Does entanglement always conserve something?

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Discussion Overview

The discussion revolves around the relationship between entanglement and conservation laws in quantum mechanics, particularly focusing on whether entanglement always implies the conservation of certain quantities such as momentum or energy. Participants explore specific scenarios, including the Hong-Ou-Mandel (HOM) experiment, to examine these concepts.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that entanglement can correlate quantities like momentum or spin, but question whether this correlation is always necessary.
  • Others argue that energy and momentum are conserved in the system of photons and detectors in the HOM experiment, regardless of the detection outcomes.
  • A participant suggests that if two photons enter different detectors, it raises questions about energy conservation, implying a potential violation.
  • Another viewpoint asserts that momentum is always conserved in entangled systems, challenging the idea of possible violations.
  • Some participants discuss the idea that conservation laws may impose constraints that necessitate certain correlations, while also suggesting that correlations can exist independently of these requirements.
  • There is curiosity about whether a conservation law exists between entangled photons produced from entanglement swapping, with a note that these photons need not be of identical wavelength.
  • A later reply mentions the possibility of entangling different types of particles, such as photons and electrons, without needing them to be the same type.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between entanglement and conservation laws, with no consensus reached on whether entanglement always implies conservation of specific quantities. The discussion remains unresolved regarding the implications of the HOM experiment and the nature of correlations in entangled systems.

Contextual Notes

Participants highlight the complexity of conservation laws in quantum systems, noting that assumptions about the behavior of particles and the specifics of experimental setups may influence interpretations. There are unresolved questions about the implications of detection outcomes in the HOM experiment.

Swamp Thing
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IIUC, entanglement sometimes plays a role in conserving come quantity like momentum or spin: the quantities measured for two particles must be correlated in order to get a certain total value.

But is this always the case? For example, what, if anything, is conserved in the Hong-Ou-Mandel experiment? Or, what conservation would be violated if one photon was detected at each output (or two electrons at the same output) ?
 
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Swamp Thing said:
Or, what conservation would be violated if one photon was detected at each output (or
Energy and momentum are conserved in the system of photon and detector.
 
anuttarasammyak said:
Energy and momentum are conserved in the system of photon and detector.

Specifically in the HOM setup, if two photons enter and end up in different detectors (instead of the same detector), how would energy be violated?

Also, electrons and photons behave in opposite ways in the HOM situation (bunching / antibunching) , so they can't both be conserving momentum / energy ...?
 
Swamp Thing said:
Specifically in the HOM setup, if two photons enter and end up in different detectors (instead of the same detector), how would energy be violated?
Now you proceed to two photons case. In the system of photons and detectors there are no violations of energy and momentum. Do you have any motivation on pursuiting the violations ?
 
I think you have it backwards. Momentum (for example) is always conserved, including in entangled systems.
 
Vanadium 50 said:
I think you have it backwards.

I guess what I'm getting at is, sometimes conservation laws put constraints on the system that make certain correlations (entanglement) necessary. But sometimes such correlations can arise / exist without the need to comply with any such requirements?
 
Right, but conservation doesn't cause entanglement. As for detecting entanglement, how would you do this without looking at a conserved quantity?
 
Swamp Thing said:
I guess what I'm getting at is, sometimes conservation laws put constraints on the system that make certain correlations (entanglement) necessary. But sometimes such correlations can arise / exist without the need to comply with any such requirements?

Just curious. Would you say that there is a conservation law between the two entangled photons that are produced from entanglement swapping?
 
  • #10
pines-demon said:
Just curious. Would you say that there is a conservation law between the two entangled photons that are produced from entanglement swapping?
Not an answer to your question, but an interesting fact: those entangled photons need not be of identical wavelength. In fact, you can even use photons to entangle (via swapping) spins of 2 distant electrons. In principle, there is no requirement that the particle types even be the same.

Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km
 

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