48 votes
Accepted

Why is UV light visible when reflected off paper?

Paper sometimes contains a fluorescer to make it look whiter. I would guess that the light you are seeing is coming from fluorescence induced by the UV light. In fluorescence some of the original ...
John Rennie's user avatar
25 votes
Accepted

Understanding the relationship between Phase Space Distributions (Wigner vs Glauber-Sudarshan P vs Husimi Q)

Let me actually collect most of my comments in an answer attempting to be more coherent than they, or your labile question. In fact, you are piling up three different questions, logically distinct, ...
Cosmas Zachos's user avatar
23 votes
Accepted

Why does light travel in a straight line if the uncertainty principle is true?

If you think about light as a particle, a photon, you can understand that it will continue on its path through space at velocity $c$ unless it scatters off another particle, is absorbed by an atom, ...
klippo 's user avatar
  • 857
22 votes
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Is it possible to create a mirror that redshifts light?

Yes, this is possible using nonlinear optics. This kind of frequency shift can be done using acousto-optic modulators and electro-optic modulators, and it is normally done in a transmission geometry. ...
Emilio Pisanty's user avatar
20 votes
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Why do coherent states have Poisson number distribution?

Short version $\newcommand{\ket}[1]{\lvert #1 \rangle}\newcommand{\Ket}[1]{\left| #1 \right>} % $Because you can use beamsplitters to split a coherent state into a tensorial product of many ...
Frédéric Grosshans's user avatar
20 votes

Is there a field of physics that attempts to understand the properties of light from a single perspective?

Yes, that theory is called quantum electrodynamics, or QED. It predicts all of the “particle” behaviors and all of the “wave” behaviors of light from a single unified mathematical model. QED was the ...
Dale's user avatar
  • 97.7k
16 votes
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What is the orbital angular momentum (OAM) of individual photons?

This is an interesting mix of misconceptions: By definition, a photon is a one-particle state with definite momentum and helicity. In quantum mechanics, the OAM doesn't commute with momentum, and ...
Emilio Pisanty's user avatar
16 votes
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Photons with half integer angular momentum - what's happening?

Nothing is happening. At least, nothing except that a new generalized quantity suggestively called "angular momentum" was defined and subsequently measured. But nothing we know about the usual angular ...
ACuriousMind's user avatar
  • 124k
15 votes

Fourier transform paradox(?) of a wave packet

1. How can we "interact" with a photon with a purely mechanical device such as a shutter? Here is the culprit - the QED vertex: The mechanical shutter, whether made from metal or plastic, contains ...
Sergei Patiakin's user avatar
15 votes

Eigenstates of the creation operator

Well, here is a seat-of-the pants "lark" formal answer, going to "rigged" Fock spaces and places you (or anybody else) shouldn't really be at; except you may already have been ...
Cosmas Zachos's user avatar
15 votes
Accepted

Why doesn't there exist a wave function for a photon whereas it exists for an electron?

Saying that a photon doesn't have a wavefunction can be misleading. A more accurate way to say it is that a photon doesn't have a strict position observable. A photon can't be strictly localized in ...
Chiral Anomaly's user avatar
15 votes
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Countable basis of coherent states used to express coherent states

A more detailed investigation of the completeness of an arbitrary countable subset $\lvert \alpha_i\rangle$ of coherent states can be found in "On the completeness of a system of coherent states" by ...
ACuriousMind's user avatar
  • 124k
15 votes
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Can we define operators like $\dfrac{1}{a^\dagger a}$?

In general, $\frac{A}{B}$ is lazy physicist shorthand notation for $B^{-1}A$. You might rightly complain that there is an ordering ambiguity and the expression could also mean $A B^{-1}$. That's ...
Andrew's user avatar
  • 47.5k
13 votes

Book Recommendation: Quantum optics

I have two go-to books. Quantum Theory of Light, by Rodney Loudon. This is your readable one. It really covers a lot of stuff, including noise (including relative intensity noise), amplitude ...
13 votes

What is the length of a photon?

A photon is not a point particle in the classical sense of the word (and neither is an electron, or any other fundamental 'particle'). Rather, it is a convenient word for describing some aspects of ...
Andrew Steane's user avatar
13 votes
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Why is Dirac's Phase Operator Non-Hermitian?

Yeah, that's a strange thing to say - indeed $an^{-1}a^\dagger$ is defined everywhere and equal to the identity (note that $n^{-1}$ itself is not defined everywhere - but it is defined on the image of ...
ACuriousMind's user avatar
  • 124k
11 votes

Why is the quantum Venn diagram paradox considered a paradox?

I think the most intuitive answer for why this is a "paradox" is because it behaves differently than other types of filters such as color filters. In the case of non-polarized filters, the behaviour ...
Sandy Chapman's user avatar
10 votes

Phase added on reflection at a beam splitter?

It actually depends on what kind of beam spitter you have. I'll give a general treatment and shows that the conclusions of both Emilio Pisanty and Steven Sagona are basically correct, corresponding ...
G.C.'s user avatar
  • 103
10 votes

What is QED about "Cavity QED"?

The terminology "cavity QED" is used to signal that one is treating the electromagnetic field as itself quantised and there is a cavity with mirrors in the problem. The alternative would be to treat ...
Andrew Steane's user avatar
10 votes
Accepted

What is the position wavefunction of coherent states?

Derivation from the eigenvalue condition The most straightforward approach is to start from the position representation of the annihilation operator $a$. I'll use the convention $a=\frac{1}{\sqrt2}(x+...
glS's user avatar
  • 14.1k
9 votes
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SPDC and single photon production

There are indeed some subtle but important differences between an SPDC-based heralded source and a true single photon source. In order to understand these differences, consider what a true single-...
Girish Kulkarni's user avatar
9 votes
Accepted

In quantum optics if you measure the energy of a thermal state does it collapse to a Fock state (Photon number state)?

Perhaps the question is, how do you measure an arbitrary number of photons without destroying the system? It's certainly the case that somehow measuring the number of photons in the cavity would ...
Guillermo Angeris's user avatar
9 votes

Why doesn't the phase operator exist?

Well there is a Susskind-Glogower operator, which is a fairly good candidate for a phase operator. The problem is that the eigenstates for this operator does not have well-defined orthogonality/...
flippiefanus's user avatar
  • 14.4k
9 votes

Can we always express the EM-Field Hamiltonian as (possibly time dependent) pair of annihilation and creation operators?

In a Fock space, every operator can be expressed in terms of creation and annihilation operators, though usually there appear nonquadratic terms. This is the subject of second quantization, and can be ...
Arnold Neumaier's user avatar
9 votes

What is the Wigner function of $|n\rangle\langle m|$?

It is, explicitly, in terms of associated Laguerre polynomials, (actually the Wigner transform of $|n\rangle \langle m|$, given its (idiosyncratic/Moyal) flipped notation, $H\star f_{mn}=E_{n}\,f_{...
Cosmas Zachos's user avatar
9 votes
Accepted

How does homodyne detection work in quantum optics?

Homodyne detection is the extraction of phase information about a laser by comparing it to a reference beam, referred to as a local oscillator (LO). This is accomplished by interfering the beams on a ...
user1588914's user avatar
9 votes
Accepted

What does it mean to Fourier transform a ladder operator (in the input-output formalism)?

I will try to give some intuitive explanations, so long answer incoming... Physical picture, Hamiltonian and operators Let us first have a look at the bath model considered by Gardiner and Collett ...
Wolpertinger's user avatar
  • 11.5k
9 votes
Accepted

What is QED about "Cavity QED"?

What would I be missing in Cavity QED if I had never learnt about quantum electrodynamcis? Excellent question, especially the way it is posed! The bottom line is that what the OP calls "standard ...
Wolpertinger's user avatar
  • 11.5k
9 votes

Countable basis of coherent states used to express coherent states

Nice question! It would surprise me that any coherent state basis would not be overcomplete, and I read your expansion a a very elegant proof of it. [Erratum: such bases exist, see the answer by ...
Frédéric Grosshans's user avatar

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