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I'm a postdoc in condensed matter theory, working mainly on cold atoms and quantum phase transitions.


Dec
14
answered Proof that quantum Fourier transform is unitary
Dec
2
answered What is the difference between quantum fluctuations and thermal fluctuations?
Nov
28
comment Peskin's book page 334 proof of $Z_1=Z_2$ to all orders in QED perturbation theory
You mean, a non-renormalized vertex diagram, don't you ? And you just give an example here.
Nov
26
comment Proving $[a_k^\dagger, a_q^\dagger]=0$
Another way to see it is to remember that $x\delta(x)=0$ (since when applied on any test-function, this will always give zero).
Nov
23
comment 1-particle non-interacting Green function
Energy conservation is related to time translation invariance of the interaction. If you have a specific question on that, I suggest that you open a new thread.
Nov
23
comment 1-particle non-interacting Green function
Yes, that's it. It's the fourier transform of a causal propagator, so there is more information than just the energy of the free particle.
Nov
21
answered 1-particle non-interacting Green function
Nov
20
comment Feynman rule for propagator with derivatives
I'm kind of confused by the fact that one field is singled out by the derivative. You should try to rewrite the lagrangian density in a more symmetric way.
Nov
20
comment Root of $i$, which one to take?
@KyleKanos: I think the OP means that you can interpret $\sqrt{i}=\pm \frac{1+i}{\sqrt{2}}$, and that there is a sign ambiguity.
Nov
20
comment Feynman rule for propagator with derivatives
What have you tried ? Best thing is to write down the vertex in Fourier space and everything should follow.
Nov
15
answered How can we differentiate between matter and antimatter?
Nov
15
revised What is the physical interpretation of a field operator
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Nov
15
comment What is the physical interpretation of a field operator
@TobiasHurth: that's just notations (think about a discretized version of space). But I just changed to integral, if that makes you feel better.
Nov
14
answered What is the physical interpretation of a field operator
Nov
12
comment masslessness of Goldstone boson, Effective action, and functional-integral measure
@TwoBs: Although my 0D case is in fact too naive (no SSB, I changed my answer) I know a 0+1D example that shows SSB. In that case it comes from the degeneracy ground-state degeneracy.
Nov
12
comment masslessness of Goldstone boson, Effective action, and functional-integral measure
@LYg: see my edit. You should start with the easy case of Ising.You can also have a look at the appendix A of arXiv:1106.5585, where the same analysis is done for a simple quantum system that exhibits SSB.
Nov
12
revised masslessness of Goldstone boson, Effective action, and functional-integral measure
added 76 characters in body
Nov
12
comment masslessness of Goldstone boson, Effective action, and functional-integral measure
@TwoBs: I think you really missed the fact that the OP's confusion is much more basic than continuous vs discrete SB, GB or gapped excitations, etc. It is about the fact that for SSB, the order parameter is not invariant (!), and how to get a meaningful answer. Furthermore, in 3D, the Ising model is still gapped in the ordered phase, and still, the partition function won't be analytic.
Nov
12
comment Does the angular momentum vector operator $\hat{\vec{J}}$ have no eigenstates?
Sure, I think its clearer now.
Nov
12
comment Does the angular momentum vector operator $\hat{\vec{J}}$ have no eigenstates?
But your justification for why it is not an observable is based on the non-commutativity of the components, which is completely different from what you just said...