Extracts from the Internet

New results of the D0 experiment

Doubly strange baryon. A new elementary particle — the Ω_b^- baryon of the quark composition ssb, about 6 GeV mass and lifetime of 1.54 ps — was discovered in the D0 experiment on the Tevatron accelerator of the Enrico Fermi Laboratory. The Ω_b^- particles were created in proton-antiproton collisions at the center-of-mass energy 1.96 TeV and identified by their decay products. All in all about 18 events of Ω_b^- creation were observed (at the 5.4σ confidence level). Source: http://arxiv.org/abs/0808.4142

Single t-quark. The D0 detector has reliably recorded the creation of t-quarks without simultaneously creating t antiquarks in the same reactions. Such processes are possible in electroweak interactions, in contrast to strong interactions which always produce t and anti-t in pairs. The detector selected hadron jets and the initial composition of the created particles was reconstructed from the structure of the jets. The measured cross section of creation of single t quarks coincides within 10% with the value predicted by the Standard model. D0 experiments are conducted by an international team that includes a group of Russian scientists. Source: Phys. Rev. D 78 012005 (2008)

Superconductivity of single crystals

High-temperature superconductors based on iron (see Phys. Usp. 51 425 (2008) as well as review papers by Yu.A. Izyumov and A.L. Ivanovskii in Phys. Usp. December (2008)) and also non-superconducting compounds of very similar structure recently became very attractive to researchers. Previous experiments studied only polycrystalline specimens of this type with crystalline granules reaching the size of at most 300 µm. Р.С. Canfield and his colleagues at the Iowa State University in USA developed a method of growing the compounds BaFe₂As₂, SrFe₂As₂, CaFe₂As₂ and (Ba_0,55K_0,45)Fe₂As₂ as single crystals as large as approximately 3×3×0,2 mm³ from a solution in liquid tin. Nearly 1% of the composition of the resulting specimens was atoms of Sn incorporated into the crystal lattice. The molecular structure of these crystals, their electric and magnetic properties were measured in detail. BaFe₂As₂ undergoes a structural phase transition from tetragonal to rhombic crystalline phase at about 85 K (SrFe₂As₂ — at 198 K and CaFe₂As₂ — at 170 K). A similar transition in polycrystalline specimens of BaFe₂As₂ occurred at a temperature of about 140 K. No transition in BaFe₂As₂ and SrFe₂As₂ to superconducting state was observed until at least 1.8 K. In contrast to this, the compound (Ba_0,55K_0,45)Fe₂As₂ showed no such phase transition but became superconducting at about 30 K. Anisotropy of superconducting properties and their dependence on the external magnetic field were studied; among other characteristics, the critical field was measured that destroys superconductivity. Critical fields along different axes of the crystal differed by a factor of 2.5-3.5 (depends on the temperature). It was also shown that CaFe₂As₂ becomes superconducting when subjected to pressure of 5 kbar, with the temperature of superconducting transition T_c ≈ 12 К. Sources: Phys. Rev. B 78 014507 (2008), Phys. Rev. B 78 024516 (2008), Phys. Rev. B 78 014523 (2008), Phys. Rev. Lett. 101 057006 (2008)

Modulation of a single photon

S. Harris and his colleagues at Stanford University succeeded in modulating an electromagnetic pulse corresponding to a single photon. Even though manipulations with single photons were practiced in a considerable number of experiments, it is for the first time that a method was worked out which imposes a prescribed shape in amplitude and phase on the wave function of a photon. The main difficulty for modulation, caused by the short pulse duration, was overcome by slowing down photons in the gas of rubidium atoms (by a factor of several thousand); this correspondingly lengthened the pulse length to several hundred nanoseconds. The method used was the splitting of photons in a nonlinear medium. The modulated photon was created in a correlated pair with a second photon whose detection was the signal that turned on the electrooptical modulator. It proved possible to create wave functions with two maximums, as well as a Gaussian and an exponential profiles. This method can be used in studying the interaction of atoms with single-photon signals of prescribed shape and also in quantum communication and quantum computation. Source: Phys. Rev. Lett. 101 103601 (2008)

Quantum repeater

Losses and decoherence in communication lines constitute a serious obstacle for implementing quantum communications. Amplification of quantum information by analogy to amplification of ordinary classical signals is impossible because this amplification involves a destructive quantum measurement. To solve this problem, H.-J. Briegel, L.-M. Duan and their colleagues suggested the concept of “quantum repeater”; its principal idea is to transfer the entanglement of a quantum state from some particles to other ones within individual segments of the data transfer channel. Z.-S. Yuan and his colleagues in Germany, China and Austria carried out an experiment in which they implemented one of the main principles of quantum repeater — entanglement transfer on a single segment. For an element of quantum memory they used an atomic ensemble as suggested by L.-M. Duan and some others. Primary photons were Raman-scattered by two gas clouds, giving rise to two pairs of correlated photons. Each cloud consisted of approximately 108 rubidium atoms cooled in a magnetooptic trap to a temperature of 100 µK. The spatial excitation modes in the clouds were quantum-correlated (entangled) with the state of polarization of the corresponding pair of photons. Then two photons, one from each pair, were made to meet, and joint measurement of their state was carried out. In this operation the gas clouds and the remaining two photons also got entangled with the primary photons. The length of the optical communications channel was 300 m; a real channel transmitting quantum information would have to consist of numerous similar segments. Source: Nature 454 1098 (2008)

Gas filaments around the NGC 1275 galaxy

Detailed observations of gaseous structures around a giant elliptic galaxy NGC 1275 were carried out using the Hubble space telescope. It was possible to distinguish, for instance, individual thin “filaments” in the filamentary distribution of gas ejected from the galactic core. The active galaxy NGC 1275 is situated at the center of the Perseus galactic cluster. The neighborhood of the central black hole ejects gas bubbles which interact with the hot intergalactic gas of the cluster (at a temperature of 4×10⁷ K) and forms filaments. The length of these gas filaments reaches 6 kpc, at filament thickness of only 70 pc. Filamentary structures were already observed in NGC 1275 earlier but it was unclear why the shape of gas filaments remained stable so they do not dissipate in the surrounding hot gas. The discovery of individual thin filaments is an indication that the observed gas configuration is maintained by the strong magnetic field in the NGC 1275 and around it. The filaments are stable as a result of the balance of gas pressure and magnetic field tension along the lines of force. Source: Nature 454 968 (2008)