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发帖数: 220 | 1 Dear Readers,
We are happy to present the first five papers that are published in our
new, open access journal, Physical Review X. These papers, together with
others that are to be published in the coming weeks into September, will
constitute the first issue of the journal. Under PRX's open access
publishing model, they are free for you to read and use.
We are very encouraged by the breadth of their topical spread. It ranges
from the well-established field of atomic, molecular and optical physics to
the still relatively new, broad and very active field that explores
magnetism or spins at microscopic level. It also extends into the
interdisciplinary area: In the paper by Belik et al., statistical physics is
applied to understand epidemic spreading; and in another by Benmore and
Weber, experimental techniques such as acoustic levitation and x-ray
scattering are used to obtain and characterize normally hard-to-make
amorphous forms of pharmaceutical drugs. We are also pleased by the high
scientific quality and potential significance of these contributions.
Equally heartening to us during the past few months has been the interest
and support that numerous researchers have shown our new journal in terms of
their submissions, their refereeing efforts and their expressions of good
will. We thank them most sincerely.
The five papers are too small a sample to be a basis for making a
generalization about the future of Physical Review X. But they reflect
APS's commitment to making Physical Review X a journal of scientific
breadth and excellence. We are confident that, with the continuing and more
focused effort of the editors and the Editorial Board to attract and select
outstanding papers and with an increasing support of the physics community,
Physical Review X can only grow stronger.
We and the Editorial Board invite you to submit some of your best works to
Physical Review X.
Best wishes.
Jorge Pullin, Editor
Ling Miao, Associate Editor
Articles
•
Natural Human Mobility Patterns and Spatial Spread of Infectious
Diseases
Vitaly Belik, Theo Geisel, and Dirk Brockmann
Planning containment strategies for emergent epidemics, as epitomized by the
recent H1N1 pandemic, requires efficient forecasts with answers to three
basic questions: How many people will be infected, where, and when? To
answer the last two questions requires the knowledge of the effective speed
of a spreading epidemic. Physical models can relate that speed to key
parameters of the underlying processes. A class of frequently used models
are the so-called reaction-diffusion models, where “reaction” refers to
infection and where the motion of people is assumed to be “diffusion—a
type of random motion.” These models typically predict that the speed
increases with the magnitude of the diffusion. Human mobility, however, is
strikingly different from the assumed diffusion. This fact challenges
predictions of these models and puts their universal features into question.
The main approach described in this paper replaces the diffusion model by a
more realistic one for human mobility patterns. In the new model,
individuals have their own home bases and typically frequent only a limited
number of places from those bases—a very different mobility pattern from
diffusion. This more realistic description leads to a number of predictions
fundamentally different from those of the reaction-diffusion models: One,
there is an upper bound on the speed of a spreading epidemic no matter of
how high the overall mobility in the system of moving/residing individuals
is. This means that the reaction-diffusion models may overestimate the
spreading speed considerably. Two, there exists a new type of outbreak
threshold in how frequently individuals travel between different places.
Both of these effects show up robustly even when the specifics are varied
about how different places or populations are connected. These insights are
not only important for the development of containment strategies, but also
lay the foundation for improved computational models designed to forecast
future epidemics. And, beyond human epidemiology, the work should also find
potential applications in a wide range of scientific problems in human or
animal ecology, population dynamics, and evolution.
Published 8 August 2011 (5 pages)
011001
•
Ensemble of Linear Molecules in Nondispersing Rotational Quantum
States: A Molecular Stopwatch
James P. Cryan, James M. Glownia, Douglas W. Broege, Yue Ma, and Philip H.
Bucksbaum
When a simple diatomic molecule such as nitrogen and oxygen is exposed to a
strong laser whose electromagnetic fields are linearly polarized, its axis
tends to line up in the direction of the electric field of the laser. In
this sense, each molecule can be thought of as a quantum rotor that
experiences a torque from the aligning laser field. This paper presents a
theoretical proposal for creating a “molecular stopwatch” by exploiting
this general principle. The essential idea is that if a laser with a
rotating, but linearly polarized electric field can be generated, then a
quantum-rotor molecule exposed to such a laser will also rotate, as its axis
will be pulled along by the rotating electric field. Specifically in the
proposal, two circularly polarized, counter-rotating laser pulses with
different frequencies, but with the same direction of propagation, are used
to generate a composite rotating field. The speed of the field’s rotation
can actually be controlled by the frequency difference between the two laser
pulses.
It turns out that more intricate control and manipulations of quantum rotors
are possible. If the two generating laser pulses are turned on in a
particularly controlled way, quantum-rotor molecules that all start out in
their zero-field, “zero-rotation” ground state get into a state of rapid
rotation through an exercise of quantum gymnastics called “rapid adiabatic
passage.” They spin around their centers, with their axes almost completely
confined in the plane perpendicular to the direction of propagation of the
lasers—like the “hands of stopwatches.” But, the molecular axes “wobble
” a bit out of the plane, and the extent of the “wobbling” in different
directions—spoken of in the sense of quantum averages—defines the shape of
the hands. By adjusting the laser parameters, the shape can be made narrow
or broad, or even to split.
The rotating “molecular stopwatch” is an example of the richness of
quantum interactions between molecules and laser fields. It can also be used
to probe the properties of molecules under external influences, such as the
collision or absorption of short x-ray pulses, or molecular dissociation
caused by intense laser pulses.
Published 8 August 2011 (6 pages)
011002
•
Landau-Zener-Stückelberg Interferometry of a Single Electronic
Spin in a Noisy Environment
Pu Huang, Jingwei Zhou, Fang Fang, Xi Kong, Xiangkun Xu, Chenyong Ju, and
Jiangfeng Du
A nitrogen-vacancy center in diamond is a defect consisting of a nitrogen
atom substituting a carbon atom and an adjacent vacant site where the carbon
atom is missing. The electronic spin localized at such a nitrogen-vacancy
center has been considered as one of the most promising candidates for a
qubit in solid-state quantum systems in quantum memory and processing.
Coherent control of single electronic spins of nitrogen-vacancy centers is
therefore a crucial step in exploiting the spins as qubits. In this paper,
we report coherent control of a single spin in a nitrogen-vacancy center by
way of an experimental realization of the general concept of the well-known
Landau-Zener-Stückelberg interferometry.
The Landau-Zener-Stückelberg interferometry refers to a completely quantum
phenomenon that occurs when a system of two separate energy levels, State “
0” and State “1,” say, is driven by an external time-dependent field.
When the field strength is adjusted over time, for example, turned up to
begin with, and then turned down, the system starting out from State 0
splits at some point into a superposition of State 0 and State 1, similar to
how an optical beam is split into two beams at a beam splitter. Further
downsweep of the field followed by a return upsweep gives the two split
states different phases—or in the analogy of “beams,” different lengths
of propagation—so that at another special point of time or field strength
the two states are brought to interfere, as the split beams do when they are
brought together again.
In the particular context of control of a single electronic spin in a
nitrogen-vacancy center, State 0 and State 1 are associated with two states
of the spin and the energy gap between them is generated by a microwave
field. A slowly varying AC field acts as the time-dependent driving field.
And the interference shows up as an oscillatory pattern of the probability
of the spin to stay in State 0, measured by recording the photoluminescence
emitted from the spin system through 105 experimental runs. The degree of
the splitting and the interference pattern can be controlled by controlling
a number of parameters in the experiment, for example, the frequency of the
microwave field. This means that the probability of State 0 becoming State 1
and the probability of retrieving State 0 in the end can all be controlled
at will.
Published 8 August 2011 (5 pages)
011003
Amorphization of Molecular Liquids of Pharmaceutical Drugs by
Acoustic Levitation
C. J. Benmore and J. K. R. Weber
Making fast acting drugs is a goal of almost every pharmaceutical company.
The route of delivering them in the forms of amorphous solids has long been
recognized as a possible way to enhance dissolution rates, increase
solubility, and bioavailability. The development in this direction is
becoming increasingly important due to the emergence of many new drugs that
are virtually insoluble in their crystalline forms. In this experimental
paper we exploit the technique of acoustic levitation of liquid droplets and
present two new methods for forming amorphous solids from molecular liquids
and solutions of a wide range of pharmaceutical drugs of varying chemical
structures and different functions. One method combines acoustic levitation
with solvent evaporation and produces amorphous gels of the drugs; the other
integrates laser-heating induced melting and subsequent cooling with
acoustic levitation and turns drugs that are usually obtained in crystalline
, functionally less effective forms to more desirable amorphous forms (a
process also known as vitrification in materials science and engineering).
Proof-of-principle applications of the two containerless methods are
demonstrated with in-situ characterizations of the samples by use of high-
energy x-ray diffraction at the Advanced Photon Source.
We anticipate that such containerless processing methods, combined with
sophisticated, high-throughput droplet-forming or dispensing methods, may
provide practical routes for scaled-up productions of amorphous drugs.
Published 8 August 2011 (7 pages)
011004
See accompanying Physics Synopsis
•
Quantum Entanglement of a Tunneling Spin with Mechanical Modes of
a Torsional Resonator
D. A. Garanin and E. M. Chudnovsky
It is well known that south and north magnetic poles of a small magnet can
interchange due to thermal noise. This effect is called superparamagnetism
and is a phenomenon of classical (as opposed to quantum) physics. In the
quantum world, magnets of smallest sizes, for example, magnetic molecules or
nanomagnets, show superparamagnetism of a different origin: Their magnetic
poles would interchange periodically as the result of quantum tunneling,
even when the temperature is at absolute zero. In this theoretical paper we
study what happens to a quantum magnet, in particular, to its magnetic poles
, when it is placed in an external magnetic field and on a quantum
nanocantilever that rotates in an oscillatory fashion about an axis aligned
parallel to the magnetic field. The magnetism of the quantum magnet comes
from aligned spins of the electrons in it, and the resulting total spin
behaves as a quantum rotator in the aligning magnetic field. The composite
system acts therefore like two quantum gyroscopes coupled together: one
being the mechanical rotational oscillator that carries the nanomagnet and
the other being the total spin of the nanomagnet.
The fundamental description of such a quantum system is a Schrödinger
equation tailored for it. Solving the Schrödinger equation leads us to
a lot of insights, not least into how the flipping of the magnet’s poles (
or its total spin) is influenced by the size and the frequency of rotation
of the mechanical oscillator. It turns out that only when the mechanical
rotational oscillator is sufficiently heavy does the nanomagnet display
superparamagnetism. In other words, when the rotational oscillator is light,
magnetic-pole flipping becomes frozen. Moreover, and interestingly, if the
rotational frequency is matched in a particular way with the difference in
the energies of the north-pole-north, south-pole-south and the north-pole-
south, south-pole-north states, the magnetic-pole flipping is predicted to
appear together with a splitting of a certain mechanical mode of the
rotational oscillator. The splitting can be detected experimentally.
This theoretical work should add fuel to the currently active research on
molecular quantum spintronics.
Published 8 August 2011 (7 pages) | x******i
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【在 w*****1 的大作中提到】
: Dear Readers,
: We are happy to present the first five papers that are published in our
: new, open access journal, Physical Review X. These papers, together with
: others that are to be published in the coming weeks into September, will
: constitute the first issue of the journal. Under PRX's open access
: publishing model, they are free for you to read and use.
: We are very encouraged by the breadth of their topical spread. It ranges
: from the well-established field of atomic, molecular and optical physics to
: the still relatively new, broad and very active field that explores
: magnetism or spins at microscopic level. It also extends into the
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