Extracts from the Internet


Bose – Einstein condensation of photons

J. Klaers and his colleagues at the University of Bonn produced the first time Bose – Einstein condensate of blackbody radiation photons. An original technique was used to solve the problems caused by absorption of photons by vessel walls during refrigiration and by the need to achieve necessary concentrations. A short (about d = 3.5λ = 1.46 µm high) cylindrical resonator with spherical mirror walls (radius of curvature ≈ 1 m) was used. The photon wavelength along the cavity axis was limited, so the dispersion relation for the transverse wave numbers was a formally correct description of a two-dimensional gas of massive particles. The resonator was filled with a solution of a pigment whose molecules absorbed and re-emitted photons, thus maintaining the photon gas at room temperature. In order to compensate for losses and increase the concentration of photons, pigment molecules were excited by laser light and emitted additional photons. The transition to the state of Bose – Einstein condensate was identified by a characteristic spectrum; it would appear at a theoretically expected photon concentration. A bright spot was observed at the center of the cavity, which corresponded to condensate of photons from the transverse TEM00 mode. Owing to the non-coherent interaction between dye molecules and photons, photons in this experiment acted as individual particles, in contrast to photons as components of polaritons whose Bose – Einstein condensate was obtained earlier (see Phys. Usp. 49 1221 (2006)). Bose – Einstein condensate of photons may find application for developing unique sources of radiation, such as UV-and X-lasers. Source: Nature 468 545 (2010)

Testing Leggett's inequalities for the orbital angular momentum of photons

The violation of Bell inequalities established in many experiments proved the absence of local “hidden parameters” in quantum mechanics. However, in 2003 A.J. Leggett (University of Illinois) suggested a non-local interpretation which preserves the Bell inequalities but whose quantitative predictions differ from those of standard quantum theory. A.J. Leggett derived new inequalities corresponding to his version of the theory, and experiments based on measuring the polarization states of photons soon proved that they are violated. S. Franke-Arnold (University of Glasgow, UK) and her colleagues tested Leggett's inequalities in a new situation — for orbital (relative to the beam axis) angular momenta of photons. Photon pairs obtained by splitting photons of laser light were investigated in a nonlinear optical crystal. Measurements found that Leggett's inequalities are violated in a wide range of angles, i.e. nonlocal “hidden variables” cannot explain the effect of entanglement of quantum states. We have just seen a new confirmation of the standard computation scheme of quantum mechanics. Source: New J. Phys. 12 123007 (2010)

Cooper pairs in the triplet state

As a rule Cooper pairs of electrons in superconductors are in the singlet state, i.e. electrons in a pair have opposite spins. K.B. Efetov (Ruhr Universitat Bochum, Germany; L.D. Landau TPI, RAS, Russia) made a theoretical prediction that Cooper pairs can also be in the triplet state with the same direction of electron spins. D. Sprungmann (Ruhr-Universitat Bochum, Germany) and his colleagues from Germany and the U.S. prepared and investigated such Cooper pairs in Josephson junctions in which two superconductors were separated by a 5 to 15 nm thick layer of ferromagnetic compound Cu2MnAl. They measured the critical current jc(d) through a contact, using a range of Cu2MnAl specimens of different thickness d. The current first behaves as it does in the case of Cooper pairs in the singlet state, i.e. it decreased exponentially with thickness, but then a plateau appeared on the function jc(d) in the range of d = 7.5-10.5 nm. This pointed to the existence of a spin-active zone in the ferromagnetic with d > 7.5 nm (transition from the spin glass state to strong ferromagnetism) in which Cooper pairs were converted to the triplet state and then easily penetrated the barrier. The possibility of a spin-active layer was predicted theoretically and some indications of the appearance of Cooper pairs in the triplet state were obtained earlier in experiments with CrO2 and Íî tunnel junctions. Source: Phys. Rev. B 82 060505(R) (2010)

Asymmetric decay of mercury-180 nuclei

An experiment on the ISOLDE mass separator (CERN) observed the effect of asymmetric nuclear decay of 180Hg nuclei which were produced in the delayed fission of neutron-deficient 180Tl nuclei. The decay of 180Hg to 100Ru and 80Kr (and their neighboring nuclei) came as a surprise since fission fragments in this case are not magic nuclei with filled shells. What appears strange is that symmetric decay would produce 90Zr nuclei which are magic in the number of neutrons and semimagic in the number of protons, so the process of decay of 180Hg to equal-mass fragments was considered dominant. The theoretical model of P. Ěoller (Los Alamos National Laboratory, USA) which takes into account the stability not only of the end products of decomposition but also of the intermediate states of decaying nuclei provides a possible explanation. In this model the asymmetric decay of 180Hg nuclei is favored energy-wise. The CERN experiment was carried out by a team including researchers from the Petersburg B.P. Konstantinov Nuclear Physics Institute. Source: Phys. Rev. Lett. 105 252502 (2010)

Gravitational lensing of gamma radiation

Researchers at the DSM/IRFU (France) and N. Copernicus Astronomical center (Poland) detected for the first time gravitational lensing of gamma radiation. The multiple image of the blazar PKS 1830-21 at the red shift z = 2.507 was formed by a galactic lens with z = 0.89. Since the spatial resolution of the space gamma telescope Fermi LAT was insufficient for observing individual images, the method used was that of searching for time correlations of time-shifted variable signals propagating along two different paths. The Fourier transform of the main and lensed signals from the PKS 1830-211 source must carry a component at a frequency corresponding to the signal delay time. This component was indeed observed and corresponds to time shift of 27.5 ± 1.3  days, which is consistent with the estimate of 26+4-5 days obtained earlier in direct observations of gravitational lensing of PKS 1830-211 in the radio band. The study of gamma-lensing of PKS 1830-211 is a successful test of the new method which can later be applied to other objects as well. Source: arXiv:1011.4498v1 [astro-ph.HE]

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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.

It is compiled from a multitude of Internet sources.

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