November 12, 2009

LX11371

Counterfactual quantum cryptography

According to quantum mechanics, events that might have occurred can have actual physical effects, even though they do not in fact occur. What has been termed as an interaction-free measurement is a typical example of such striking counterfactual phenomena: the presence of an object can be determined without a photon being scattered by the object. It has also been shown that the outcome of a quantum computation can sometimes be inferred without the running of a computer. This counterfactual computation exhibits a surprising counterintuitive quantum computational effect, but it seems that it does not have a practical advantage for a specific computational purpose in its present form. Here we apply the fundamental concept of quantum counterfactuality to a real-world communication task in what may be called a ‘counterfactual communication.’ We present a novel class of counterfactual protocols of quantum cryptography that relies on the ‘non-transmission’ of a signal particle (the carrier of secret information): the
mere possibility for signal particles to be transmitted is sufficient to create a secret key.

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LJ11767

Realization of Universal Ion Trap Quantum Computation

Any residual coupling of a quantum computer to the environment results in computational
errors. Encoding quantum information in a so-called decoherence-free subspace provides means to avoid these errors. Despite tremendous progress in employing this technique to extend memory storage times by orders of magnitude, computation within such subspaces has been scarce. Here, we demonstrate the realization of a universal set of quantum gates acting on decoherence-free ion qubits. We combine these gates to realize the first controlled-NOT gate towards a decoherence-free, scalable quantum computer.

November 11, 2009

LD12736


Big surprises come in small nuclei

Electron scattering measurements from nuclei have shown a clear difference in
how quarks are distributed in large nuclei compared to small nuclei, a
phenomenon called the EMC effect. Our understanding of the origins of the
effect is limited by the difficulty of modeling large, complex nuclei.
Therefore, even basic information about the effect has been elusive, such as
the underlying mechanism that causes the difference and whether it depends on
the mass or density of the nucleus.

Jefferson Lab experiment E03-103 made precise new measurements of the EMC
effect for four light nuclei. The large difference between 3He and 4He, the
lightest nuclei measured, rules out mass-dependent explanations. The large
difference between 3He and 9Be, which have similar densities but very
different masses, rules out density-dependent models. The low density of 9Be
is related to its unusual structure; most of the time it is in a configuration
with two 4He-like clusters and an additional neutron orbiting around each
other. The orbiting clusters yield a large radius and an anomalously low
average density, even though most nucleons are contained within the high local
densities of the clusters. This clearly demonstrates that the microscopic
structure of nuclei, usually neglected in high energy measurements, is of
critical importance. The results also suggest that the EMC effect may be
entirely generated within these small, high-density clusters, where densities
can briefly approach those in a neutron star.

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lg12369

Blast Shocks and marbles falling upon sand

In the 1940s, G. I. Taylor, a famous physicist and hydrodynamicist,
managed to deduce the energy released by a nuclear bomb blast,
classified information then, from photographs of such explosions.
Since then, blasts have been studied in a variety of settings and are
believed to arise in the explosion of stars raising important
questions about the relevance of the analysis developed by Taylor. In
a recent paper to be published in Physical Review Letters, we show
that such a complex phenomenon can be produced in the lab by simply
letting a marble impact a sand layer. If the sand layer is made to
flow, the impact of the marble makes a hole in the layer. This hole
expands rapidly as if it were a small explosion. And the expansion of
the hole indeed follows the rules for blast shocks. With one major
difference, energy losses are present and can be accounted for in a
simple way. It may well be that such experiments help in the
understanding of phenomena as far away as a dying star and provide a
simple system to further study the blast problem.

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LG12995

Taming the charmonium wilderness

Since its discovery in the early 70s until the beginning of the new century,
charmonium spectroscopy was considered a safe haven in the tumultuous
landscape of hadronic physics. It seemed to be fairly well understood and no
unusual or unexplained features seemed to exist. However, during the last
years experiments have proved how utterly wrong we were. No less than
eighteen new mesons have been recently reported in this once upon a time
quite and peaceful energy region. Their understanding as simple
quark-antiquark pairs has proved to be nearly impossible and therefore
several different attempts to dissect them have been performed. Their success
in the ambitious task of obtaining a full picture of the whole charmonium
spectra has been, at best, limited.

In our work we have attempted a different approach. Making use of standard
few-body techniques developed, for example, to study the deuteron, we have
examined the possible existence of meson-meson molecular states hidden within
the charmonium spectra. Hence, our idea was not to describe the properties of
a carefully chosen set of states, but to determine the possible existence of
molecular structures. We have carefully examined all possible quantum
numbers establishing boundaries for the existence of molecular states
that might be identified with some of the new reported mesons. This is
the first time such a global and ambitious study has been performed
and we hope it will encourage an experimental effort to confirm or rule out
the molecular character of some particular states.


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LK11836

Refrigerated ions last longer!

We have measured the lifetime of negative helium ions stored in an ion trap
at very low temperature to avoid any influence on the result by the thermal
radiation from the surroundings. Even though no visible light is seen in the
thermal radiation of an object at room temperature, the infrared radiation
is sufficiently intense and energetic to destroy a weekly bound system like
negative helium rapidly enough to severely disturb a measurement of the
intrinsic lifetime (found to be 359 microsecs in the present experiment). By
building a very small electrostatic ion-beam trap and mounting it in a
vacuum chamber which can be cooled down to 10 Kelvin, we have for the first
time reached a situation where the thermal radiation can be neglected and
the lifetime determined directly without the need for any assumptions or
corrections.

November 9, 2009

LG13016

Movies of Quantum Decoherence

Complex superpositions of photonic quantum states that display strong
quantum interference have been “filmed” as they evolve and decohere in
time, allowing detailed observations of their decay as they interact with
their environment.

Using a superconducting phase qubit, experimenters at UC Santa Barbara
have created a range of quantum superpositions of photon states in a
microwave resonator. They can analyze the photon states by employing a
technique known as Wigner tomography, from which two-dimensional Wigner
quasiprobability functions can be calculated; these display the
artistically appealing characteristics of quantum interference. By
performing tomography on the states as they evolve in time, the
experimenters can construct “movies” of the state evolution, providing
striking illustrations of the decoherence and decay of the delicate
quantum states in the resonator. These detailed measurements also allow
the experimenters to extract the time-dependent density matrix for the
resonator, with non-zero on- and off-diagonal elements, and they find good
agreement with theory for the detailed time evolution of every element in
the density matrix. This paper and its accompanying movies present an
appealing and intuitive demonstration of quantum state evolution and
decay.

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BG11217

Atomic configuration, conductance and tensile force of platinum wires with single-atom width

An atomic-scale direct visual experiment was realized for possible smallest artificial structures, i.e., single-atom-width wires. Only six platinum atoms were pulled out from a substrate in line using a nanotip. For individual atomic wires, the dynamic process was directly observed by transmission electron microscopy while simultaneous measurement of electric conductance and mechanical strength. Thus, now, our method allowed us to produce smallest artificial structures, to observe them, and to analyze electrical and mechanical properties one by one. Metallic atom wires have been classified into two large groups: those which exhibit the quantization of conductance or not. Platinum atomic wires typify non-quantized conductance group. Due to deficiency of method, the picture of wires exhibiting non-quantized conductance have not been imaged, although their electricity have been intensively studied. The present study demonstrated an experimental evidence for the picture of atomic wires exhibiting non-quantized conductance.


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AH10402

Magic travel on "Poincare sphere"

Researchers recently investigated evolution of the optical polarization in periodically poled optical superlattice with electric field by observing polarization trajectories on Poincare sphere and novel behaviors of optical polarization were surprisingly discovered.
Optical polarization which reflects the vector nature of the electromagnetic field has been attracting researchers' curiosity for centuries and many remarkable effects regarding optical polarization has been discovered. Recently, researchers from Shanghai Jiao Tong University introduced such striking topic into an optical superlattice. Here, the superlattice functioned as a "magic box" which was able to flexibly manipulate the polarization of a light through electric field. To get an insight into such "magic", researchers observed the evolution trajectories of polarization using "Poincare sphere".
Experiments have revealed that after passing through the "magic box", optical polarization of some wavelengths motivated by electric fields will travel along a closed path parallel to the equator of Poincare sphere like planets around stars; perturbation of the initial optical polarization may force the evolution to take a different orbit. More interestingly, optical polarization of other wavelengths will however travel along a series of discrete paths with a common point of tangency. Here, the superlattice provides a spaceship for polarization and the electric field functions as an energy booster which supports the travel.
This study will promote a novel method for precise and flexible polarization control and may find applications in many scientific realms.

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[BBR1149] R19942PRB

Doppler-Effect in Superconductivity

If waves like light or sound are reflected from a moving surface a shift of wavelength and frequency occurs, which is known as the Doppler-effect. In a recent experiment, we demonstrated that the Doppler shift also occurs in the wave mechanics of electrons, i.e. in the Andreev reflection at a normal-metal/superconductor interface. Andreev reflection means that the incoming electron is reflected as a hole, while a Cooper pair joins the superconducting condensate. Andreev reflections allow supercurrent flow through a Josephson junction formed by normal metal sandwiched between two superconducting contacts. If the condensate in the contacts is set in motion, e.g., by an external magnetic field, the wavelength of the reflected hole is drastically affected. As a consequence the allowed energy levels in the normal metal shift. In our experiment the Doppler-shift results in an unanticipated sensitivity of the supercurrent capacity of the junctions and other properties, which may be exploited in the construction of ultra-sensitive magnetic field detectors.

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LJ11730

Dark matter nuggets shine brightly in gamma-rays

Ultracompact nuggets of dark matter formed when the Universe was in its infancy could be detected by existing high-energy telescopes. If the nuggets (“ultracompact minihalos”) are made of weakly-interacting massive particles (WIMPs), they should produce gamma-rays when WIMPs annihilate with one another. Our paper shows that depending on exactly when the nuggets formed, they could produce enough gamma-rays today to be detected by the Large Area Telescope aboard the Fermi satellite, or existing ground-based gamma-ray observatories such as the HESS telescope, in Namibia.

Dark matter makes up almost a quarter of the Universe, but so far we have no idea what it really is; WIMPs are the leading theoretical candidate. A detection of gamma-rays from WIMP annihilation would not only solve the dark matter problem, but revolutionise our current understanding of particle physics. Ultracompact minihalos are a recently-proposed dark matter structure; our paper shows that their compactness results in very strong gamma-ray signals.