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

[BOTW]

Basic Intro to BEC (http://www.colorado.edu/physics/2000/bec/)

[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

[wacki]

[ QUOTE ]
Basic Intro to BEC (http://www.colorado.edu/physics/2000/bec/)

[/ QUOTE ]

Eh.. I know all of that stuff. That is an excellent website btw. Still, it doesn't explain how you keep cesium or sodium as a gas at 1 billionth of a degree Kelvin.

[DrPublo]

Yes and no. The page you linked shows solutions to a particular model problem, known as the particle-in-a-box (PIAB). PIAB is basically the first thing you learn in most QM courses, as it's a pretty easy example that naturally leads to the idea of energy quantization. I'd go through the particle in a box derivation here but its not really well suited to an internet forum. Just search "one dimensional particle in a box" and I'm sure you'll find a ton of 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]

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

[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

[DrPublo]

Laser cooling, then evaporative cooling.

The Doc

[gumpzilla]

[ QUOTE ]
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.

[/ QUOTE ]

What's not to like about Griffiths? Most people I know greatly appreciate the amount of English he uses, though I've heard of people before who don't like him. I think it's a highly recommended book.

wacki, unless you're going to give it several years of commitment, don't even bother worrying about M theory. The amount of mathematics and further physics training you'd need are going to be extensive. I don't really see interested amateurs trying to slog through quantum field theory, for example. And, at the end of the day, you're going to have an understanding of a mathematical formalism which has yet to be convincingly demonstrated to correspond to reality. It gets a lot of mainstream press because it sounds so weird, but it's not what the entirety of physics is about right now.

Nonrelativistic quantum mechanics isn't super hard, and it's the stuff that most people are talking about when they talk about quantum mechanics. The uncertainty principle, the measurement problem, indistinguishability - all of that comes in here.

[Chairman Wood]

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


[/ QUOTE ]
Here (http://www.colorado.edu/physics/2000/bec/) is a link to an explanation that everyone on this board should be able to understand. Carl E. Wieman, who won the 2001 Nobel Prize in Physics was the graduate advisor of one of my professors. He, Chris Monroe, had the fortune of working for him when they finally achieved the condensate. A few years ago, I also got a chance to hear Professor Wieman, give a lecture on this. Quite a cool guy that is really interested in teaching science, maybe moreso than his own research. That is rare among professors.

[tolbiny]

I love reading about physics from a guy wiht the hamburgler as his avatar.

[bholdr]

ok i see now, but it still looks like info is being moved FTL, which is supposed to be a no-no.

[DrPublo]

Even scientists like hamburgers.

Robble, robble.

The Doc

[wacki]

[ QUOTE ]
I love reading about physics from a guy wiht the hamburgler as his avatar.

[/ QUOTE ]

You say a lot of random crap that tends to be rather funny.

Don't stop.

[gumpzilla]

[ QUOTE ]
ok i see now, but it still looks like info is being moved FTL, which is supposed to be a no-no.

[/ QUOTE ]

I should be able to tell you more about this, but my knowledge of BEC is weak. However, the notion of rigidity is generally not relativistically acceptable; this isn't a special problem with BEC, but any solid object you might name. So "pushing" on one end of the condensate is going to lead to some kind of wave that will propagate across the condensate, and that resolves your problem.

[BOTW]

[ QUOTE ]
Eh.. I know all of that stuff. That is an excellent website btw. Still, it doesn't explain how you keep cesium or sodium as a gas at 1 billionth of a degree Kelvin.

[/ QUOTE ]

I figured you did, but I can't pass up an opportunity to pimp my alma mater. That [censored] is colder than my ex.

I'm bumping this cuz it deserves it.

[EliteNinja]

[ QUOTE ]
Even scientists like hamburgers.

Robble, robble.

The Doc

[/ QUOTE ]

Classic.

[wacki]

[ QUOTE ]
Laser cooling, then evaporative cooling.

The Doc

[/ QUOTE ]

Cesium and sodium are solids at room temperature. How do you turn them into a super cold gas? I understand laser cooling and the magnetic field evaporative cooling, but why doesn't it solidify?

[edtost]

[ QUOTE ]
Even scientists like hamburgers.

Robble, robble.

The Doc

[/ QUOTE ]

shouldn't you be thesis-ing?

[kevyk]

Griffiths's QM book is great, and his E+M book is even better.

[Voltron87]

I don't care about BECs, atoms, or optical computes. What I want to know is, are we any closer to building a lightsaber? That is all that matters to me.

[Sweaburg]

[ QUOTE ]
Griffiths's QM book is great, and his E+M book is even better.

[/ QUOTE ]

I agree totally with this. You can read Griffiths books (Linky (http://www.amazon.com/exec/obidos/tg/detail/-/0131118927/qid=1113580814/sr=8-1/ref=pd_csp_1/104-3518174-3783901?v=glance&s=books&n=507846)) pretty much like novels and can probably pick a copy up at pretty much any college/universities used book store for pretty cheap. Shankar is an okay book too, but its huge and will be too intimidating as a first QM book.

Sweaburg

[CrazyN8]

[ QUOTE ]
[ QUOTE ]
Even scientists like hamburgers.

Robble, robble.

The Doc

[/ QUOTE ]

Classic.

[/ QUOTE ]

that will have me laughing for a few days - robble, robble.

the rest will have my head hurting....damn you guys!

[DrPublo]

10 days to go....and so much left to do /images/graemlins/frown.gif

The Doc

[DrPublo]

Low pressure. Think about what the phase diagram looks like.

The Doc

[InchoateHand]

[ QUOTE ]
What do you want to know about BECs?

The Doc

[/ QUOTE ]

Can you make popsicles out of it?

[Jazza]

[ QUOTE ]
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

[/ QUOTE ]

Wait just a minute there fella!

are you saying that you could send a signal from one side of the condesate and it instantaneoulsy can be detected from the other side?

or is this one of those things were something is traveling faster than light, but no information is traveling faster than light?

[DrPublo]

The idea of particle indistinguishability has to do with the difference between Bose statistics (or Bose-Einstein statistics, because apparently when Bose was trying to get this stuff published, no one believed him so he had to enlist Einstein's help) and Fermi-Dirac statistics. If you've ever heard of Fermions and Bosons, this is what the terms refer to.

Thus when I write that the atoms in the BEC are indistinguishable, it's because at very low temperatures they obey Bose statistics. A consequence of that is something you do to one atom must be done to all atoms, because you can't really label the atoms as atom 1, atom 2, etc.

Get it?

The Doc

["spaceman"Bryce]

whoa this thread talks about science.I better run away from it before i say something stupid.....

[Jazza]

yeah i think i do, but my question was would this allow signals to travel faster than light?

you do something to one atom, and some one can measure this change a finite distance away an infintesimal time in the future?

cause that would be a big deal if true yes?

[Jeff W]

I don't know much about Bose-Einstein condensates, but from what I've read in DrPublo's description, it seems like this is just a routine case of quantum entanglement. If so, information is not travelling faster than the speed of light.

If you're interested in quantum entanglement, then you should google "Bohm's Paradox".

[DrPublo]

Don't you mean Bell's paradox?

The Doc

[Jeff W]

Nope. I meant Bohm's Paradox--reformulation of the EPR paradox for spin instead of momentum.

Edit: EPR and Bohm's Paradox. (http://en.wikipedia.org/wiki/EPR_paradox)

[DrPublo]

In 1982 (or thereabouts), Nick Herbert published a paper in Foundataions of Physics describing a thought experiment that he thought was a means of using quantum entanglement to produce faster-than-light communication. The idea was to entangle the state of two quantum particles (in quantum computation speak, "q-bits"), make a copy, and send the copies in opposite directions at some speed close to the speed of light to two observers. Call this speed s. After time t, when the observers receive the particles, the distance between them is 2st. Then, if the observers at either end of the experiment could determine something about how the other person measured the quantum system through the entangled quantum state, faster than light communication occured.

It turns out his reasoning is flawed because it can be shown that making a copy of an arbitrary quantum state destroys the state you're copying, so you're left with only one copy still. The speed of light was saved as the ultimate speed limit again.

I would venture that if you tried to pass a signal through the entangled quantum state of a BEC, some assumption we have implicitly made somewhere would be violated and in the same way the speed of light would again be vindicated.

Get it?

The Doc

Edited for the journal title.

[DrPublo]

Nice link, thanks.

The Doc

[wacki]

[ QUOTE ]
Low pressure. Think about what the phase diagram looks like.

The Doc

[/ QUOTE ]

http://www.chm.bris.ac.uk/motm/uf6/phasediag.gif

I guess this phase diagram wouldn't apply then. I get your point, I just didn't think sodium and cesium behaved that way at low pressures.

[wacki]

http://boojum.hut.fi/research/theory/Phasehe3log.gif
Or this helium one.

[wacki]

Ok, what is a superfluid?

[DrPublo]

A superfluid is a state of matter that exhibits 0 internal resistance to flow, just like a superconductor exhibits 0 internal electrical resistance.

The Doc

[Voltron87]

Like I said, are we any closer to a working prototype of a lightsaber?

[Jazza]

yah that was my first guess