Talk:Hong–Ou–Mandel effect

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I think that in the image of the possibilities there should be a + sign before the fourth case not a minus — Preceding unsigned comment added by 2001:718:1401:58:0:0:2:DEBE (talk) 11:38, 30 August 2013 (UTC)

Nice article! I made some minor changes: --J S Lundeen (talk) 14:15, 10 July 2008 (UTC)
 * 1) The HOM does have a well known classical analog - now explained.
 * 2) The HOM dip has zero visibility for completely distinguishable photons
 * 3) The mathematical relationship with purity has been added
 * 4) "exit the same port" can be misconstrued to mean always exit port 1 (or port 2), rather than exit randomly but together.

Thanks Jeff! I corrected a small typo in the displayed equation, but I really like your additions. QuantumCyclops (talk) 15:17, 17 July 2008 (UTC)

Indeed, a nice article. I have two comments and one question:

Comment 1: The wording "creation and annihilation operators $$\hat{a}$$, $$\hat{a}^{\dagger}$$, and $$\hat{b}$$, $$\hat{b}^{\dagger}$$ " should be changed to "annihilation and creation operators $$\hat{a}$$, $$\hat{a}^{\dagger}$$, and $$\hat{b}$$, $$\hat{b}^{\dagger}$$ ", i. e. exchange the words "creation" and "annihilation" to make them correspond to the order of the operators.

Comment 2: I like your taking into account phase shifts at the beam splitter. However, I suppose that we could make a "symmetric" beam splitter which does introduce equal phase shifts for reflections off its top- and bottom-side. A simple pellicle beam splitter made out of a high-index material would be an example. What happens to the HOM-effect? Do we really need this phase shift? I suspect that we only need a relative phase shift between reflection and transmission. This phase shift is the same for a mode entering the beam splitter from either side.

Question 1: I'm aware of the standard treatment of the HOM-effect. I do not understand why the HOM-dip does not reverse and become a peak if a delay of lambda/2 between the two beams is introduced. My reasoning is based on the existence of a photon wave function (see publications by Scully et al.) and the assumption that the phase of the wave function is identical to the phase of the electromagnetic wave that describes the photon wave packet.

Photonics-UW (talk) 16:07, 2 January 2010 (UTC)

Comment 1: Has been taken care of by another user.

Comment 2: In the case of the symmetric beam splitter the HOM effect still appears. The reflected photons acquire a phase shift i, regardless which side of the beam splitter they reflect of. In the case of two reflected photons the phase shifts multiply, and we get the overall -1 phase, while the case of two transmitted photons gives an overall phase of +1. Again, there will be cancellation. The difference is that the relative phase between |2,0> and |0,2> is now +1 instead of -1. When I have more time I may expand the article to include this.

Question 1: A phase of lambda/2 on one mode will be a global phase for the input state |1,1>, and is therefore unobservable. The only way to increase the coincidence rate is to "unbalance" the beam splitter.

QuantumCyclops (talk) 16:03, 5 February 2010 (UTC)

I have removed a line in the second paragraph, which claims that the Hong-Ou-Mandel effect is not due to the overlap of the wavepackets (bunching), but rather due to the indistinguishability of the wave packets. The reference that supports this claim described an experiment in which an entangled input state was used, as opposed to a separable state in the HOM effect. I think that this is therefore altogether a different two-photon interference experiment. QuantumCyclops (talk) 14:12, 24 October 2010 (UTC)

Thought: To me saying that the 2 photons "extinguish each other" seems misleading because they do of course leave one of the ports together... Perhaps it was meant that the coincident count is extinguished...?

Regarding addition of HOM sCMOS paper
On 31. March an addition was made which adds a paper published 30. March (dated 1. April) (https://en.wikipedia.org/w/index.php?title=Hong%E2%80%93Ou%E2%80%93Mandel_effect&type=revision&diff=654408383&oldid=640940521). Judging from the IP, I suspect it was added by the authors or people close to the authors. Unfortunately I don't quite see how the article benefits from the addition -- we shouldn't list every new detection for the well established HOM effect. I'm interested in opinions. In wikipedia we need to find a balance between papers that are relevant as sources to the article, and papers that merely try to gain public exposure by appearing in the article. DerManu (talk) 09:16, 26 May 2015 (UTC)

Regarding first names
I did some googling for my PhD-Thesis on author's first names: Drahflow (talk) 11:14, 2 March 2016 (UTC)
 * Ching Ku Hong: http://wwwhome.postech.ac.kr/html/portlet/ext/person/popup.jsp?sLocale=en&pernr=00020189&orgeh=00032000&prefLocale=en
 * Zhe Yu Ou: http://physics.iupui.edu/people/zhe-yu-jeff-ou-0

Dangerously misleading conceptualization
The invoked concept in the quantum mechanical description is dangerously misleading. When a photon enters a beam splitter it is not "either reflected or transmitted". Alluding to this (in my view) incorrect concept is likely to produce misunderstandings when looking at Mach-Zehnder interference experiments and at delayed choice experiments. In the latter two, invoking this concept leads to a contradiction with what these experiments show.

However, when a photon enters a beam splitter it is transformed into a superposition of being in the upper and being in the lower arm of the splitter. It is only much later, in a position measurement, that this superposition is resolved again.

Similarly, the description used in "The photon coming in from above is reflected and the photon coming in from below is transmitted." is dangerously misleading, since the Hong Ou Mandel effect demonstrates that there is no such thing like "the photon coming in from above" and "the photon coming in from below". This language suggests a possibility of distinguishing these two photons, which is not consistent with the results of the experiment described in this article. — Preceding unsigned comment added by 217.95.163.3 (talk) 09:09, 21 August 2020 (UTC)

Why is the minus sign required by reversibility?
The minus sign for reflecting off the bottom plate is essential for the effect. Currently the article simply states it's required. Why is that? Ionsme2 (talk) 20:02, 24 October 2022 (UTC)
 * If you look at the transformation matrix below you can try to check that in order for it to be a unitary matrix, its needs the determinant to be 1, thus it needs the -1.--ReyHahn (talk) 20:28, 24 October 2022 (UTC)

Wrong interpretation
The paragraph quoted below is quite wrong, especially the first sentence. relative phases between input beams are not defined. The correct comparison to the classical case is to calculate the expectation value of the product of the two output intensities /(*). The value of this expression is 1 for two light pulses with different arrival times, different frequencies or otherwise orthogonal spatio-temporal modes. For identical spatio-temporal input modes, the value drops to 0.5, while in the quantum two-photon case, it drops to 0. That is the essence of the quantum result! If one uses coherent state inputs, coincidence counts due to higher photon number Fock state terms |n>1> of the input states will also lead to a result of 0.5.

"The result is non-classical: a classical light wave entering a classical beam splitter with the same transfer matrix would always exit in arm c due to destructive interference in arm d, whereas the quantum result is random. Changing the beam splitter phases can change the classical result to arm d or a mixture of both, but the quantum result is independent of these phases." 128.243.187.232 (talk) 09:45, 8 February 2023 (UTC)


 * Could you please elaborate how the value drops to 0.5%? So, how can there be a HOM dip visibility of 50% in the classical case? If you could just point me to some good sources / papers, that would be already more than helpful. Thank you in advance! ~ Sterafix (talk) 15:39, 23 November 2023 (UTC)