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Extracts from the Internet | ||
Radiative β-decay of free neutrons1 July 2016In addition to proton, electron and antineutrino, photons can be emitted in rare cases under β-decay of the free neutron. The main mechanism of their production is bremsstrahlung of an electron, while the contribution of the other processes does not overstep 1%. The radiative decay n → p+e-+(anti)νe+γ was first registered reliably in the 2006 RDK I experiment. At the National Institute of Standards and Technologies NIST (USA), a new experiment RDK II was performed in which the branching ratio of neutron radiative decays was measured to the highest accuracy, and the energy spectrum of newly born photons was measured for the first time. Progress was achieved through an increase in the statistical data volume and refinement of the measurement methods. A neutrino beam from the nuclear reactor was transmitted through superconducting electromagnets where p and e- produced in the decays declined to charge-sensitive detectors while the photons flew straight and were registered by two photon detectors using the method of temporal coincidence with charged particles. One array of avalanche photodiodes registered photons directly (in the energy range of 0.4 to 14 keV) and the other one by the scintillator luminescence (14.1-782 keV) excited by these photons. In both cases the measured shape of the photon spectrum and the probability of their emission agree well with the predictions of the Standard Model. The results of this experiment are important for verification of the Standard Model and the search for effects beyond it. Source: Phys. Rev. Lett. 116 242501 (2016) Squeezed quantum states in atomic clock1 July 2016Precision of atom interferometers is restricted by the noises associated with the quantum-mechanical uncertainty principle. I. Kruze (Institute of Quantum Optics, Leibniz University, Germany) et al. proposed and demonstrated experimentally a new configuration of atom interferometer in which the standard vacuum limit is overcome by vacuum squeezing in the empty interferometer input channel. The interferometer operates by the Ramsey principle in the regime of atomic clock. At first 104 87Rb atoms in the state of Bose-Einstein condensate in an optical dipole trap were transferred to certain electronic states through exposure to a sequence of microwave pulses. Given this, one of the interferometer channels was cleaned so that on the average 0.75 atoms remained in it. The microwave pulses squeezed the vacuum in this channel, that is, reduced the corresponding rms value of the quadrature below the standard quantum limit. This improved the quantum state purity of 104 atoms located in other channels and heightened the interferometer sensitivity by 2 dB. This quantum state squeezing method may help in creating the next generation of atomic clock. Source: arXiv:1605.07754 [quant-ph] “Schrödinger’s cat” in two boxes1 July 2016C. Wang (The University of Yale, USA) with colleagues performed an experiment in which an electromagnetic analogue of “Schrödinger’s cat” was simultaneously located in two boxes in a superposition of “alive-” and “dead-cat” states. Such kind of experiment was proposed theoretically in 1993. It had already been shown before that the role of “Schrödinger’s cat” can be played by a harmonic oscillator in superposition of two vibrational states with large occupation numbers. The experiment described was carried out with coherent microwave photons in quasi-classical states excited in two superconducting cavities joint by a tunneling contact. Several tens of such photons were simultaneously present in the cavities. The “cat’s life” states corresponded to identical directions of electromagnetic oscillations simultaneously in two cavities. The states were measured and the Wigner functions reconstructed by the quantum tomography method. It was shown that realized in the experiment was actually a “Schrödinger’s cat” which simultaneously lived in two boxes (in two cavities in this case). Source: Science 352 1087 (2016) Tunneling time in strong field ionization1 July 2016It is a known fact that quantum tunneling is not instantaneous but takes some time. However, the time delay involves the problem of definition of quantities and their relation with the observed characteristics. The tunneling time can only be well determined in the Bohm approach, where it is simply the time during which the particle trajectory lies inside the potential barrier. T. Zimmermann (Swiss Federal Institute of Technology, Zurich) et al. calculated the tunneling time for the case of hydrogen and helium ionization in a strong field. The ionization theory formulated by L.V. Keldysh in 1964 (see, e.g., the reviews in Phys. Usp. 47 855 (2004) è Phys. Usp. 58 3 (2015))) was used in various tunneling models and the results obtained were compared with the available experimental data. The best agreement of time delay measurements taken on attosecond time scales was obtained with Larmor determination of the tunneling time, whereas the Bohm approach showed poor agreement with experiment. The authors of the paper believe that the Bohm time corresponds not to the tunneling time but rather to the lifetime of the bound state. Source: Phys. Rev. Lett. 116 233603 (2016) Supermassive black holes in early Universe1 July 2016F. Pacucci with colleagues from Italy and Japan discovered two remote objects which may appear to be supermassive black holes (BH) due to a direct collapse of gas clouds, i.e., to gas cooling and compression without its fragmentation and star formation. The mechanism of supermassive BH origination on the red shifts z≈6, where they are sources of quasar radiation, has not yet been completely clarified. This can be helped by a search for BH at the stage of their formation and early growth due to matter accretion. Young BH are very likely to reside in gas-dust clouds so that prevalent in their spectrum is IR radiation. The authors of the paper constructed the accretion model and predicted the spectrum shape and X-ray luminosity of such objects. The joint review CANDELS/GOODS-S containing the Hubble, Spitzer and Chandra data reported on two such objects – candidates with z>6. If their spectrum is assumed to be formed by young stars then the star formation time seems to be unrealistic (>2000M☉year-1). Therefore, the most probable for these objects is the model of accreting BH with masses ≈105M☉ produced in the course of direct collapse. Sourse: MNRAS 459 1432 (2016) |
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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