# Extracts from the Internet

## Neutrino oscillations: νe appearance in the νμ beam

The T2K (Tokai to Kamioka) experiment recorded electron neutrinos emerging in the beam of muon neutrinos. This is an indication of the reality of neutrino oscillations νμ → νe. Six such candidate events have been identified, with the expected number of background events at the level 1.5 ± 0.3. By the T2K data, the mixing angle $\theta_{13}$ (one of the parameters characterizing oscillations) is not zero, namely, 0.03(0.04) < sin2(2θ13) < 0.28(0.32) for the direct (inverse) hierarchy of neutrino masses. The beam νμ was produced at the accelerator complex J-PARC in Tokai (Japan). One of the two detectors recorded the neutrinos at the base of the beam while the second detector which detected the appearance of νe in the beam was the SuperKamiokande 22.5-kiloton Cherenkov detector located at a distance of 295 km from the accelerator. Russian scientists from the RAS Institute for Nuclear Research participate in the T2K experiment. Electron neutrinos νe were also indentified in the MINOS experiment conducted on the territory of the USA, in the beam of νμ at confidence level 2.7 σ. By the MINOS data, the mixing angle was bounded from above by sin2(2θ13)<0.12, and these values are consistent with those of the T2K. For details of the long-base accelerator experiments see a review by Yu.G. Kudenko in the Phys. Usp. Issue 8 (2011). Sources: Phys. Rev. Lett. 107 041801 (2011), www.kek.jp

## Cooling to the quantum ground state

J.D. Teufel at the National Institute of Standards and Technology (NIST, USA) and his colleagues cooled a microscopic membrane to energy below one quantum of its mechanical vibrations. An aluminum membrane consisting of about ≈ 1012 atoms was placed in a superconducting microwave resonant circuit which created strong coupling of mechanical vibrations to the electromagnetic field. After preliminary cooling with liquid helium to a temperature T = 20 mK the membrane contained ≈ 30 phonons, i.e. quanta of mechanical vibrations. The resonant mechanical frequency of the membrane Ωm ≈ 10 MHz was slightly lower than the electromagnetic frequency of the resonator and for this reason microwave photons were mostly carrying energy away from the membrane; as a result, it was possible to cool it ultimately down to T = 400 K. This method of cooling is known as the sideband cooling. An analysis of the spectra of photons emitted by the resonator established that on average the cooled membrane contained 0.34 ± 0.05 phonons, i.e. the condition kBT < ћΩm was satisfied. Owing to the strong concentration of the electromagnetic field close to the mechanical system, it proved possible in this experiment to achieve 104 times longer duration of the membrane staying in the quantum ground state than in the earlier experiment carried out in the University of California in 2010 (where a different method was used). Source: Nature 475 359 (2011)

## Quantum superposition of frequency states of the photon

E. Zakka-Bajjani (NIST) and her colleagues have obtained for the first time photons of radio emission in a quantum superposition of two states corresponding to different frequencies, or, by analogy to optics, in states of different colors. They used two harmonics of a quarter-wavelength waveguide shorted by a superconducting SQUID whose state fixed the boundary conditions at the end of the waveguide. They first prepared single microwave photons in the Fock state, which were then coherently distributed between the two states having different frequencies. A superposition of these states was created by parametric frequency down-conversion: the SQUID induction was modulated at a frequency of about 7-12 GHz, equal to the difference between the frequencies of the harmonics. The photons in this experiment could have different contributions from the two frequency states, e.g. in ratios of 1:1 or 1:3. In some sense this system resembles a pair of coupled harmonic oscillators for which the superposition of frequency states has previously been realized for ions and for a number of other quantum systems. Nature Physics (2011), to be printed.

## Fluorescence in clusters of nanoparticles

It is a known fact that fluorescence emission of semiconductor nanoparticles is intermittent: dark and light periods last from microseconds to hours, and their distribution follows the Levy statistics. It is assumed that the emission “turns off” when one of the charges of the excited exciton is trapped into the surface layer or escapes from the nanoparticle. In contrast to spherical particles, elongated nanorods emit for a shorter time since charges in them are bound less strongly. S. Wang (University of Pennsylvania, USA) and her colleagues discovered that if nanorods are collected into a compact cluster, then the emission from nanoparticles becomes more stable: duration of bright periods of individual rods increases roughly in proportion to the number of nanoparticles in the cluster N; in this experiment it was N = 2-110. Cadmium selenide nanorods of length ≈ 5 nm were deposited on a substrate and illuminated by a blue laser. Their red fluorescence was video recorded through an optical microscope, after which the lengths of dark and light intervals were measured and recorded. The arrangement of nanorods in clusters was mapped as a preliminary procedure using an electron microscope. One likely explanation of the dependence of the characteristics of the observed emission on N is the interaction between electrons in different nanorods. This study can help in creating fluorescent biomarkers for studying processes inside cells because the intermittency of emission from the nanoparticles is now a serious impediment to using this method. Sources: Nature Communications 2 364 (2011), www.sciencedaily.com

## Gravitational lensing of the cosmic microwave background

The effect of gravitational lensing of cosmic microwave background (CMB) has been measured at high accuracy by means of the Atacama Cosmology Telescope (6-meter radiotelescope) located in Chile at the altitude of 5200 m above sea level. The measurements were made at a frequency of 148 GHz, and observations covered the area on the sky of 324 sq. deg in total. On average, cosmic background photons deviated by three angular seconds so on smaller angular scales the response was somewhat smoothed. The biggest contribution to the lensing effect originated with matter density inhomogeneities at redshift z ≈ 2 with characteristic scales of ≈ 300 Mps. The effect of gravitational lensing was detected at the 4 σ level as a non-Gaussian contribution to the four-point correlation function of CMB fluctuations. The background Gaussian component was calculated by randomization of the phases of the measured signal at each point. Weak indications to gravitational lensing of the CMB have previously been obtained by observing cross-correlations of the WMAP and galaxy data and by measuring attenuation of acoustic peaks. The combination of the Wilkinson Probe polarization data and the Atacama Cosmology Telescope lensing data made it possible to overcome the well-known geometric degeneracy and come up with an independent confirmation based exclusively on CMB data, of the existence in the Universe of dark energy with the equation of state p ≈ - ρ. Sources: Phys. Rev. Lett. 107 021301 (2011), Phys. Rev. Lett. 107 021302 (2011)

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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