Boffins build 'slow glass' light-trapping nanodoughnut
Exciton in tight ring = fast photonic computing?
Warwickshire boffins believe they may be on the track of science-fiction "slow glass", through which light might take a long time to travel. The scientists think that such light-storing materials might be fashioned using excitons mounted inside unfeasibly tiny "quantum doughnuts".
In essence it seems that an exciton is an electron which has been jazzed up a bit energy-wise by absorbing a photon of light. The idea is that if you could prevent such excitons immediately re-emitting their photons and so turning back into electrons at once as they normally do, you could effectively store, or "freeze" light.
Such technology was imagined by renowned sci-fi scribe Robert Shaw in his novel Other Days, Other Eyes and its precursor works. "Slow Glass", through which light would take hours, days or years to travel, was used for many things in the book: stored daylight offered free lighting, for instance. (The omnipresence of slow-glass lights also delivered an equivalent of a CCTV panopticon, as images of anything happening within view of a pane of slow glass would later be visible on its other side.)
According to Warwickshire physicists Andrea Fischer and Rudolf A. Roemer, excitons - properly handled - could offer something on these lines, and might also be extremely handy in light-based (photonic) computing. With electronics approaching its limits, many researchers believe that photonics could deliver the next wave of big performance increases.
Normally an exciton, fairly uselessly, simply releases its photon almost at once. But Fischer and Roemer, trying out notions one day in the lab, decided to try slotting excitons into some quantum nano-doughnuts they had lying around.
Such "Aharonov-Bohm nano rings" were originally created as a by-product of manufacturing comparatively humdrum quantum dots. It seems that sometimes even the most skilful boffin, knocking out a batch of quantodots in a hurry, will inadvertently splash the material onto the receiving surface too hard and make a doughnut rather than a contiguous nano-blob dot.
Fischer and Roemer decided to slot an exciton into the middle of such a nano-ring, in the 10-100 nm size range. That in itself - as one would naturally expect - achieved nothing. But the addition of "a combination of magnetic and electric fields" makes it possible to trap the slippery exciton in one's unfeasibly minuscule quantum ring, at which point it is entirely at one's bidding. The exciton can then be made to hold onto its photon, "freezing" it in place, or collapse back in electronhood and emit the light on command.
"This has significant implications for the development of light based computing," says Roemer.
Though other scientists have slowed light down using various techniques which might also be significant in the possible future wave of photonic IT, Roemer and Fischer consider that theirs is the first which properly locks a photon down for release on demand.
The Warwickshire brainboxes, allied with others, publish their paper Exciton storage in a nanoscale Aharonov-Bohm ring with electric field tuning here (subscription required). ®
Maybe not such a new idea?
Is this not a similar concept to electron trapping in phosphor crystals? We have cassettes with phosphor screens in them in our lab. You put in a dried electrophoresis gel labelled with a radioactive isotope (say P32 or S35) and the radiation promotes electrons to higher energy electron orbitals (e.g. makes "exitons") which due to the nature of phosphor electron orbitals stay there until given an extra jolt of light in a phosphorimager (something like a giant digital scanner that you put the screens onto), after which they release a perfect image of the radioactive areas of the gel.
A similar concept is used for image intensifiers (such as found in real-time x-ray machines and night vision goggles), although in this case phosphor crystals are used to convert light to a high energy electron beam in a vacuum tube, and then back to light after amplification.
These concepts have been in general use for decades so maybe not quite so sci-fi after all, just a higher-tech version of an old invention?
That's the issue Pete was discussing in his previous two posts:
"Since all the light from everything the glass can "see" impinges on the whole pane and then travels through and will be emitted omnidirectionally on the other side, there is no possibility of extracting an image from it"
Pete's hypothesis analyses the light striking the outer surface of the glass as a wave, which is then propagated through the slow glass and transmitted omnidirectionally when it emerges on the other side. Considered as a particle, this means that any given photon would be transmitted in either a random direction or from a random location in relation to its location and the direction it was travelling when it entered the glass.
Given the Heisenberg Uncertainty Principle, which in essence states that you cannot simultaneously observe the momentum and position of a quantum particle, Pete's hypothesis would be correct.
Consider a photon travelling at C through a vacuum in a certain direction: this photon has a determinate momentum. Therefore, it can be predicted to pass through a given point in space at a given time. However, when the photon enters the "slow glass", it combines with an electron to form an exciton, as the article states. The momentum of this exciton is equal to the vector sum of the momentum of both electron and photon, and is thus changed from both. Furthermore, because this exciton is being constrained to follow a curved path in space by means of the "quantum doughnut" or torus, its momentum is constantly changing as long as it is in the torus - because the direction component of the momentum vector is constantly changing as it moves along the curve of the torus.
So in order to release the photon so that it is travelling in a determinate direction from a determinate location, which is required to recreate the image "seen" at the entrance point, would require that both the position *and* direction (that is, momentum) be determined at the same time - the very proposition that the Uncertainty Principle states cannot be achieved. Since this is the case, either the photon would be released in a determinate direction from an indeterminate position, or it would be released from a determinate position in an indeterminate direction.
Therefore, to answer your question, it is possible that the outgoing photon could travel in the same direction as the incoming one, but then it would be from a different location. If it were in the same location, it would then be a different direction. Ergo, no image, either way.