Scientists have found a way to turn iron nuclei transparent to X-rays, using lasers. And besides the coolness of making things disappear using lasers, this could have huge applications in optical computing.
One of the fun things about science is every now and then something pops up that is extremely cool and that you were never even aware existed as a concept, let alone a practice. For me, electromagnetically induced transparency (EIT) is one of those wonderful surprises. Apparently, it's been known for years that, with a properly applied laser of one wavelength of light, it was possible to make objects transparent to other wavelengths of light. It's been used in lightweight atoms, but recently they brought in a heavy hitter.
By all accounts, iron is a heavy substance. Its bulk is the reason why it shows up on X-rays at all. The high-energy waves pass right through lighter material, which is why x-ray pictures only show the heavy calcium skeleton that people have, and not the lighter flesh. Iron-57 atoms are not going to be missed by an X-ray — until scientists used EIT to wink them out of existence, as far as the X-rays were concerned. Scientists took two thin sheets of the material and held it in place with carbon, which is invisible to X-rays. They placed two platinum mirrors to either side of the iron sheets. They then fired a beam of low-energy X-rays into the set-up. The beam was reflected by the platinum mirrors back and forth again and again. Trapped, it set up a standing wave in the sandwich of equipment. Standing waves have peaks and troughs, where the most violent activity takes place, and things called 'nodes,' where there is no motion at all. You've probably seen this when you've made a wave using a jump rope — the middle part, between the peak and trough of the jump rope's wave, will always stand still. That's a node.
When one sheet of iron was at a node, and one sheet was at an antinode (the peak or the trough), another, higher energy x-ray was shot through the entire contraption. The x-ray moved through them without interacting with either sheet of iron. It was like they weren't there. Scientists believe that the iron 'disappears,' because of what's called a quantum-optical effect. The platinum mirrors form an optical cavity — a little light trap. When this happens, the atoms all absorb and radiate energy in a synchronized way. More energy being sent through there will, if the system is placed exactly right, no longer interact with a bunch of different atoms in different states causing a few of them to randomly absorb a photon and pop an electron up to a higher orbit. Instead, it will deal with a synchronized team of atoms. If it hits this wall of atoms in the one way, all the atoms will react as one. If it hits one group of iron atoms at a node and the other at an antinode, the oscillations it causes will simply cancel each other out, and it will be as if nothing is there at all. It's kind of as if the two pieces of metal "cancel each other out" on a quantum level.
Obviously, it would be cool if we could get this in macro size, making solid objects vanish from X-ray scans at will with the flick of a laser. More practically though, any light-controlled on-off switch that incorporates metal is a promising step for entirely 'optical' computers operated with beams of light. Maybe scientists could go for broke and create invisible optical computers, operated with beams of light. The best of both worlds!
Top Image: Dr. Ralf Roehlsberger, DESY