Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yor_on on 17/02/2009 13:27:36

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: yor_on on 17/02/2009 13:27:36

If you read http://www.abc.net.au/science/news/stories/2007/1843012.htm you will be flabbergasted :) At least I was.
There was so many 'strange' statements in it.

That it is possible to 'stop' light is one thing, but to say that its 'imprinted' information on 'matter' then can jump between 'condensate clouds'?
As what?

If it gets 'imprinted' on the condensate and then leave it? 'Jumping' that one millimeter to the other one.
In what form did it leave that first condensate cloud?
And how did it keep its 'energy' intact?

Do you know?


-------

" when the laser was turned off, the light pulse made an imprint – “like a hologram,” said Hau – that started moving slowly until it exited the condensate cloud into free space.

“What you wind up with is an absolute perfect copy of the light pulse, but in matter form,” she said.

And then, in a true “quantum leap”, the transformed photons entered into the neighbouring condensate cloud a fraction of a millimetre away, where the original light field reappeared.

“It is one of those things that are known from theory but are still counter-intuitive,” said Fleischhauer, who wrote a commentary, which, like the study, was published in the British journal Nature on Thursday.

The data are transferred from one cloud to the next because the optical pulse is converted into a wave of travelling matter, the authors say.

These experiments hold the promise of “very real technological benefits,” said Fleischhauer."

----

Read this instead, it got 'Pictures' ::))
http://harvardmagazine.com/2007/07/fantastic.html
And a lot of really cool links:)

It seems as the thing 'wandering' here consists of sodium atoms?
How do those atoms keep their 'order', and what would happen if they didn't, like us 'changing' that order backwards?
And why does it propagate between those clouds at all?

----------

" The light drives a controllable number of the condensate's roughly 1.8 million sodium atoms to enter into quantum superposition states with a lower-energy component that stays put and a higher-energy component that travels between the two Bose-Einstein condensates. The amplitude and phase of the light pulse stopped and extinguished in the first cloud are imprinted in this traveling component and transferred to the second cloud, where the recaptured information can recreate the original light pulse.

The period of time when the light pulse becomes matter, and the matter pulse is isolated in space between the condensate clouds, could offer scientists and engineers a tantalizing new window for controlling and manipulating optical information; researchers cannot now readily control optical information during its journey, except to amplify the signal to avoid fading. The new work by Hau and her colleagues marks the first successful manipulation of coherent optical information. "

Ah, that's my kind of girl:)

And light then connects/adheres to 'matter' in some strange way?
But it doesn't 'become' new matter, it just uses what already is there, right?

Title: Re: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: swansont on 17/02/2009 21:25:31

One of the reasons that people want to do things with quantum computers is that you can store lots of information that way — the phase of the light that is stored is one of the reasons that holograms behave the way they do.

What's happening here is the atomic states are able to record the phase information of the light when the photons are absorbed.  The atoms recoil when doing so, so they move toward the other condensate, and when they interact, they force the second condensate to have the same phase.  So the information has been transferred, and you can recreate the original light pulse.

Title: Re: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: yor_on on 17/02/2009 23:55:18

So what exactly are this phase information stored and then retrieved?
"a wave function phase carries no physical information, it just describe that the wave function is a wave. the magnitude of the wave function carries information about the probability finding the particle in some points in space. the magnitude of the wave function in a point in space is proportional to the probability of finding the particle in that point"

And the information discussed here is then the possibility to store one of two phases.
That then will correspond to a 'one' or a 'zero' digitally.

So what is then moved here from one condensate to another, not the wave itself.
Just the information of that waves phase, and that phase of a lightwave corresponds to its position in the cycle between the crest and trough.

A atom treats phase differently. If I get it right you can think of the atom (for this thought experiment) as a transparent globe. inside it you have this invisible 'top' spinning, the degree to how it is leaning (or how you choose to lean it) as it spins around will be the phase seen for the atom.

And that is what is transfered here, not so much any information as an imprint of that 'spin' translated into the atoms phase, What we call 'information' here is no more than us giving the atom a new 'nudge' by exciting it with the original light wave, and then quenching the original 'coupling laser',  and then turning a new coupling laser 'on' to the other condensate, after those atoms 'drifted' over to it. So the light here still needs a transference of energy from outside to be reconstructed.

Btw: they don't stop the light, they slow it down, but it still has some momentum left that, I think(?), is transfered to those atoms that receive that 'imprint'. And that's why the atoms move over.

Ahh, at last I think I understand it somewhat:)
Probably not:::)))

---------
So the light seen at that other condensate is not so much 'resurrected' as ¨'reconstructed' by using the new phase transfered to those atoms by the original light wave. At least it seems more reasonable than saying that we get the 'original' wave back.

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: yor_on on 18/02/2009 13:22:49

So in a way this is a 'letdown' from my original impression of my dear Professor Lene Vestergaard Hau original experiment in which she was said to first stop the light and then 'resurrect' it.
In fact you could do this if I'm right:) with a laser without needing to 'imprint' anything on anything with a fifty percent success.
Just have a laser and 'something' to detect its phase with. turn it on, off, on, off, on, off, ad infinitum into that BEC.
And then, Voila, 50 percent of the cases should give you the same 'phase', that you once decided to be your reference 'phase'.
And if you just could get rid of those other fifty percent you could say...
-This 'machine/system' my friend, truly is 'resurrecting' the light.
Well, at least you would be able to tell it to a gullible 'science reporter':)

----

To be honest I can't really see Professor Hau doing this 'on purpose', it must have been a 'misunderstanding' from those reporters side, that then just kept on growing.
And she, and her compatriots, have a very good imagination, to be able to see those possibilities.

But I'm wondering if there is some research on how fast a 'rumor/misunderstanding' becomes a fact on the Internet.
Anyone?

-----------------

I can see the importance of it for our next (next next next?) generation of computers, but they won't be quantum computers will they?
As that needs the computers 'spin(s)' to have a 'superposition' without us knowing which way it will fall out.
So my next adventure, I think, is to try to understand what a quantum computer 'really' should be seen as, when all the 'hype' is taken away:)
And to try to understand some more of that strange subject, waves::))

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: Vern on 18/02/2009 13:45:06

Quote from: yor_on

So my next adventure, I think, is to try to understand what a quantum computer 'really' should be seen as, when all the 'hype' is taken away:)
And to try to understand some more of that strange subject, waves::))

Most of the info I glean out of quantum-computer studies is that the memory will not be simple binary information. The phases stored (remembered) by excited atoms will be like colour. Colour is infinitely variable over a wide range so each atom could store more than just binary information.

I'm still trying to understand the article you linked. I couldn't figure out whether the info was duplicated (copied) or whether the info was suddenly available at another place, but no longer available in its original place.

 

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: swansont on 18/02/2009 17:32:54

The phase information is stored because you are putting the system into a superposition of the two states, but you can do this with the wave function having different phases; this represents the phase of the incoming light.  It's not a binary state in that sense, since any phase can be represented.  However, storing information this way is going to depend on how you get the information out — it's not about what state that atom ends up in (that would be binary) but if you interfered the atoms with a reference, then the interference pattern can tell you more about the system, and you can possibly use that to store/retrieve information.

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: Vern on 18/02/2009 18:31:35

Looks a little like a holographic process, as you explain it. It will be interesting to see how the computer model develops.

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: yor_on on 26/02/2009 17:36:01

I'm having a discussion elsewhere touching this subject :)

In it Chantal writes

" By the way, I hope you all realize that if you "believe" in the "probability" version of Schroedinger, you trade one weird thing with another! Namely that the cat doesn't interfere with itself but that it could be both alive and dead at the same time ;-).

Which one is weirder ;-)))

Yes, atoms can actually self interfere, but you have to cool down the atoms significantly. "

---

So I need to ask, isn't the definition of a BEC...

" When the condensate is produced, virtually all the atoms in the gas fall into the lowest-energy quantum mechanical state. Spread out in space, they become superimposed on one another, each indistinguishable from the other, creating what has been called a "superatom." A collection of millions of atoms, extending over the entire gas, behaves like a single atom described by the "matter wave" picture of quantum mechanics."

And that is a coherent 'system' and not individual 'atoms' any longer, right?

Looking at the experiment done here by Lene Vestergaard Hau we do have atoms moving freely between two BEC:s but how should one see them?

As something 'superimposed ' without retaining any individuality or..
But after getting that 'nudge' they are unique, even it that BEC, right?
Or are the whole first BEC getting that 'imprint' as they are one 'superatom'?

--
And if it is 'a few' atoms what does that mean?
That we have an entanglement between the 'resurrected' light wave and that first BEC.
And what about the second BEC if it has another phase initially, then we have two phases in 'one' BEC or...


Welcome to my confusion:)


--------------------
The wireless telegraph is not difficult to understand.
The ordinary telegraph is like a very long cat.
You pull the tail in New York, and it meows in Los Angeles.
The wireless is the same, only without the cat.

A.E
-----------
Those who lost dreaming are found

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: Vern on 26/02/2009 18:52:29

You're not alone in your confusion; I'm still trying to figure out how hydrogen 4 can be a boson. [:)]

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: swansont on 27/02/2009 17:19:40

Quote from: Vern on 26/02/2009 18:52:29

You're not alone in your confusion; I'm still trying to figure out how hydrogen 4 can be a boson. [:)]

Who's claiming that it is, and how long does hydrogen-4 exist that anyone could confirm that?

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: swansont on 27/02/2009 17:36:20

@ yor_on,

Atoms do interfere, and it does not require cold atoms, though making them cold usually makes it easier to see.

The whole BEC is getting the "imprint" of the phase in the Hau experiment, which happens when the coupling laser is turned on.  So it sounds (qualitatively) somewhat like stimulated emission of light, where the phase of the incoming photon dictates the phase of the stimulated photon (i.e. they are the same) even though the excited atoms need not have the same phase.  Only in this case it's the incoming atom dictating the phase of the other atoms in the BEC.

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: Vern on 27/02/2009 19:02:18

Quote from: swansont on 27/02/2009 17:19:40

Quote from: Vern on 26/02/2009 18:52:29

You're not alone in your confusion; I'm still trying to figure out how hydrogen 4 can be a boson. [:)]

Who's claiming that it is, and how long does hydrogen-4 exist that anyone could confirm that?

This Wikki (http://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_condensate) seems to indicate that H4 is a boson when cooled to a low enough temperature.

Edit: Oooops: I meant helium 4, not hydrogen 4. Missed that extra proton.

Quote from: Wikki

The slowing of atoms by use of cooling apparatuses produces a singular quantum state known as a Bose condensate or Bose–Einstein condensate. This phenomenon was predicted in 1925 by generalizing Satyendra Nath Bose's work on the statistical mechanics of (massless) photons to (massive) atoms. (The Einstein manuscript, believed to be lost, was found in a library at Leiden University in 2005.[6]) The result of the efforts of Bose and Einstein is the concept of a Bose gas, governed by the Bose–Einstein statistics, which describes the statistical distribution of identical particles with integer spin, now known as bosons. Bosonic particles, which include the photon as well as atoms such as helium-4, are allowed to share quantum states with each other. Einstein demonstrated that cooling bosonic atoms to a very low temperature would cause them to fall (or "condense") into the lowest accessible quantum state, resulting in a new form of matter.

This transition occurs below a critical temperature, which for a uniform three-dimensional gas consisting of non-interacting particles with no apparent internal degrees of freedom is given by:

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: yor_on on 01/03/2009 11:35:57

Thanks Swansont
So what we might end up with then, are two 'entangled' condensates?

Yeah Vern:)
It's strange, but cool.

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: swansont on 02/03/2009 01:06:11

AFAIK they are not entangled. 

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: yor_on on 02/03/2009 13:48:26

Sorry, you are so right there Swansont.
It's me not thinking it through.

Entanglement will always have two opposite spins and here there are only one as the 'imprint' includes the whole 'super atom'.

How the he** do a beamsplitter produces two spins, thinking of it again?
It's by 'splitting one light quanta 'beam/photon' in the beam splitter (X) begetting two copies, having opposite spins. (had to rewrite that one, as not even I could understand what I meant there:) Or send in two beams, each one with its own spin, and then get entangled spins out from the beam splitter. But it would still mean getting just two spins, even though I in this case started with two photons instead of just one. But that's not phase..

So could we have two BEC:s in a etanglement and then by 'forcing' a 'known spin' (is that what this first experiment was doing, forcing a 'known' spin?) on one BEC then get the opposite spin on that other BEC without doing any observations?

Would the BEC:s still be super positioned if so, or would us knowing the spin, even if not observing, resolve the situation into one 'state'? And then by transferring those sodium atoms, would give us yet another state :) as they are 'super atoms'

I need to get this straight in my head here :)
I'm not saying that it's possible though...

To get it straight in my head, that is :)

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: swansont on 03/03/2009 14:23:28

It may be possible to entangle two condensates (or it may not), but if it turns out to be possible I'm not sure how you'd do it.  You need a two-state system, where the atoms in one BEC would have to be in one state while the other BEC is in the other state.  The trouble is that BECs form in particular states — if you e.g. change the spin orientation, the magnetic trapping doesn't work anymore.  So this would require some techniques more advanced than what I'm familiar with.

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: yor_on on 28/03/2010 22:34:02

Okay, really needed to look at that to see what 'phase' meant here. Am I right in understanding the phase as being where you measure one sine wave relative another in a 'beam'?

"The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0."

And does it make any sense to speak of the phase of one sine wave? As one seem to chose this 'place' of measurement arbitrarily? I mean, depending on where you place t=0 you will get a different value won't you? For one sine wave? So in this case? How did that phase get reconstructed. I can easily see the frequency getting reconstructed though. Or in the case of two sine waves, offset against each other in that light pulse, get the same 'displacement' at t=0 when measuring them again..

Or am I bicycling in the blue younder here?
===

mechanical waves (http://id.mind.net/~zona/mstm/physics/waves/introduction/introductionWaves.html#) 
Phase waves. (http://en.wikipedia.org/wiki/Phase_%28waves%29) 
Frequency. (http://en.wikipedia.org/wiki/Frequency)   

===Quoted from Properties of Waves. (http://simple.wikipedia.org/wiki/Wave_%28physics%29)

 [ Invalid Attachment ]


Quote;

"Waves have properties that can be measured. All waves are made by adding sine waves... The shape of a sine wave is given by its amplitude, phase, wavelength and frequency. The speed that the sine wave moves can be measured. The amplitude and wavelength of the sine wave is shown in the picture. The highest point on a wave is called the peak. The lowest point is called the trough.

The peak of a wave and the trough of a wave are always twice the wave's amplitude apart from each other. The part of the wave half way in between the peak and the trough is called the baseline. All waves are made by adding up sine waves. Waves also have amplitudes, phases, wavelengths, frequencies and speeds that can be measured."

But where would the Phase of this sine wave be?
And furthermore, out of sheer curiosity, how do you measure that phase exactly, in the new sine wave created by this experiment, if so?

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: yor_on on 29/03/2010 00:04:22

Let's look at it again. To do so we first need to understand coherent light. As i see it :) coherent light is a wave made up from a lot of waves 'falling in line' so to speak.

"It's incorrect to say that "in laser light the waves are all in phase." When two light waves traveling in the same direction combine, they inextricably add together, they do not travel as two independent "in-phase" waves. The photons in laser light are in phase, but the WAVES are not. Instead, ideal laser light acts like a single, perfect wave."

And here we see that word again 'in phase' ?
Ah yes?

"We say that laser light is "in phase" light. However, in-phase photons are nothing unique, and they don't really explain coherence. Any EM sphere-wave or plane-wave is made of in-phase photons. For example, all the photons radiated from a radio broadcast antenna are also in phase, but we don't say that these are special "in phase" radio waves, instead we just say that they are waves with a spherical wavefront. Even if all the photons in laser light are in phase, it is still incorrect to say "all the WAVES are in phase." Photons are not waves. They are quanta, they are particles, and they do not behave as small, individual "waves." Yes, all the photons are in phase, but only because they are part of a single plane-waves."

So what did it really mean? Talking about light having different 'phases' then?
Can't be the lasers 'wave(s)' can it?
Photons then?

"The light from a laser is basically a single, very powerful light wave. Single waves are always in phase with themselves, but it's misleading to imply that a single plane-wave or sphere-wave is something called an "in phase" wave. Laser light could more accurately be called "pointsource" light. Sphere waves or plane waves behave as if they were emitted from a single tiny point. The physics term for this is "spatially coherent" light. Light from light bulbs, flames, the sun, etc. are the opposite, and are called "extended-source" light.

Extended-source light comes from a wide source, not from a point-source, and the waves coming from different parts of the source will cross each other. Starlight and the light from arc welders is "point-source" light and is quite similar to laser light. Light from arc-welders and from distant stars has a higher spatial coherence than light from most everyday light sources. (Note: the sun is a star, correctly implying that light becomes more and more spatially coherent as it moves far from its source. This is a clue as to the REAL reason that lasers give spatially coherent light!"

Have to admit that I'm still not sure how to understand the concept of all those 'phases' existing in the coherent beam made by an laser?
====

Okay Photons :)



"Coherent light is composed of photons (no, not waves this time.) that all have the same wavelength and phase." So does a 'photon' have a phase? Not as I understands it? Does it? Don't you need a wave, or rather two, to create what's called a 'phase'? Does it have a wavelength then? Well, that depends on who you talk with.

Diverse quotes..
==
1. Yes. It's energy, frequency and wavelength are related by: E=hf=hc/lambda.

2. I Bragg-diffracted a few x-rays per hour in my Ph.D. thesis, and counted them individually in a sodium-iodide detector. So wavelength is a property of individual photons. Also, there was no obvious correlation of "streams of photons".

But..

3. The diffraction pattern shows up when a lot of photons are measured. I don't think the wavelength of a single photon has ever been measured before. So all this is just "quantum fairy tales"..

4. You can arbitrarily define a wavelength of a photon by placing it in a resonant cavity of a certain size (In cavity QED ) and the resonance condition is just lambda/2. If you send in photons of other energies they just decay immediately. I'd say this is a pretty good way of measuring wavelength (adjusting the length of a resonator while looking for a resonance is a pretty common technique for measuring wavelength in microwave engineering, this is essentially the same thing).
==

I don't know :)

And looking at it as a 'wavepacket' I've always seen that as making a best approximation (cut off) of that idea of a photon. Treating it as a 'mix' of different 'wavelengths'. "the wavelength varies over the distance of the wave packet. The range in wavelength corresponds to the range in energy, since the energy of a particle cannot be definitely determined (the velocity of the particle is never known to 100% accuracy). So, the wavelength is ill-defined."

And lastly. "A field has a given wavelength as long as it does not interact. The quanta related to this field (Photon) has a momentum given by the wavelength of that fields times the Planck constant. Thus to quanta with the same momentum there is associated a wavelength. However the number of quanta is not an observable since it is not well defined for distance smaller that the Compton length."

So here we have an added problem, Treating it as photons there are difficulties defining just how 'many' they are at any given instant. And that naturally will create a problem defining what the wavelength might be. It seems as HUP all over again, doesn't it?

So what is this 'phase' associated to a laser beam, ah photons?
I'm stymied here :)

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: yor_on on 29/03/2010 01:54:58

Maybe I will see it if I look at the definitions for phase?

==Quote.

Phase - A description for the relative position in a cyclical or wave motion. Because one complete wavelength is described as 2p radians or 360 degrees, the phase of a wave is given in radians, degrees, or fractions of a wavelength. The term in phase refers to phase angles between two wavefront occurrences of zero and 360 degrees or a whole number multiple of these.

Phase Difference - The phase angle by which one periodic disturbance or wavefront lags behind or precedes another in time or space. Phase differences are usually described in terms of fractions or multiples of a wavelength.

Phase.

In wave motion, the fraction of the time required to complete a full cycle that a point completes after last passing through the reference position. Two periodic motions are said to be in phase when corresponding points of each reach maximum or minimum displacements at the same time.

If the crests of two waves pass the same point at the same time, they are in phase for that position. If the crest of one and the trough of the other pass the same point at the same time, the phase angles differ by 180° and the waves are said to be of opposite phase. Phase differences are important in alternating electric current technology

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fcontent.answers.com%2Fmain%2Fcontent%2Fimg%2FMcGrawHill%2FEncyclopedia%2Fimages%2FCE505400FG0010.gif&hash=4860699d37869163c53eb583665f61fd)

An illustration of the meaning of phase for a sinusoidal wave. The difference in phase between waves 1 and 2 is φ and is called the phase angle. For each wave, A is the amplitude and T is the period.

==End of quote.

Looking at this last description laser light could indeed be seen as being 'in phase' as it will represent one single wave as it comes out, depending on definition of course. But then there will be only one 'phase'? But if I regulate the strength of that light, won't I get a new 'phase' then? Is that what this phase discussion is about? As for phase difference? There can't be any such in coherent light, can there?

So what happens when I regulate the strength of it? I control its energy right?
Isn't that its frequency I'm regulating there?

===Quote.

The relationship between energy and frequency relates to the energy of the photon for that particular frequency. The constant of proportionality is known as Planck's constant, and is denoted in Physics by the letter h. The formula E=hf gives the energy of a photon in Joules for a wave of frequency f Hertz.

Where E= energy, h= Planck's constant and f= Hertz

The wavelength (lambda) does not appear in this equation, as wavelength = speed of light(c) divided by frequency(f).

Or frequency = speed of light divided by wavelength,so the equation could be written as E = h times c divided by lambda, which would yield the same result

===End of quote.

Let me see if I got it straight. The energy of a photon can only be defined if treated as a wave?

==Quote

"The energy and momentum of a photon depend only on its frequency (ν) or equivalently, its wavelength (λ):"

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fmath%2Fd%2F0%2F7%2Fd07def13d6f88776fe72fd064c75f820.png&hash=9bdf848919da9fb7fc24560423dc1247)

where k is the wave vector (where the wave number k = |k| = 2π/λ), Ω = 2πν is the angular frequency, and ħ = h/2π is the reduced Planck constant

====End of quote.

So yes, energy is wavelength or, equivalently, frequency here.
So am I getting closer to the idea?

By varying the intensity/energy/wavelength/frequency you will create a different phase?
But why not call it wavelength if so?

As we're looking on one wave here, not many?
I'm not sure I really have understood this concept when it comes to the experiment.
You will not be able to give the condensate different 'phases', it seems to me, without finding a way to 'separate' that same condensate into different, sorry, don't remember the right word here, let's call it 'areas' if so?

As I said, I could be bicycling in a blue blue younder here :)
===

Swanson wrote "The phase information is stored because you are putting the system into a superposition of the two states, but you can do this with the wave function having different phases; this represents the phase of the incoming light."

I think I will have to reread it and see where I did go wrong :)
Superpositioning of two states have to mean something else than what I'm discussing here.
As this 'phase' I am seeing is just one piece of information?

Title: Can light stopped in a Bose Einstein Condensate "jump" between condensates?
Post by: yor_on on 29/03/2010 03:19:42

Okay, I see it :)

What you have is the ultracold condensate cloud equivalent to a 'superatom' . This 'superatom' is in one state already before the experiment (predefined state?). When injecting the short laser pulse in the cloud you will place the 'superatom/condensate' in a super position defined by the 'state' it had itself before, combined with the 'state', (amplitude/shape) and phase, of the incoming laser light. So there is your superposition.

"By blocking the control laser beam when a co-propagating probe light pulse has slowed down and is compressed enough to be contained within the atom cloud, this light pulse can be halted and extinguished, while the atomic hologram stays in the cloud. When the control laser is turned on again, the process is reversed; the probe light pulse is regenerated and moves on. Such storage of optical information has been demonstrated for classical light pulses containing many photons, for singlephoton light pulses, and for entangled light fields.

Instead of storing just a single light pulse in an atom cloud, a threedimensional image may also be injected, slowed and stored. During the slowlight process, the image information is compressed in the direction of propagation. Two- and three-dimensional images have indeed been slowed and stored in a warm alkali gas. In these experiments, the optical phase parameter was used to compensate for the thermal smearing of the stored patterns, that result from atomic motion in the hot gas during the storage time. With the use of cold atoms, such thermal smearing can be minimized. It is particularly interesting to use ultracold atom clouds, in the form of Bose–Einstein condensates, for storing information.

A Bose–Einstein condensate is fundamentally different from even very cold, non-condensed clouds. Bose–Einstein condensates are phase-coherent objects, and changes of the stored holographic imprint owing to atom dynamics do not lead to loss of information as in non-condensed clouds; rather, they lead to processing. As very rich nonlinear dynamics can occur in superfluid Bose–Einstein condensates, and as the dynamics can easily be controlled by controlling atom-atom interactions (for example, with external magnetic fields), Bose–Einstein condensates can be very powerful processors of optical information."

So, maybe I have understood what 'phase' means here then?
If you feel I'm missing out on it, do correct me.
Want to read the paper :)

slow light (August, 2008). (http://www.seas.harvard.edu/haulab/publications/pdf/HauNaturePhotonics2008.pdf)