|MadSci Network: Chemistry|
[Mad Scientist’s note: as a laser physicist I view the term ‘laser,’ which is an acronym for Light Amplification by Stimulated Emission of Radiation, applied to ‘matter laser’ with a rather lopsided smile; there’s obviously no _light_ amplification in a matter laser … but then again, as a laser physicist I’m pretty biased!]
“What are matter lasers?” Very good question. The answer lies at the heart of modern physics, and leads towards some of the most advanced ideas on the technological front. Like many revolutionary ideas, it’s not too hard to explain either, once you understand a couple of basic concepts. So, I’ll tell you what a matter laser is, explain the concepts behind its operation, discuss how they’ve been demonstrated, and talk a little bit about how they might lead to better computers.
The simple answer to the question, “What is a matter laser?” is that it is a device that generates a beam of coherent particles. Because most of the demonstrations for doing so right now produce coherent atoms, you’ll probably have more luck searching under ‘atom laser’ for further information. What do I mean by “coherent particles”? Ah – there’s the trick.
“Coherence” is a property that relates a wave to some other wave or collection of waves. For a good description of coherence you can go here. Basically, if you have a bunch of waves that are coherent with one another, you get strong interference effects between them. In particular, there can be areas exposed to strong wave sources that never see a measurable wave because of the destructive interference between the different sources. An example of destructive interference is produced by noise-cancellation headphones. Electronics inside the headphones sense the sound waves coming in from the outside and create new waves that, in principle, just cancel the outside ones at your ear via destructive interference. If the headphones worked just right, you wouldn’t even be able to hear an airplane taking off from 50 meters away. This only works if the headphones generate waves perfectly coherent with, and exactly out of phase with, the incident sound waves.
“But if coherence is a property of waves, how can we have coherent _particles_??” Here’s the part where we delve into modern physics. While a more complete description can be found here the basic point is that in 1925 a scientist, Louis DeBroglie, suggested the stunning hypothesis that all matter can be considered wavelike. This would mean that all matter could exhibit interference phenomena. He even postulated the means to compute a particle’s wavelength, which he claimed was inversely proportional to its momentum. Since momentum is (for nonrelativistic particles) the product of mass times velocity, DeBroglie’s statement is that the heavier the particle and the faster it moves, the smaller its associated wavelength. Amazingly, this hypothesis has been verified in the lab. The simplest experiment to show interference between particles demonstrated interference between electrons. Indeed, the very existence of atom lasers is another successful demonstration.
Having discussed a bit of what a matter laser is – a device for producing coherent streams of atoms – we’re now on to how to build one. The necessary condition for producing a coherent matter laser is the creation of particles with DeBroglie wavelengths that extend beyond their ‘hard’ edges. For most particles, this is simply impossible. For example, you and I can be considered particles, but we sure don’t appear to show any ‘wavelike’ properties. Why is this? Because we’re way too massive to have an appreciable wavelength. For some examples of calculating DeBroglie wavelengths, you can go to this site.
Creating large DeBroglie wavelengths is difficult even for very tiny particles. A common candidate for atom lasers is the sodium atom, so we’ll look at the DeBroglie wavelength for these particles. We need to know their mass and velocity. Sodium atoms are, obviously, very light. Any periodic table will show their mass to be 23 atomic mass units or about 38 x 10^-27 kg. At any temperature, sodium atoms have a thermal velocity proportional to the square root of the absolute temperature. At room temperature, their average velocity is 570 m/s. When we plug these values into the DeBroglie relationship we find that the wavelength is 0.03 nanometers (3 x 10^-11 meters). How large is this compared to the size of the atom itself? A quick Google search shows that the electronic radius of a sodium atom is about 0.15 nm, which is 5 times greater than the room-temperature DeBroglie wavelength. We’ll have to slow the atom down if we want DeBroglie wavelengths large enough to exhibit atom-atom coherences.
So, even for particles as light as atoms, the secret to creating atomic coherences is to make the atoms very, very cold. To get the DeBroglie wavelength equal to the atomic radius of a sodium atom the atoms must be cooled down to 12 K, or only twelve degrees above absolute zero. To observe the coherence effects in a gas of atoms, the DeBroglie wavelength must extend to the nearest atom. The highest densities attained in a “dilute” gas are around 10^15/cm^3, where atom separations are 100 nm. To get these wavelengths, sodium atoms must be cooled to lower than 27 x 10^- 6 K!
As amazing as it seems, it is possible to cool atoms down to these temperatures – and below. Using precisely aligned and precisely tuned lasers, it is possible to generate a very special group of cold atoms called a Bose-Einstein condensate (BEC). Beyond the fact that this condensate is _extremely_ cold, the special properties of BECs and how they’re formed are beyond the scope of this answer. (For further information you can go here or here.) Average atomic temperatures inside a BEC are less than 100 nK, or one tenth of one millionth of a degree above absolute zero!
Ultra-cold atoms provide the basis for a matter laser, but not a way of generating ‘beams’ of atoms in analogy to beams of laser light. The remaining steps needed to generate such beams are technical (cool, but technical) and can be checked out from the references provided here.
More importantly, if you want to prove that your atomic beams are coherent, the best way to do so is to create two beams that will interfere with one another. If you can measure sharp interference fringes in the overlap region, you’ve demonstrated coherent atoms – a matter laser. One of the first successful demonstrations was performed at MIT and was written up in PhysicsWeb and Popular Mechanics. Refinements to the output coupling, resulting in controllable beam directions and velocities, were attained at NIST . Each of these groups has some general overviews of their matter laser work (NIST Matter Laser Overview MIT Atom Laser Overview). Exploring their sites provides a wealth of information on the topic.
OK – so that’s a matter laser. What’s it good for? Obviously, future applications are limited at the moment by the technology needed to cool atoms to sufficiently cold temperatures. With present techniques, you’d need several stable, accurately-tuned lasers, an ultra-high vacuum chamber, and significant graduate-student slave labor to produce a BEC. This isn’t going to get packaged into a DVD player anytime soon.
However, some ideas have been proposed nonetheless. To get to your question of faster computer chips, one application which has been discussed is very high-resolution lithography. In lithography, a technique used to fabricate computer chips, the smaller the wavelength the smaller the minimum feature size possible in your chip. An atom laser could, potentially, have wavelengths 100 times shorter than those of current UV (~200 nm) lithography sources. Since current wisdom says smaller elements in an electronic chip mean greater speed, atom lasers could mean computer chips many times faster than those fabricated from optical lasers.
Once again, you asked a very good question. I hope the answer stimulated at least a few brain cells – it certainly encouraged me to dig into the topic!
Atomic Physics Links
Matter laser articles
Coherence & Interference
DeBroglie & Wave-Particle Duality
Bose-Einstein Condensates (BECs)
Try the links in the MadSci Library for more information on Chemistry.