Separation device

A newly proposed separation devices has come out (http://physics.aps.org/articles/v5/6).

Rubí and Peter Hänggi of the University of Augsburg, Germany, led a team that has developed a new approach to these ratchet sorters. They start with a mathematical framework in which the entropy of the system is treated like potential energy, with entropy “barriers” that repel particles. These are regions where particles are restricted to a small space, which reduces the number of states (locations and velocities) that a particle can occupy. Fewer states means lower entropy. Like balls rolling down a hill, particles tend to move away from these low entropy spots.

The team applies this formalism to a tube with walls that periodically ramp from a narrow diameter to a wide diameter and back, with an asymmetric or “sawtooth” profile. This shape forms distinct but still connected chambers, or segments, each of which is a few microns long. Entropic barriers inhibit travel between segments; however, the barriers are steeper going to the left, so the net motion of the particles is to the right.

In order to clearly see the entropic effect in their computer simulation and analytical calculations, the researchers apply an oscillating force that essentially shakes the particles back and forth inside the tube. In a real experiment, this force could be an oscillating electric field.

Resistance becomes lower under pressure

The property of matter often changes drastically as external knobs such as temperature (goes lower) and pressure (gets raised) are tuned (http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.108.026403).

Wustite (FeO) is a prototype for the iron-bearing minerals found in the Earth. Though FeO is insulating at ambient conditions, in the late 1980s researchers observed it undergo a transition to a metallic state when compressed by shock waves. The nature of this transition has, however, been unclear.

In a paper in Physical Review Letters, Kenji Ohta of Osaka University, Japan, and colleagues report their combined theoretical and experimental attack on the problem. The research team measured high-temperature resistivity and structural x-ray diffraction patterns of FeO in a diamond anvil cell to simulate conditions in Earth’s mantle. At a temperature of 1900 kelvin and pressure of 70 gigapascals, Ohta et al. were able to watch as FeO in a rocksalt atomic structure became metallic without any structural changes.

To understand these findings, Ohta et al. performed density-functional calculations of electrical conductivity as a function of temperature and pressure. The results suggest that their observations are consistent with a new kind of insulator-metal transition involving fluctuations between a high-spin state to a low-spin state in the FeO. For geophysicists, this makes the picture of conductivity deep in the Earth richer: both insulating and metallic phases must be added to the phase diagram, with potential implications for thermal and electrical conductivity, and in turn models of the planetary magnetic field. –David Voss

Classical Not Always Lose

It was shown that, classical physics does as efficiently as quantum physics in energy transfer in biological systems [http://physics.aps.org/synopsis-for/10.1103/PhysRevE.83.051911].

A prominent goal of quantum information and computing is to be able to exploit quantum entanglement in qualitatively new devices, such as massively parallel computers. Has biological evolution already harnessed entanglement for its own purposes? Recent studies have indeed suggested that electronic excitation transfer (EET) in photosynthesis benefits from quantum entanglement. Now, a paper appearing in Physical Review E is likely to stimulate further investigation and controversy on this question. Based on calculations, John Briggs and Alexander Eisfeld, of the Max Planck Institute for the Physics of Complex Systems in Dresden, Germany, assert that under the conditions prevailing in photosynthesis (in particular, in the so-called Fenna-Matthews-Olson complex that lies at the heart of the process), energy transfer in a classical system is just as efficient as in its quantum counterpart.

To model the photosynthesis that occurs in plants, Briggs and Eisfeld study a collection of monomers, each possessing a single electronic state and coupled to its neighboring units by a dipolar interaction. The authors find that for dipolar interactions similar to those found in real molecular aggregates, the coherences in quantum transport (from the Schrödinger equation) are identical to those occurring in classical transport according to Newton’s equation. Although their analysis neglects the influence of the environment, the authors report that calculations including dephasing processes in the quantum and classical equations lead to the same conclusion. – Ron Dickman

Spins coupled to a mechanical resonator

Employing the spin-phonon coupling, they argued that, a tiny magnet welded with a torsional mechanical oscillator can be described by a spin-boson model [PRL, 106:147203(2011)]. What interests me is actually this, is it possible to filter spins using magnetoptical coupling schemes ? Especially, what is the implication for the DNA filtering effects that were reported earlier in this blog. See also the review [Phyics, 4:28(2011)].

The orbit of photons around black holes

Black hole distorts the space-time on its periphery drastically. This distortion is manifest in everything moving nearby, including photons. A photon is a spin-1 boson, and its orbit can be computed using geodesic equation. Due to the distortion, a photon shall gain excess angular momentum in the course of orbiting. And the trajectory can be very spiral, as this numerical study exposes [http://www.nature.com/nphys/journal/v7/n3/full/nphys1938.html?WT.ec_id=NPHYS-201103].

A photon emitted near a rotating black hole feels the ground beneath it swirl around. Try to run over a rotating surface, such as the platform of a merry-go-round, and you will not only find yourself fighting the Coriolis force; your body follows the rotation and you stagger and stumble. A photon does not stumble, but rotating spacetime can impart to it an intrinsic form of orbital angular momentum (OAM) distinct from its spin. Like other forms of orbital angular momentum, the photon's OAM is quantized by integer multiples of ħ, not just ±ħ. One can visualize OAM by the wavefronts of this twisted light⁷, which are not planar but rather resemble a cylindrical spiral staircase, centred around the light beam (Fig. 1). The intensity pattern of twisted light transverse to the beam shows a dark spot in the middle — where no one would walk on the staircase — surrounded by concentric circles. The twisting of a pure OAM mode can be seen in interference patterns, which show a fork-like structure of partially broken mirror symmetry.

Trapped ions realize coupled harmonic oscillators

I highlight this work just because it was done at nearly the same time by two distant groups, one in US and the other in Austria. Both published their work in Nature. They demonstrated the potential of trapped ions in quantum computing.

[doi:10.1038/nature09800] More than 100 years ago, Hertz succeeded in transmitting signals over a few metres to a receiving antenna using an electromagnetic oscillator, thus proving the electromagnetic theory¹ developed by Maxwell. Since this seminal work, technology has developed, and various oscillators are now available at the quantum mechanical level. For quantized electromagnetic oscillations, atoms in cavities can be used to couple electric fields^{2, 3}. However, a quantum mechanical link between two mechanical oscillators (such as cantilevers^{4, 5} or the vibrational modes of trapped atoms⁶ or ions^{7, 8}) has been rarely demonstrated and has been achieved only indirectly. Examples include the mechanical transport of atoms carrying quantum information⁹ or the use of spontaneously emitted photons¹⁰. Here we achieve direct coupling between the motional dipoles of separately trapped ions over a distance of 54 micrometres, using the dipole–dipole interaction as a quantum mechanical transmission line¹¹. This interaction is small between single trapped ions, but the coupling is amplified by using additional trapped ions as antennae. With three ions in each well, the interaction is increased by a factor of seven compared to the single-ion case. This enhancement facilitates bridging of larger distances and relaxes the constraints on the miniaturization of trap electrodes. The system provides a building block for quantum computers and opportunities for coupling different types of quantum systems.

[doi:10.1038/nature09721] The harmonic oscillator is one of the simplest physical systems but also one of the most fundamental. It is ubiquitous in nature, often serving as an approximation for a more complicated system or as a building block in larger models. Realizations of harmonic oscillators in the quantum regime include electromagnetic fields in a cavity^{1, 2, 3} and the mechanical modes of a trapped atom⁴ or macroscopic solid⁵. Quantized interaction between two motional modes of an individual trapped ion has been achieved by coupling through optical fields⁶, and entangled motion of two ions in separate locations has been accomplished indirectly through their internal states⁷. However, direct controllable coupling between quantized mechanical oscillators held in separate locations has not been realized previously. Here we implement such coupling through the mutual Coulomb interaction of two ions held in trapping potentials separated by 40 μm (similar work is reported in a related paper⁸). By tuning the confining wells into resonance, energy is exchanged between the ions at the quantum level, establishing that direct coherent motional coupling is possible for separately trapped ions. The system demonstrates a building block for quantum information processing and quantum simulation. More broadly, this work is a natural precursor to experiments in hybrid quantum systems, such as coupling a trapped ion to a quantized macroscopic mechanical or electrical oscillator.

Relativity and the Lead-Acid Battery

This relation may seem at first glance quite unusual. For most, relativity needs be concerned only when the considered speed becomes comparable to that of light. The Lead -acid battery seems having no attachment to this speed. However, if you look into the electrons that are busy inside the substance, you may change your mind. Lead is a heavy element, and relativity effects, as you can derive from Dirac's equation, scale as the quartic power of the atomic numbers. That is how relativity plays a role, which proves crucial for your cars to start, in Lead-Acid Battery [http://prl.aps.org/pdf/PRL/v106/i1/e018301].

The energies of the solid reactants in the lead-acid battery are calculated ab initio using two different basis sets at nonrelativistic, scalar-relativistic, and fully relativistic levels, and using several exchange correlation potentials. The average calculated standard voltage is 2.13 V, compared with the experimental value of 2.11 V. All calculations agree in that 1.7–1.8 V of this standard voltage arise from relativistic effects, mainly from PbO2 but also from PbSO4.

Polar thin film is not ferroelectric

Epitaxial thin film of strontium titanate on a silicon substrate is now demonstrated non-ferroelectric, despite its spontaneous polarization [PRL 105, 217601 (2010)]. This polarization is not due to spontaneous symmetry breaking, rather it is a property of the interface ground state. The symmetry is absent from the outset in the presence of the substrate, which strains the film. Therefore, the polarization is not switchable. This finding is made by DFT computations and STEM techniques.

We use SrTiO3=Si as a model system to elucidate the effect of the interface on ferroelectric behavior in epitaxial oxide films on silicon. Using both first-principles computations and synchrotron x-ray diffraction measurements, we show that structurally imposed boundary conditions at the interface stabilize a fixed
(pinned) polarization in the film but inhibit ferroelectric switching. We demonstrate that the interface chemistry responsible for these phenomena is general to epitaxial silicon-oxide interfaces, impacting on the design of silicon-based functional oxide devices.

Thermofluidic effects in nanochannels

When a fluid is subject to a temperature gradient, a velocity field can be generated, a phenomenon known as convection. It should be noticed that, such phenomena parallel what happens to electrons in a metal: temperature gradient drives electrical current under the name of thermoelectric effect. Now as things can be made smaller and smaller, it becomes an interesting subject to investigate the so-called micro- or nano-fluidic flow: liquid flow through a micro-size or nano-size channel. In this study by researchers from Hong Kong [PRL 105, 174501 (2010)], they used molecular dynamics simulations to examine a nano fluid housed in a nano-channel with particularly designed walls: the wall consists of two parts, the left and the right one, with respective surface energies, and a temperature gradient is held symmetric with respect to the border between the left and right wall. Their study showed that, an asymmetric flow can be generated with this temperature gradient provided the variance of surface energies is big enough. This is funny and many possibilities can be imagined to broaden their studies.

Simulation of Dirac equation

This is an experimental work [1] on mimicking the motion of a particle that is subject to Dirac-type model. It managed to detect a quivering motion, the so-called Zitterbewegung, which is understood to arise from the interference between positive energy components and negative energy components. Such quivering occurs at very high frequency (ten powered to 21 Hz) and with very tiny amplitude (of the order of Compton wavelength) for electrons in vacuum, and very difficult to observe. In this work, the authors considered the Dirac equation in 1+1 dimensions, which was realized with a single trapped ion. For such a 'relativistic' ion, the oscillation period is couple of microseconds.

Experimental recipes:
(1)trap a single 40Ca1 ion in a linear Paul trap22 with axial trapping frequency vax52p31.36MHz and radial trapping frequency vrad52p33 MHz;
(2)prepare the ion in the axial motional ground
state and in the internal state jS1/2,mJ51/2æ (mJ, magnetic quantum
number).
(3)identify spinor states.

How to measure the average position of the trapped ion without a full reconstruction of the state:
To measure the position for a motional state rm, we have to
(1) prepare the ion’s internal state in an eigenstate of sy,
(2) apply a unitary transformation, U(t), that maps information about rm onto the internal states;
(3) record the changing excitation as a function of the probe time t, by measuring
fluorescence22.

Results are displayed on the figures.


[1]Vol 463|7 January 2010| doi:10.1038/nature08688