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Questions about Delayed-Choice Quantum Eraser

  1. Mar 5, 2004 #1
    I was reading about Scully and Druhl's delayed-choice quantum eraser in Brian Greene's Fabric of the Cosmos today and I had a couple of questions that maybe you guys can help me with.

    As I understand the idea, in one type of this of experiment a photon is fired(?) to a beam splitter the photon then has a 50% probability of going to one two down-converters which then separates the photon into two different photons. The signal photon continues on to a reflective surface and then to a screen while the idler photon then goes to a detector. Since we know which way the original photon went at the beam splitter there is no interference pattern shown at the screen.

    In another version of the experiment instead of the idler photon going directly to the detector another beam splitter is added which gives the idler photon a 50% chance of going to a detector (d1) or to another beam splitter which gives the photon a 50% chance of going two different detectors (d2, d3). If the idler photon is detected at d1 (or d4 if went the other way at the first beam splitter) there is no interference pattern at the screen for the signal that corresponds to the idler photon detected at d1 (or d4). However, if the idler photon is detected at d2 or d3 there will be an interference pattern since it cannot be determined where the original photon went at the first beam splitter.

    Now for my questions.

    1. If there are no idler photon detectors will there only be an interference pattern?

    2. What if idler photon detectors are there but are not setup to detect idler photons (there are turned off) will there only be an interference pattern?

    3. What if the idler photon detectors are far enough away from the beam splitter that the signal photon will hit the screen before the idler photon reaches any detector, will there be an interference pattern? What if the detectors are tuned off after the signal photon has hit the screen but before the idler photon reaches the detector, will there be an interference pattern?

    4. What if more beam splitters are added so that no idler photon detector will show which way the original photon went at the first beam splitter, will there only be an interference pattern? If detectors are added that will determine the which way the original photon went at the beam splitter will there be an interference pattern? What if the new detectors are are left turned "off"? What if they are set turned "off" but will turn on after the signal photon hits the screen but before the idler photon will reaches the new detectors, will there be an interference pattern?

    I hope this makes sense and thank you.
    Last edited: Mar 6, 2004
  2. jcsd
  3. Mar 6, 2004 #2


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    I don't know the answers to these questions myself, but here is a discussion of the experiment by its authors, and perhaps that will help.
  4. Mar 23, 2005 #3
    It seems to me that this experiment could be used to communicate faster than the speed of light.

    Maybe this shows not only the non-local mechanisms through space at work but non-local mechanisms through time.

    Does this make any sense or am I talking gibberish? :confused:
    Last edited: Mar 23, 2005
  5. Mar 24, 2005 #4
  6. Mar 24, 2005 #5
    Let me rephrase. Modify the experiment so the experimenter decides whether or not to destroy the which-path information.

    Now, an interference pattern will be detected before the experimenter chooses whether or not to destroy the which-path information. This means someone observing the interference pattern (or lack of) will know whether or not the experimenter chose to destroy the which-path information before the experiment makes the choice. If you have a little code with the experimenter you could talk in binary and get an immediate response even if the experimenter is in another galaxy.

    Since Bell's Theorem and Aspect's experiment provides strong evidence for the non-local (a.k.a. spooky) actions through space. Since the outcome of this experiment is decided by the future does that mean there are also spooky actions through time.

    Still gibberish? Let me know what I can clarify, I want to get to the bottom of this. :shy:
  7. Mar 25, 2005 #6
    Very interesting experiment. From the link given.

    "When the incoming photon hits the crystal, it is destroyed and a pair of entangled photons is generated by the crystal at the spot where it hit."

    I guess my question is I would like to know more about this process which goes unexplained as if it were common sense that would occur.

    Edit: this is what I was looking for 'spontaneous parametric down conversion'
    Last edited: Mar 25, 2005
  8. Mar 26, 2005 #7
    If it Just SPDC your questioning - it's not that it is "Common Sense". Just a tested observation that shows a SPDC by a crystal "divides" a beam of light into two beams of light with the SAME amount of light in each of the Two beams as was in the starting ONE beam.
    However, since the HZ of the two new beams is 1/2 that of the starting beam the each have only 1/2 the starting energy (total E still same).
    If how this works is ?, you need ask for someone to talk to crystal structure etc.

    If your asking about "entangled photons" and if these are: Well that's a debate.
    Most think they are entangled, I do not.
    Many Bell tests have given "Proof" they are. (Lots of threads - enjoy)

    Using an "Eraser" experiment is the hardest to use, I'd avoid. Without a very detailed explianation including diagram of set up and every possible condition of a hard test to run - you cannot know what may be assumed by someone. Thus I've never seen an "Eraser" question in text alone that didn't seem like gibberish. At least understand the other tests first.
  9. Mar 26, 2005 #8
    I know all that. Heres a question though, In the context of a single photon being fired, does the split happen? The way I see it, It all comes down to interpretation or definition of what a photon consists of, is it a single particle is it capable of being split, or is it a packet of smaller quanta whereby theoretically it could split into smaller parts? If the split of a single photon happens we can assume that it consists of + quanta, and what you get is the sum of 2 parts, what do we want to call these and do they really have a link to each other in the form of entanglement whereby a change in polarization of 1 equals a change in the system? Experiments show that yes, a change in 1 part changes the system, so for all practical purposes I think the word entanglement is appropiate for at least what it tries to imply - a link between the 2 parts.
  10. Mar 26, 2005 #9
    I see at least 3 questions - only partly related to each other and a conclusion that doen't make sense if you already "know all that".
    You need to be a bit more clear or any answer you get will sound like gibberish.

    But as before:
    No there is only one partical going into the crystal.
    Two particals come out each with 1/2 the energy due to one half the HZ.
    ..Energy, spin, etc. is all conserved.
    And for me, NO entanglement is involved - Most others will disagree on this point.
    Pleanty of good posts detail why - just "search forum" for entanglement or EPR.
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