LQ12029

WHY KICKED BLACK HOLES SOMETIMES KICK‑BACK

The merger of two black holes leads in general to a third black hole with
a recoil velocity, ie a "kick". The generation of a large kick has a
direct impact in astrophysics: Depending on its size, in fact, it
determines the number of galaxies containing supermassive black holes at
their centres. The generation of this kick can be understood in terms of
an unbalanced emission of gravitational radiation. Sometimes, however,
the merged black hole shows a deceleration, ie the "anti‑kick", before
reaching a final constant velocity. No straightforward conceptual
explanation was found for this deceleration. This paper provides such an
explanation and a simple interpretation of the physics of this process.
In essence, the anti‑kick is due to the radiation from a deformed black
hole where the anisotropic curvature distribution on the horizon
determines the direction and intensity of the kick. This work is
important because it gives a simple and intuitive explanation to a
process whose mathematical details are horribly complex. Also, it
suggests a methodology to probe the physics around a black hole by
monitoring the geometry near its horizon. This approach may help
understanding some fundamental aspects of black‑hole physics.


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LM12274ER


Fluctuation-induced Casimir-like forces between membrane inclusions fluctuate

Two uncharged metallic plates placed in vacuum attract each other.
This attractive force, named after its discoverer Casimir, originates
from the boundary conditions imposed by the plates on the fluctuations
of the electromagnetic field. Similar fluctuation-induced forces exist
in many other contexts.
Inclusions in a cell membrane, for instance proteins, impose
constraints on the thermal fluctuations of the membrane. This gives
rise to a long-range force between these inclusions, which is
analogous to the Casimir force. Although this fluctuation-induced
force is by essence inseparable from its fluctuations, only its
average value is well-known.
In this paper, we study the fluctuations of the Casimir-like force
acting between two membrane inclusions. We show that this force
fluctuates strongly: in fact, it is dominated by its fluctuations.
Furthermore, we find that these fluctuations depend on the distance
between the two inclusions. This distance dependence shares a common
physical origin with the Casimir force itself.



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LN11801

Enhancing proton migration with laser light.

In this paper we show that resonant infrared laser light increases the proton hop rate in oxides by nine orders of magnitude. This giant enhancement is significant because proton conduction is the underlying process behind important technologies such as fuel cells and hydrogen production by electrolysis. The mechanism behind this photo-enhanced effect is vibrational excitation of proton-oxygen (O-H) bonds in the oxide material. When a photon's wavelength is resonant with the stretching motion of the O-H bond, the photon is absorbed and adds energy to the proton. This excitation effectively helps the proton become mobile and contribute to bulk diffusion or conduction. Ultimately this work provides new fundamental insight into the microscopic dynamics of hydrogen in solids.

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LK12367

Amplification of photon pairs in semiconductors

Two-photon gain is a unique process predicted by quantum mechanics, in which photons are duplicated in pairs, causing light to be amplified with nonlinear intensity dependence. We directly observed the phenomenon of two-photon gain in a semiconductor miniature device, designed to enable direct conversion of electrical current into photon-pair amplificaiton. The nonlinear intensity-dependence enables giant pulse generation in a chain-reaction like process. Since gain is the fundamental ingredient for lasing, it may now be possible to realize two-photon lasers with exceptional classical and quantum charcteristics. Previously, two-photon gain was realized in dilute atomic systems in a maser-like configuration with low powers and optical pumping. Achieving two-photon gain in solids, in particular semiconductors, has several benefits, similar to those which stimulated the evolution of the maser to a diode laser: orders of magnitude higher emitter densities, micro-fabrication technology, and electrical pumping. The observation of semiconductor two-photon gain paves the way for the realization of efficient and miniature devices significant for bio-imaging, spectroscopy, quantum-information and fundamental light-matter interaction studies.


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LN12122


Wireless goes to the nanoscale!

Wireless gadgets such as cell phones and Bluetooth are all around us,
and ensure flexible and long‑range connectivity. More difficult has been
to fully translate these concepts to the visible: can we have a wireless
link with light at the nanoscale? In their upcoming paper in Phys. Rev.
Lett, researchers from the University of Pennsylvania and the University
of Texas at Austin have theoretically proposed and modeled a wireless
link between optical nanoantennas. By suitably loading and matching
silver‑nanowire antenna pairs with optical nanocircuits, they have
numerically demonstrated an optical wireless connection between
nanoscale emitters and receivers, which may have several advantages,
such as reduced absorption loss, over regular plasmonic waveguide links.
This approach may one day compete with current optical interconnects in
chip‑scale data processors and computational systems.


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LM12327


Testing quantumness

Statistical uncertainties occur in any physical measurement
and the question whether or not a discovery is significant is
of central importance in fields like astrophysics or particle
physics. In quantum physics, many experiments nowadays aim at
the generation of multiparticle entanglement, which is considered
to be a central resource for quantum information processing.

In this paper, we investigate theoretically and experimentally
the significance of entanglement detection. Theoretically, we
point out which kind of measurements one has to perform in
order to detect the quantum correlations in a physical system
unambiguously with a small statistical uncertainty. Experimentally,
we observe the predicted phenomenon in a four‑photon experiment.

Our results provide novel theoretical ways to detect entanglement
with high statistical significance, allowing for unambiguous
statements on quantum correlations. This is important for future
experiments, since the event numbers decrease with increasing
the numbers of particles, so the statistical uncertainties
become more relevant.

LM12564

Water molecules queue into nanotubes

Researchers have, for the first time, demonstrated that
water can enter ultrathin carbon nanotubes ‑ tubes with a wall consisting of a
single layer of carbon atoms and a diameter down to half of a billionth of a
meter (0.548 nanometer) ‑ thin enough to prevent water molecules from passing
each other inside the tube. This first experimental proof of such single‑file
transport of water occurring in nanotubes holds promise for the design of
ultraselective filter membranes and, eventually, nanofluidic devices where
water or other fluids would be transported and manipulated at the molecular
scale in a "first‑in‑first‑out" manner.

While intuitively unexpected because of the highly water‑repellent carbon
surface of the tubes, transport of water through carbon nanotubes has been the
subject of many theoretical studies. In the present work, the researchers
caused nanotubes of specific diameters and structures to vibrate, using a wide
range of lasers of different wavelengths (colors). They could distinguish
water‑filled and empty nanotubes, as these vibrate at different frequencies ‑
in much the same way as a water‑filled glass resonates at a higher pitch than
an empty glass.

The team found water‑filling in extremely thin tubes ‑ thinner than generally
predicted by theory ‑ and the results even indicate that the water molecules
are forced to adopt specific orientations and arrangements depending on the
exact nanotube structure.


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EL10710

Solving the mystery of raindrop formation

We have experimentally discovered a new physical mechanism for the formation of local small-scale concentrations of inertial particles (e.g. droplets) suspended in turbulent nonisorthermal flow. The mechanism of rain formation is not yet sufficiently understood and remains an outstanding problem in atmospheric physics. Calculations based on the assumption of uniform spatial distribution of droplets yield unrealistically long times for rain formation. One of the most important mechanisms of rain formation is associated with appearance of small-scale clusters of droplets ("inch" clouds) due to cloud turbulence. All previous studies of inertial particle clustering were performed for isothermal turbulence, while temperature distribution in clouds is inhomogeneous. The new effect of small-scale clustering in the presence of non-uniform mean temperature distribution is much stronger than inertial clustering in isothermal turbulence and leads to formation of small-scale concentrations even of very fine droplets. This effect elucidates the mechanism of rain formation in turbulent clouds and can be also significant in various industrial multi-phase turbulent flows (e.g. internal combustion engines).

BG11574

Arranging atoms in a nanowire to halt heat flow

The lowering of the thermal conductivity, which describes the ease of heat flow in a substance, is of interest for many practical applications such as thermoelectric energy conversion. It has been known since the 90's, from the works of Dresselhaus and Hicks, that lower‑dimensional structures such as nanowires have drastically lower thermal conductivities compared to bulk materials. Before that, the mixing of different types of atoms in a material had been used to lower the thermal conductivity. But what about a combination of nanostructuring and atomic arrangements? Through computational techniques of molecular dynamics, cluster expansion and genetic algorithm optimization, researchers have tackled the problem of finding optimal arrangements for atoms in a nanowire to obtain the lowest possible thermal conductivity. They investigated silicon‑germanium nanowires which are promising for thermoelectric applications. They used molecular dynamics simulations which model the dissipation of random heat currents in equilibrium, in turn giving the resistance of the nanowires to heat flow. Using insights from previous experimental and computational work, they reduced the thermal conductivities of the nanowires by roughening the nanowire surfaces in the simulation. Then the astronomical combinatorial problem of arranging two types of atoms in the nanowires is reduced to a tractable problem by the cluster expansion technique. The best arrangements of silicon and germanium atoms in the nanowire are obtained using the cluster expansion from a "survival of the fittest" genetic algorithm optimization. The resulting nanowire structure, showing complete planes of Ge, is predicted to have a thermal conductivity more than 100 times lower than bulk silicon.


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AN10495

Counting Photon Holes

This paper introduces a new device that is capable of detecting whether a photon has been removed from a certain place in a weak laser beam,while affecting neither the laser beam, nor the photon hole. It has been found that such photon holes can show nonclassical characteristics, such as entanglement, making them an alternative to photons in future quantum computers.

Thus far, photon hole signals have been detected by photon detectors, which absorb the background photons, demolishing the signal. The proposed method is based on a phenomenon which renders a dense medium, in this case a cell filled with rubidium atoms, completely transparent to a probe pulse, only if the signal contains no photon hole. As soon as a photon hole enters the cell, the probe pulse stops propagating, a phenomenon known as light stopping. This results in a delay of the probe pulse which can be detected. Still, the signal beam exits the cell without any significant distortion. The measured photon hole signal can then be reused for other purposes. Such "nondemolition measurements" play a key role in the implementation of future quantum computing circuits.