Can two photons interfere with each other

  1. Can two photons interfere with each other, or is interference strictly limited to a photon (et cetera) "interfering with itself"?
  2. jcsd
  3. Sure, for example in the Michelson Interferometer two different photons interfere to make a dark or a light region in the pattern you see.
  4. Of course they can, dispersion-fenomena like the rainbow, or all known diffraction patterns are a proof of that.

  5. Hello εllipse,

    unlike the previous two posters, I have another opinion. Photons do not interfere with each other.

    Example: Double slit experiment
    Here, one can show that a photon so to say "interferes with itself". You can read the Marcella paper, where the interference pattern is explained quantum mechanically as single photon interference (it's a very good paper! :biggrin: ):

    Thomas Marcella, "Quantum interference with slits", European Journal of Physics 23 (2002), pp. 615-621
    Try to download it here:

    Diffraction can be explained by single photon interference, it's not a result of two photons interacting with each other (see example above).

    In the Michelson interferometer two photons also do not interact.
    Another example is the Mach Zehnder interferometer, where single photons show "interference". There's a nice paper on it:

    “Photon Quantum Mechanics and Beam Splitters,” C.H. Holbrow, E.J. Galvez, and M.E. Parks, American Journal of Physics 70, 260-265 (2002)
    You can download it here for free:
  6. I do not think you got my point. The observed diffractionpatterns all arise because of the combination of diffraction and interference. that was my point

  7. Actually we need to make something clear here. Interference is a wave like property while a photon is a the mere particle-interpretation of a wave. So saying that two photons interfere with each other is incorrect since you are combining two concepts that are different in nature. It is like trying to express or explain 24 in terms of the colour red.

    To first order, photons do not interact with each other but in higher order perturbationtheory photons DO mutually interact (indirectly though) via a process called hadronization.

  8. I think εllipse is asking whether photons interact in such a way that they annihilate each other like electromagnetic or sound waves.
    The post by mmwave for example shows that there's this misconception.

    But let's ask εllipse what exactly he means with his question.


    side note for Marlon:

    Classically, where you have electromagnetic waves, I agree with you Marlon that there might be a difference between the terms "diffraction" and "interference".
    For the double slit we have:
    a) Two infinite narrow slits => only interference pattern
    b) Two finite narrow slits => diffraction (in the classical sense) pattern also occurs, which can be explained by single slit with finite width

    However, there might be misunderstanding: When I mention interference I am talking about quantum interference where you add probability amplitudes if indistinguishable ways occur.
    Then one explains the double slit interference pattern not as a combination of diffraction and classical interference but only as a result of quantum interference (adding probability amplitudes like in the Marcella paper).
  9. Hmm, I don't really know; I'm confused on this, so that's why I'm asking :confused:.

    In my physics class, my teacher teacher mentioned a little about the wave properties of light, and I assumed it was similar to how two out of phase sound waves can have destructive interference, but in reading about quantum mechanics I've learned that the wave properties of photons, and everything else, are waves or probablity, and then you have the Feynman interpretation, where one particle takes all paths and "interferes with itself"; is that right? So I was asking if two particles will interfere with each other or if the interference is limitted to one particle interfering with itself. Maybe I don't know enough of what I'm talking about to ask the question right.

    Let me try asking another way, and hopefully it'll make sense. Can two light waves which are out of phase destructively interfere and produce no light? And is this a different question from asking, "Can two photons which are out of phase destructively interfere and produce no photons?"

    Why is quantum mechanics so much more difficult for me to get a handle on than relativity?
  10. Hello εllipse,

    I'd like to know in which grade you are, then we know how far we can go with our explanations.

    Yes, that's correct. If you don't use quantum mechanics, then you can compare the wave properties of light with that of sound waves. And there's nothing wrong with this model, it's a wonderful theory which allows you to calculate the pattern behind a double slit, single slit etc..
    It only becomes a problem when you turn down the laser intensity and you see single photons behind the double slit:

    Yes, correct. Your "waves of probability" are called "probability amplitudes" in quantum mechanics. And the Feynman interpretation says that the photon interferes with itself.

    In my understanding it's only the latter, namely interference is limited to one particle with itself.

    Two photons, which are out of phase do not interfere with each other destructively. You have two photons, each of them with a probability amplitude. And these probability amplitudes (or probability "waves) do not interfere with each other.
    It's like the photon says: "Heh, there's another photon, but I don't care. I only interfere with myself and only listen to MY propability amplitude."

    Hey no problem, that's normal. Everyone finds quantum mechanics difficult at the beginning. To be honest, I only understood in 5th semester university what was meant by "the photon interferes with itself".

    The quantum effects seem so weird, but that's what makes quantum theory so interesting.

    And a question to you: Do you know the double slit experiment, and what problem comes up when you see single photon dots on the screen?
    This experiment really helped me to understand what is meant by "the photon interferes with itself".
    Last edited: Aug 27, 2005
  11. One more thing:

    There's a book by Richard Feynman with the title "QED: The strange theory of light and matter". It's a book for everyone. And in the first chapter Feynman gives an example of the photon interfering with itself (glassplate, reflectivity changes with thickness). The photon's probability amplitude is illustrated as arrow in this book, just check it out.
  12. Yes.
    It's probably the more difficult for everyone. There's just more
    stuff to learn to "get a handle on" quantum theory. I don't have
    a handle on it yet, being only an intermittent student -- just some
    of the basics.

    Below are some links to papers and lists of references. one-photon and two-photon interference.pdf
  13. Ok, thanks.

    12th (US)

    Ok, so what you're saying is although this can model reality, it's not at all what really happens. And I know physicists shy away from saying what does "really" happen, but there are times when you can say there are certain things that obviously don't "really" happen (like the Bell inequality, which puts a restricts certain ways the world can't be (local realism, etc)). So two sources of light won't interfere with each other in the sense of destructive interference and wave patterns? The Michelson-Morely experiment stuff I've read has talked about it as if two photons interfere with each other, but I am assuming that in the context of the quantum mechanics, what's really happening is an individual photon is travelling both paths and interfering with itself?

    Yes, I think so. I know about the double slit experiment and have read that it has been performed by sending single particles through at a time, so the conclusion is that there is some way the particles take both paths, because when they end up on the other side it's just a dot, although statistically it makes an interference pattern as more and more dots build up.
  14. Do you mean that two probability waves from the same photon which go through different slits in the double slit experiment will ineterfere, or are you saying that two "light waves" of some other meaning from different sources can interfere?

    I'm sorry if the answer to this was addressed in the links you provided. It is late here, and I need to get some sleep. I will read through the links in the morning, thanks.
  15. Before everything I would like to apologize if I write something false because I don't have a university level in physics. But your discussion seems to be a central pattern relatively to my own approach. As I remark, a part of the discussion is turning around a definition of concept: diffraction, interference, ... By side, everything in the experiment seems to indicate a possible self interaction for the photon. This fact is discussed here with the usual binoculars. May I introduce some new remarks: It is well-known that every light can be considerred as the superposition of two ellipsoidal polarized waves (see Landau and Lifschitz). What I mean with this: every light (photon) has from the very beginning a dual nature; a photon is a dual thing. Can not this intrinsic duality be the deep reason explaining why a photon can act (interfere) with itself?
  16. fibre optics, anyone?
  17. In my opinion what "really" happens is that single photons interfere with themselves.
    But I want to stress that I would never tell somebody that the wavemodel is "wrong", since it makes correct predictions.

    What you probably read two electromagnetic waves interfering, NOT two photons interfering with each other. You can use the wave model too.
    But then like in the double slit, when you have a weak laser such that you see single photons, you must use quantum mechanics and you have individual photons that interfere with themselves.
    For example read the abstract here:
    The physicist Paul Kwiat uses a Michelson interferometer to make "quantum mechanical bomb tests"
    See also here, figure 6:
    where a Michelson interferometer is shown.

    I am a little careful here. I did a little search on the internet and read about an experiment where you have two independent(!) laser beams that show "some sort of" interference effect. I can't tell you what kind of interference exactly was observed in the experiment (I think beat signal), since I haven't read the papers yet [1],[2],[3] (see at the bottom of this post).
    But from what I read from another discussion board and a paper [3], it's not to be understood as an interference of two photons:
    I typed in google interfere "Two lasers" and found the following websites:

    Some quotes from them:
    I've given you links above, but always be careful about what you read from other websites. There's much crap on the internet. I only would trust the peer reviewed papers (see [1],[2],[3] at the bottom).

    Besides, it's nice that you already understand the problem with the double slit experiment.



    [1] "Interference of Independent Photon Beams", R.L. Pfleegor and L. Mandel, Physical Review Letters, Volume 159, Number 5, 1967

    [2] "Quantum Theory of Interference of Light from Two Lasers", Jordan and Ghielmetti, Physical Review Letters, Volume 12, Number 22, June 1964

    [3] "Quantum effects in one-photon and two-photon interference", L. Mandel,
    Review of Modern Physics, Volume 71, Number 2, 1999 one-photon and two-photon interference.pdf
    Last edited: Aug 28, 2005
  18. I took your question about "light waves" as referring to classical
    light waves. You can do this experiment at home and see the
    regions of constructive and destructive interference.

    Get recent textbooks on the classical and quantum theories of light
    if you can. I have to do this myself. :-) There are lots of
    online sources for good condition used books that will probably
    fit your budget.

    I don't know what to recommend at this time since I'm still
    looking -- maybe some of the science advisers here can let us
    know what they think are some good ones.
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