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The average trajectory of photons in the two-slit experiment
1 July 2011
A. Steinberg (University of Toronto, Canada) and his colleagues have determined
by way of weak quantum measurements the average trajectories of photons in an
interference experiment similar to Young's two-slit experiment. An experiment
is referred to as weak if it yields only partial information about the quantum
state of the system in a way that does not destroy this state. For the light
source A. Steinberg et al used a quantum dot of low luminosity; only one photon
was in the beam at any moment of time. Photons propagated through the optical
splitter and entered two arms of the Brown – Twiss interferometer. A calcite crystal
placed in the path of the two beams generated phase shift and rotation of the
polarization vector by the amount dependent on the direction of the photon
momentum. Therefore selection of photons with a specific polarization allowed
conduction of weak quantum measurements of photons' transverse momenta at
different distances from the screen (the CCD matrix). After a large number of
measurements of information on the average momenta and coordinates of interactions
between photons and the screen, it was possible to calculate the set of average
photon trajectories. The concept of average photon trajectories was introduced
operationally; they are not real trajectories, and carry only a limited meaning
in quantum mechanics. Compressions and rarefactions of the bunch of average trajectories
was found to correspond to maxima and minima of the interference pattern on the screen; the
pattern was not destroyed by weak quantum measurements of pulses. The results of
the experiment fully comply with the standard Copenhagen interpretation of
quantum mechanics.
Source: Science 332 1170 (2011)
Dynamic Casimir effect
1 July 2011
An experiment has been carried out at the Chalmers University of Technology in
Gothenburg, Sweden in which C.M. Wilson and his colleagues observed for the first
time the dynamic Casimir effect which was theoretically predicted by G. Moore
in 1970. The dynamic Casimir effect, in contrast to the usual static Casimir
effect with two plates, occurs for a single mirror moving at relativistic speed.
It changes the spectral composition of zero-point quantum fluctuations in the
vicinity of the mirror, and the pairs of virtual particles arising from the
vacuum at sufficiently high velocity can separate and convert into real
particles at the expense of energy obtained from the mirror. Ñ.Ì. Wilson and his
colleagues built an aluminum waveguide approximately 100 micrometers long,
plugged by a superconducting contact — a SQUID. The SQUID's inductance was
modulated within 10 % by the microwave field frequency of ≈ 11 GHz. The modulation produced periodic changes in boundary conditions at the end
of the waveguide whose effective electric length correspondingly oscillated with
the amplitude of ≈ 1 nm. This produced an analog of a mirror oscillating at a
speed of about ≈ 5 % of the speed of light. Observations recorded emerging
microwave photons. This radiation was composed of two correlated modes with
different frequencies, which pointed to its quantum origin from split pairs of
virtual photons in agreement with the theoretical calculations for the dynamic
Casimir effect.
Source: arXiv:1105.4714v1 [quant-ph]
“Friction” between holes and electrons in a semiconductor
1 July 2011
Researchers from the University of California, Berkeley, and the Sandia and Lawrence
Berkeley National Laboratories investigated the motion of electron-hole
excitations in a single quantum wall. Unlike the case with non-interacting
electrons and holes, exchange of momentum between them reverses the direction of
flow of charge waves, namely, the flow of electrons drags holes in the direction
of its motion. A similar but weaker effect has already been observed earlier in
multilayer systems in which the interaction was between electrons and holes from
neighboring layers (parallel quantum walls). In this experiment the quantum wall
was an n-doped 9 nm wide GaAs/AlGaAs structure. The wave of electron-hole
density was excited by laser pulses and created variations of the refractive
index in two-dimensional gas. These variations were recorded by optical
measurements. These measurements by phase-resolved transient grating
spectroscopy yielded the values of the ambipolar diffusion coefficient and charge mobility,
and this data allowed calculation of the characteristic value of the interaction
(“friction”) between the fluxes of electrons and holes in charge waves.
Source: Phys. Rev. Lett. 106 247401 (2011)
Helical structures in a magnetized plasma
1 July 2011
Ì. Schwabe (Max-Planck-Institute for Extraterrestrial Physics, Germany) and his
colleagues found new ordered structures in the plasma in strong magnetic fields.
Plasma from neon, argon, krypton, and from the air was studied at low pressure
at room temperature. The plasma with ionization level 10-7-10-6 was
created by radio frequency discharge at the center of a superconducting solenoid
whose magnetic field (vertically oriented) was uniform to within 0.65 %. To
achieve visualization, microscopic dust particles (2.55 micrometers in diameter)
were injected into the plasma and illuminated by a horizontal laser beam; the
observations were conducted through a transparent top electrode and through the
side wall of the vessel. If the solenoid magnetic field was low, the plasma-dust
crystal filling the vessel rotated in almost solid-body fashion. As the field
grew above ≈ 1.3 T at pressures less than 20 Pa, plasma filaments emerged
parallel to the magnetic field and their common rotation separated into
individual vortices of about 1 mm radius around the axes of filaments. At the
same time, microscopic particles in the plasma formed patterns of spirals and
concentric circles in the transverse direction. No detailed theory of filament
rotation and pattern formation has been developed so far. The authors of the
experiment hypothesize that the phenomenon responsible for this effect is
probably the motion of positive ions. In contrast to electrons, they are not
trapped in the filaments and can travel through the vessel, are deflected by the
magnetic field and affect the motion of dust microparticles.
Source: Phys. Rev. Lett. 106 215004 (2011)
Galactic cluster PLCK G266.6-27.3
1 July 2011
Observations with the Planck Space Radio Telescope (based on the
Sunyaev – Zeldovich effect) and the X-ray Observatory ÕÌÌ-Newton revealed that
the cluster of galaxies PLCK G266.6-27.3 belongs to a rare type of distant
massive clusters with high X-ray luminosity. The iron emission line in the
spectrum yielded the red shift z = 0.94 ± 0.02. The mass of the cluster,
based on its X-ray luminosity ≈ 1.4 × 1045 erg s-1 in the
range 0.5-2 keV and on dynamic models, is found to be (7.8 ± 0,8) × 1014M☉. This cluster is thus a member of the three known clusters at
redshift z > 0.5 with the largest X-ray luminosity, and has become one of
the two most massive clusters with z ≈ 1. The cluster PLCK G266.6-27.3
is unique in that it appears to be very regular: spherically symmetric with a
power-law density cusp at the center and a cold core. In other words, even
though PLCK G266.6-27.3 is observed in a fairly early cosmological epoch when
most of the clusters are only just beginning to form, the cluster
PLCK G266.6-27.3 had already undergone dynamic relaxation. One possible explanation
of these properties is that this cluster grew out of a very rare large density
perturbation.
Source: arXiv:1106.1376v2 [astro-ph.CO]
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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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