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23 votes
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Why are Alkali atoms used in many Cold Atom experiments?

Alkali atoms have several benefits! The one outer electron makes them "hydrogen-like". Therefore, it is "easy" to calculate the energy levels which makes predictions and ...
kai90's user avatar
  • 618
22 votes
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What is the difference between superfluidity and Bose condensation?

Short answer: the terminology is not always used consistently. A superfluid and a BEC are certainly logically different. Superfluidity refers to the ability of some liquid to flow without any ...
Rococo's user avatar
  • 7,720
17 votes

Gas or liquid? a Bose-Einstein condensate

Neither gas, nor liquid, nor something in between. In a classical phase scheme, there are 4 states of matter: Solid Liquid Gas Plasma Bose-Einstein condensate is a non-classical state of matter (...
Agnius Vasiliauskas's user avatar
15 votes
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How is a Bose-Einstein condensate produced from sodium atoms that do not have an integer spin?

The protons, neutrons, and electrons that make up any atom are all Fermions with spin $1/2$. You can't make a Bose-Einstein condensate out of just electrons, for instance. But a composite particle ...
Paul T.'s user avatar
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13 votes
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Why is Rubidium-87 used for Bose-Einstein condensates?

For a BEC, you want atoms to be in the same quantum state, not necessarily at the same position. For a BEC, the temperature is low enough so that the de Broglie wavelength $\lambda_{\mathrm{dB}} \...
SuperCiocia's user avatar
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12 votes
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How can helium be liquid at absolute zero, given the Third Law of Thermodynamics?

Interesting question. The "third law" of thermodynamics actually states that The entropy of a perfect crystal at absolute zero is exactly zero. Since many systems in nature crystallize at some ...
valerio's user avatar
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12 votes

Why zero chemical potential does not allow the Bose-Einstein Condensation of Phonons?

There is an important distinction between Bose-Einstein condensation of atoms and quasiparticles, which I will describe below. Bose-Einstein condensation of an ideal (atomic) Bose gas For bosons, ...
Yoseob Yoon's user avatar
11 votes
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Aether existance in alternate universe made of Bose-Einstein condensate

I believe this question is a reference to the superfluid vacuum theory. First and foremost, I believe the Gross-Pitaevskii equation in the problem statement is incorrect. It seems to have mixed up ...
Elliot Yu's user avatar
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11 votes
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How is it possible a quadratic Hermitian Hamiltonian $H = b^\dagger b^\dagger + b b$, with $b$ boson, cannot be diagonalized?

Essentially, you are trying to prove that your initial Hamiltonian can be rewritten as $$H = r a^\dagger a + sI\tag{1}$$ where $r$ and $s$ are reals and $a^\dagger$ and $a$ satisfy bosonic ccrs and ...
Valter Moretti's user avatar
11 votes
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Why can't a gas of photons reach a Bose-Einstein condensate?

It can, it's just hard as you have to engineer it. See, for instance, here. A Bose-Einstein Condensate is more than just a state described by a macroscopic wavefunction. It's the result of a phase ...
SuperCiocia's user avatar
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11 votes
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Why do we cool atoms with laser light opposed to normal light?

Doppler shift is not a problem, but the essential element in laser cooling mechanism: the laser frequency is chosen in such a way that it is just below the Doppler shifter natural frequency of the ...
Roger V.'s user avatar
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10 votes
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Why is the density of a BEC so low?

The first thing to realise is that, although we often talk about BEC as a low-temperature equilibrium state, in dilute alkali gases a BEC is a highly excited, non-equilibrium, metastable state of the ...
Mark Mitchison's user avatar
9 votes

Interactions and scattering length in Feshbach resonances

I was confused about this point too when I first learned about Feshbach resonance. Eventually I realized that my confusion had more to do with scattering theory than Feshbach resonance per se, so I'...
Tim Goodman's user avatar
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9 votes
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Negative mass, or just the 'appearance' of negative mass? What's the acid test?

You're right, there's some confusion due to the way this paper has been publicized. Negative effective mass The 'negative mass' referred to in the paper is effective mass. The idea is that while ...
knzhou's user avatar
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9 votes
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What is "quantum gas"?

To put it simply, a quantum gas (or quantum fluid) is a system made of indistinguishable particles, as opposed to a classical gas where you assume that you can differentiate one component from other (...
Mat's user avatar
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9 votes

Why are Alkali atoms used in many Cold Atom experiments?

This is probably related to the presence of a Feshbach resonance, that allows to tune the scattering length via an externally applied magnetic field. A key ingredient to have a Feshbach resonance is ...
Matteo's user avatar
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8 votes
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Does Bose-Einstein Condensate other than liquid helium exist?

Yes. Bose-Einstein condensation was experimentally achieved in systems of: Rubidium atoms (first experimental realization, 2001 Nobel prize) Potassium atoms Cesium atoms Lithium atoms Sodium atoms ...
valerio's user avatar
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8 votes

Quantum fields and condensates

If $\langle 0|\phi (x) |0\rangle = c $, then $\phi $ is related to the quantum field $\Phi $, whose $|0\rangle $ is the vacuum state, by means of $$ \phi (x) = \Phi (x) + cI\:.$$ In other words, $$\...
Valter Moretti's user avatar
8 votes

Is every spontaneous symmetry breaking connected with a second order phase transition?

No, first-order symmetry-breaking transitions are common as well. For example, the 2D $q$-state Potts model has a first-order symmetry-breaking transition for $q \geq 5$.
tparker's user avatar
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8 votes
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Is every spontaneous symmetry breaking connected with a second order phase transition?

The answer above is great example. I would give another simple situation. If the system have $Z_2$ symmetry, and the free energy could be written in the form: $$f(m)= a(T-T_c)m^2+ b m^4$$ then as we ...
xjtan's user avatar
  • 618
8 votes
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Why do composite bosons form a BEC?

A bound state of two fermions is, among other things, a state in which the two fermions are highly entangled with each other, in the sense that the bound-state creation operator can't be factorized ...
Chiral Anomaly's user avatar
8 votes
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What is difference between Bose-Einstein condensation in 3D and 2D?

In principle you can get a condensate in an infinite 3D system. In practice however it is problematic, as the requirement is that you need a certain finite density and therefore an infinite number of ...
John's user avatar
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8 votes
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Tricky Integral: Evaluating Renormalized Ultraviolet "Divergent" Integral

Let's factor out $\frac{\hbar^2}{2M}$ from the integrand and take: $$\lambda\equiv\frac{g n\big(1+ \epsilon (3 \cos^2\theta-1)\big)}{\hbar^2/2M}$$ Then the integral is rewritten as $$\Delta E=\frac{1}{...
Navid's user avatar
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8 votes

Gas or liquid? a Bose-Einstein condensate

It is possible to distinguish between a gas and a liquid only below the critical point, where the coexistence between two phases of different densities is present. Indeed, above the critical point, ...
GiorgioP-DoomsdayClockIsAt-90's user avatar
7 votes
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Symmetry breaking and Superfluid - Mott Insulator transition

$\mathrm{U}(1)$ symmetry is not broken, it is spontaneously broken, meaning that although the Hamiltonian/Lagrangian might enjoy the symmetry, the ground state does not. For example, the ferromagnetic-...
Aaron's user avatar
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7 votes
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Soliton solutions of the Gross-Pitaevskii equation

If you relabel $x$ as "$t$" can think of it as mechanics problem $$ \frac{d^2 \psi}{dt^2}= g \psi^3 -\mu \psi. $$ As $$ g \psi^3 -\mu \psi = \frac{d}{d\psi}\left(\frac 14 g \psi^4 -\frac 12 ...
mike stone's user avatar
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6 votes

Superfluid $\Leftrightarrow$ Bose-Einstein condensate?

In my opinion the wikipedia quote in the question is inherently misleading. The two concepts (superfluidity and condensation) are certainly related, but also very much different, as they describe ...
jarm's user avatar
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6 votes
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Temperature of Bose-Einstein-Condensate in space

The temperature of a BEC formed from a dilute atomic gas (e.g. Rb87) isn't determined by the ambient radiation field, as the vast majority of the photons don't interact with the atoms. BECs are also ...
Gremlin's user avatar
  • 2,323
6 votes

Are gravastars observationally distinguishable from black holes?

Fact: quantization of gravity is still at a research stage, only effective quantized field theories are used. Fact:gravastars ( had to look it up) belong to one proposal for quantizing gravity, which ...
anna v's user avatar
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6 votes

Why zero chemical potential does not allow the Bose-Einstein Condensation of Phonons?

I think you pretty much have all the right ideas in your question. In a conventional BEC, particle number is conserved. Therefore, as T goes down towards 0, $\mu$ goes up towards the lowest state to ...
Steve Byrnes's user avatar
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