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Bose – Einstein condensation of photons
1 January 2011
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
1 January 2011
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
1 January 2011
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
1 January 2011
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. Ìoller (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
1 January 2011
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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