3 ms·

Entanglement doesn't violate locality, it's measurement that does that. And that's because we don't have a rigourous handle on what measurement actually is, and

Entanglement doesn't violate locality, it's measurement that does that. And that's because we don't have a rigourous handle on what measurement actually is, and why we call it "the measurement problem"!

westurner 1y ago
Didn't they originally use polarizing filters to measure photonic phase? If it were possible to measure the phase of a photon after a beam splitter in a nondestructive way, shouldn't it be possible to determine whether measuring one causes state collapse in the other? This says that photonic entanglement is polarization, and that photonic phase can be inferred from second order of intensity, IIUC: "Bridging coherence optics and classical mechanics: A generic light polarization-entanglement complementary relation" (2023) https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.5.033110 https://journals.aps.org/prresearch/abstract/10.1103/PhysRev... Shouldn't it then be possible to nondestructively measure photons and thus entanglement?
- naasking 1y ago
  > If it were possible to measure the phase of a photon after a beam splitter in a nondestructive way "Non-destructive measurement" is an oxymoron. It's not a real measurement if it doesn't destroy the coherence of entanglement. Weak measurements do destroy some entanglement, just not "all" of it.
  - westurner 1y ago
    > "Non-destructive measurement" is an oxymoron. It's not a real measurement if it doesn't destroy the coherence of entanglement. If there were no loopholes to Bell's theorem I would agree. > Weak measurements do destroy some entanglement, just not "all" of it. IDK if that's true. Are all methods of observing probabilistic states destructive forms of measurement? Does a camera on a candle diminish the candle, or does it take energy (and information) from the "wake" of the field moments or field disturbances? I don't think that anyone realizes that it's possible to infer photonic phase from intensity (by Huygens-Steiner).
    - westurner 1y ago
      > Weak measurements do destroy some entanglement, just not "all" of it. Which measure of degree of entanglement best characterizes state linkage across spacetime? Depending on definition, doesn't a laser pointer entangle phase states across spacetime, but only slower than c (the speed of transverse photonic waves in a total vacuum)? Are the states synchronized with a constant delay? Quantum discord: https://en.wikipedia.org/wiki/Quantum_discord https://en.wikipedia.org/wiki/Quantum_discord : > In quantum information theory, quantum discord is a measure of nonclassical correlations between two subsystems of a quantum system. It includes correlations that are due to quantum physical effects but do not necessarily involve quantum entanglement. Quantum mutual information: https://en.wikipedia.org/wiki/Quantum_mutual_information https://en.wikipedia.org/wiki/Quantum_mutual_information

westurner 1y ago

Didn't they originally use polarizing filters to measure photonic phase? If it were possible to measure the phase of a photon after a beam splitter in a nondestructive way, shouldn't it be possible to determine whether measuring one causes state collapse in the other? This says that photonic entanglement is polarization, and that photonic phase can be inferred from second order of intensity, IIUC: "Bridging coherence optics and classical mechanics: A generic light polarization-entanglement complementary relation" (2023) https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.5.033110 https://journals.aps.org/prresearch/abstract/10.1103/PhysRev... Shouldn't it then be possible to nondestructively measure photons and thus entanglement?

naasking 1y ago
> If it were possible to measure the phase of a photon after a beam splitter in a nondestructive way "Non-destructive measurement" is an oxymoron. It's not a real measurement if it doesn't destroy the coherence of entanglement. Weak measurements do destroy some entanglement, just not "all" of it.
- westurner 1y ago
  > "Non-destructive measurement" is an oxymoron. It's not a real measurement if it doesn't destroy the coherence of entanglement. If there were no loopholes to Bell's theorem I would agree. > Weak measurements do destroy some entanglement, just not "all" of it. IDK if that's true. Are all methods of observing probabilistic states destructive forms of measurement? Does a camera on a candle diminish the candle, or does it take energy (and information) from the "wake" of the field moments or field disturbances? I don't think that anyone realizes that it's possible to infer photonic phase from intensity (by Huygens-Steiner).
  - westurner 1y ago
    > Weak measurements do destroy some entanglement, just not "all" of it. Which measure of degree of entanglement best characterizes state linkage across spacetime? Depending on definition, doesn't a laser pointer entangle phase states across spacetime, but only slower than c (the speed of transverse photonic waves in a total vacuum)? Are the states synchronized with a constant delay? Quantum discord: https://en.wikipedia.org/wiki/Quantum_discord https://en.wikipedia.org/wiki/Quantum_discord : > In quantum information theory, quantum discord is a measure of nonclassical correlations between two subsystems of a quantum system. It includes correlations that are due to quantum physical effects but do not necessarily involve quantum entanglement. Quantum mutual information: https://en.wikipedia.org/wiki/Quantum_mutual_information https://en.wikipedia.org/wiki/Quantum_mutual_information