Ok physics guys, explain this..... Frozen light.

[wacki]

http://www.physorg.com/news3679.html

Looks like more of the Bose-Einstein condensation crap.... J.A. Sucker, I'm still waiting on an explanation for that one.


This new research could be a major breakthrough in the quest to create super-fast computers that use light instead of electrons to process information. Professor Lene Hau is one of the world's foremost authorities on "slow light". Her research group became famous for slowing down light, which normally travels at 186,000 miles per second, to less than the speed of a bicycle.

Using the same apparatus, which contains a cloud of ultra-cold sodium atoms, they have even managed to freeze light altogether. Professor Hau says this could have applications in memory storage for a future generation of optical computers.

But Professor Hau's most recent research addresses the issue of optical computers head-on. She has calculated that ultra-cold atoms known as Bose-Einstein condensates (BECs) can be used to perform "controlled coherent processing" with light. In ordinary matter, the amplitude and phase of a light pulse would be smeared out, and any information content would be destroyed. Hau's work on slow light, however, has proved experimentally that these attributes can be preserved in a BEC. Such a device might one day become the CPU of an optical computer.

Traditional electronic computers are advancing ever closer to their theoretical limits for size and speed. Some scientists believe that optical computing will one day unleash a new revolution in smaller and faster computers.

Professor Lene Hau is Gordon McKay Professor of Applied Physics & Professor of Physics at Harvard University.

[DrPublo]

What do you want to know about BECs?

The Doc

[wacki]

1) What is the BEC?
2) Did the nuclei all fall in to one spot?
3) Is it like one giant atom that is held loosely together?
4) Why does it condense?
5) How does it condense?
6) What are these "strange properties" I keep hearing about?

[DrPublo]

How much physics do you know already?

Let's see, going from the top. A BEC is a new state of matter (states of matter: solid, liquid, gas, etc) where all the atoms collapse into the same quantum state. What does this mean? It depends on how detailed you want to get, but think of a quantum system (read: atom) as having lots of discrete energy states. These states are populated depending on the ambient energy (temperature), internal energy, etc. More energy = more population in higher (more energetic) states.

As a bunch of atoms get REALLY, REALLY cold (within a few billionths of a degree of absolute zero or so), the atoms have less and less internal energy (the internal energy is both radiated to the environment and also removed via laser cooling). This means theres less energy to populate the various states, and so all the atoms lie in the same energetic state, known as the ground state (ground = lowest).

When the atoms are all in the same quantum state, they become indistinguishable. That means no measurement can determine between them, and they can do all sorts of cool things like swap positions in the condensate because there's nothing stopping them from doing so (again, indistinguishable). This means that a force acting on one part of the condensate will simultaneously act on ALL parts of the condensate.

What, you ask?

Think of the condensate as a big ball of jello sitting on your desk. If you poke jello, with a fast enough camera you could detect waves starting from your finger and traversing the jello, coming back, etc and causing it to "jiggle." Not so for a BEC. If you poke a BEC, that force will simultaneously act on all the atoms. Neat! This is especially cool if you consider what happens if you take a BEC and push it through a little hole or something, so that theres a divider between the halves. Then a force acting on one side of the divider must simultaneously act on the other side of the divider as well! It can do this because once the atoms are all in the same quantum state, they become entangled (another quantum buzzword), which basically means that the atoms collectively form a state that no individual group of atoms could make.

Did I do a good job explaining?

The Doc

[DrPublo]

Some people have heard of BECs and wonder why it appears as a big "smear". I mean, atoms are supposed to be little tiny things you can't see, right?

It turns out that quantum mechanics predicts several uncertainty relations (known as Heisenberg's uncertainty principle), which basically state the more you know one thing the less you know about a conjugate thing. The most famous relation is between momentum and position, but there are others.

In particular, it turns out that (using LaTeX notation)

\Delta E \Delta t \ge \hbar/2,

where E is energy and t is time. Since when we make an atom really really cold we know more about its energy, we must therefore know less about the time at which we are making the measurement, and thus the BEC appears as a smear.

I'll probably post more thoughts as they come to me.

The Doc

[wacki]

[ QUOTE ]
How much physics do you know already?

[/ QUOTE ]

You won't lose me on classical newtonian mechanics. However, mention the word quantum state.....

[ QUOTE ]

Did I do a good job explaining?


[/ QUOTE ]

Excellent. Also, the fact that you were able to explain highly technical concepts in incredibly simple manner shows me you are a very bright individual.

Do you have any suggestions for learning Quantum physics? I'm completely naive in this area but I'd like to learn. Is there anything simple out there. I know about the M theory, but without understanding the fundamentals, it sounds more like a religion to me.

[wacki]

Also, is this what they mean by quantum state?

http://www.chemistry.ohio-state.edu/betha/qm/5afr.html

[DrPublo]

As for learning QM, I think the standard text in most college courses is by Griffiths. I'm not a huge fan of it but it's pretty standard.

Other books I've used:
Quantum Mechanics, by Leonard I. Schiff.
Principles of Quantum Mechanics, by R. Shankar. <--Way recommended, although maybe not as a first treatment.

If you want a pretty general introduction, I find that most physical chemistry books actually do a better, clearer job of explaining particle in a box, energy quantization, etc. than most physics books. I suggest "Physical Chemistry: A Molecular Approach" by McQuarrie and Simon. You'll only need the first 8 chapters or so, the rest is other P-chem stuff.

The Doc

[bholdr]

[ QUOTE ]
Not so for a BEC. If you poke a BEC, that force will simultaneously act on all the atoms. Neat!

[/ QUOTE ]

very neat. do the vibrations now take place at some sub-atomic level, or not at all?

what would happen if the BEC was very large? say, one light-second in length? obviously, it couldn't move faster than light, but your post and the other info i've seen seems to indicate that, lacking condensation and expansion of the distances between the atoms, etc, it would. that's O.K, if any information contaied within it's movenemts was somehow destroyed. if it's not, big trouble.



please educate this layman (albeit a well read one).

[DrPublo]

Good question. Obviously there are practical limits to this stuff, including how you keep something one light-second long at a few billionths of a degree kelvin!

I don't quite know the theoretical limits to the size of BECs, but I can take a few quesses. The atoms in a BEC form one large entangled quantum state. Things we have learned from studies in quantum computation is that its pretty hard to maintain an entangled state over any substantial timeframe or distance. Decoherence effects set in, which basically mean that the atoms prefer to get entangled with the environment rather than each other after awhile. So my guess is it would be tough to set up an enormous BEC.

With regard to the vibrations, you can't take the anology too seriously because its never a good idea to try and directly compare classical and quantum phenomena. In jello, the vibrations are the means by which the poking force is transmitted throughout the solid. In a BEC, the reason an external force acts everywhere at once is because the atoms are identical and thus interchanging freely, so the atom you poke on the end of the condensate might as well be the atom buried deep in the middle, etc. There's not really any "vibration" about it.

The Doc