What happens to electronic heat loss if the sample becomes so thin that
the lattice vibrations (phonons) can only move in the 2D plane?
This question was answered in an experimental study by Karvonen and
Maasilta with a somewhat counterintuitive result: The electrons can emit
their energy
to the lattice more effectively if the lattice is a thin 2D membrane.
To see this effect, the authors studied thin copper wires (down to 15
nm) on dielectric silicon
nitride membranes of varying thickness down to 30 nm at sub-Kelvin
temperatures. In the thinnest samples below 0.5 K, the electrons of the
Cu wires stayed
much cooler when external heat was applied, showing that the heat flow
between electrons and the lattice was enhanced. This improved dissipation
efficiency could help delicate low-temperature circuits such as quantum
bits, and may also be benefitial for the operation of solid-state
coolers. LD10965
***
Two of Einstein's discoveries related in an unexpected way
Einstein introduced in 1925 the notion of what is now called Bose-
Einstein condensation in gases, which was observed for the first time
in 1995 by a group in JILA. This started a whole new field of
research that is presently very active and led to two Nobel prizes in
the last ten years. In 1935, in a totally different context, namely
the foundations of quantum mechanics, Einstein, Podolsky and Rosen
(EPR) showed in a famous article that, if one accepts natural
notions of realism and locality, quantum mechanics does not describe
the whole of physical reality; additional variables (often called
"hidden variables") are necessary to make this description complete.
Bell showed later that, if one accepts the existence of these
variables, and if the predictions of quantum mechanics are always
correct, the variables have to evolve in a non-local way in some
cases. On the other hand, the "orthodox" interpretation of quantum
mechanics rejects the EPR reasoning, since it introduces the notion
of physical reality at the level of microscopic quantities, which is
considered as ill-defined. Bohr, the founder of this interpretation,
emphasized that microscopic objects in themselves do not possess
physical reality, which applies only to the whole system including
the microscopic particles and the measurement apparatuses.
A new twist in the story is that Bose Einstein condensates shed new
light on this celebrated debate. It has been pointed out recently (1)
that, if one transposes the EPR reasoning to interference between
Bose-Einstein condensates, the EPR additional variable is no longer
microscopic but macroscopic, since it describes the relative phase of
the arbitrarily large condensates. In this case, Bohr's argument does
not apply directly; one can then naturally wonder if this macroscopic
phase still gives rise to quantum non-local effects, or if non-local
effects are limited to microscopic physical variables such as the
spin of a few isolated particles. In a recent letter (2), Laloë and
Mullin have shown that the Bell theorem can be extended to Bose-
Einstein condensates and that quantum non-local effects do occur in
the context of the macroscopic phase, while it remains true that the
measurements themselves have to test this variable with microscopic
sensitivity.
What is surprising is that the non-local effects predicted by these
calculations do not vanish in the limit of very large Bose Einstein
condensates; actually, they remain as large as in the usual case with
two microscopic particles. Nevertheless, their observation requires
that the number of particles in both condensates be strictly equal,
and that the spins of all particles be measured. With present
experimental techniques, experiments with double condensates in ultra
cold gases each containing about 10 particles seem feasible. LD10985
***
Entanglement Percolation in Quantum Magnets
Quantum entanglement in interacting many-body systems has gained great
interest recently because of common aspects of quantum information,
condensed matter physics and quantum field theory. A fundamental
question is concerned with the scaling of the entropy quantifying the
degree of entanglement between two regions in a bipartite system. This
issue is well understood for one-dimensional systems, but only few
results are available in higher spatial dimensions and are mostly
restricted to non-interacting fermions or bosons. In this work the
entanglement entropy is determined, using an asymptotically exact
renormalization group treatment, for a two-dimensional quantum system
that consists of spin-1/2 Ising degrees of freedom coupled through
random nearest-neighboring interactions on a square lattice in a
transverse magnetic field. This system undergoes a quantum phase
transition at a certain transverse field strength, at which point the
von Neumann entanglement entropy of a subsystem violates the "area law",
providing evidence for non-trivial and long-range quantum entanglement
in the ground state. The entanglement entropy per surface area of a
subsystem diverges in a double logarithmic form, indicating a type of
percolation of the critical ground-state wavefunction which is
fundamentally different from classical percolation. The latter can be
found in an analogous quantum system with random bond dilution; here the
area law is valid at the quantum critical point, which implies
entanglement cannot be regarded as an indicator of criticality for
higher dimensional systems in the way as for one-dimensional cases. LD10982
***
The symmetry of aleatory gene expression
Randomness is a remarkable feature of molecular dynamics of cells and sometimes
is, naively, considered the opposite of symmetry. However we found a symmetry
principle governing the stochastic equations for the genetic expression of a
gene. A binary or spin-boson model for the gene is the main theoretical
framework. The ?spin up? state describe the unrepressed configuration in
which protein synthesis occurs freely. The presence of a regulatory protein
partially blocks the transcription-translation process corresponding to the
?spin down? state. The spin is surrounded by the synthesized proteins and
the fundamental quantities are the probabilities to find the ?spin up? or
?down? as a function of the time and protein number. Master equations
similar to birth and dead random process, common in the theory of chemical
reactions, are selected to describe the dynamics. Symmetries are not apparent
in the traditional discrete form of the equations, although they show up when
the master equations are recoded in terms of differential equations. The
dynamics takes an analogous form to the angular momentum theory. The invariance
here is a Lorentz-like symmetry and the invariant is the decay time of the
switch to equilibrium configuration. The traditional classification of noise in
terms of the Fano factor has an astonishing interpretation involving the
?angular momentum? components. Anyway, despite the elegance of the
formalism and the analyticity of the solutions the main point here is the
possibility to use the well established group theoretical machinery to deal
with complex networks. LD11411
***
Curious behavior of spin revisited
In 1975, the beautiful neutron interference experiments showed that when
neutrons are rotated and brought back full circle, they are not as they
used to be but acquire minus sign in their phase; a quite wired feature
of spinor (half-integer spin particle) wave function in quantum mechanics.
In this paper, we demonstrate the similar phase shift but for seemingly
trivial integer-spin particles, specifically, laser-cooled Rb atoms. For
the m=0 state of Rb atoms, i.e., the state with no net magnetization
along a given axis (quantization axis), there is no preferred direction,
i.e., the ``north pole” and ``south pole” are identified. Accordingly,
when m=0 atoms are flipped, they are exactly as they used to be except
for the phase factor. The phase factor is non-trivial -1 or trivial 1
depends on whether total spin is odd or even, i.e., their parity. This
is a quite curious feature of m=0 integer-spin wave function in quantum
mechanics as predicted by Robbins and Berry in 1994.
Our measurement reveals these features, which may provide a new scheme
for testing parity non-conservation in atoms thanks to the
parity-dependence of the phase. LE11527
***
Imaging electron flow in a ring
Imaging a system is an essential step towards its understanding. For example, the visualization of electron systems at the surface of metals, obtained by scanning tunneling microscopy (STM), yields a decisive insight into their physics. Here, we show the counterpart of STM for the case of a confined electron system. We use a new scanning probe technique, based on a very weak electrostatic perturbation of the electron system by a charged tip. We provide experimental images of the behavior of electrons inside a model device: a quantum ring buried several nanometers below the sample surface. Using quantum mechanical simulations, we demonstrate that our experimental data are directly connected to the local density of states inside the ring. Our investigation technique will help to design new kinds of quantum nanodevices based on a precise spatial control of electron interferences and trajectories. LE11496
***
Numerical evidence points to the inefficacy of travel restrictions for the containment of the global spread of pandemic influenza. A recent theoretical study explains why.
A key issue dominating the recent discussions on containment plans against a possible pandemic influenza is the effectiveness of restriction measures on the international air travel. Recent findings based on detailed data-driven numerical simulations pointed out the limited benefit provided by feasible travel flows reductions in slowing down the global spread of an infectious disease not contained at the source. The study by Colizza and Vespignani introduces a theoretical framework for the understanding of this phenomenon. They study the propagation of epidemics in reaction-diffusion systems that model spatially separated urban areas, connected by mobility patterns, e.g. travel flows of individuals between cities. Given an outbreak of an emerging infectious disease in a city, the virus may then propagate to several other cities brought by infected travelers. Travel restrictions are therefore aimed at halting the spread by reducing the amount of travelers. The study by Colizza and Vespignani quantitatively shows that the complex structure of realistic mobility patterns, characterized by the presence of highly connected cities, would favor the global spread of epidemics, notwithstanding huge reductions in the travel flows. The complex nature of our increasingly interconnected world is responsible for the failure of feasible travel restriction measures. LF11665
***
Quantum Dots are set in motion.
Dynamic quantum dots are created when surface acoustic waves pass across a
chip. Electrons can be confined within a dynamic quantum dot and carried
through a device at single-electron precision. This ability to move
individual electrons across a device may be used to transfer qubits around
a quantum computing circuit, as well as increasing our understanding of
fundamental electronic phenomena at sub-nanosecond timescales. But despite
their potential applications, the underlying physical properties of dynamic
quantum dots are not well known. In this paper we have measured the
non-equilibrium escape of electrons from dynamic quantum dots; this is the
first measurement of the quantum behaviour of electrons within a
dynamic-quantum-dot system. We have demonstrated that electrons can be
confined in the dynamic dot throughout a complex circuit for the first
time. By applying simple models to the device we were able to estimate
fundamental properties such as the tunnelling probability of electrons and
the addition energy of the dynamic quantum dot.
Image: Artistic impression of the device in operation. Surface gates and
the surface acoustic wave confine electrons (blue) into dynamic quantum
dots. At the tunnel barrier electron probability may tunnel out of the dot. LE11340
***
Accelerating & Shape Changing analytic Solitons can Describe DNA Promoter
Dynamics
Solitons are fascinating particle-like stable localized solutions of
nonlinear integrable systems like sine-Gordon (SG) model, which move without
changing their
velocity and shape. However, in realistic situations like DNA-promoter
dynamics, fluxons in Josephson junction, spin waves in ferromagnet, the
observed solitons
usually have more complex motion with acceleration and change of shape.
This happens due to inhomogeneities like
impurity, defect, nonuniformity of the media, which break the
inegrability of the system and therefore solutions can be extracted only
numerically or at best perturbatively.
We show in this paper that if the
shape of the soliton, induced by a variable mass, changes simultaneously
with its velocity
maintaining a certain relation between them, we can restore the
integrability of the variable mass SG model, both at the classical and exact
quantum level. Thus we can explain the complex dynamics of realistic
solitons through our exact analytic solutions, at certain limits
(see the enclosed figure: (file fig1c.jpeg)
Shape and velocity changing exact soliton of integrable variable mass
sine-Gordon equation).
Our explicit analytic result reveals, that the
position-dependent mass in integrable variable mass SG
model, can control the behavior of the soliton by
changing the boundary condition of the field. This in turn can change, most
unexpectedly, the
associated topological charge of the solution. Thus
the kink-solution with such inhomogeneity instead of its usual charge
Q=1 can exhibit the charge Q= 1/2, proposing a possible exact
integrable theory
of semi-fluxon, which has been observed recently in several experiments,
but not yet have any exact theoretical solution.
***
Catch a wave of the magnetic "snake".
Novel self-assembled snake-like patterns are created when magnetic
micro-particles floating on water are exposed to an alternating magnetic
field. These magnetic "snakes¿¿ possessing unique dynamic and magnetic
properties, are spontaneously created from short chains of magnetic
micro-particles as a result of the competition between magnetic and
hydrodynamic forces. Their internal structure and magnetic order can be
tailored with an external magnetic field. The birth and existence of the
¿snake¿ is driven by pattern induced waves on the water surface. Strong
induced vortex flows on the surface of water controllable through the
parameters of the external excitations finalize the rich hydrodynamic
picture of the self-assembled magnetic ¿snake¿.
The self-assembled materials that automatically arrange themselves into
useful patterns in a controlled electric and/or magnetic field
environment have tremendous potential as components in micro- and
nano-scale devices. They can be applied to fabricate the next generation
of magnetic recording media, transparent conductors based on
self-assembled conducting networks of magnetic micro-particles and
micro-fluidic devices for bio-analysis where an alternating magnetic
field can be used to assist transportation of micro-particles through
micro-channels in biochips.
The attached image demonstrates self-assembled magnetic "snake" and
velocity pattern associated with it. LE11317
***
Bistability in the shape transition of strained islands
In 1993 Tersoff and Tromp at the IBM T.J. Watson Research Center Yorktown Heights proposed a novel route to create nanowires on a surface. In a frequently cited PRL publication [PRL 70, 2782] they demonstrated that when a growing epitaxial island is lattice mismatched with respect to the substrate, the island will initially have a compact shape governed by the ratio of the step energies. However, beyond a critical size the strain energy resulting from the lattice mismatch becomes the dominant factor. As a result the island prefers to maximize its perimeter, resulting in extremely elongated island shapes, i.e. nanowires. The transition from the compact to the elongated shape has always been assumed to be a smooth transition. Li, Liu and Lagally [PRL 85, 1922] provided a quantitative description of the energetics of a two-dimensional strained island in the vicinity of the critical island size. Based on this model Zandvliet and van Gastel demonstrate that under a particular set of conditions the transition from compact to elongated is more complicated than previously anticipated. In fact, compact and elongated shapes can coexist at one and the same temperature. Their results demonstrate that the precise shape of a strained island can depend on its history and how it was formed, leading to hysteresis. This fundamental conclusion implies that a novel route is available to create two-dimensional nanostructures that would otherwise not be thermodynamically stable.
***
Continuous macroscopic limit of a discrete stochastic model for interaction of living cells
The Cellular Potts Model (CPM) is a widely used method for modeling
multicellular biological systems. The CPM is a mesoscopic model,
representing cells as individual extended entities, reversibly adhering
to one another and moving in chemical fields so as to minimize free
energy. In this paper the authors present a macroscopic model
based on the continuous limit of the CPM under certain
biologically applicable conditions. The derived macroscopic model
facilitates parameter search and estimation of qualitative behaviors
of large numbers of cells, which are computationally prohibitive in
implementations of the classic CPM. Numerical analysis employing the
macroscopic model resulted in determination of conditions promoting
formation of a lattice-like aggregation pattern of a nonconfluent field
of cells, an arrangement applicable to development of spongy bone via
the intramembranous route. In contrast to earlier suggestions
that the trabecular arrangement of spongy bone is based on pre-existing
vascular patterns, or later-forming patterns of mineral deposition, the new
model suggests that it can arise from the self-organizing behavior
of mesenchymal cells interacting with their extracellular matrix. LC10892
***
The early promise of nanofluids (colloidal suspensions of nanosized particles),
as an advanced nanoengineered coolant, has been recently tempered with a
persistent lack of understanding of the thermal conduction mechanisms. In a
recent microconvection theory, it is hypothesized that the random motion of the
diffusing nanoparticles is an efficient source of fluid convection that can
increase the heat transfer capability of the surrounding fluid. A striking
simplicity and a similarity to the macroscopic heat transfer concepts have made
the theory popular in the engineering world. In our experimental work, we
disprove the central tenants of the microconvection hypothesis and show that
the convection effects at the microscales cannot influence the thermal
conductivity of a nanofluid. Our collaborative effort affirms the molecular
nature of nanofluid thermal conduction through a close match of the
experimental data against the predictions of classical mean-field theory and
extensive molecular dynamics simulations. LB10849
***
Ultracold polar molecules confined with a TWIST
The Thin WIre electro-Static Trap (TWIST) is a recently implemented design that
enables electrostatic trapping of polar molecules spatially superposed to
a magneto-optical trap (MOT). As the name suggests, the TWIST is made of thin
wires, minimally perturbing the light fields of the MOT.
This unique feature makes it possible to instantaneously trap molecules created
via photoassociation from the ultracold atoms confined in the MOT, without the need
for a loading scheme.
The complementary nature of the trapping mechanisms of TWIST (electric fields)
and MOT (magneto-optical fields) allow for independent optimization and operation,
as the molecules are not affected by the magnetic field and neither the atoms by the
electric field.
The spatial overlap of TWIST and MOT is also crucial for the next step:
We plan to cool the remaining atoms that are not photoassociated into molecules further
via evaporative cooling and use them as a thermal bath to reduce the temperature
of the trapped molecules further,possibly all the way into the quantum degenerate regime. LE10868
***
Will chromium ions replace silicon in future computers?
Current semiconductor computer technology is slowly reaching its limits of performance. The new magic term for future alternatives is ¿quantum computing¿,
where we enter the fascinating realm of quantum mechanics, and of quantized properties of materials. Here we show that a material containing chromium, a
transition metal ion, is one of the candidates to replace silicon in future computers.
Current processors are based on 65 nm technology (about 2.5 millionth of an inch) which corresponds to about 400 times the typical distance between atoms
in a material. The limits of this technology, which is based on classical charges and currents, are on the horizon. At these very small scales quantum
effects start playing a role, and whether we like it or not we need to take quantum mechanics into account. While this may be not such a good thing for
current semiconductor technology, it also opens the door to fascinating new possibilities, and the field of quantum information technology is thriving.
In a classical computer information is stored in bits, and can have the value of 0 or 1. In a quantum computer on the other hand, quantum bits (qubits)
are the elementary units and they are described using a set of two quantum wavefunctions, |0> and |1>. Due to its quantum nature, a qubit can exist in
any superposition of |0> and |1> and thus can have an infinite number of possible states instead of just 1 or 0. This opens powerful possibilities for
a computer based on these qubits.
However, interactions with the environment can change the state of this qubit, which would lead to the wrong answer. It is therefore crucial to find
qubits that have very small interactions with their environment, so that the state of the qubit stays preserved over a sufficiently long time
(coherence time). A very promising candidate for a qubit is the magnetic moment (electron spin) associated with an electron.
In this paper, we introduce a new solid state material in which qubits interaction with their local environment, mostly the magnetic fields of
neighboring nuclei and electrons, can be suppressed to such an extent that quantum superpositions are detectable even at room temperature.
Starting from solution, crystals of K3NbO8 doped with Cr5+ ions have been obtained. Cr5+ ions carry a spin with projection ±1/2 defining the two
qubit states. Rabi oscillations are observed for the first time in a spin system based on transition metal oxides up to room temperature. At liquid
helium temperature the coherence time reaches ~ 10 microseconds and enables about 500 operations before the qubit state is too much disturbed by its
environent. This shows that a diluted ensemble of Cr:K3NbO8 is a potential candidate for solid-state quantum information processing.
Figure Caption: Crystalline structure of Cr doped K3NbO8 with only electronic spins S=1/2 at Cr sites. Pulsed temporal variations of local electromagnetic
field (suggested in black) are used to control the quantum superposition of the two spin states known as |0> and |1>. The background shows measured Rabi
oscillations (between states |0> and |1>) at 4 K. LC11463
