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.