportation between qubits equal to 0.845-0.872. Source: Science 364 875 (2019)

Soliton gas

Solitons are stable configurations of a medium moving almost without changing their profile. A multisoliton system can be represented as a gas of some particle-like objects. In 1971, V.E. Zakharov (Lebedev Physical Institute RAS, Landau Institute for Theoretical Physics RAS, and Novosibirsk State University) derived a kinetic equation for the gas of interacting solitons and in 2009 he formulated a theory describing solitons in the wave theory. N. Mordant (Grenoble Alps University, France) and his colleagues examined experimentally a one-dimensional soliton gas in water. A sinusoidal wave excited in a 34-m pool fell to solitons of different sizes. When reflected from the pool nosing and undergoing oncoming and accompanying collisions, the solitons formed a chaotic gas that was observed by video cameras. The stationary state of the soliton gas was in good agreement with theoretical calculations and with the results of numerical simulations conducted earlier by E. Pelinovskii and A. Sergeeva from IAP RAS and the NSTU. The study of soliton gas in water is a step towards understanding of the properties of such a gas in plasma and in nonlinear optics. For solitons and wave collapses see Phys.-Usp. 55 535 (2012). Source: Phys. Rev. Lett. 122 214502 (2019)

A cold gas at the Galactic center

The distribution of hydrogen with temperature T≈104 K around the supermassive black hole in the center of our Galaxy was investigated using the ALMA complex of radio telescopes in Chili. The radiation was registered on the wave of 1.3 mm generated under transitions n = 31 → 30. E. Murchikova (Princeton Institute for Advanced Study and California Institute of Technology, USA) with coauthors managed to single out the indicated recombination line, although its intensity is only 0.1 % from the radiation in continuum. The cold gas appeared to form an accretion disc around the black hole. The disc radius is 0.004 pc and its mass is (10-5-10-4)M. The measured Doppler gas velocities are much lower than the calculated Kepler velocities. This is indicative of a small slope angle of the disc rotation axis to the view axis or of a great role of pressure and/or magnetic fields in the disc dynamics. Source: Nature 570 83 (2019)

A search for non-Gaussianity of cosmological perturbations

According to many-year observations, the perturbations of the microwave background (relic) radiation filling the Universe obey the Gaussian statistical distribution law. Some small deviations from the Gaussian law are however not excluded. This would mean the presence in the early Universe of exotic objects such as cosmic strings, or else non-Gaussianity might be a consequence of more sophisticated inflation models or a new physics. The Planck results of the search for non-Gaussianity of perturbations over the whole period of observations are presented. In particular, the E-mode of radiation polarization sensitive to non-Gaussianity was examined in detail. Compared to the previous studies, the analysis included low multipoles of 4 ≤ l ≤ 40. At the state-of-the-art high precision level, no non-Gaussian contribution to the perturbations was found. This, in turn, imposes restrictions on the parameters of the processes that could in principle lead to non-Gaussianity. For the methods of the search for non-Gaussianity see Phys.-Usp. 55 1098 (2012) and for quantum fluctuations and relic radiation perturbations see Phys.-Usp. 59 1021 (2016). Source: arXiv:1905.05697 [astro-ph.CO]

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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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