Microwaves to entangle trapped ions

This is definitely a very important step toward practical quantum computing. Rather than using laser, which are not easy to set up as required, these groups [The work is described in Nature 476 181 and Nature 476 185.] use microwaves that can created by just alternating currents, to entangle ion spins.

However, entangling two trapped ions had required a pair of carefully aligned ultraviolet laser beams – which cannot be produced easily on an integrated circuit. To entangle two pairs of ions, two pairs of laser beams were needed and so on. A practical quantum computer would need a processor containing thousands or even millions of qubits, so scientists have long sought a way to manipulate many trapped ions without large numbers of laser beams.

In 2001 Christof Wunderlich and a colleague of the University of Hamburg had the idea of replacing the lasers with microwave and radio sources, which can be produced and controlled much more easily. Such radiation had previously been used in other trapped-ion experiments, but using it to implement quantum logic operations was a highly revolutionary suggestion. This is because the type of interaction required for quantum logic is usually very weak for this radiation. However, the researchers suggested adding a magnetic field gradient to stimulate the interaction.

Unfortunately, the need to use states that are sensitive to static magnetic fields makes the quantum states vulnerable to the magnetic noise found all around us, and the technique proved problematic. In 2008 physicists at the Ion Storage Group at the National Institute for Standards and Technology (NIST) in Boulder, Colorado, proposed eliminating the static magnetic-field gradient and using instead the oscillating field produced by the microwave source itself. The benefit being that the quantum states used in this scheme are less vulnerable to magnetic noise and more robust.

Both research groups now report significant advances in the journal Nature. Wunderlich's group, now at the University of Siegen, together with colleagues from the Institute of Theoretical Physics in Ulm, have come up with a way to produce states that, while still sensitive to the applied magnetic-field gradient, are far less vulnerable to noise and thus can be preserved more than 100 times longer. In a commentary accompanying the papers, Winfried Hensinger of the University of Sussex compares the group's scheme to a car's suspension system, which decouples the body from the wheels so that bumps in the road do not disturb the driver.

The NIST group, meanwhile, goes further and performs all of the essential quantum logical operations (albeit on only two qubits) using microwave radiation delivered via a waveguide integrated into a chip. "We've integrated the mechanism that does the entanglement between the two ions into the trapping structure," says Christian Ospelkaus, who built the experiment together with colleagues at NIST. "We no longer need to build a really complex and sophisticated laser system around the whole camp: we just send an electric current through the trap structure and that generates oscillating fields and it does all the other coherent quantum operations we need to do."[http://physicsworld.com/cws/article/news/46826]

Subwavelength focus of sound

In focusing waves, one is often faced with the so-called diffraction limit as a result of the wave nature, which limits the resolution when seeing objects using waves. Now there came an interesting study beating this limit by focusing sound into a 1/25th wave length spot. Remarkably, this is attained with Coke cans !

Sound, like light, can be tricky to manipulate on small scales. Try to focus it to a point much smaller than one wavelength and the waves bend uncontrollably — a phenomenon known as the diffraction limit. But now, a group of physicists in France has shown how to beat the acoustic diffraction limit — and all it needs is a bunch of soft-drink cans.

Scientists have attempted to overcome the acoustic diffraction limit before, but not using such everyday apparatus. The key to controlling and focusing sound is to look beyond normal waves to 'evanescent' waves, which exist very close to an object's surface. Evanescent waves can reveal details smaller than a wavelength, but they are hard to capture because they peter out so quickly. To amplify them so that they become detectable, scientists have resorted to using advanced man-made 'metamaterials' that bend sound and light in exotic ways.

Some acoustic metamaterials have been shown to guide and focus sounds waves to points that are much smaller than a wavelength in size. However, according to Geoffroy Lerosey, a physicist at the Langevin Institute of Waves and Images at the Graduate School of Industrial Physics and Chemistry in Paris (ESPCI ParisTech), no one has yet been able to focus sound beyond the diffraction limit away from a surface, in the 'far field'. "Without being too enthusiastic, I can say [our work] is the first experimental demonstration of far-field focusing of sound that beats the diffraction limit," Lerosey says.

Lerosey and his colleagues took a similar approach to an experiment they performed in 2007 and later described theoretically for electromagnetic waves^1,2. The group generated audible sound from a ring of computer speakers surrounding the acoustic 'lens': a seven-by-seven array of empty soft-drink cans. Because air is free to move inside and around the cans, they oscillate together like joined-up organ pipes, generating a cacophony of resonance patterns. Crucially, many of the resonances emanate from the can openings, which are much smaller than the wavelength of the sound wave, and so have a similar nature to evanescent waves.

To focus the sound, the trick is to capture these waves at any point on the array. For this, Lerosey and his team used a method known as time reversal: they recorded the sound above any one can in the resonating array, and then played the recording backwards through the speakers. Thanks to a quirk of wave physics, the resultant waveform cancels out the resonance patterns everywhere — except above the chosen can.

After the playback, the can continues to resonate by itself, scattering out the sound energy left inside. Normal waves scatter efficiently, so they disappear quickly. However, the evanescent-like waves are less efficient at scattering, and take roughly a second to make it out of the can — a prolonged emission that allows the build up of a narrow, focused spot. In fact, Lerosey's group found that the focused spot could be as small as just 1/25th of one wavelength, way beyond the diffraction limit. The results are due to be published in Physical Review Letters³.

There is some debate among acoustic scientists as to whether this is the first time anyone has truly beaten the acoustic diffraction limit. Mechanical engineer Nicholas Fang at the Massachusetts Institute of Technology in Cambridge thinks that the results are a first because the focal point is away from the lens, in the far field. But John Page, a physicist at the University of Manitoba in Winnipeg, Canada, who has published evidence for sub-wavelength focusing in the near field⁴, disagrees. "Super-resolution is super-resolution, no matter in what regime it is obtained," he says.

Still, Page calls the Lerosey group's work "a very important accomplishment" and believes it could find many applications, such as feeding energy to tiny electromechanical devices so they can operate.

Lerosey himself thinks that the simplicity of the apparatus is what bodes so well for applications. "To me, this experiment says, 'we can do it easily, even with Coke cans,' and it opens a door."

[http://www.nature.com/news/2011/110708/full/news.2011.406.html]

Light passes through without reflection

This is not an old concept: a light beam may not be reflected if the thickness of the glass it shines upon is carefully chosen so that the reflected wave from the second surface goes out of phase with the one from the first surface. Now that the proper thickness is proportional to the wavelength of the incident light, it is not possible to use a single piece of glass for reflectionless control of light with various colors. But, nature offers much more. One can make a more delicate refractive index profile of medium so that it invisible to a wide spectrum [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.193903].

Abrupt interfaces disrupt wave propagation. For example, light passing from air into a sheet of glass will partially reflect backwards. When the light exits back into air, there is a second reflection that can cancel the first for light of just the right frequency, given the refractive index and thickness of the sheet. The wave nature of electrons creates similar effects when they encounter a region with a changing electrostatic potential. But early in the history of quantum mechanics, theorists realized that certain smoothly varying potential profiles could eliminate the reflection of electrons over a wide range of frequencies.

As it turns out, the same concepts work for light: intense light pulses known as solitons create precisely this kind of profile in the refractive index of the surrounding medium, eventually becoming trapped. Creating permanent versions of such “reflectionless potentials” has, however, proved difficult. In Physical Review Letters, Alexander Szameit of the Technion in Haifa, Israel, and colleagues in Germany and Australia at last implement the lack of light reflection in the laboratory.

In their experiments, a beam of light travels along an array of closely spaced, parallel waveguides created in a glass sample through direct laser-writing. By changing the spacing between some of the waveguides, the researchers construct a stripe along the length of the array that has a different refractive index modulation relative to the rest of the array. For almost any change in spacing, light traveling diagonally across the stripe is partially reflected, as usual. But a stripe having the special variation suggested by theory generates almost no reflection. The technique adds to the bag of tricks that researchers have for manipulating light. – Don Monroe

On the properties of wave functions

In standard textbooks on preliminary quantum mechanics, it is usually stated that, any physical wave function must be single-valued, which is surely all the time true, as long as the wave function is interpreted as the probability amplitude. Besides, it is usually also stated that, the wave functions (together with its first derivatives) have to be continuous. Obviously, the single-valued-ness does not warrant any continuity. Here I just want to emphasize that, the former property is a direct sequel-a of the Born interpretation, whereas the latter is never a must. Actually, the latter is model dependent: different Hamiltonian can lead to different matching conditions that may not necessarily demand the wave function itself or its derivative be continuous. Indeed, in the conventional p^2/2m case (free from singular potentials), from the Schrodinger equation, H wavefunction=E wave function, directly follows the continuity of the first derivative of the physical wave functions, from which follows the continuity of wave functions themselves. On the other hand, for Dirac-type Hamiltonian that is linear in p (also free from singular potentials), one can only derive from the corresponding Schrodinger equation the continuity of the wave functions, and none can be imposed upon their derivatives. Even more, in the case of singular potentials, for the Dirac (conventional) case, the wave functions (the first derivative) must be discontinuous to satisfy the Schrodinger equation.

Definitely, the above discussions apply to any kind of wave equations, such as Maxwell equations. In summary: (1) single-valued-ness is a must; (2) continuity is not.

Cloak for sounds

The idea of cloaks that make objects disappear is really capturing. It has been realized at least in the lab by a theory called "transformation optics", which relates to the transformation properties of Maxwell's equations. However, this idea not only blossoms in optics but also in acoustics, where similar equations exist in 2D. A new paper in PRL designs a cloak for ultrasonic waves[Phys. Rev. Lett. 106, 024301 (2011) – Published January 10, 2011]. The difficulty lies in varying the density of materials in a desired way[Physics 4, 2 (2011)].

Invisibility devices based on coordinate transformation have opened up a new field of considerable interest. We present here the first practical realization of a low-loss and broadband acoustic cloak for underwater ultrasound. This metamaterial cloak is constructed with a network of acoustic circuit elements, namely, serial inductors and shunt capacitors. Our experiment clearly shows that the acoustic cloak can effectively bend the ultrasound waves around the hidden object, with reduced scattering and
shadow. Because of the nonresonant nature of the building elements, this low-loss ( 6 dB=m) cylindrical cloak exhibits invisibility over a broad frequency range from 52 to 64 kHz. Furthermore, our experimental study indicates that this design approach should be scalable to different acoustic frequencies and offers the possibility for a variety of devices based on coordinate transformation.

Insights of the decade from Science

Now we are coming to the end of not only this year but also the first decade of this century. Science has its list of the insights of this decade in science. In materials physics, the meta-material and the related conformal optics which underlies the operation of these materials are enlisted. The ground breaking papers are as follows: [http://www.sciencemag.org/site/special/insights2010/]

• U. Leonhardt, "Optical Conformal Mapping," Science 312, 1777 (2006).
• J. Pendry et al., "Controlling Electromagnetic Fields," Science 312, 1780 (2006).
• D. Schurig et al., "Metamaterial Electromagnetic Cloak at Microwave Frequencies," Science 314, 977 (2006).
• T. Ergin et al., "Three-Dimensional Invisibility Cloak at Optical Wavelengths," Science 328, 337 (2010).

Friction not so simple

Friction is certainly a standard part of middle school physics courses. It is observed that, to move an object in contact with another one, a force must be applied larger than the static friction, which is supposed to be uniform across the interface. However, this picture is inadequate. Actually, it was perceived that non-uniformity occurs at least locally. Understanding the nature of friction and how to model it better is not only theoretically interesting but practically imperative, because friction is relevant to a plenty of phenomena, such as rampant earthquakes and snow ruptures. Friction is the force that holds those events from bursting out. On the hand, it is also desirable to gain insight into how slip occurs locally when friction fails. This is key to modeling. This latest publication investigated this problem.

The way in which a frictional interface fails is critical to our fundamental understanding of failure processes in fields ranging from engineering to the study of earthquakes. Frictional motion is initiated by rupture fronts that propagate within the thin interface that separates two sheared bodies. By measuring the shear and normal stresses along the interface, together with the subsequent rapid real-contact-area dynamics, we find that the ratio of shear stress to normal stress can locally far exceed the static-friction coefficient without precipitating slip.
Moreover, different modes of rupture selected by the system correspond to distinct regimes of the local stress ratio. These results indicate the key role of nonuniformity to frictional stability and dynamics with implications for the prediction, selection, and arrest of different modes of earthquakes.

Simulating metric signature effects with metamaterials

A funny work here.

We demonstrate that the extraordinary waves in indefinite metamaterials experience an effective metric signature. During a metric signature change transition in such a metamaterial, a Minkowski space-time is created together with a large number of particles populating the space-time. Such metamaterial models provide a tabletop realization of metric signature change events suggested to occur in Bose-Einstein condensates and quantum gravity theories.

A review on cloaking theory

Scientists and novelists have been intrigued for centuries by the possibility of hiding an object so completely that neither trace of the object nor of its cloak is to be found. Recent theoretical developments show that cloaking is, in principle, possible for electromagnetic waves and to a limited extent for other types of wave, such as acoustic waves. An energetic program of experimental research has shown some of the schemes to be realizable in practice.

We have a touching faith in the ability of our eyes to tell the truth. No other sense has such confidence invested in it, so when our eyes deceive us the result is bewilderment, giving rise to appeals to magic or even the supernatural. This explains the enormous interest aroused by recent work on invisibility and the cloaking of objects from electromagnetic radiation. In this article we review the theories and experiments behind the hype and suggest what devices might realistically be expected in the near future and what is likely to prove impossible.

Hard wired into our brains is the expectation that light travels in straight lines. Mostly this is true, but there are well-known exceptions, such as mirages, which occur when a hot surface heats the air above, reducing its density and hence creating a refractive index gradient immediately above the surface (Fig. 1, top). Such a gradient bends the trajectories of light rays so that an observer misinterprets where the light is coming from. Typically, light from the sky is refracted by the gradient, giving the appearance of water shimmering in the distance—hence a cruel illusion seen by a thirsty traveler in the desert or, more prosaically, the appearance of a wet road on a hot day.

It is the ability of refractive index gradients to bend light that the invisibility engineer exploits. Light is steered around the hidden object by a cloaking device, and then returned to the same straight line trajectory, rather as a skier would make a chicane around a tree (Fig. 1, bottom). The observer’s brain is unaware of the possibility of chicanes and sees only that which is behind the cloak and nothing of the cloak itself or of its contents. The real challenge of cloaking lies in deriving a theoretical prescription for the optical properties of the cloak and, even more challenging, realizing these properties in a material. Transformation optics provides the theoretical background and metamaterials provide the means of achieving the prescribed parameters.

shielding earthquakes

Earthquakes tend to cause disasters to humans. It is desirable to screen them. Seismic waves are generally composed of two components, the transverse one (i.e., the S waves, which represents the up-down vibrations of crust) and the longitudinal one (P waves, left-right vibrations). The latter travels faster and can reach more distant places, while the former is more fierce. Here is a piece of work coming up with a design, which assumes a concentric structure, to shield P waves. Their numeric simulations show that, this design is efficient with a broad frequency band.
(1)Ultrabroadband Elastic Cloaking in Thin Plates
(2)brief introduction to seismic waves