in
quantum information processing, which highlight measurements and
multipartite
cluster states in a striking way. Though linear optical realization using
multi-photon has been demonstrated recently, both speed and fidelity output
of
computing are rather low and not fully satisfactory. In this work, we
develop a scheme for creation and characterization of a two-photon
four-qubit cluster
state. Furthermore, we have designed and demonstrated successfully the first
proof-of-principle experimental realization of one-way quantum computing
employing such a source. The excellent quality and high generation rates of
our
source enables high efficient computing (which is 4 order of magnitude more
than the usual multi-photon realization), including implementation of
Grover's
algorithm and high fidelity quantum gates that are at the hear of universal
quantum computing. Our results could help to make a significant advancement
of
quantum information processing and goes largely beyond the limitation
imposed
by employing technique of multi-photon entanglement. In addition, this
source
could serve as a very promising candidate, and complement current progresses
toward rapid and precise optical quantum computing. LE10869
***
Signatures discovered of a new type of superconducting phase
High temperature superconductivity was first discovered in La2-xBaxCuO4 by Bednorz and Mueller, for which they received the Nobel Prize in 1987. Soon thereafter, superconductivity with higher transition temperatures, Tc, up to a current record of around 150K, was discovered in closely related cuprates, as the field of high temperature superconductivity grew into one of the most exciting subjects in all of physics. The common feature of all these materials is that they consist of stacks of planes of copper and oxygen atoms which form a square lattice . Slightly after the initial discovery, a strange anomaly was observed in the variation of Tc with the Ba concentration, x, in this self-same material originally studied by Bednorz and Mueller: while Tc achieves values in excess of 30K for x slightly larger and slightly smaller than x=1/8, in a very narrow range of x near x=1/8, Tc plummets to very low values, Tc(x=1/8) \approx 4K. This anomaly was later shown to be the result of the occurrence of another, extremely interesting so-called “stripe phase,” which competes with superconductivity. In the stripe phase, the electron density, rather than being uniformly distributed throughout the crystal, spontaneously form stripes of alternating high and low density with a period of four lattice constants. The orientation of the stripes rotates by 90o from plane to plane, so macroscopically no difference between the two directions can be detected. It is still not known to what extent stripe related phenomena occur in other cuprates.
Yet another striking new discovery has just been reported in La2-xBaxCuO4 with x=1/8 in a forthcoming paper in Physical Review Letters by Li et al: Although Tc is only 4K, direct evidence of a strange form of local superconducting correlations are seen in the form of a dramatic factor of 100 drop in the in-plane resistivity below a much higher temperature, Tdrop ~ 42K. This temperature, not coincidently, is the same temperature at which the electron spins freeze into a stripe-like pattern, as well. However, the resulting state is not like any conventional superconducting state. Although the resistivity is much smaller below Tdrop, it is not zero, as in a true superconducting state. Moreover, the resitivity for currents in the direction perpendicular to the Cu-O planes continues to grow larger below Tdrop, so this three dimensional material acts more and more as if it consisted of completely decoupled two-dimensional layers. A natural explanation for all these unprecedented observations has been proposed in another forthcoming paper in Physical Review Letters by Berg et al. They postulate that in the striped phase, which is known to occur in LBCO, that the superconducting order is also striped in the sense that it forms stripes of alternating positive and negative superconducting order throughout the plane. Because of the crystalline geometry, the coupling between the positive and negative regions on neighboring planes exactly cancel each other, thus explaining the dramatic interlayer decoupling that occurs in this state. Moreover, because of the sign changes, any imperfections in the crystal cause, at large enough distances, glassy behavior of the superconducting order even within a plane, which accounts for the fact that resistance does not actually vanish. If further experiments confirm this interpretation, this striped superconducting state constitutes the third new state of matter discovered in this one, fascinating material. LD10997
***
Laser oscillation controls electron motion in a molecular bond
We have shown in quantum mechanical simulations that it is possible to
steer the electron in a dissociating hydrogen molecular ion and locate it
at one of the two nuclei using the oscillating electric field of a laser.
It is well known that chemical reactions can be controlled using
femtosecond laser pulses by changing electronic quantum states in a
molecule on the time scale of the nuclear motion. The advent of even
shorter laser pulses in the attosecond regime has paved the way to steer
the electrons in a molecular bond directly by the oscillating electric
field of the laser. Our simulations of the dissociation of the hydrogen
molecular ion show how this can be acchieved using two laser pulses with
a time delay. The first attosecond pulse initiates the dissociation
process. While the distance between the protons increases, the
internuclear Coulomb barrier acts as an ultrafast shutter and traps the
electron at one of the two nuclei. We have shown that by applying a
second ultrashort laser pulse during the shutter time it is possible to
drive the electron by the oscillating electric field and locate it at
either one of the nuclei with a high probability. LC10888
***
Washboard Road: Why wheels make ripples
A bane to motorists and road commissioners alike, the causes of washboard roads are surprisingly difficult to untangle. Researchers at the University of Cambridge showed that contrary to the beliefs of some physicists and engineers, the diameter of the wheel, average size of the granules, compression of the gravel, and separation of differently sized granules play no apparent role in the development of washboard patterns. Using a simplified system (a hard-rubber wheel with no tire or suspension), washboard ripples appear spontaneously on their granular road, which shows that the washboard pattern is a fundamental instability and contradicts the common belief that they are caused by the suspension of the vehicle. The authors have also showed that the washboard instability (the spontaneous formation of ripples) occurs for a wide range of parameters. They were able to obtain ripples with a blocked or square wheel (i.e. a plough), using various grains (fine and coarse sand, wet and dry sand, rice), and various masses, radii and velocities for the wheel, and finally using our 2D simulations. The practical conclusion is a sad one for motorists: it’s virtually impossible to avoid the formation of washboard ripples on unpaved roads. LB10838
***
Controlling nature's self-organization
Atom vacancies in a monatomic layer of gallium atoms on a silicon surface are shown to "selforganize" into a nanoscale pattern of almost perfectly straight lines, and the average line spacing can be controlled with a precision better than 0.05 nm.
Self-organization is a promising route for the "bottom-up" fabrication of functional nano-devices; arrays of self-organized vacancy lines in hetero-epitaxial growth ( i.e., growth of material A on B) may be used as templates for fabricating nanowires. However, the laws of thermodynamics dictate that the structural uniformity of such nanostructures will be compromised by entropy, nature's tendency to maximize disorder.
Here, we carefully controlled the line spacing by adjusting the chemical potential of the gallium atoms. We used atomic-resolution imaging to study the resulting line patterns. Where current mean field models describing vacancy line arrays break down in the limit of ultrasmall (~2 nm) line spacings, we were able to quantitatively analyze the energetic driving force of the self-organization process, as well as the entropic factors causing the observed deviations from perfect line structures. Our theoretical analysis involves a novel hybrid approach of first-principles density functional theory calculations and statistical mechanical modeling, and may be applicable to similar problems in thin film growth and nanoscience. LE11169
***
Cylindrical cloak: An engineering challenge
Based on a coordinate transformation of Maxwell's equations, Pendry et al. (Science 312, 1780, 2006) recently proposed a method for constructing a cylindrical invisibility cloak. In this paper, we confirm, analytically, that such a two-dimensional cylindrical cloak with ideal material parameters is truly a perfect invisibility cloak. Our method is based on the cylindrical wave expansion method in which electromagnetic fields are represented by Bessel functions. However, we also discover that any tiny perturbation of the location of the cloak's inner boundary will induce noticeable field scattering and penetration due to the slow convergence of the zero-th order scattering coefficient. This, together with the fact that the extreme material parameters (infinity values of permeability or permittivity) are required for the ideal cylindrical cloak at its inner boundary, illuminate the great challenge in realizing a perfect cylindrical invisibility cloak. LE11086
***
Labyrinthine island growth on surfaces
Most theories of epitaxial growth neglect the effects of deposited atoms alloying into the substrate. Our work shows that this omission misses phenomena that are important for determining the morphology of epitaxial films. Using a combination of microscopy and electronic structure calculations we observe and explain a distinctive new growth mode of alloy surfaces. When Pd is deposited on Ru, two-dimensional islands initially nucleate. Remarkably these islands grow only from special regions along their periphery, causing a snake-like motion and giving rise to labyrinth patterns in the Pd film. Atomically resolved scanning tunneling images detect alloying around the non-moving sections of the islands. Ab-initio calculations indicate that the alloying can hinder the growth of the islands. Real-time observations by low energy electron microscopy corroborate the predictions of our model: that some atoms of the substrate mix with the islands sides, "poisoning" them and preventing attachment of additional deposited atoms on these "contaminated" areas. Then only areas of the islands moving fast enough can remain more pure, so that new atoms more easily attach to them. This phenomena should often appear when surface alloying occurs during deposition. LD11538
***
Water waves have been a subject of research for almost 200 years.
A particularly interesting type of waves are the "line solitons",
which propagate for long distances without change in shape, and
which have fascinated many photographers, see for example
http://www.amath.washington.edu/~bernard/kp/waterwaves.html.
Because solitons are nonlinear objects, they don't simply pass through
each other, but they interact, resulting in a mutual displacement.
A famous photograph illustrating this phenomenon is at
http://www.cirrus-digital.com/Toedtemeier.html
As it turns out, however, this is only one of the possible scenarios.
In this article we take a new look at line soliton interactions, taking
advantage of a new class of exact solutions of the equation that
describes these phenomena, the Kadomtsev-Petviashvili equation.
The result is that the interaction are very different depending on the
amplitudes and directions of the individual solitons. In particular,
in some cases the solitons interact "resonantly", creating an array of
intermediate (or "virtual") solitons, see Fig. 1.
When more than two solitons interact, there is a dazzling variety of
interaction patterns [the exact number being (2N-1)!!, where N is the
number of solitons and the double factorial is (2N-1)*(2N-3)*...*5*3*1],
which are again classified in terms of the amplitudes and directions of
the solitons. Fig. 2 shows a catalog of the 5!!=15 different types of
solutions obtained when N=3. LD10989
