Violation (and Restoration) of Einstein’s Equivalence Principle by the Hawking/Unruh effect

LC13069

- Einstein’s Equivalence Principle, the principle which forms the conceptual basis of General Relativity, says that an observer can't (locally) distinguish between acceleration or being in a gravitational field. Einstein imagined comparing an observer in an accelerating elevator with an observer in the same elevator at rest on the surface of the Earth; in each case the observer feels an identical downward force. In our work we’ve found that an observer confined to such an elevator can distinguish between the two cases by using two related quantum phenomena: the Hawking effect and the Unruh effect.

Due to quantum fluctuations of fields an observer in the gravitational field of a black hole will measure a temperature – the Hawking temperature. Similarly an accelerating observer will measure a temperature – the Unruh temperature. We have shown that if the two different observers measure the same local acceleration they will measure different temperatures – the observer in the gravitational field will measure a higher temperature.

Surprisingly in regions of stronger gravitational field the two temperatures approach the same value. Thus as the gravitational field becomes stronger the Equivalence Principle is restored, hinting that gravity and quantum mechanics are more compatible (not less) for strong gravitational fields.

Imaging atomic interactions

LE12909

- We have developed an atom-imaging technique that allows us to detect the
positions of individual Rydberg atoms, which are atoms with a highly
excited outer electron. Using this technique, we provide the first
spatially resolved images that demonstrate the “Rydberg blockade.” This
effect is at the core of proposals for a quantum computer architecture
based on neutral atoms

A Rydberg atom has such a tenuous grasp on its excited electron that the
atom is extremely sensitive to external electric and magnetic fields,
and interacts very strongly with other Rydberg atoms. The interaction
between Rydberg atoms is so strong that a Rydberg atom can “block” the
laser-excitation of another Rydberg atom by shifting the energy levels
of the second atom out of resonance with the laser. This is termed the
Rydberg blockade effect. This process leads to quantum entanglement,
which can be used in quantum computation algorithms.

A second Rydberg atom can only be excited if it is farther than a
“blockade radius” from the first atom. We directly measured this
blockade by laser-exciting Rydberg atoms in a cold atomic vapor and
measuring the Rydberg atom positions. We observe a blockade radius of
about 10 microns, which is about 100,000 times larger than the radius of
a ground state atom.

Information could escape from black holes after all

LB12889

- New research suggests that information could escape from black holes after all. The research, which appears in the latest issue of Physical Review Letters, uses the basic tenets of quantum mechanics to give a new description of information leaking from a black hole.

Our results didn’t need the details of a black hole’s curved space-time geometry. That lends support to recent proposals that space, time and even gravity itself may be emergent properties within a deeper theory. Our work subtly changes those proposals, by identifying quantum information theory as the likely candidate for the source of an emergent theory of gravity. The results actually extend the predictions made by standard techniques that rely on a detailed knowledge of space time and black hole geometry.

We cannot claim to have proven that escape from a black hole is truly possible, but that is the most straight-forward interpretation of our results. Our results suggest that quantum
information theory will play a key role in a future theory combining quantum mechanics and gravity.

ELECTROMAGNETIC WAVE PROPAGATION EXCEEDING THE SPEED OF LIGHT

LX12496B

- This work demonstrates theoretically and experimentally that the peak of a pulse can emerge from a metallic plate perforated with tiny holes and sandwiched between dielectrics before the pulse enters the sandwiched subwavelength-hole array. The effect known as superluminality arises because of the coupling interference between two states: one coming up from a mode supported by the grounded dielectric slab, and the other emerging from the periodic pattern. While being counterintuitive, this phenomenon is physical and does not violate the causality principle, which is defined with respect the pulse turn-on time occurring before the peak time. In contrast previous works, the presented superluminal mechanism is scalable to any wavelength regime from radio frequencies to visible light, it uses a simple structure without any special (e.g. gain or absorptive) materials, and it occurs for substantial values of the transmittance. This
work expands our fundamental understanding of periodic gratings and may find applications in pulse re-shaping, tunable delay element with canceled group velocity dispersion, and beam spectral analysis.

Single-photon router

LB13197

- In recent years, quantum information science has advanced rapidly, both at the level of fundamental research and technological development. For instance, quantum cryptography systems have become commercially available. These systems are examples of quantum channels, serving mainly to distribute quantum information. There is a significant effort to combine these quantum channels with quantum nodes that would offer basic processing and routing capability. The combination of these channels and nodes would create a quantum network enabling applications simply impossible today. Quantum networks connecting simple quantum processing nodes are also a promising architecture for a scalable
quantum computer.

In this letter, we demonstrate an example of a rudimentary quantum node, a single-photon router. The active element of the router is a single ”artificial atom”, a superconducting qubit, strongly coupled to a superconducting transmission line. Exploiting the phenomenon of electromagnetically induced transparency (EIT), we show that we can route a single-photon signal from an input port to either of two output ports with an on-off ratio of 99%. The switching time of the device is shown to be a few nanoseconds, consistent with theoretical expectations and the device parameters. The device is a nanofabricated circuit offering a clear path to scalability. For instance, it is straight forward to extend this router
to select between multiple output channels.

Signature of hydrogen-bonded supramolecular assemblies at dye-sensitized solar cell interfaces

LC13086B


- Due to the growing global demand for energy, the development of efficient ways of harnessing solar power has become a key scientific challenge. Among promising low-cost alternatives to silicon photovoltaics, dye-sensitized solar cells based on mesoporous TiO2 films sensitized with the dye Ru(dcbpyH2)2(NCS)2 (N3 dye) have gained prominence due to their relatively high energy conversion efficiencies. In dye-sensitized cells the photocurrent is generated via ultrafast electron transfer from the photoexcited dye sensitizer to the semiconductor. As a result the atomistic nature of the semiconductor/dye interface plays a critical role in the performance of these solar cells. Substantial efforts have been devoted to elucidating the structure of this interface, and yet its atomistic nature remains highly controversial. Here we perform a systematic comparison between measured core-level photoemission spectra at the TiO2/N3 interface and the spectra calculated by us from first principles for a variety of atomistic interface models. This analysis suggests that systematic
hydrogen-bonding between dyes occurs on the TiO2 surface, leading to a supramolecular assembly. The present finding hails a paradigm shift in our understanding of dye-sensitized solar cells and bears on the design of more efficient nanoscale photovoltaics.

Physicists Change the Color and Shape of Single Photons

LE13282

- Physicists have simultaneously changed the color and shape of a single photon, the smallest unit of light. The work represents an important step towards implementing communication over long distances with privacy secured by the laws of quantum physics. The photon was extracted from a quantum dot, a semiconductor version of an atom that emits photons one at a time, using a specially-designed optical fiber. Then, the single photon was combined with a much stronger, pulsed laser beam inside a crystal that enables the two light beams to interact efficiently. After exiting the crystal, the color or wavelength of the single photon had been shifted by almost 600 nm, an amount greater than the size of the entire visible spectrum. Because the researchers were using a pulsed laser, its pulse shape became imprinted on the single photon during the color-conversion process. Researchers utilizing different quantum technologies, which often require single photons of a specific wavelength and shape, can now use this approach to link their systems together in a large-scale network for quantum information processing applications.

Why can't we avoid queues in MANHATTAN-like road networks?

LE13030

- As a Mayor of a growing city with traffic issues: would you choose
Manhattan-like road patterns or more “disordered” old European styles?
As a cellular biologist: do you understand how active transport on
biofilament networks organizes inside a cell?

The question of how the structure of a network affects its transport
properties dates back to 19th century Kirchhoff's work on the
conductance of resistor networks. Kirchhoff's well-known linear laws are
nowadays the basis of any electric circuit “current-voltage” analysis.

Network transport characteristics, however, are trickier if the conveyed
species reciprocally interact in narrow channels or on filamentous
structures: non-linear collective phenomena such as queues or jams can
appear.

A paradigmatic model to study traffic phenomena is the Totally
Asymmetric Simple Exclusion Process: particles move stochastically along
one-way lanes and cannot occupy the same position in space.

Via this model, we show that connectivity is very important for traffic
issues on networks. "Regular" connectivities, with junctions having an
equal number of incoming and outgoing segments, produce fluctuating jams
at each junction. Differently and surprisingly, in “irregular”
connectivities traffic jams as such disappear altogether, leaving high
or low dense traffic lanes with small transport fluctuations.

In urban traffic, these results would suggest that rationally designed
Manhattan-like road layouts could lead to traffic jams everywhere,
whereas the anarchy of historically grown cities could help avoiding
this problem. Speculations on the complex layout of cytoskeletal
transport in living cells are tempting…

Beautiful physics in a cup of water

LE13271

- Everyone who has ever observed the surface ripples that form in a simple vibrating cup of water served in an airplane or a train has unconsciously performed the experiment carried out for the first time in 1831 by Faraday. It consists in the rhythmical vertical shaking of a recipient filled with water and ever since Faradays first observations the developing surface waves are known as Faraday Waves. Only a few researchers have paid attention to another phenomenon which seems to be linked to these surface waves: the induced movements of tiny particles added to the fluid. In this paper, we determine for the first time the whole surface velocity field and its statistics using a technique called Particle Image Velocimetry (PIV). Surprisingly the energy spectra of this fluid flow show characteristics typical for 2-dimensional turbulence which is an important approximation for natural systems such as currents in the ocean or circulations in the atmosphere. In contrast to three-dimensional turbulence it includes the possibility of passing energy from small to large spatial scales, a process which explains a seemingly contradiction: the creation of large structures such as eddies when stirring only happens on much smaller scales.

Critical behavior and transmission of information of a swarm of units

LE13424

- This paper shows how a few lookout birds transmit to the whole swarm their reactions to either environmental scares or attractions. Cooperation generates organization as a phase transition from the random motion of single independent individuals to an order condition where all the units fly in the same direction. However, at criticality the flock organization is not permanent and undergoes collapses allowing the single individuals to recover their independence, thereby making the flock select randomly new directions. The danger/opportunity perceiver birds act as nucleation seeds generating a bias turning the free-will condition into wise decisions, namely the choice of the most convenient flying directions, according to the environment state: As a result of criticality the swarm of cooperating units behaves as a single "intelligent" individual. This article shows that criticality turns local interaction into a long-distance communication, with a time delay determined by the distance between two consecutive organizational collapses. The time delay is proportional to the flock size, thereby generating two challenging issues, (a) the choice of the optimal size for the swarm function and (b) the assessment of whether the occurrence of organizational collapses is related or not to the process of information transmission diffusion via diffusion.

Is religion doomed?

LA12632

- Much like animals competing for a limited supply of food or water,
social groups can be thought of as self-perpetuating entities that
compete for members. The group of religiously-unaffiliated individuals
has grown rapidly in recent times. We use research on social
conformity, together with methods from nonlinear physics, as the basis
for a mathematical model of this growth. We come to a startling
conclusion: coexistence of religious and irreligious segments of
society may be unstable! That is, our model suggests that religion
may be driven toward extinction in the long run. We test the model
with historical census data drawn from 85 regions of nine modern
secular democracies, and find that it fits data well. According to
the model, a slight advantage in the perceived utility of religious
non-affiliation should lead to the long-term decline of religion.

New metamaterial cloaking concept promises vessel motion without resistance or wake

LF13222

- When a vehicle or a vessel moves through air or water at a steady speed, most of its fuel is spent on fighting the hydrodynamic resistance. This is because the vessel needs to push the fluid out of its way in order to move forward. The disturbance of the fluid caused by the vessel motion normally spans the distances greatly exceeding the size of the vessel; enormous amounts of energy are needed to displace the huge mass of fluid in that range. Using computer fluid dynamics (CFD) simulations, we show that the volume where the fluid is pushed by a vessel can be reduced to a thin layer filled with a properly designed fluid-permeable porous metamaterial with anisotropic permeability. The structure, dubbed “fluid flow cloak,” moves through the fluid without generating any wake behind it, and it experiences zero resistance force, known as the drag force in hydrodynamics. The drag force normalized to the size of the vehicle, or the drag coefficient, is one of the most important measures of fuel efficiency. For relatively slow motions, we find that the drag coefficient can be made to vanish.

A new kind of nanoscale laser emits coherent photons with ultra-low energy consumption

LD13537

A thresholdless laser, which would produce coherent light the instant it is switched on, has been a concept pursued by many research groups for the inherent advantages in terms of energy consumption. The threshold represents the amount of energy expended before coherent light is emitted by a laser. Researchers have now reported a single nanowire polariton laser with an ultra low threshold energy of 92 nJ/cm2 at room temperature. This translates to a very small threshold optical power density of ~7W/cm2 or an electrical current density of ~1.75A/cm2, the lowest achieved so far in any laser. Polaritons are admixed particles resulting from the strong coupling between cavity photons and excitons. Polariton lasers, operating in the strong coupling regime may prove to be a new source of coherent light representing a regime in efficiency and performance beyond that of conventional semiconductor lasers. Coherent emission in a polariton laser results from stimulated scattering of polaritons into quantum degenerate polariton states and subsequent spontaneous radiative recombination. The polariton laser consists of a single defect-free and GaN nanowire of length and diameter equal to 750nm and 60nm, respectively, enclosed in a dielectric microcavity. The nanowires are epitaxially grown on silicon substrates before being selectively dispersed in the microcavity. The polariton dispersion characteristics and the nonlinearity, spectral linewidth narrowing, polarization, and coherence of the output were measured and analyzed. The threshold carrier density for polariton lasing is three orders of magnitude lower than that of photon lasing in the same devices.

How do our organs acquire their shape?

LD13645

Morphogenesis is the evolutionary process leading from the few cells in an
embryo to the very complex shapes of organs or animals; how it is guided
remains an open and fascinating question. In this paper, we show that
complex patterns observed in the intestine can be understood from
simple mechanical arguments.

When cells divide in an epithelial monolayer, they exert a pressure on
their surroundings, which is at the origin of the intestinal tube
wrinkling: the monolayer buckles. By forming large wrinkles, the area of
the tube is increased. This buckling theory predicts patterns strikingly
similar to those observed in the small intestine.

The shape of a tissue also has important effects on division : shape
can feedback growth. Adding this ingredient to the model allows us to
explain the patterns observed in the large intestine as well,
providing a comprehensive physical theory of the morphogenesis of the
intestine.

Intestinal shape and renewal seem to be at least partially controlled by a
mechanical balance. This fine balance is disrupted in the case of
intestinal diseases and should be studied more carefully in the future,
as it may shed some new light on these diseases.

FOREVER ENTANGLED

LD13369

Quantum entanglement between two separate macroscopic objects which is
maintained for as long as electricity runs in the lab is demonstrated.
This achievement disproves a popular belief that quantum entanglement,
the main component in quantum information processing, is a fragile
property which can only exist for a limited time. The novel method
used to produce this unusual result is based on employing dissipation
for generation of entanglement. Dissipation, or, in other words,
uncontrolled interaction with the environment, so far has been the
major reason for ruining quantum entanglement. We engineer dissipation
for atomic ensembles containing thousands of billions of atoms so that
it generates entanglement rather than impairing it. Our work
demonstrates that dissipation can be used for quantum information
processing. Applications range from quantum communication to quantum
sensing.

Graphene Spintronics: Realization of Nano-second spin relaxation times at room temperature.

LY12671

- Conventional electronic transistors involve the control of electronic charge at the nanoscale to realize memory, logic and communication functions. All these electronic charges, however, also carry a spin that remains unutilized in present commercial devices. This has motivated the search for new materials that propagate spin-polarized currents over large distances. Among them the most promising materials for spintronics has been graphene, a truly two-dimensional crystal of carbon atoms with relativistic carriers. Micron-scale spin relaxation lengths have been previously demonstrated in single-layer graphene. In this paper, we show that the two-layered cousin of graphene (called bilayer graphene) is a far more interesting candidate for spintronics. By fabricating spin valves on bilayer graphene we have achieved record room temperature spin relaxation times up to 2 nanoseconds, which are significantly higher than for single layer graphene. Furthermore, the presence of interlayer interaction between the two layers of bilayer graphene makes it a particularly lucrative (profitable) nanoscale material for spintronics – particularly since this interaction can be tuned by electric-field. Spin devices made from exactly two stacked layers of graphene turn out to be remarkably different not only from single-layer graphene but also from all other multilayer graphenes.Our work provides fundamental insight into the unique properties of bilayer graphene for spintronic applications.

Layers of rapidly moving media may repel or attract each other with Casimir force

AB10715

- When trains pass each other at a high speed, the air pressure
vibrations in the gap between the train bodies may not only be audible
by the passengers, but, in a worst case scenario, may push the trains
off track or pull them too close resulting in a crash. Now scale
everything down to microns and replace the trains with rapidly
moving layers of a dielectric, and the air pressure vibrations with
the quantum fluctuations of the electromagnetic field, and you will
get an idea of what is studied in this paper. With a rigorous
treatment we theoretically demonstrate a possibility for the Casimir
force to be repulsive in dielectric layers that rapidly slide one with
respect to another. The repulsion occurs in a triple-layer structure
where the exterior layers move in the same direction with respect to
the stationary middle layer of the same material. When the exterior
layers move in the opposite directions (like the trains in the
example) the Casimir force is attractive and is stronger than the same
force between stationary layers.

First Electrical Cable Built of a Single Molecule

BER1188B

- Can single molecules be deployed as reliable and effective conductors in the future electronic devices? Seeking the answer to this question we succeeded for the first time to create an electric cable from a single molecule in a systematic and reproducible way. The obtained wires are only 10 Ångstrom (1 nanometer) long, but they represent a prototypical example of a true molecular conductor clamped to the electrodes via the two well-defined “crocodile” clamps of atomic size. We create the wires by lifting one end of the single molecule lying on the atomically clean metal surface with the sharp metal tip vibrating with sub-Ångstom amplitudes. The vibrating tip on one hand plays a role of the second electric contact used for the conductance measurements at the same time it allows us to measure the stiffness of the wire thus controlling its conformation during the liftoff process.

Soda cans squeeze sound on tighter spots

LD13104


- On top of being convenient soda containers, cans are also good acoustic
resonators. Anyone can verify this by blowing air inside an empty can: an
almost pure tone can be heard. In this letter, we prove that a packed
bunch of soda cans behaves as a new sonic material with fascinating
properties. Subject to various tones, the ensemble of cans responds
collectively and creates complex sound patterns. Very interestingly, those
patterns oscillate on spatial dimensions of the order of a soda can, that
is, much smaller than the wavelength of the acoustic waves that excite
them. Generating broad spectrum sounds with low cost computer
loudspeakers, we demonstrate that 1 meter wavelength audible acoustic
waves can be squeezed thanks to the cans onto spots as tight as a few
centimetres. This experiment opens up the possibility to control acoustic
power or send audio messages on dimensions much smaller than the
wavelength. Our approach, evidenced with soda cans for the sake of
simplicity, is very general. It can be realized using many resonator and
is valid for acoustic and elastic waves at any frequency. We believe it
opens up exciting possibilities for the control of audible sound in new
exotic ways, but also more pragmatically for the design of smart sensors,
actuators and MEMS.

Stressed Workers Lead to Relaxed Colleagues

LC13228

- We study the nature of workplace stress from the aspects of human-human interactions. We investigated the distribution of Center for Epidemiological Studies Depression Scale scores, a measure of the degree of stress, in workplaces. We found that the degree of stress people experience when around other highly stressed people tends to be low, and vice versa. A simulation based on a model describing micro-level human-human interaction reproduced this observed phenomena and revealed that the energy state of a face-to-face communication network correlates with workplace stress macroscopically.

Bacteria Swim Faster in Groups

LD13593

- It is a well known fact that in nature many animals exhibit collective behaviours to move more efficiently and save energy. For example, birds fly in flocks when they migrate, fish swim together when they feed, and cyclists ride in a close group during a race to dramatically reduce drag. However, little is known about what happens when we consider collective behaviour in the microscopic world of bacteria. In particular, whether collective swimming is also the most efficient way of moving at the micro-scales, where viscous effects are dominant, is a question that until recently had no answer.

The mystery has been solved by researchers who investigated the collective swimming of bacteria. By measuring the three-dimensional velocity field, they discovered that by moving together bacteria can swim three times as fast, and that the mass transport in the suspension can be considerably improved.

Surprisingly, to achieve this enormous increase in speed, the bacteria used only a tiny amount of additional energy. The study was the first to discover that also for microorganisms collective swimming is the most energy-efficient way of moving and of absorbing oxygen and nutrients in nature.

A record-size quantum computing register

LC13841

- Quantum computing holds the revolutionary promise of exponentially speeding up such calculations as integer factoring and the simulation of quantum mechanical systems. Building a practical quantum computer will require a scalable number of individual quantum memory units ("Qbits" or "Qmodes"), all individually addressable and controllable with no error. In this paper, we demonstrated an experimental breakthrough: the generation of a record-size quantum register of 60 Qmodes, in 15 independently entangled (i.e. specifically correlated) sets, called cluster states. This massively scalable implementation of a quantum register was all-optical and based on an exotic laser, an optical parametric oscillator, which emitted quantum electromagnetic fields (the Qmodes) at equally spaced optical frequencies over an "optical frequency comb." Technical constraints limited the measured number of generated Qmodes to the reported 60 but we estimated the actual size of our quantum register to be of 180 to 600 Qmodes. This work is a major step toward entangling all these Qmodes together in a single cluster state, in order to achieve a scalable platform for a quantum computer.

Time-reversed optical parametric oscillation

LC13088

- The invention of the antilaser has attracted recently a considerable
attention from the scientific community and general media as well. The
antilaser is a device that acts like a laser in reverse and is thus
capable of completely absorbing coherent light beams instead of
scattering them as most other things do. Such a device might provide a
way to build miniature silicon optical switches or lead to new types
of photonic sensors.

In this Letter the author has shown rather generally that the main
idea of time-reversing a laser to perfectly annihilate coherent light
quanta is not peculiar to a laser system, and can be applied to other
kinds of optical instabilities. In particular, the time-reversed
process of optical parametric oscillation in a nonlinear crystal can
realize a kind of coherent perfect absorber for colored incident
signal and idler fields. In this device, two coherent light waves of
different colours can be fully annihilated in the crystal. As compared
to the antilaser, here photons are not converted into heat or some
internal energy of the medium, rather they are converted into photons
of a different colour. The idea of time reversing a generic kind of
optical instability is expected to be fruitful to built up a novel
class of optical devices similar to the antilaser but operating with
e.g. multicoloured light.

Solid-State Refrigerators and Materials to Convert Waste Heat to Energy

BB11707

- Innovations could improve the conversion of waste heat to energy.

By substituting some of the indium in CuInSe2 with manganese the thermoelectric properties of CuInSe2 have been improved by over two orders of magnitude. Thermoelectric materials work in two ways, by creating either electricity (Seebeck effect) or a temperature gradient (Peltier effect). These improved thermoelectric substances may be key in converting waste heat to energy, for example in automobile engines. Additionally, these materials may prove useful in developing new solid-state refrigerators, due to their advantages of being more compact, portable and environmentally friendly as compared to traditional liquid, ozone-depleting refrigerants. In this work, increased electrical conductivity has been observed in CuInSe2 by adding manganese, thus the thermoelectric figure of merit (ZT) of the material is increased. At the same time, the addition of manganese disrupts the normal atom locations in the material, resulting in a decrease in the thermal conductivity. This is exciting because most often the improvement in one of these properties is at the expense of the other. This work demonstrates that CuInSe2-based systems could be viable thermoelectric materials if the electrical conductivity could be even further enhanced. The improved system would be of interest to the automobile industry, for example, for the purpose of converting waste heat in the internal combustion engine to electricity.

Turbidity breaks the diffraction barrier

LB13391

“Turbidity” caused by multiple scattering is normally considered detrimental to optical imaging. For example, translucent media such as biological tissues and a ground glass make an object underneath invisible. In our study, we developed a method of extracting the original image information from the multiple scattering. This enabled us to see through the turbid media. More importantly, we made a counter-intuitive finding that optical turbidity, rather than being a hindrance to imaging, can in fact dramatically improve both the spatial resolution and the field of view of the target images. In essence, our method turns a turbid medium into a unique and unconventional lens. We believe that this will lead to great important applications in deep-tissue biological imaging and super-resolution imaging.

Figure caption:

Live cell imaging under a rat skin tissue. (a) the image of a microglia cell hidden under a skin tissue. (b) reconstructed image from (a).