Quantum Entanglement Communication

An attempt to send information using quantum entanglement based on an experiment done by Birgit Dopfer in 1998

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Dopfer's experiment sent one beam from an entangled pair to a double-slit to get an interference pattern, while the other beam went to a lens with a screen behind it. Depending on where the screen was, the entangled parters of the photons that went through the slits could be focused to a point or not, either preserving the momentum information or destroying it. If the information was preserved, the interference pattern was destroyed, otherwise interference was maintained.

One critical feature of her experiment was a coincidence detector - an AND gate - to filter out only pairs of photons detected simultaneously. If there is a way to get rid of the coincidence detector, this scheme could be used for communication - faster than light and retrocausal. It's very like not possible, but I would at least like to explore and understand the reasons why.

Dr. John Cramer explored this over about a decade and came to the conclusion that it was impossible. He said there is an "anti-signal" which fills in the otherwise dark spaces of the interference pattern such that one can't tell whether the pattern is there. The coincidence detector filtered out those photons so it did get a signal. It seems, therefore, like these anti-photons are not from entangled pairs, yet they respond to the setting of the measurement done on the entangled beam. Also, how can a photon go through two slits and get an inverse of the normal interference pattern? The phase at each slit would have to be different, but that makes absolutely no sense at all to my admittedly somewhat untrained mind. The conclusion I keep coming to is that I just need to build the damn thing.

There are other configurations that show interference patterns that are affected by the "which way" information being preserved or not. Mach-Zehnder interferometers are good and light isn't lost in the space between slits. However, they're very sensitive to coherence. Double-slits should not be. Matthew and I have talked at length about different configurations to avoid the double-slit and not lose most of the light.

We need to be able to very carefully align optics, so I am working now on designing mounts that can be adjusted in two directions plus rotation. We have the most of the equipment, such as a Beta-Barium-Borate (BBO) crystal to downconvert light into entangled pairs, plus detectors to register the (very faint) light. I built counting circuits and interfaced those to a PIC18F2550 and through there to Linux in order to read the counts. Once I have the optics mounts working, we will also need to mount dichroic mirrors and IR filters to cut out all the left over visible light from the pump laser (~1 in a billion photons are downconverted, and shining what amounts to the full laser beam into the detectors is a bad idea).

  • 1 × BBO Crystal Non-linear optic to downconvert 405nm to pairs of 810nm photons
  • 2 × Single photon detectors Avalanch photo doides to detect single photons

  • Some Theory

    mindwalker5912/07/2015 at 22:58 0 comments

    In 1805 or thereabouts, Thomas Yound performed the famous double slit experiment:

    The purpose of the first slit is to make the light "spatially coherent," meaning it comes from a point-source, of line-source in this case. At the slit, the light diffracts so that is spreads out and covers both of the next two slits. The waves from each of the two slits on the rightr interfere with each other, creating the light and dark bands seen on a screen to the far right.

    The waves, in this case, are not electromagnetic waves but rather are quantum waves of the square-root of the probability of finding a photon anywhere on the wave. The double-slit experiment can be done with electrons, protons, atoms, or buckyballs (which have a very small wavelength).

    What this all means is that a photon (or electron, proton, buckyball) is initially in a state of superposition with respect to its position. when "measured," the photon stops being in many places and instead is in one place where it gets measured.,

    Where is gets interesting is when two photons (or other quanta) are created in such a way that there is a relationship between them. When a violet photon is absorbed by a beta-barium-borate (BBO) crystal and two infrared photons are emitted, their momentum must add up to the momentum of the original violet photon. Also, their polarizations can be related - either the same or orthogonal. These properties are in a state of superposition until they interact with stuff, whereby one photon takes on a value and the other photon takes on the corrosponding value.

    If the two photons didn't take on correlated values, it would be possible to violate conservation of momentum. When measured, each photon takes on a value instantaneously regadless of separation distance and light speed, and since the universe is relativistic, the idea of "instantaneous" can get very interesting.

    The obviousl question to ask is whether the photons really do have values for their properties and that we just can't predict them. The answer to that question is definitely "NO!" John Bell came up with a theorem that showed the correlation rates could only reach a certain percentage if they did actually have values, and quantum mechanics predicted a higher rate. See Bell's Inequality. Many experiments show that the rate of correlation is what QM predicts, ruling out these "hidden variables" theories.

  • BBO Crystal

    mindwalker5911/19/2015 at 19:01 0 comments

    I do have a BBO (bet-barium-borate) crystal. It's cut for type II, 405nm, colinear spontaneous parametric down conversion (SPDC), which means that it will produce two cones of 810nm light from a 405nm source, and those cones overlap along a line. In that overlap will be pairs of photons that are momentum and polarization entangled. They can be separated with a polarizing beam splitter (PBS) which will break the polarization entanglement but should leave the momentum entanglement intact. That will be great for sending one beam through the double slit while the other goes to the lens and screen/detector.

    I have a dichroic mirror to reflect off the non-downconverted light, leaving just the IR light which includes the entangled pairs.

    I haven't messed with this yet and am moving >:[ What I want to do is get stuff set up to send a beam from a cheap, ebay laser diode through the BBO and PBS, then detect photons from each and count both raw detections and coincidence detections. That should give me an idea about how many entangles pairs and what percentage of the total I get.

    Next is the critical experiment: send one beam through a double-slit. Dr. Cramer said the "anti-signal" will cause photons (that would be filtered out by the coincidence detector - thus, not entangled) to fill in the otherwise dark parts of the interference pattern. So putting one of these beams through a double-slit will NOT produce an interference pattern, if I don't filter with a coincidence detector? That's really hard to believe. Essentially, each photon's wave function goes through each slit, diffracts into concentric arcs from each slit where the arcs are maxima of the wave function. Those arcs overlap and interfere constructively at various angles. It seems to me that in order to get an inverse interference pattern, which is what this "anti-signal" light would do, the phases of the waves at each slit would have to be different by pi, and I can't see how/why that would happen.

    There's no help but to build it. Moving. And I need optical mount that I can adjust, which I can't work on because I'm moving! However, I can post the designs for those, possibly.

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