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19
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3answers
601 views

Are the recently observed Dirac monopoles separable?

I just went through Observation of Dirac monopoles in a synthetic magnetic field. What exactly has been observed? More importantly, are these monopoles localized inside the apparatus (no stray ...
18
votes
2answers
722 views

Can bosons that are composed of several fermions occupy the same state?

It is generally assumed that there is no limit on how many bosons are allowed to occupy the same quantum mechanical state. However, almost every boson encountered in every-day physics is not a ...
12
votes
3answers
1k views

trying to understand Bose-Einstein Condensate (BEC)

I am a computer scientist interested in network theory. I have come across the Bose-Einstein Condensate (BEC) because of its connections to complex networks. What I know about condensation is the ...
10
votes
4answers
452 views

What happens to those electrons of BEC cold atoms?

BEC cold atoms occupy the same ground state. But what about the electrons or other fermions of the BEC atoms? Are they in the same state? Do electrons of one atom interact with those of another?
10
votes
1answer
156 views

How to exchange light and matter?

A while ago an experiment demonstrated that it is possible to stop a light pulse in a supercooled sodium cloud, store the data contained within it, and totally extinguish it, only to reincarnate the ...
9
votes
4answers
1k views

Bose-Einstein condensate in 1D

I've read that for a Bose-Einstein gas in 1D there's no condensation. Why this happenes? How can I prove that?
9
votes
1answer
154 views

Why is the BCS trial function valid across the BEC-BCS crossover?

In one of the two main theoretical approaches used in describing ultracold Fermi gases and the BEC-BCS crossover, the so-called BCS-Leggett approach, the starting point is the BCS trial wavefunction: ...
9
votes
1answer
709 views

Bose-Einstein Condensate with T>0 in Theory and Reality

I am interested to understand how positive entropy Bose Einstein condensation for cold atoms (say) behave. The way I think about it is as follows: We have an ideal pure state where every atom is in ...
8
votes
2answers
462 views

Could the chemical potential of a Bose gas be zero?

Could the chemical potential of a Bose gas be zero ? If it was the case, we will have an infinite number of particles in the ground state ! No ? But I've heard that for $T < T_c$, $\mu = 0$, so I ...
8
votes
1answer
720 views

Possibility of Bose-Einstein condensation in low dimensions

I remember having a problem (for practice preliminary exams at UC Berkeley) to prove that Bose-Einstein condensation(BEC) is not possible in two dimensions (as opposed to three dimensions): For ...
7
votes
4answers
9k views

Practical applications for a Bose-Einstein condensate

What are the main practical applications that a Bose-Einstein condensate can have?
7
votes
3answers
262 views

Why do Photons want to be together?

So I've heard that when a photon flies by a atom excited enough to release a photon there's a good chance it will. Because Photons want to be together and have the same direction etc? Is this true? ...
6
votes
1answer
421 views

Expansion of multi-particle state vector as a sum of n-entangled states

Physically, quantum entanglement is ranged from full long-range entanglement (Bose-Einstein condensate), described by a basis of states that look like this: $$ |\Psi\rangle = |\phi_{i_{0} i_{1} ... ...
6
votes
2answers
300 views

Change of basis in non-linear Schrodinger equation

At the mean-field level, the dynamics of a polariton condensate can be described by a type of nonlinear Schrodinger equation (Gross-Pitaevskii-type), for a classical (complex-number) wavefunction ...
6
votes
1answer
183 views

BEC for holography?

I am spending some time reading about Bose-Einstein condensation. I want to know if it is possible to use atom lasers to realize the kind of holography traditionally associated with nano-fabrication. ...
5
votes
2answers
547 views

What prevents bosons from occupying the same location?

The Pauli exclusion principle states that no two fermions can share identical quantum states. Bosons, one the other hand, face no such prohibition. This allows multiple bosons to essentially occupy ...
5
votes
1answer
119 views

Bose-Einstein condensate for general interacting systems

There is Bose-Einstein condensate (BEC) for non-interacting boson systems. Can we prove the existence of BEC for interacting systems?
5
votes
3answers
84 views

In a radioactive Bose-Einstein condensate, would all the atoms disintegrate simultaneously?

They're all supposed to do the same thing - so I suppose they would? Also, would the resulting half-life be the same as that of the individual atoms it is made up of?
5
votes
1answer
121 views

What do we mean when we say Bose condensation? And why can't fermions condense if they don't pair?

In common textbooks, we are told that bosons can condense in a single-particle state because of bose statistics and when the system undergoes a bose condensation, the bose field operator obtains a ...
5
votes
0answers
86 views

BEC in a rotating disc

Goodmorning everybody, I have to run a numerical simulation of a Bose-Einstein condensate on a rotating disc. Now, my problem is that I became suspicious about the equation I'm using, since the final ...
4
votes
1answer
703 views

Are all bose-einstein condensates superfluid?

I feel like the answer should be "no" since all superfluids are not strictly BEC since they can undergo a Kosterlitz–Thouless transition in 2D, for example. I believe the ideal gas isn't superfluid, ...
4
votes
1answer
550 views

BCS theory, Richardson model and Superconductivity

I'm studying Richardson Model in second quantization. There are many initial points that I don't understand: We supposed that an attractive force between 2 electrons exists, due to electron-phonon ...
4
votes
1answer
522 views

Gross-Pitaevskii equation in Bose-Einstein condensates

I was hoping someone might be able to give a approachable explanation of the Gross-Pitaevskii equation. All the sources I've been able to find seem to concentrate on the derivation, and I don't have ...
4
votes
1answer
155 views

First order coherence function in terms of momentum distribution function

Can someone show me how the first order coherence function $G^1(r,r')\equiv \left \langle \hat{\Psi}(r)\hat{\Psi}(r') \right \rangle $ for a system of bosons is related to the momentum distribution ...
4
votes
1answer
814 views

Looking for a complete review of the BEC-BCS crossover

I'm looking for comprehensive review of the BEC-BCS crossover, both from a theoretical point of view, and from a experimental one. Even something at textbook level, but exhaustive, would be OK, but I ...
4
votes
0answers
350 views

Toward the establishment of non-equilibrium (quasi-equilibrium) magnon BEC theory

In 2006, Demokritov et al have reported that they have achieved the observation of quasi-equilibrium magnon Bose-Einstein condensation (BEC) in YIG at finite (room) temperature by using the method ...
4
votes
1answer
297 views

Hamiltonians, density of state, BECs

When working with Bose-Einstein condensates trapped in potentials, how can one tell what the density of state of a system of identical bosons given the Hamiltonian, $H$? (I have been told that it is ...
3
votes
2answers
807 views

Can a system entirely of photons be a Bose-Einsten condensate?

Background: In Bose-Einstein stats the quantum concentration $N_q$ (particles per volume) is proportional to the total mass M of the system: $$ N_q = (M k T/2 \pi \hbar^2)^{3/2} $$ where k ...
3
votes
3answers
202 views

Bose-Einstein condensation in systems with a degenerate ground state

I understand that when a system enters the BEC phase a sizable fraction of the total number of particles enters the ground state, until at some point almost all of your particles are in the ground ...
3
votes
2answers
924 views

Why water is not superfluid?

My question is in the title. I do not really understand why water is not a superfluid. Maybe I make a mistake but the fact that water is not suprfluid comes from the fact that the elementary ...
3
votes
1answer
136 views

Hamiltonian with Dirac Delta function

I've to compute this expression $$ \hat{H} ~=~\frac{1}{4}g_2\int d^3R\int d^3r\ \bar{\Psi}(\vec{R}+\frac{\vec{r}}{2})\bar{\Psi}(\vec{R}-\frac{\vec{r}}{2}) $$$$ \times \left[ ...
3
votes
1answer
42 views

Superfluid $\Leftrightarrow$ Bose-Einstein condensate?

On the Wikipedia page on superfluidity one can find the sentence not all Bose-Einstein condensates can be regarded as superfluids, and not all superfluids are Bose–Einstein condensates. So I was ...
3
votes
1answer
116 views

Simple & intuitive explanation of superfluidity?

I know that superfluidity is caused by the fluid having zero viscosity. This only happens at very low temperature, so the fluid (e.g. Helium-4) is a Bose-Einstein condensate. I also know that in a ...
3
votes
0answers
90 views

positronium BEC stability

After reading this article regarding Positronium BEC formation (for lasing purposes), there is a mention in there regarding Ps "up" atoms not annihilating with "down" atoms, the article is pretty ...
3
votes
0answers
175 views

Matter-wave interference from free falling cold atoms

and another exam question, this is about current research: Interference of matter waves has been studied using ultra-cold atoms. The phase of a matter wave for free-falling cold-atoms at time $t$ ...
3
votes
0answers
652 views

How do I derive the critical temperature for bose condensation in two dimensions?

In class we derived the 3D case, but there's a step I don't understand: $$ N = g \cdot {V \over (2 \pi \hbar)^3} \cdot \int\limits_{0}^{\infty}{1 \over{e^{\left( E_p \over{K_B T}\right)}-1}} d^3 p = ...
2
votes
1answer
193 views

periodic boundary conditions for vortex in a square lattice

I am trying to follow this paper and track the dynamics of vortex motion on a discrete (square) lattice. The idea is to simulate the time evolution of the Gross-Pitaevskii (GP) equation, which reads ...
2
votes
1answer
171 views

How does an atom look like in momentum space?

I am reading an article in a popular science magazine about atoms in extremely low temperature (condensation?). It's interesting and I've got few questions regarding this phenomenon. 1) In the ...
2
votes
2answers
233 views

existing bounds on maximum density achieved by a Bose condensate

As we know, fermions are subject to exchange interactions that limit the densities they can achieve. However bosons (simple or composite) are not constrained by this, which implies physical phenomena ...
2
votes
1answer
137 views

Bose-Einstein condensate and nonlinear waves

Can Bose-Einstein condensate be written as non-linear wave equation (in terms of mean field approximation theory)? the equation is: source: http://xxx.tau.ac.il/abs/1308.2288 What I do ...
2
votes
1answer
132 views

Why People talk so much about Feshbach resonance while dealing with Bose-Einstein Condensate (BEC)?

Why People talk so much about Feshbach resonance while dealing with Bose-Einstein Condensate (BEC)? Why not tune the system near the resonance and measure the effect on BEC formation?
2
votes
2answers
389 views

Has Bose-Einstein theory been considered for dark matter?

Has Bose-Einstein theory been considered for dark matter? The theory would explain why no measurable radiation is emitted due to zero temperature--its lack of interaction with other matter and its ...
2
votes
1answer
217 views

True Ground State Population of Ideal Bose-Einstein Condensate at Critical Temperature

I'm supposed to demonstrate that although we make the assumption in an ideal BEC that the ground state population follows $N_0 = N\left[1-\left(\frac{T}{T_c}\right)^{3/2}\right]$ in reality the true ...
2
votes
1answer
90 views

Proof of the conservation of the energy functional for the Gross-Pitaevskii equation?

From the Gross-Pitaevskii equation \begin{equation}i\hbar\frac{\partial\psi}{\partial t}=\left(-\frac{\hbar^2}{2m}\nabla^2+V+g|\psi|^2\right)\psi\end{equation} using the variational relation ...
2
votes
1answer
104 views

Correct way to do a Thomas-Fermi approximation for cold gases

I have calculated the total Gross-Pitaevskii energy for a 2D Bose-Einstein condensate in an harmonical trap, using a variational gaussian wave function with a variational parameter b. Now I want to ...
2
votes
0answers
125 views

Bose-Einstein condensation in 3D

I have read in many books that BEC takes place in momentum space and in only 3-dimensions. What is meant by this statement?
2
votes
0answers
135 views

Deriving the “total” Bose Einstein density of states, including the condensate

Is is possible to derive the Bose-Einstein density of states containing the delta function representing the BE condensate?
1
vote
1answer
98 views

Some conceptual questions in BEC

In Bose-Einstein condensate (BEC), people often say there is a well defined macroscopic phase. What exactly the macroscopic phase is? (a phase factor $\mathrm{e} ^{i\phi}$ in a many-body wavefuction?) ...
1
vote
1answer
58 views

How do you actually cool the atoms to create the Bose-Einstein Condensate?

What is the actual way you cool atoms to a low enough temperature that you can observe their quantum behavior at a macroscopic level, like in the Bose-Einstein Condensate? ...
1
vote
1answer
41 views

Formalism for BEC with short-distance sub-structure “corrections”

It's simple to write down a Bose-Einstein Condensate wavefunction---in the position basis, $$\Psi(r_1,\ldots,r_N)=\psi(r_1)\cdots \psi(r_N) \qquad \qquad (1)$$ But in experiments the boson is never ...