Posts Tagged ‘invisibility

Quantum Invisibility Cloak Hides Objects from Reality

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Physicists have worked out how to cloak a region of space from the quantum world, thereby shielding it from reality itself

A core-shell nanoparticle with different effective masses and potential energies defined in each region. Quantum matter wave of the transport electron, Ψi,is assumed to propagate along the z-axis

A core-shell nanoparticle with different effective masses and potential energies defined in each region. Quantum matter wave of the transport electron, Ψi,is assumed to propagate along the z-axis

Invisibility cloaks are all the rage these days. Over the last few years, this blog has followed various attempts to develop invisibility cloaks for earthquakes, acoustics and for various parts of the electromagnetic spectrum. In the last few days, we’ve seen the emergence of a new generation of cloaks that can hide large, everyday objects across the entire optical spectrum.

Today, Jeng Yi Lee and Ray-Kuang Lee at the National Tsing-Hua University in Taiwan take the idea of cloaking to its ultimate limit. These guys have worked out how to build quantum invisibility cloaks. These are cloaks that shield objects from the quantum properties of the world outside. That’s not so much an invisibility cloak as a reality cloak.

The idea is simple in essence. Ordinary invisibility cloaks work by steering light around a region of space to make it look as if it weren’t there. The mathematical approach that describes this is called transformation optics. It starts with Maxwell’s equation which govern the behaviour of light as it passes through space.

One way to think of light is as a field in space. In transformation optics, this field can be stretched and squeezed like a rubber sheet when it passes through certain types of material. The goal is to engineer this material so that it stretches the sheet around regions of space and so make them invisible.

The approach developed by Jeng Yi and Ray-Kuang is mathematically identical to this. But instead of starting with Maxwell’s equations, they start with the Schrodinger equation which governs the probability of an object being present in a region of space.

Their idea is to treat this probability field like a rubber sheet that can be stretched and squeezed.  So the goal in designing a quantum invisibility cloak is to stretch this sheet around a region of space so that the probability of existing inside it is zero. In effect, they’re designing a cloak that shields its contents from reality.

There are several important caveats here. The first is that the shield would be designed to shield from a particular form of Schrodinger equation, perhaps associated with electrons for example. That would shield a region of space from the quantum properties of nearby electrons but not necessarily from other things.

So such a cloak would provide shielding from certain aspects of reality rather than all of it. Making a “broadband” reality cloak that hides from many quantum objects at once would presumably be a much bigger challenge, if it is possible at all.

Another important point is that this paper is entirely theoretical. Jeng Yi and Ray-Kuang provide a theoretical treatment of a nanoshell that cloaks the region of space within it from the effects of matter nearby.  This approach demonstrates that the idea is possible on the nanoscale, at least in principle.

The big challenge of course will be to find ways of implementing the technique for real. That will take some impressive engineering but Jeng Yi and Ray-Kuang say it ought to be possible using today’s semiconductor technologies.

So the idea would be to create a hollow nanoparticle, perhaps out of silicon,  that would shield its contents from electrons passing nearby. That’s something that could be useful for quantum information storage or processing, for instance.

That’s an interesting idea that could stimulate an entirely new approach to shielding. If it sounds a little far-fetched, it’s worth bearing in mind the timescale over which conventional invisibility cloaks have emerged. The original theoretical paper proposing the idea appeared on 25 May  2003 and the paper describing the first working cloak appeared on 19 October the same year.

If the development of quantum invisibility cloaks is anything like that, we could see one in operation before the end of the year.

Ref: Hide The Interior Region Of Core-Shell Nanoparticles With Quantum Invisible Cloaks


Written by physicsgg

June 14, 2013 at 7:43 am


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Video: New Material for Invisibility Cloaks

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invThe new material’s artificial “atoms” are designed to work with a broad range of light frequencies. With adjustments, the researchers believe it could lead to perfect microscope lenses or invisibility cloaks.

One of the exciting possibilities of metamaterials – engineered materials that exhibit properties not found in the natural world – is the potential to control light in ways never before possible. The novel optical properties of such materials could lead to a “perfect lens” that allows direct observation of an individual protein in a light microscope or, conversely, invisibility cloaks that completely hide objects from sight.
Although metamaterials have revolutionized optics in the past decade, their performance so far has been inhibited by their inability to function over broad bandwidths of light. Designing a metamaterial that works across the entire visible spectrum remains a considerable challenge.
Now, Stanford engineers have taken an important step toward this future, by designing a broadband metamaterial that more than doubles the range of wavelengths of light that can be manipulated.
The new material can exhibit a refractive index – the degree to which a material skews light’s path – well below anything found in nature.
“The library of refractive indexes that nature gives us is limited,” said Jennifer Dionne, an assistant professor of materials science and engineering and an affiliate member of the Stanford Institute for Materials and Energy Sciences at SLAC National Accelerator Laboratory. “All natural materials have a positive refractive index.”
For example, air at standard conditions has the lowest refractive index in nature, hovering just a tick above 1. The refractive index of water is 1.33. That of diamond is about 2.4. The higher a material’s refractive index, the more it distorts light from its original path.

All natural materials have a positive index of refraction — the degree to which they refract light. The nanoscale artificial “atoms” that constitute the metamaterial prism shown here, however, were designed to exhibit a negative index of refraction, and skew the light to the left. Technology that manipulates light in such unnatural ways could one day lead to invisibility cloaks.

Really interesting physical phenomena can occur, however, if the refractive index is near-zero or negative.
Picture a drinking straw leaning in a glass of water. If the water’s refractive index were negative, the straw would appear inverted – a straw leaning left to right above the water would appear to slant right to left below the water line.
In order for invisibility cloak technology to obscure an object or for a perfect lens to inhibit refraction, the material must be able to precisely control the path of light in a similar manner. Metamaterials offer this potential.
Unlike a natural material whose optical properties depend on the chemistry of the constituent atoms, a metamaterial derives its optical properties from the geometry of its nanoscale unit cells, or “artificial atoms.” By altering the geometry of these unit cells, one can tune the refractive index of the metamaterial to positive, near-zero or negative values.
One hitch is that any such material needs to interact with both the electric and magnetic fields of light. Most natural materials are blind to the magnetic field of light at visible and infrared wavelengths. Previous metamaterial efforts have created artificial atoms composed of two constituents – one that interacts with the electric field, and one for the magnetic. A drawback to this combination approach is that the individual constituents interact with different colors of light, and it is typically difficult to make them overlap over a broad range of wavelengths.
As detailed in the cover story of the current issue of Advanced Optical Materials, Dionne’s group – which included graduate students Hadiseh Alaeian and Ashwin Atre, and postdoctoral fellow Aitzol Garcia – set about designing a single metamaterial “atom” with characteristics that would allow it to efficiently interact with both the electric and magnetic components of light.
The group arrived at the new shape using complex mathematics known as transformation optics. They began with a two-dimensional, planar structure that had the desired optical properties, but was infinitely extended (and so would not be a good “atom” for a metamaterial).
Then, much like a cartographer transforms a sphere into a flat plane when creating a map, the group “folded” the two-dimensional infinite structure into a three-dimensional nanoscale object, preserving the original optical properties.
The transformed object is shaped like a crescent moon, narrow at the tips and thick in the center; the metamaterial consists of these nanocrescent “atoms” arranged in a periodic array. As currently designed, the metamaterial exhibits a negative refractive index over a wavelength range of roughly 250 nanometers in multiple regions of the visible and near-infrared spectrum. The researchers said that a few tweaks to its structure would make this metamaterial useful across the entire visible spectrum.
“We could tune the geometry of the crescent, or shrink the atom’s size, so that the metamaterial would cover the full visible light range, from 400 to 700 nanometers,” Atre said.
That composite material probably won’t resemble an invisibility cloak like Harry Potter’s anytime soon; while it could be flexible, manufacturing the metamaterial over extremely large areas could be tricky. Nonetheless, the authors are excited about the research opportunities the new material will open.
“Metamaterials will potentially allow us to do many new things with light, things we don’t even know about yet. I can’t even imagine what all the applications might be,” Garcia said. “This is a new tool kit to do things that have never been done before.”

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Written by physicsgg

May 7, 2013 at 3:44 pm

Posted in Materials Science

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‘Antimagnet’ joins list of invisibility approaches

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The design may lead to shields that protect pacemaker wearers during MRI scans

Researchers have designed a “cloak” that is invisible to magnetic fields both coming in and coming out.

The idea of blocking magnetic fields has been proposed before, but the new design, in the New Journal of Physics, could even hide magnetic materials.

It could thus find application in security or medical contexts, such as those surrounding MRI scans.

The approach uses superconductor layers and the “metamaterials” familiar from recent invisibility cloak research.

Metamaterials are artificially designed materials designed to guide electromagnetic waves – like light or magnetic fields – in a way that natural materials do not.

Much research in recent years has attempted to put metamaterials to work in Harry Potter-style invisibility cloaks that guide light waves around a cloak’s wearer – although experiments have only demonstrated such effects on tiny items, or for a limited range of colours.

But because light and magnetism are two facets of the same physical force, many of the same principles apply for demonstrating a magnetic cloak, as the report’s lead author Alvaro Sanchez explained….. Read the rest of this entry »

Written by physicsgg

September 26, 2011 at 12:52 pm