Are Bose Eistein Condensates Classical Objects

[bhobba]

This came up in a thread discussing if quantum effects occur at the macro level. I always thought it was pretty much standard wisdom that Bose-Einstein condensates show quantum effects at the macro level. I mean for such states atoms loose their individuality and behave like one large quantum system. Am I missing something here?

Thanks
Bill

 

[Maui]

This came up in a thread discussing if quantum effects occur at the macro level. I always thought it was pretty much standard wisdom that Bose-Einstein condensates show quantum effects at the macro level. I mean for such states atoms loose their individuality and behave like one large quantum system. Am I missing something here?

Thanks
Bill

As far as i can understand, the Bose condensate is observed only indirectly(inferred) as interacting with the outside macro world raises the temperature and destroys coherence. Feel free to correct me if i am wrong but direct observation is needed to justify the claim that quantum effects can be observed on the classical scale.

It is my understanding that brains are not suited to observe quantumness, though i agree with you that the world is entirely quantum. This is supported by the latest understanding of the HUP and some experiements like the DCQE. I am not claiming that i know that brains destroy coherence and single out outcomes but i am genuinely interested if direct quantum behavior can be experimentally observed(even in principle).

 

[JK423]

Why would you think that a BEC is a classical object? In the sense, that all the atoms of a BEC are delocalized and in the same quantum state, hence coherent. But i think you already knew that, so i suppose you should elaborate a little bit more on what you're trying to say because i don't undestand.. :)

 

[bhobba]

As far as i can understand, the Bose condensate is observed only indirectly(inferred)

I believe they were actually created in 1995. But how they determined its properties I have zero idea.

Thanks
Bill

 

[DrDu]

Do you consider classical electromagnetic fields to be classical objects?

 

[stevendaryl]

This came up in a thread discussing if quantum effects occur at the macro level. I always thought it was pretty much standard wisdom that Bose-Einstein condensates show quantum effects at the macro level. I mean for such states atoms loose their individuality and behave like one large quantum system. Am I missing something here?

Thanks
Bill

A large part of the weirdness of Bose-Einstein condensates have nothing to do with quantum mechanics (in the sense of the uncertainty principle, non-commuting operators, etc.) but just follow from the differences between Bose-Einstein statistics and Maxwell-Boltzmann statistics. This difference is purely due to a difference in how you count the number of microstates associated with a macrostates. If you think of particles as in principle distinguishable, then you get Maxwell-Boltmann, and if you think of them as in principle indistinguishable, then you get Bose-Einstein. I'm not sure what the connection is between quantum mechanics and indistinguishability of particles. They are usually taught together, but that might be just because both come into play at the level of atomic particles, and are unimportant for large objects.

 

[bhobba]

Do you consider classical electromagnetic fields to be classical objects?

Yes - but Quantum Fields - no.

Thanks
Bill

 

[bhobba]

Why would you think that a BEC is a classical object?

I don't - I think its QM writ large - the particles have lost their individuality and is simply one large quantum object.

Thanks
Bill

 

[bhobba]

A large part of the weirdness of Bose-Einstein condensates have nothing to do with quantum mechanics (in the sense of the uncertainty principle, non-commuting operators, etc.) but just follow from the differences between Bose-Einstein statistics and Maxwell-Boltzmann statistics. This difference is purely due to a difference in how you count the number of microstates associated with a macrostates.

Wait a minute - I don't think that classical systems have the property when you exchange two particles it is exactly the same - that's the reason for the different counting and why it doesn't follow the classical Gaussian Boltzmann law which is based on the binomial distribution.

Thanks
Bill

 

[JK423]

A large part of the weirdness of Bose-Einstein condensates have nothing to do with quantum mechanics (in the sense of the uncertainty principle, non-commuting operators, etc.) but just follow from the differences between Bose-Einstein statistics and Maxwell-Boltzmann statistics. This difference is purely due to a difference in how you count the number of microstates associated with a macrostates. If you think of particles as in principle distinguishable, then you get Maxwell-Boltmann, and if you think of them as in principle indistinguishable, then you get Bose-Einstein. I'm not sure what the connection is between quantum mechanics and indistinguishability of particles. They are usually taught together, but that might be just because both come into play at the level of atomic particles, and are unimportant for large objects.

Just one particle bounded inside a potential well shows full quantum behavior. N indistinguishable particles in the same state in a potential well show similarly full quantum behaviour that doesn't have -only- to do with the statistics. The statistics may give you extra features, but the quantum features of just one particle are also adopted by the N particles.

 

[JK423]

I don't - I think its QM writ large - the particles have lost their individuality and is simply one large quantum object.

Thanks
Bill

Then who does?

 

[stevendaryl]

Wait a minute - I don't think that classical systems have the property when you exchange two particles it is exactly the same - that's the reason for the different counting and why it doesn't follow the classical Gaussian Boltzmann law which is based on the binomial distribution.

Thanks
Bill

Okay, I think that's what I intended to say. The difference is due to a difference in how you count states. If you want to call that "quantum mechanics", I guess you can, but it seems to me that it's separable from the rest of quantum mechanics.

 

[stevendaryl]

Just one particle bounded inside a potential well shows full quantum behavior. N indistinguishable particles in the same state in a potential well show similarly full quantum behaviour that doesn't have -only- to do with the statistics. The statistics may give you extra features, but the quantum features of just one particle are also adopted by the N particles.

I guess I need to get down to specifics. What specific behavior of Bose-Einstein condensates are we talking about?

 

[bhobba]

Then who does?

It arose in the thread I mentioned in my first post. In that thread it was thought ny some, not by me, all macro objects were classical. I mentioned liquid helium, and some other stuff, and thought it pretty much settled the issue - but some thought it didn't. I thought I would do a post to see what people generally thought with BEC's which is even more obvious to me its not classical. Looks like you are in my camp.

Thanks
Bill

 

[JK423]

I guess I need to get down to specifics. What specific behavior of Bose-Einstein condensates are we talking about?

Well i talk generally, but in your previous discussion did you discuss about a specific behaviour of a BEC that depends only on statistics? From the OP i understand that someone implied that BECs do not show quantum behaviour..? I don't know where that came from..
If you could elaborate a bit it would be very helpfull.

 

[bhobba]

From the OP i understand that someone implied that BECs do not show quantum behaviour..? I don't know where that came from..

It arose in a general sense in this thread:
https://www.physicsforums.com/showthread.php?t=690802&page=3

Perhaps you can contribute to it. I really don't know what else I can say - to me its totally obvious size has nothing to do with quantum behavior and BEC's are a prime example of that.

Thanks
Bill

 

[Salman2]

If we consider that the quantum state is primary at origin of universe (not classical), then the question becomes when (and why and how) do quantum entities begin to show classical behavior..correct ? In this view, the BEC states (and superfluid states) would be on the cutting edge of the transition when quantum entities begin to show classical behavior. The question is: why do classical events spontaneously appear from primary quantum entities, not the reverse..correct ?

So, can someone explain what new attributes are found in classical states that are missing in quantum states...the experimental confirmation of these 'new' attributes not found in quantum states should help us better understand where any entity of interest falls along the continuum from quantum <--> classical...correct ?

Edit: Perhaps we can start with He-3 and Li-6 isotopes for examples. Let us assume both are primarily quantum entities. It is known that both can show quantum superfluid behavior under certain conditions. So, what new attributes are added to He-3 and Li-6 as quantum entities that also let us call them classical entities ?

 

[bhobba]

So, can someone explain what new attributes are found in classical states that are missing in quantum states.

I think a classical object has certain properties you expect in classical mechanics. One is it has a definite position and momentum. BEC's as a whole have that - at least I think they do. But another property is you can break it down into constituent parts and those parts are distinguishable - BEC's are not like that - if they even have the property of parts is open to question - they behave as a single large quantum object. I am sure people more expert than me in condensed matter physics can elucidate even more weird totally counter intuitive behavior not explainable classically.

Thanks
Bill

 

[stevendaryl]

Well i talk generally, but in your previous discussion did you discuss about a specific behaviour of a BEC that depends only on statistics? From the OP i understand that someone implied that BECs do not show quantum behaviour..? I don't know where that came from..
If you could elaborate a bit it would be very helpfull.

I'm saying that the existence of Bose-Einstein condensates depends only on statistics. I didn't make any claims about behavior.

 

[stevendaryl]

I think a classical object has certain properties you expect in classical mechanics. One is it has a definite position and momentum. BEC's as a whole have that - at least I think they do. But another property is you can break it down into constituent parts and those parts are distinguishable - BEC's are not like that - if they even have the property of parts is open to question - they behave as a single large quantum object. I am sure people more expert than me in condensed matter physics can elucidate even more weird totally counter intuitive behavior not explainable classically.

Thanks
Bill

I did a quick Google search to try to find examples of weirdness associated with BECs, and I couldn't find a good page describing them. But an example I remember is in the case of superfluids (not all BECs are superfluids, and not all superfluids are BECs), the liquid flows without friction, can squeeze through tiny cracks, and can flow uphill on its way to a lower elevation. If these properties are explained by quantum tunneling, then I would agree that's a macroscopic quantum effect.

 

[Charles Wilson]

From Astronomy Picture of the Day Archive:

http://apod.nasa.gov/apod/ap100228.html

CW

 

[Salman2]

I think a classical object has certain properties you expect in classical mechanics...

OK. But I was thinking along the line of how these classical properties arise from a more primary quantum state. For example, in this paper, the transition from quantum to classical is suggested based on scale of measurement:

http://prl.aps.org/abstract/PRL/v99/i18/e180403

How would this hypothesis apply to proper classification of a BEC state as quantum or classical, I have no idea ?

 

[stevendaryl]

OK. But I was thinking along the line of how these classical properties arise from a more primary quantum state. For example, in this paper, the transition from quantum to classical is suggested based on scale of measurement:

http://prl.aps.org/abstract/PRL/v99/i18/e180403

How would this hypothesis apply to proper classification of a BEC state as quantum or classical, I have no idea ?

I'm trying to get an intuitive idea about what is different about a BEC. If you have a bottle filled with a regular, classical fluid, the fluid is made of particles, each of which is localized. Some particles are at the top of the bottle, some are in the middle, some are at the bottom. If instead you have a bottle filled with a superfluid made of BEC, then the particles making up the fluid are not localized (right? I'm not sure). Rather than having some particles at the top of the bottle and other particles at the bottom, EACH particle in the fluid is in a spatially distributed state. Each particle has a wave function that is spread throughout the bottle. Is that correct?

 

[Cthugha]

In terms of maths one often treats BECs in terms of the complex Ginzburg Landau equation or the Gross-Pitaevskii equation. Here you indeed treat the wavefunction of the ground state as a classical field (which is simultaneously the order parameter). You usually do not do this for the excitation spectrum. The reason why that works is simply the large occupation number. Putting it a bit oversimplified: As you can go from QED to classical electromagnetism for large photon numbers you can do the same here as non-commutativity of field operators loses importance.

That does not exclude that there are situations where this treatment might may not be adequate, but it captures a lot of things.

As far as i can understand, the Bose condensate is observed only indirectly(inferred) as interacting with the outside macro world raises the temperature and destroys coherence.

Well, for example people working on photon BECs in the lab have definitely already directly observed a BEC with their own eyes. I have seen one, too, although not on photons, but a different system.

 

[Maui]

Well, for example people working on photon BECs in the lab have definitely already directly observed a BEC with their own eyes. I have seen one, too, although not on photons, but a different system.

I thought photons do not have coupling/bonding and do not constitute a solid and hence cannot become a BEC? I could be wrong though.
Where can i read about this experiment and system that you have seen? Thanks!I just saw a video on youtube of superfluid helium running upwards as if gravity didn't exist. Is this due to quantum coherence or is there a classical explanation?

 

[Charles Wilson]

If we consider that the quantum state is primary at origin of universe (not classical), then the question becomes when (and why and how) do quantum entities begin to show classical behavior..correct ? ...So, can someone explain what new attributes are found in classical states that are missing in quantum states

In setting up a solution for the Schrödinger Equation,
"The general form for the such waves would be Psi = A sin (n π X / L)".
Wolfson and Pasachoff, Physics, ISBN 0-673-39836-6

"L" is the length of the "infinite square well paradigm" where a "Particle" moves in a "constrained path" in one dimension.
May I suggest that the BEC is giving us a look at what a Quantum "L" might be. In a "higher energy" environment, a quantum "L" is easily found and a "Particle" appears and that particle appears in a very small space, limited at the smallest volume by the Uncertainty Principle. A Quantum "L" shows a variable "largest length" that increases as energy decreases:

"...He3 cannot become superfluid as easily as its boson sibling [[He4]]. Instead at a transition temperature roughly 1000 times lower than that of He4, a weak attraction between He3 atoms begins to make itself evident. Atoms with equal and opposite momenta tend to form pairs in the two particles orbit each other at a distance".

Lounasmaa and Pickett, "He3 Superfluids, SciAm, June 1990.

See also (again): http://apod.nasa.gov/apod/ap100228.html

Finally, the higher Noble Gasses are suggestive. Helium is everyone's favorite Classical Quantum entity, but why then does "Neon" not exhibit the same amount of Macro properties as Helium?

I suggest that our Decoherence Friends should have something to say here. Neon and Argon and on up the scale have more interactions in the nucleus that (How I hate to use this word) "Collapse" expanding Quantum "L's" and force the reset, by virtue of greater energy exchange, of what might be a BEC into a tightly compacted collection of interacting particles.

CW

 

[Cthugha]

I thought photons do not have coupling/bonding and do not constitute a solid and hence cannot become a BEC? I could be wrong though.
Where can i read about this experiment and system that you have seen? Thanks!

It does not work for bare photons. You need to place them into a cavity filled with some dye, so they can thermalize via scattering. Cavity photons also have a small effective mass (before discussions start: having an effective mass does not imply that photons are indeed massive).

First realization of photon BEC: J. Klaers et al., "Bose–Einstein condensation of photons in an optical microcavity", Nature 468, 545–548 (25 November 2010). Arxiv version: http://arxiv.org/abs/1007.4088

Another system, where photons are a direct part of the condensate wave function:
J. Kasprzak et al., "Bose–Einstein condensation of exciton polaritons", Nature 443, 409-414 (28 September 2006). This manuscript is not on the Arxiv, but you should be able to find some free copies of it hosted by some of the institutes for which the authors work just by googling for the title.

 

[bhobba]

OK. But I was thinking along the line of how these classical properties arise from a more primary quantum state. For example, in this paper, the transition from quantum to classical is suggested based on scale of measurement:

http://prl.aps.org/abstract/PRL/v99/i18/e180403

That's how this discussion arose. Someone thought the classical realm was when Planck's constant went to zero for macro objects. I believe its when entanglement and decoherence is removed - that's what gives objects its classical behavior - not size. If you go to a low enough temperature - and thermal excitement is a form of entanglement - then quantum properties emerge eg atoms loose individuality and you get things like BEC's which are to my way of thinking not classical - but it may depend on your view of what classical is.

Thanks
Bill

 

[bhobba]

I just saw a video on youtube of superfluid helium running upwards as if gravity didn't exist. Is this due to quantum coherence or is there a classical explanation?

If I remember correctly its got something to do with van der walls attraction between it and the container and it no longer having constituent parts. If you pour water in a jar then pour it out you see drops stuck to the side held there by that slight attraction. But because super-fluid helium has no friction and it really is a single object it more than sticks to the side - the slight attraction causes it to run up the side of the container.

Something interesting occurred to me though. Is super-fluid helium a BEC? If so why did I read somewhere BEC's weren't observed until 1995?

Added later:

Found a nice paper on it - it seems it may be more complex and interesting than I thought:
http://arxiv.org/ftp/arxiv/papers/1103/1103.0517.pdf

Thanks
Bill

 

[DrClaude]

Is super-fluid helium a BEC?

Not exactly, see
http://www.eng.fsu.edu/~dommelen/quantum/style_a/nt_sfnebec.html
and references therein.