Ultracold disappearing act: 'Matter waves' move through one another but never share space

Ultracold disappearing act: 'Matter waves' move through one another but never share space
Physicists (from left) De Luo, Jason Nguyen and Randy Hulet observed a strange disappearing act during collisions between forms of Bose Einstein condensates called solitons. In some cases, the colliding clumps of matter appear to keep their distance even as they pass through each other. CREDIT: Jeff Fitlow/Rice University

A disappearing act was the last thing Rice University physicist Randy Hulet expected to see in his ultracold atomic experiments, but that is what he and his students produced by colliding pairs of Bose Einstein condensates (BECs) that were prepared in special states called solitons.

Hulet's team documented the strange phenomenon in a new study published online this week in the journal Nature Physics.

BECs are clumps of a few hundred thousand lithium atoms that are cooled to within one-millionth of a degree above absolute zero, a temperature so cold that the atoms march in lockstep and act as a single "matter wave." Solitons are waves that do not diminish, flatten out or change shape as they move through space. To form solitons, Hulet's team coaxed the BECs into a configuration where the attractive forces between lithium atoms perfectly balance the quantum pressure that tends to spread them out..

The researchers expected to observe the property that a pair of colliding solitons would pass though one another without slowing down or changing shape. However, they found that in certain collisions, the solitons approached one another, maintained a minimum gap between themselves, and then appeared to bounce away from the collision.

"You never see them together," said Hulet, Rice's Fayez Sarofim Professor of Physics and Astronomy. "There is always a hole, a gap that they must jump over. They pass through one another, but they never occupy the same space while they're doing that.

"It happens because of 'wave packet' interference," he said. "Think of them as waves that can have a positive or negative amplitude. One of the solitons is positive and the other is negative, so they cancel one another. The probability of them being in the spot where they meet is zero. They pass through that spot, but you never see them there."

Hulet's team specializes in experiments on BECs and other ultracold matter. They use lasers to both trap and cool clouds of lithium gas to temperatures that are so cold that the matter's behavior is dictated by fundamental forces of nature that aren't observable at higher temperatures.

To create solitons, Hulet and postdoctoral research associate Jason Nguyen, the study's lead author, balanced the forces of attraction and repulsion in the BECs.

"First we make a Bose Einstein condensate and then we use a sheet of light to split the condensate in half and push the two halves apart," Nguyen said. "We hold them apart and turn each of them into solitons, and then we take the sheet away and let them fall back toward one another and collide."

Cameras captured images of the tiny BECs throughout the process. In the images, two solitons oscillate back and forth like pendulums swinging in opposite directions. Hulet's team, which also included graduate student De Luo and former postdoctoral researcher Paul Dyke, documented thousands of head-on collisions between soliton pairs and noticed a strange gap in some, but not all, of the experiments.

"One of the defining features of a soliton is that they are supposed to be able to pass through one another and emerge unfazed," Hulet said.

"Some of the collisions are consistent with that," he said, pointing to images of two solitons oscillating, meeting, emerging and continuing on their cycle. "These two solitons certainly appear to have passed through one another.

"In another set of collisions, there's always this gap between them," he said, pointing to a different set of images. "It doesn't look like they ever close that gap to be able to pass through. In fact, it looks like they've come together and then bounced off one another."

Hulet said the idea of solitons bouncing away from one another had been around for about 40 years, based on longstanding observations of optical solitons in fiber-optic cables. In this scenario, the gap is viewed as evidence of a force that is pushing the solitons apart.

To probe more deeply, Hulet's team needed to conduct a new set of experiments that focused on the one defining feature of a soliton that they couldn't control—its phase.

The first soliton was observed in a canal in Scotland in 1834 and they've since been observed in magnets, fiber-optic cables, atomic nuclei and even swimming pools. Hulet's team was among the first to report BEC "matter-wave bright solitons" in 2002.

Like a wave in the ocean or a light beam in a fiber-optic cable, solitons have a characteristic amplitude, frequency and phase. Hulet's team could control the amplitude but they could not control the soliton's phase.

"All waves oscillate in time," Hulet said. "They have a frequency at which their amplitude becomes positive, negative, positive, negative and so on. The rate of that oscillation, how often it switches, defines their frequency. Where they begin that cycle is something we refer to as 'the phase.' It's a kind of starting point."

The wave's phase is an angle that can vary between zero and 360 degrees. Waves that are "in-phase" have the same starting point, and waves that are "out-of-phase" are 180 degrees off, meaning that one begins at its peak while the other starts at its trough.

"When we saw the initial data we said, 'This doesn't make sense, because solitons are always supposed to pass through one another and these look like they're bouncing instead,'" Hulet said. "So we began thinking about how we could tag one of the solitons to make it distinct so that we could follow its trajectory in time and see what it did."

The team found a way to "tag" one soliton by making it larger than the other. In the next round of experiments, Nguyen and Luo captured pictures of collisions between different-sized solitons.

"We did that experiment over and over for many different relative phases, and we looked for two cases, one where the relative phase was zero, or in-phase, and another where it was 180 degrees, or completely out-of-phase," Hulet said.

For the in-phase case, the team saw the two solitons pass through one another and emerge, just as predicted by theory.

"In the out-of-phase case, the one with the gap, where it appeared that they had been bouncing off of each other, we still saw the gap but we also saw the larger soliton emerge unfazed on the other side of the gap. In other words, it jumped through the gap!"

Hulet said the experiment confirmed the theory that solitons do pass through one another, even in cases where they are out-of-phase and only appear to bounce away from each other.

Many of the events that Hulet's team measures occur in one-thousandth of a second or less. To confirm that the "disappearing act" wasn't causing a miniscule interaction between the soliton pairs—an interaction that might cause them to slowly dissipate over time—Hulet's team tracked one of the experiments for almost a full second.

The data showed the solitons oscillating back and fourth, winking in and out of view each time they crossed, without any measurable effect.

"This is great example of a case where experiments on ultracold matter can yield a fundamental new insight," Hulet said. "The phase-dependent effects had been seen in optical experiments, but there has been a misunderstanding about the interpretation of those observations."


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Everlasting Quantum Wave: Physicists Predict New Form of Soliton in Ultracold Gases

More information: Nature Physics, dx.doi.org/10.1038/nphys3135
Journal information: Nature Physics

Provided by Rice University
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Nov 02, 2014
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Nov 02, 2014
This seems to remind me of my high school physics days, when I learned that an electron can jump from one orbital shell, to another, but doesn't exist in between. I thought how can that be?? Later I learned how to calculate a simple emission line for the hydrogen atom using first principles. ie, consider an electron to be a wave of certain frequency at a given radius. A slighter bigger radius means the electron has to travel a bit further, and hence goes out of phase with itself, and thus cancels itself as the wave goes around the circle. Get the radius just right though, and the wave is back in phase, and the electron can exist, ie another orbital shell. So there we have it, the electron DOES travel through the gap between orbitals, just it's out of phase, and you only "see it" when it is back in phase!!! Yeah I know I'm thinking classically here, but it is a good mental picture for me to have, and this article on Solitons seems to back this up. I realise this only works for Hydrogen

Nov 03, 2014
More proof of multiple dimensions.
This was very expected, welcome to go back in my posts where I talked about this. The first person to find this was an electrical engineer working with a nano-transistor.

I understand people don't get this stuff, but it's time to break down some barriers and forget everything you learned in high school physics.
If you put all waves in the multi-verse together the net effect would be 0. In essence, everything is nothing or zero.

Nov 03, 2014
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Nov 03, 2014
I think maybe "erson", in his comment above, is saying what occurred to me... that perhaps we shouldn't be surprised that a BEC (albeit "halved") briefly exhibits the published properties of a BEC?

I have a sneaking suspicion that, even in a discussion among the "experts", it is painfully obvious that we simply don't have the language nor the comparative analogies at our disposal to even have a very intelligent discussion about what is being observed. Case-in-point is found in "tritace"'s comment above... his/her use of "dark matter" is a bit comical to me since the term (in my understanding) is merely a placeholder for some theoretical thing/situation that we don't have any real clue about but adopted to fill-in a gap so that we can forge ahead blindly. Dark matter/energy don't seem like effective variables to be used in "solutions", right?
I'll close with a chuckle about how said dark "stuff" seems shared by both cosmology and quantum theory... Now who's the "kooky cousin"?

Nov 03, 2014
Circa 1967, Art Winfree proposed his law of coupled periodic oscillators, which revolves around the relative phases of the periodic oscillators in the system to be organized.

The BEC waves described in this Physorg article are two periodic oscillations, in Winfree terms. Winfree showed that such a system could couple in two ways: either exactly in phase or exactly out of phase. This BEC experiment involves all of the elements of Winfree's law, with the results that Winfree predicted. Even the illusion of "bouncing." The 180 degree phases make the two waves precise mirror images of each other, indistinguishable unless tagged.

I have made many Physorg posts, over the last four years, showing that Winfree's law applies to physics. It must: Max Planck's quantum of energy is a periodic oscillation.

Winfree has several followers, including Steve Strogatz of Cornell. But none are physicists.

Nov 03, 2014
...I have a sneaking suspicion that, even in a discussion among the "experts", it is painfully obvious that we simply don't have the language nor the comparative analogies at our disposal to even have a very intelligent discussion about what is being observed. ...


Okay, okay... Either "I have a sneaking suspicion" or "it is painfully obvious", not both! You decide. ;)

Nov 03, 2014
jdavis417 made my day with
Okay, okay... Either "I have a sneaking suspicion" or "it is painfully obvious", not both! You decide. ;)
Its like "I like this tech stuff, I have a sneaking suspicion I am intelligent enough to understand this so its obvious I can because I have more than a sneaking suspicion I am conscious of intelligence" or words to that effect - lol !

I read those some years ago on some other forum & your comment jdavis417 on that which you read reminded me, I am still shaking my head wondering what background, education, experience could possibly lead to the type of quote I mentioned in last para.

I wonder if it was uttered by the same person you commented on, presumably some years after the event so its had time to "settle" sufficiently to make it more succinct :-)

Nov 03, 2014
If you put all waves in the multi-verse together the net effect would be 0. In essence, everything is nothing or zero.


Or - one...

Nov 03, 2014
Whydening Gyre offered me more chance to apply chuckle force re numerical/philosophical paradigms with
If you put all waves in the multi-verse together the net effect would be 0. In essence, everything is nothing or zero.
Or - one...
Ah yes but, one what ?

One abstract notion held in consciousness by one deity of some (one's) sort.

ie. One as in not abstract a unitary thing of what attribute then singular or plural attributes.

If there is no-one to conceive a numeral (or abstract notion of a numeral) then does that numeral exist, who would be there to confirm, deny or even communicate it & to whom ?

Ain't stuff fun.

Nov 03, 2014
One of the basic tenants of the new model for an underlying driving mechanism capable of uniting the fundamental forces as well as providing an updated physical mechanism for the formation of the universe based on well known nuclear processes scaled to higher energy levels the further back in history they extend, is that it's not only particles that obey the laws of dispersion, but also waves as well. The results of this study provide a strong indication that I'm on the right track. Waves are ultimately what gives all matter it's structure and the presents that it needs to interact with the rest of the physical world.

Nov 04, 2014
...

If there is no-one to conceive a numeral (or abstract notion of a numeral) then does that numeral exist, who would be there to confirm, deny or even communicate it & to whom ?

Ain't stuff fun.


The SPCA is at the door... do you want to recant your story of placing that cat in that box? ;)

Nov 04, 2014
...I wonder if it was uttered by the same person you commented on, presumably some years after the event so its had time to "settle" sufficiently to make it more succinct :-)


I hope I don't come across as either "butt hurt" or defensive of my id, but since the comment you replied to was a correction of my own grammar, (although I did reference others in the grammatically offensive one) you've given me an opportunity to further my point (which was admittedly, weakly stated).

1.) I'm no genius and I can prove it (if I didn't already). I do not intent to feign any understanding of this... that would counter my contention that...
2.) These subjects seem to be beyond normal comprehension... At least in the way that we can comprehend that the stove is hot & we shouldn't have a lie-down up there.
3.) In these experiments and observations we seem to be utilizing a very loose "scientific method" (perhaps necessarily).
4.) "Right thinking" may come from outside the field. ;)

Nov 04, 2014
...Waves are ultimately what gives all matter it's structure and the presents that it needs to interact with the rest of the physical world.


I'd like to hijack your comment to further my own, due to character limitations for replies. I hope my comments are taken as intended... as the light-hearted, inane musing's of a simpleton. Thanks to those past-and-future for "making it fun".

We could live in a kind of "Flat Land" where the waves that you reference are something quite different from what we can observe.
We ought to admit that our "dark stuff", & perhaps most theories, are like jumping on the back of the cowardly lion because he thinks he might be able to clear the chasm. I'm not saying this is wrong... I'm saying that we could have very little idea what we're observing and it's very difficult (perhaps impossible) to express in terms that we are accustomed to. Keys to their expression may come from "children". Comments from the uninitiated should be considered.

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