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B Are these good analogues for entanglement

  1. Feb 10, 2016 #1
    In his book on quantum physics, "The theoretical minimum" Leonard Susskind says that if Alice and Bob get their different coins from the same Charlie who mixes them up behind his back so he won't know which coin ends up in whose hands, the coins become entangled.

    If Alice gets her coin from Charlie-A who mixes two different coins and then discards one and Bob gets his coin from Charlie-B in the same way, the coins will not get entangled because they are mixed up by different sources.

    So if I send two photons on the same beam splitter which mixes up the photon identity and then get them absorbed by two resonant crystals, the crystals get entangled, but if I use two beam splitters, the crystals will not get entangled.

    Did I understand correctly?
  2. jcsd
  3. Feb 10, 2016 #2


    If we detect light with two photodetectors, then we’ll either get one click on detector 3, or one click in detector 4. Classically, in contrast, we would get half the intensity in each. An experiment with an appropriate nonclassical light source (not a laser!) and a 50/50 beamsplitter can thus “prove” the existence of photons: no two clicks are simultaneously observed (see Kimble, Dagenais, and Mandel, Phys Rev Lett 39, 691 (1977).)


    Nonclassical light is light with nonclassical quantum noise properties, which can be understood only on the basis of quantum optics.

    squeezed light, fock states

    Do they require nonclassical light for entanglement and coincidence counting?
  4. Feb 10, 2016 #3


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    No. You have been told repeatedly that you must consider the entire context of an experimental setup. You cannot pick and choose elements and expect to get the desired result.

    First: Polarizing Beam Splitters (PBSs) do not mix up anything. If anything, they usually do the opposite: sorting a mixed beam into components.

    Second: To get entanglement, you need a few things. One of the requirements is fixed photon number. You cannot shine a beam of light (which lacks a fixed photon number) into a PBS and expect to see pairs of entangled photons coming out. And just saying you have 2 photons is meaningless, you may as well say you are starting with 2 unicorns. Getting 2 suitable photons together requires its own additional mechanism.

    Third: even when you have 2 photons going into a PBS, the PBS outputs do NOT magically entangle those photons. There are other factors that are important too, and those critically determine what happens. The PBS is just one component.

    Fourth: If you think that your example above is viable, perhaps you can give a reference. I think you will see that actual experiments creating entangled crystals do not look anything like what you describe.

    I will re-recommend that you stop and take time to read the references you have been previously supplied (by myself and others as well as yourself) rather than starting new threads with similar themes. There are many ways to create entanglement, but they are difficult to execute and attention to detail (and theory) are absolutely required.
  5. Feb 10, 2016 #4
    First, PBS and BS are not the same thing. PBS are polarizers and BS are half-silvered mirrors.Half- silvered mirrors do not distinguish between color,polarization and they reflect and transmit any photon with 50% probability. And you said just yesterday BS create path entanglement. I did read all about the role each element plays on Wikipedia. Only 50-50 beam splitters entangle, filters and polarizers do not.

    I have referenced Leonard Susskind's book which talks about a thought experiment of mixing up coins of different type and gives a random one to each of the two people so they wont know which coin they posess. He says the correlation obtained between the coins is not factorizable.
  6. Feb 10, 2016 #5


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    Is there a point here? I repeat every one of my comments with half silvered mirrors. You don't just send a couple of photons to a half silvered mirror and get entanglement out. Not sure where you get this from.

    So: Got references?
  7. Feb 10, 2016 #6

    Both speak of the half-silvered mirror or the symmetric beam splitter as an alternative to SPDC.

    Beam splitters erase path information, the indistinguishability of paths between two particles is the source of entanglement from the beam splitter, rather than the energy and momentum conservation which create entanglement in particle decay.

    Last edited by a moderator: May 7, 2017
  8. Feb 10, 2016 #7


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    Last edited by a moderator: May 7, 2017
  9. Feb 10, 2016 #8
    Might I say the experiment is being described? Zeilinger says two pairs of entangled photons from nonlinear crystals are being entangled with each other with the use of beam splitter and detectors.

    As of entangling crystals,


    The signal photon now traverses the switch, a polarizing beam splitter (PBS) and a Faraday rotator (FR), before a 50/50 beamsplitter (BS) creates single-photon entanglement between spatial modes A and B. This entanglement is, upon absorption, mapped onto the crystals MA and MB.

    By means of absorbtion of entangled photons, I see from above setup matter particles become entangled
    Last edited: Feb 10, 2016
  10. Feb 10, 2016 #9
    Your logic should connect what Zeilinger said about photons forgetting where they came from when they meet a half-silvered mirror with that Nature figure, where it says lack of information about the path of photons after they meet on a beam splitter creates an entangled state.

    As you see, erasing path information creates the entanglement. And this erasing is done by beam splitter.
    Also, it says on Wikipedia that fiber optic couplers entangle photons by confining and mixing them


    "Entanglement is usually created by direct interactions between subatomic particles. These interactions can take numerous forms. One of the most commonly used methods isspontaneous parametric down-conversion to generate a pair of photons entangled in polarisation.[68] Other methods include the use of a fiber coupler to confine and mix photons, the use of quantum dots to trap electrons until decay occurs, the use of the Hong-Ou-Mandel effect, etc., In the earliest tests of Bell's theorem, the entangled particles were generated using atomic cascades."
  11. Feb 10, 2016 #10


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    No, you are mixing and matching snippets of text to support a position that is incorrect. You said "So if I send two photons on the same beam splitter which mixes up the photon identity and then get them absorbed by two resonant crystals, the crystals get entangled, but if I use two beam splitters, the crystals will not get entangled."

    Sending 2 photons to a single beam splitter do not, on their own, result in 2 crystals being entangled in any meaningful basis. If you have some reference for this, please provide it.

    And despite what you think is a reference to something similar (a single photon entangling 2 crystals), you will quickly see that there is no experimental realization of that as you envision it. It is something else entirely, you can get an idea from this work:


    Again, I caution you from using terminology out of context.
  12. Feb 11, 2016 #11

    "To entangle qubits in separate pieces of diamond, the team uses lasers to entangle each qubit with a photon at temperatures of 10 kelvin. The photons meet midway through a fibre-optic cable, where they are themselves entangled."

    What about an entire computer made of ions, beam splitters and detectors?

    "The individual particles of light (photons) emitted by each atom are entangled with the quantum state of the atom. Light is collected with large microscope lenses and focused into fiber optic cables. The other end of the fiber optic cables are pointed at a thin piece of glass- a beam-splitter. When the photons hit the beam splitter, the wave-like part of the two photons interfere. After passing through or bouncing off of the beam splitter, the photons pass through thin film polarizers. These polarizers reflect or transmit the photons depending on the direction the photons' electric fields and strike single photon counters. Simultaneous detection of single photons on certain detector pairs heralds entanglement between the remotely trapped atoms."
    Last edited: Feb 11, 2016
  13. Feb 11, 2016 #12


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    The original question has been answered, so this thread is closed.
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