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I Bell's inequality experimental data

  1. Feb 15, 2017 #1
    Everything I've seen about Bell's inequality has had the setup of 120 degree angles between the axis of measurements. The experiment then proves that the basic hidden variable theory can't be true. But the actual measurement has always been told to me as a 0.5 correlation. 50% of the time the two particles are reported in the opposite state, 50% of the time they are not. Which is exactly what entanglement predicts.

    But it's also what you'd expect to see if the particles were not entangled.

    First question, and the easiest way around this: Do the experiments keep track of individual results at individual measurement settings? Collectively, it's 50%. But can we look at the data and see that anytime the measurement axis line up the entanglement is 100%? If the particles are entangled, and the axis of measurement is the same, then the measurements will give the opposite value. Do we have that data? Do you know where I can see it for myself?

    If we don't have that data, then I'm quickly going to despair of ever understanding why we're so convinced that the particles are still entangled at the time of measurement. I do not have a doctorate in theoretical physics, and I'm not about to get one. I don't even understand how we write entangled states or how to do math with them. The other option is if we know exactly what it takes to break an entangled state, such that we could be certain that the particles were entangled at the time of measurement.

    So, essentially: If the only way to know whether or not two particles are entangled is to measure them both, then a 50-50 split in a two-state system doesn't prove that they are entangled.
  2. jcsd
  3. Feb 15, 2017 #2


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    Staff: Mentor

    That's not correct. With entangled particles in a Bell state and 120° between the detectors, the quantum probability that the particles are measured with opposite spins is 1/4. Edit: to clarify, I am talking about measurement pairs along different angles.

    Have a look at figure 3 in https://arxiv.org/pdf/1508.05949.pdf
    Last edited: Feb 15, 2017
  4. Feb 15, 2017 #3
    Yup, that answers my question. Fig 3 (C) is what I was looking for. Thanks!
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