Recent content by Q.B.

Thanks for your answer. Actually I heard about the maser example, but as you said, it involves a magnetic extraction of the excited state of the two level system. However, I had in mind a "local" population inversion, where one leaves the atoms (excited or not) where they are, but the dynamics...

Thanks for your answer. For instance, I thought that for laser diodes, the population inversion was realized thanks to some electric current flowing into a PN junction.

Hi everyone, I would like to understand the answer to the question in the title. I got that when we use optical pumping of a laser, the probability, given an incoming photon of adequate energy, for an atom to absorb it and for an other to produce some stimulated emission out of it are equal...

It's "created" by the decay. This should be taken carefully since virtual particles are a way of representing an interaction within a perturbation approach in QFT, and are not to be thought of as temporary propagating particles.

Hi, 1) you only need the W boson to turn one quark into the other. W bosons are electrically charged, and while turning a down quark into a up type quark, they leave with both the charge and the "downness"-"upness" difference (even if it's a very unclear way to put it - but these...

Hi, yes I think it is correct! Note that you didn't need to do a contrapositive proof for the first part, you could just build ##L## and ##\epsilon## from the ##M## you would have had (which looks very much like what you did). But it doesn't make your proof less right.

To estimate this, you need to know: -the dependence of the temperature ##T## of the CMB with time/scale factor ##a## of the universe, which is roughly ##T \propto \frac{1}{a}## -the evolution of the scale factor with time, from the Friedman equation which gives for a matter-dominated universe...

Hi, ##(1)## is verified by ##x_n=L## when ##n## is even and ##x_n=L+n## when ##n## is odd (indeed, for ##\epsilon>0## and ##n_0##, define ##n=2n_0## which satisfies the condition) but this sequence is clearly unbounded. ##(3)## leads to ##x_n=L, \forall n \geqslant n_0## (indeed, take given...

Hi, ##f^2## or ##abs(f)## both send ##a## and ##b## to ##1##. So ##1## has ##a## and ##b## as antecedents, and any other real has no antecedent. So if ##S## contains ##1## it has ##X=\{a,b\}## as reciprocal image, and the empty set if it doesn't contain it.

Hi, Your statement must be wrong since ##a=5,b=1## and ##m=1## clearly violate it.

Yes! (EDIT: sorry, it seems according to Orodruin's reply to this post that I haven't made my yes clear enough: let's suppose you had the normalisation right, it would indeed be the way you would calculate the charges: the diagonal values give you the ##T_3## charge of each component, to which...

Hi everyone, I'm trying to understand why laser diodes need a stronger input current to start lasing when their temperature increases. If I tried to add thermal transitions to the rate equations governing the evolutions of an atom quantum level populations (let's say ##n_{up}## and...

Hi, Q9: I think he meant ##\gamma \rightarrow \phi \phi##. The factor 2 might be due to a symmetry factor that has to be taken into account when outgoing particles are identical (a consequence of Pauli's own principle). Q10: I would say that indeed fields have their interest when particles...