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| Quantum conjurers from Imperial College London: Dr Mark Frogley and Prof Chris Phillips |
It sounds like magic: walls, curtains, even dresses could be rendered transparent by bathing them in a specially crafted beam of light. Rescuers could use the beam to peer through rubble after an earthquake, while doctors could gaze at a damaged lung after making a patient's skin and ribs vanish.
This remarkable disappearing trick is the kind of sorcery that would grace the pages of a Harry Potter book, yet it is a prediction of real-world science. Magic and science do, after all, have their roots in a fundamental urge to make sense of the world so that we may manipulate it to our own ends. And, in the sense that a wizard is a wise man, they still exist today; wizards (and a few witches) can be seen in the Royal Society, Britain's national academy of science.
Next week, a team led by Prof Chris Phillips, a quantum conjurer from Imperial College London, will join an elite coven at the Royal Society's summer exhibition (see below for details), where they will present tantalising evidence of how to make objects disappear. At the flick of a switch, he and his colleague Dr Mark Frogley can make something invisible, albeit just a fraction of a millimetre square of a special material and only for a one ten thousandth of a millionth of a second.
Things are visible because of the way that their atoms interact with a beam of light. When the beam, an electromagnetic wave, hits an atom on this page the electrons in the lowest energy state of the atom absorb the wave's energy and rise to higher energy levels. Only light of exactly the right colour, the one that corresponds to the energy difference between the two levels, will be absorbed - all other colours pass through.
To show that it is possible to stop absorption from happening, so this light beam can also pass through, Prof Phillips, Dr Frogley and Swiss colleagues at the University of Neuchatel created an idealised atom. They put down single layers of atoms, one at a time, to create specially patterned crystal wafers only a few billionths of a metre deep. These two-dimensional sandwiches, dubbed nanostructures, behave like "artificial atoms": within them, electrons rattle around in energy levels that are customised to respond in a predictable way to laser light.
In this way, they could exploit an effect predicted by quantum mechanics, the baffling theory that rules the atomic domain: an electron can be prevented from absorbing a particle of laser light and jumping to a higher energy level if a second laser beam is used to link or "couple" the two energy levels to a third one.
To perform this conjuring trick in his lab at Imperial, Prof Phillips uses intense beams of infra-red light from lasers that rely on special semiconductor crystals grown in the former Soviet Union. Although the laser is rated at 10 million watts, it is surprisingly safe: he encourages me to put my hand in the invisible beam: with each pulse of laser light, I feel a tiny pinprick as some of my skin cells are vaporised.
Using two powerful beams made this way, the team performed its vanishing trick: the artificial atoms became transparent to one beam when a second - coupling - laser illuminated them at the same time. "By shining an invisible powerful laser onto these 'artificial atoms', we have learnt how to control the motion of the electrons so they no longer absorb light - when the laser is switched on, the crystals instantly become invisible, only to return to their normal opaque state when the laser is switched off."
As Prof Phillips says, "we have proved the physics". Although this was achieved with an idealised material, it suggests that by carefully designing a wand of laser light it may be possible to make anything transparent. "The effect has the potential to lead to all sorts of new applications. You can imagine a laser that works at frequencies we can't see and, when it shines on your hand, it would open up a transparent hole."
Called "dressing" by quantum boffins, coupling can do more than stop materials from absorbing light. Turn up the coupling laser and the light passing through an object is amplified. "If you made my hand transparent so I could see something the other side, like your face, I could make it appear brighter and brighter," he says.
The laws of physics predict that something strange occurs at the same time: as the image brightens, there is a dramatic slowing down, by a factor of almost 40, of the speed of light inside the artificial atom. "This may hold the key to ways of storing and manipulating information in a new and entirely optical way," says Prof Phillips.
Scientists fantasise about a "quantum computer", one that is predicted to have stupendous power - if only it could be built. "A quantum computer made from just 1,000 of our nanostructures could perform calculations in a second which would take a normal computer longer than the age of the known universe, so although it will be very difficult, it's a goal well worth chasing."
The Invisible Man comes a step closer to reality
What is perhaps even more remarkable is that this is one of no fewer than four approaches to invisibility to emerge in the past couple of years. On the floor below Prof Phillips is the office of Sir John Pendry, a theoretician who has taken a different approach to invisibility that puts emphasis on creating materials to ply light, so it can bend around an object, rather than pass through. Working with colleagues at Duke University, North Carolina, Sir John has found a way to manipulate light using "metamaterials", whose properties derive from their microscopic structure rather than their chemical composition.
Their first application was in realising bizarre optical properties that were first postulated in 1968 by the Russian physicist Victor Veselago . He had been studying the laws that describe light and other electromagnetic radiation which were laid down in the 19th century by the great Scottish mathematical physicist James Clerk Maxwell. Veselago wondered what would happen if two of the constants in Maxwell's equations were flipped from positive to negative.
Peculiar optical properties resulted but Veselago's work remained obscure until six years ago, when his mathematical fantasy was realised by the newly invented metamaterials. They could be made with a negative refractive index - a measure of the ability to warp light - so that light entering them bends in the opposite direction to light passing through normal materials. Then Sir John found that Veselago materials could make a "superlens" that can create a perfect image. This discovery has many practical uses; for example, light beams can be focused as small as the features on microchips, making electronics faster and smarter.
At this point, Sir John waves around what looks like a pizza base. It is a metamaterial lens, consisting of copper wires and loops, smaller than the millimetre scale wavelength of radar, that can focus a radar beam. Now he has built on this idea in a proposal for a cloaking device.
To achieve invisibility, the secret is to use metamaterials with a gradient of refractive indices to make light waves flow around an object, as happens in a mirage, when blue light from the sky is bent by heated air to create the impression of a blue pond in the desert. In this way, "electromagnetic fields can be dragged into almost any desired configuration", says Sir John.
His invisibility cloak would be woven of metamaterials based on metals, because of their extraordinary responsiveness to light (which is why they make such good mirrors), but fashioned into structures at near-atomic dimensions. When electromagnetic waves encountered this cloak they would produce neither a reflection nor a shadow but skirt around so that whatever lies inside the cloak remains out of view.
There are rival stealth theories. Prof Ulf Leonhardt, a black hole theorist at the University of St Andrews, has come up with a variant of this idea. Prof Graeme Milton of the University of Utah, Salt Lake City, and Nicolae-Alexandru Nicorovici of the University of Technology, Sydney, suggest that superlenses can make things placed nearby invisible. And in another proposal, Dr Andrea Alù and Prof Nader Engheta of the University of Pennsylvania devised a way to make objects disappear by, in effect, cancelling the light they scatter by using a "plasmonic cover".
The cover is awash with plasmons, ripples in the ocean of electrons flowing across the surface of nanostructures of silver and gold. When light of a given frequency strikes a plasmon that oscillates at a compatible frequency, the energy from the light is harvested by the plasmon and eventually converted back to light, cancelling light scattered by the object and rendering it near invisible.
The are drawbacks to these theoretical invisibility cloaks - a plasmonic cover would have to be delicately tuned to hide each different object, for example. But we have come a long way since The Hitchhiker's Guide to the Galaxy, when the late Douglas Adams alluded to how Hades' "cap of darkness", which made the wearer vanish, and the Romulan cloaking device were firmly the stuff of myth and science fiction.
According to Adams, objects could be made invisible by using a SEP-field, where SEP stands for Someone Else's Problem. This problem may now have been solved - four times over.
# The Royal Society's summer exhibition takes place at 6-9 Carlton House Terrace, London SW1 on July 3 (from 6pm to 9pm) and on July 4-6 (from 10am to 4.30pm). It is open to all and entrance is free. If you can't make it to London there will be a second chance to see it at the Glasgow Science Centre, September 12-14. See here for more details.
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