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location United Kingdom
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visits member for 2 years, 1 month
seen 1 hour ago

Currently doing a PhD in Controlled Quantum Dynamics at Imperial College and the University of Oxford.


22h
comment How to rebut denials of the existence of photons?
@WetSavannaAnimalakaRodVance I'm afraid I know nothing about SED. What I mean is the following. Many states of the radiation field can be described by a classical electric and magnetic field distributed according to a positive semi-definite quasi-probability distribution, e.g. the Wigner function. Perhaps such a description is equivalent to some formulation of stochastic electrodynamics. However, Fock states cannot be described in this way: their quasi-probability distributions will be negative or singular.
22h
answered How to rebut denials of the existence of photons?
Nov
22
comment Calculating a resonance fluorescence spectrum (Mollow Triplet)
@user129412 Okay, well then if the operators you are dealing with are not Hermitian it's probably the case that you have the Fourier transform of an anti-commutator.
Nov
22
answered Calculating a resonance fluorescence spectrum (Mollow Triplet)
Nov
21
comment What is the entropy of a pure state?
I like the idea behind your first sentence, but it does not really follow that entropy is not observer-dependent (it is observer-dependent!). Rather, you are defining a "standard observer" who has access to a given set of observables (or technically, a thermodynamic state space). Then, for all standard observers, the entropy is uniquely defined. However, you can still imagine observers with access to more information (e.g. Maxwell demons) for whom the observed entropy is different.
Nov
21
comment How can it be that the sun emits more than a black body?
@RobJeffries Great, thanks very much for the extra info.
Nov
21
comment Why is (von Neumann) entropy maximized for an ensemble in thermal equilibrium?
@WetSavannaAnimalakaRodVance Thanks :) Glad that someone still occasionally reads these old posts. I agree, the Jaynes paper is one of the best pieces of scientific prose I have ever read, and it was also rather a personal revelation when I first encountered it.
Nov
14
comment How is the scattering length in 2d defined?
Could you comment on the relationship between the scattering length and the scattering cross section in $d$ dimensions? Also, do you have a reference where the derivation of the scattering length from the self-energy is discussed in more detail?
Nov
13
comment How large can an atom get? What's the farthest an electron can be from its nucleus?
One of my favourite illustrations of just how comparatively large Rydberg atoms are is that their typical length scales would be more familiar to a biologist than an atomic physicist. For example, a Rydberg atom in an $s$ state with principal quantum number $n = 508$ can hold about 100 human red blood cells within the mean radius of the electronic orbital.
Oct
21
comment What is the Quantum Transition Time for Photon Emission?
@CuriousOne To be clear, I was agreeing with you; time dependence is not the explanation. I was just pointing out that the Jaynes-Cummings model is a good place to start along the lines you were suggesting. And indeed that the answer I linked already contains a lot of the necessary elements.
Oct
21
comment What is the Quantum Transition Time for Photon Emission?
@CuriousOne BTW, the purpose of my comment was simply to point out that the Jaynes-Cummings Hamiltonian is easy to understand but also gives extremely good agreement with experiments at optical frequencies, so no need for a contrived model Hamiltonian. I think this answer gives a nice exposition of the relevant details.
Oct
21
comment What is the Quantum Transition Time for Photon Emission?
@CuriousOne The standard description of this process in quantum optics goes via the Jaynes-Cummings model, which is time-independent. Talking about the time-dependent field only makes sense when the light field is in a semi-classical (e.g. coherent) state. In the full quantum treatment the role of the oscillating electric field is replaced (formally speaking) by the oscillatory time dependence $e^{-iEt/\hbar}$ of the photon states. Nevetheless, the Hamiltonian is time-independent; the point is that the interaction mixes the $n$-photon and atomic eigenstates so transitions occur between them.
Oct
17
awarded  Good Answer
Oct
17
comment Entanglement Hamiltonian for two 1/2 spin system
@Antonio_phy Setting $k_B T=1$ just means that you use units of energy equal to $k_B T$, it doesn't necessarily mean that the temperature cannot change. However, if you like you can think of $T$ being constant (but finite, importantly), and imagine varying the external magnetic field instead of the temperature. The results are the same, because the only thing that matters is the dimensionless ratio $\mu/k_BT$.
Oct
17
revised Entanglement Hamiltonian for two 1/2 spin system
added 22 characters in body
Oct
17
revised Entanglement Hamiltonian for two 1/2 spin system
added 400 characters in body
Oct
17
answered Entanglement Hamiltonian for two 1/2 spin system
Oct
16
comment Why does an external laser drive only couples certain levels?
@garyp I think the OP refers to an atomic level scheme similar to the first figure on this page.
Oct
16
comment Why does an external laser drive only couples certain levels?
@KobyYavilberg Often the two ground states will have non-zero spin projections in different directions. You can selectively couple to just one transition by using polarised light, which changes the angular momentum projection by a definite amount according to the direction of polarisation and the selection rules Emilio has described. So even if the ground sub-levels are completely degenerate, and the two lasers are at the same, resonant frequency, each laser can be made to couple to only one transition by choosing different polarisation states.
Oct
16
awarded  Yearling