Why do all experimental realizations of BEC use alkali metal gases?

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Discussion Overview

The discussion revolves around the reasons why Bose-Einstein Condensation (BEC) is primarily realized using alkali metal gases, such as sodium (Na) and cesium (Cs), and the nature of coherence in BEC. Participants explore the interactions between atoms, the significance of phase factors, and the implications of superposition in quantum mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why BEC experiments are exclusively conducted with alkali metals, suggesting that BEC can theoretically occur with various atoms, including helium-4 and composite bosons like helium-3.
  • Another participant explains that alkali metals are favored due to their magnetic trapping capabilities and the simplicity of their electronic structure, which avoids complications from other orbital types.
  • There is a discussion about whether atomic interactions can be neglected, with one participant asserting that interactions are crucial for condensation due to spin symmetry, while another emphasizes that at low temperatures, thermal agitation and repulsion are minimized.
  • Coherence in BEC is described as all atoms being in the same state with a single wavefunction, leading to a discussion about the nature of phase differences among atoms.
  • A participant proposes that if individual atomic wavefunctions are summed, they might cancel each other out due to random phase distributions, prompting further clarification on the significance of coherence versus superposition.
  • Another participant challenges the idea of summing individual wavefunctions, suggesting that BEC should be viewed as a many-body problem where coherence is more relevant than superposition.

Areas of Agreement / Disagreement

Participants express differing views on the role of atomic interactions in BEC and the implications of coherence and superposition. There is no consensus on the interpretation of these concepts, indicating ongoing debate and exploration of the topic.

Contextual Notes

The discussion highlights the complexity of many-body quantum systems and the challenges in applying principles of superposition to BEC, suggesting that assumptions about individual wavefunctions may not hold in this context.

frankchen
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I just read a review paper on Boson-Einstein condensation for the first time. I have 2 questions.
1.Can anyone tell me why all experimental realization of BEC are done in alkali metal gas, like Na, Cs? Can the interaction between atoms be totally neglected?
2.A facinating property of BEC gas is its coherence, i.e. all atoms are precisely in the same state(ground state) with the same phase factor. I don't know why. Suppose as the tempreture is lowered, the atoms go to the ground state one by one, why they will neccessarily be in the same phase?
 
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frankchen said:
I just read a review paper on Boson-Einstein condensation for the first time. I have 2 questions.
1.Can anyone tell me why all experimental realization of BEC are done in alkali metal gas, like Na, Cs? Can the interaction between atoms be totally neglected?

In principle, Bose-Einstein condensaton (BEC) can occur with almost any atom, be it due to the spin of the whole atom itself (He4), or the formation of composite boson of 2 or more atoms (such as in He3 or Y). The reason why the first realization of BEC for atoms was achieved for alkali metals is due to the nature of the magnetic trapping being used to cool the gas. Alkali metals have 1 unpaired s-orbital valence shell electron and can be trapped in a magnetic field. It also doesn't have the added complications of p and d-orbital valence shell, but it doesn't mean such atoms cannot achieve BEC.

Now I'm not sure what you mean by "interaction between atoms" can be totally neglected. After all, they all condense BECAUSE of the "interaction" of the spin symmetry. At such low temperatures, thermal agitation and coulombic repulsion have been significantly lowered so that the spin interactions dominates.

2.A facinating property of BEC gas is its coherence, i.e. all atoms are precisely in the same state(ground state) with the same phase factor. I don't know why. Suppose as the tempreture is lowered, the atoms go to the ground state one by one, why they will neccessarily be in the same phase?

They don't condensed to the "same phase". It is just that the whole glob can be described via a single wavefunction. The coherence can be either temporal or spatial. This means that one atom differs from another only by a phase, but can still be described by the same wavefunction. That is typically what is meant by coherence.

Zz.
 
can i say this way? The state of the whole system is the superposition of all single atom wavefunctions. since all atoms are in the same state( on the same ray), but their phase are randomly distributed. we sum up all the wavefunctions, we get zero?the atoms cancel each other because of their randomness in phase?
 
frankchen said:
can i say this way? The state of the whole system is the superposition of all single atom wavefunctions. since all atoms are in the same state( on the same ray), but their phase are randomly distributed. we sum up all the wavefunctions, we get zero?the atoms cancel each other because of their randomness in phase?

Eh?

Forget about the atom's individual wavefunction. That is no longer significant. Consider it as simply one entity - a boson (this will work only for non-composite boson case). You now have a particle of an integer spin. This is what is condensing. Everybody that condenses can be described crudely as "sin(kx - phi). The only difference is that particle as phi=12, the other particle phi=23, and the other particle phi=374, etc. But they all have the same wavefunction. They only differ in phase.

There is also no reason why you have to "sum" this.

Zz.
 
yeah, i know what i was saying is wrong(it doesn't happen in experiment). but since the principle of superposition is a fundamental rule in QM and we did it in other occasions, like the two slit experiment, why "sum" all the individual wavefunctions leads to trouble here?
 
frankchen said:
yeah, i know what i was saying is wrong(it doesn't happen in experiment). but since the principle of superposition is a fundamental rule in QM and we did it in other occasions, like the two slit experiment, why "sum" all the individual wavefunctions leads to trouble here?

Superpostion of what?

The principle of superpostion is a PART of QM, but it isn't QM.

You need to keep in mind that when we have BEC, we are dealing with a many-body problem and not just one, two, three, twelve, 36, etc particles. You cannot write a wavefunction to account for each individual consitutents of the glob - that will get you nowhere fast.

I think the problem we have here is the meaning of "coherence", not "superposition".

Zz.
 

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