What is Entanglement: Definition and 864 Discussions

Quantum entanglement is a physical phenomenon that occurs when a group of particles are generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the group cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical and quantum physics: entanglement is a primary feature of quantum mechanics lacking in classical mechanics.
Measurements of physical properties such as position, momentum, spin, and polarization performed on entangled particles can, in some cases, be found to be perfectly correlated. For example, if a pair of entangled particles is generated such that their total spin is known to be zero, and one particle is found to have clockwise spin on a first axis, then the spin of the other particle, measured on the same axis, is found to be counterclockwise. However, this behavior gives rise to seemingly paradoxical effects: any measurement of a particle's properties results in an irreversible wave function collapse of that particle and changes the original quantum state. With entangled particles, such measurements affect the entangled system as a whole.
Such phenomena were the subject of a 1935 paper by Albert Einstein, Boris Podolsky, and Nathan Rosen, and several papers by Erwin Schrödinger shortly thereafter, describing what came to be known as the EPR paradox. Einstein and others considered such behavior impossible, as it violated the local realism view of causality (Einstein referring to it as "spooky action at a distance") and argued that the accepted formulation of quantum mechanics must therefore be incomplete.
Later, however, the counterintuitive predictions of quantum mechanics were verified in tests where polarization or spin of entangled particles was measured at separate locations, statistically violating Bell's inequality. In earlier tests, it couldn't be ruled out that the result at one point could have been subtly transmitted to the remote point, affecting the outcome at the second location. However, so-called "loophole-free" Bell tests have been performed where the locations were sufficiently separated that communications at the speed of light would have taken longer—in one case, 10,000 times longer—than the interval between the measurements.According to some interpretations of quantum mechanics, the effect of one measurement occurs instantly. Other interpretations which don't recognize wavefunction collapse dispute that there is any "effect" at all. However, all interpretations agree that entanglement produces correlation between the measurements and that the mutual information between the entangled particles can be exploited, but that any transmission of information at faster-than-light speeds is impossible.Quantum entanglement has been demonstrated experimentally with photons, neutrinos, electrons, molecules as large as buckyballs, and even small diamonds. The utilization of entanglement in communication, computation and quantum radar is a very active area of research and development.

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  1. DrChinese

    A Entangling a photon from the Sun with one from Earth

    NOTE: This isn't actually Quantum Foundations or Interpretations, but I thought the audience might be best here. Or perhaps regular Quantum Physics or Astronomy? Not sure. Please move if appropriate. A fun entanglement experiment! How about entangling light emitted from the sun with...
  2. D

    I Exploring Measurements in Quantum Field Theory: From Light Cones to Bell Tests

    Hello, I'm interested in how measurement, entanglement, bell test etc are handled in QFT. It seems most QFT texts are being quite light on details on the subject. There would be is a preparation step as the start followed by some interaction and a measurement at the end. Interaction is usually...
  3. W

    B How are the spins of two entangled photons measured at light speed?

    and how is it known that the two photons are entangled in the first place? I mean before measuring how do you know that you have the correct two photons?
  4. B

    B Entanglement & Superposition Probabilities

    I cannot find a clear answer on the following beginner’s question on some QM fundamentals: Suppose we have two particles, A and B. Let’s say we generated these as (or otherwise entangled them as) an entangled pair with opposite/orthogonal states. Perhaps horizontally and vertically polarized...
  5. cianfa72

    I Understanding Bell Inequality Proof: Explained from a Probability Perspective

    Starting from this link my understanding of Bell inequality proof goes as follows: Suppose we have a model of local pre-determinate hidden variables for QM. This amounts to say QM objects are in pre-determinate given states even if we do not measure it. Locality just means that spacelike...
  6. G

    I Entangled, mixed state with conditional entropy zero

    Hi. The classical (Shannon) conditional entropy is never negative. It can be written as ##H(Y|X)=H(X,Y)-H(X)## which allows for a quantum generalization using von Neumann entropy. In the case of entangled states, it can become negative. I guess it should be possible to construct an entangled...
  7. vanhees71

    I The notion of locality in (Quantum) Physics should be clearly defined

    Unfortunately one of the threads about entanglement and Bell tests has again been closed prematurely. It has not been clarified what "locality" means. In the physics community, not involved in philosophical arguments about foundations of QT, it's clearly defined as the property of a...
  8. G

    I General argument that entanglement can only be created locally

    Hi. As far as I know, entanglement can initially only be created locally, for example by creating two photons at one place in a crystal, or by local forces. Sure, there's entanglement swapping, but the initial entanglement was still created locally. But I only know examples. Is there a general...
  9. K

    I Spooky action at a distance not allowed?

    Sabine Hossenfelder implies spooky action at a distance is wrong. She says “They seem to think if you do something to one particle in an entangled pair, then that will immediately affect the other one. But this isn’t so. It’s only when you measure one particle, then you have to update the...
  10. RobbyQ

    I Photon Entanglement: Identifying & Using an Entangled Pair

    In this video how are the entangled photons later used and actually identified as an entangled pair amongst billions of others. Also does he really mean the photon is split or is the quantised energy split with half frequencies?
  11. RobbyQ

    I Generating Entangled Electron Pairs

    I have read, what I believe, misleading articles about generating entangled electron pairs. Some suggesting the electron is split. But this isn't possible because it's an elementary particle with charge/mass and Spin properties. So how do we achieve generating entangled electrons with opposite...
  12. jaketodd

    I A hurdle in quantum computing I've never been able to figure out

    So the whole idea with quantum entangled computing, is that particles in superposition can compute more than one thing at the same time, right? But how does a system know which computed result is which? Maybe like a hashtag that separates one from another? But wouldn't that get jumbled, and...
  13. vibhuav

    I Physical meaning of two independent, non-interacting parts

    I keep coming across this descriptor, "two (or three) independent, non-interacting parts," in many books on QM (for example, Penrose's Shadows of the Mind). It is usually followed by a mathematical description (for example, state vector |A>|B>). I can wrap my mind around the quantum paradox of...
  14. giodude

    I Quantum Entanglement and Parallel Worlds/Existence

    Hey! I'm new to the forums so its nice to be here. I don't have a deep deep background in physics (I plan to self study after I finish my math studies). However, I recently learned about the notion of quantum entanglement. My basic understanding of it is that quantum entanglement (will use QE...
  15. Kostik

    B Entanglement inside a black hole

    Suppose two electrons are entangled with opposite spins. Electron #1 passes through the event horizon of a black hole, together with Laboratory Assistant #1. Suppose the assistant measures electron #1's spin after they pass through the event horizon (according to #1's [proper] time) and measures...
  16. L

    B Quantum Entanglement & Hybridization

    I'm conflicted about how hybridization and quantum entanglement can simultaneously co-exist. I'm first confused about how quantum entanglement was proven. I tried to read to proves (I'm in grade 11 and planning on writing an ee on this) and it flew relatively over my head. Hybridization states...
  17. F

    B Quantum entanglement and simultaneity

    I know very little about quantum physics. I was looking up the definition of quantum entanglement and asked ChatGPT to explain it. Here is an interesting phrase in its answer: "Once the particles are entangled, measurements made on one of the particles will instantaneously affect the state of...
  18. PainterGuy

    B Quantum entanglement and hidden variables

    Hi, I have a basic understanding of quantum physics. I was reading a Wikipedia article on hidden variables, https://en.wikipedia.org/wiki/Hidden-variable_theory . The article says the following. I was confused about the words "local" and "nonlocal" in the quote above so I checked out another...
  19. DrChinese

    A Can entanglement be detected in a single Bell pair?

    Can entanglement be detected in a single Bell pair? The canonical answer is NO, you can only see the entanglement in an ensemble of Bell pairs. For example, you could look at a stream of photon pairs coming from a PDC source. If you apply the CHSH formula (usually presented as an "S" value)...
  20. L

    B Quantum Connection vs Entanglement: What's the Difference?

    quantum connection vs. quantum entanglement Do they mean the same? Are there any differences? Is the second one more common/newer/more standard than the first one? Could someone please answer these questions or recommend something to read?
  21. K

    I Entanglement swapping, monogamy, and realism

    For this discussion I would like to continue referring to the entangled pairs 1,2 and 3,4 as we have in previous discussions. For background on this discussion refer to post-selection: pre-existing correlations or action at a distance where @DrChinese describes the entanglement swapping...
  22. DrChinese

    A How Many Hidden Variables Would It Take to Model Photon Polarization?

    Suppose the EPR* concept were true as it would be applied to entangled photon polarization. (Please note: I am not saying it is.) They thought QM was incomplete, because there must exist "elements of reality" (hidden variables) that supplied the highly correlated results on entangled particle...
  23. Seyara

    Quantum entanglement question

    Ok so just tell me this, if you flip a coin with your eyes closed and it lands on your hand and then you look at the coin and it is heads… there was a time in the duration of the coin being in the air at which the coin was in a state of neither heads or tales. But only once it hits your hand it...
  24. H

    A A question about entanglement swapping

    Happy new year for all the forumers, My question is about a modification of entanglement swapping: We use to have two sources of maximally entangled pairs (a and A) and (b and B) when a is up then A is also up, same thing for b and B and up and down refer to a measurement along a given direction...
  25. S

    I Experiment with gamers, random numbers and entanglement

    https://www.wired.com/story/this-random-video-game-powers-quantum-entanglement-experiments/ I don't understand the principle of this experiment. The gamers produced random numbers, and what was done with these numbers then? Was the value like <S> in CHSH inequalities computed, and was it...
  26. physicsclaus

    How to calculate gating time from the rate of the random coincidence?

    Hello everyone, I am now doing experiment related to quantum erasure. After plotting the correlation measurement with and without blocking one of the polarization from the SPDC source (say, V polarization), I do not know how to work further on the gating time from the rate of the random...
  27. StevieTNZ

    I What does delayed choice entanglement swapping require?

    A follow-up question from a previous thread: https://www.physicsforums.com/threads/is-entanglement-swapping-a-result-of-post-selection-or-a-result-of-a-quantum-operation-called-a-bsm.1047772/. So my point of view is this: there are systems 1-4 (which refer to photons 1-4 in...
  28. DrChinese

    A Exploring Entanglement Swapping: Post-Selection vs. BSM?

    I am starting this thread to focus on a point discussed in a QP thread. The relevant experiment is referenced below, which is a very complex experiment intending to remove some of the "loopholes" in Bell tests. For this thread, those loopholes don't matter and we won't reference that element...
  29. S

    B Is there any practical proof that quantum entanglement really works?

    What i mean if we change state/spin at one end it will immediately effect the other. Can we see that live using two camera which may be 10 meter apart so that minium time delay. Is there any video proof exist such kind?
  30. M

    I Entanglement and Born rule

    Let be an entangled pair of photons 1 and 2, with the same polarization. The wave function is ##|12>=\cos\psi|HH>+\sin\psi|VV>## with ##\psi## the angle of polarization. The first ##H## (##V##) in ##|HH>## (##|VV>##) is photon 1, and the second one is photon 2. Alice observes photon 1 with a...
  31. J

    I Exploring Entanglement Through Laser Light Mixing Experiments

    https://quantum.phys.lsu.edu/old-website/seminars/abstracts/Kaushik10.pdf I have discovered an experiment in the link above where you get NOON entangled states by mixing classical laser light with quantum light from SPDC. But I cannot understand the mathematics behind it. Can you explain it to...
  32. T

    I How does a process of a Raman transition break quantum entanglement?

    I'm reading this article about quantum entanglement, and the author writes about a process in a Raman transition which would break the entanglement, and I'm interested about how it breaks the entanglement. So the passage which I'm interested in begins with: "The answer is to do an operation...
  33. E

    I Wondering why quantum entanglement occurs

    hi there I am an engineer but have a huge interest in physics and wanted to ask anyone who has any knowledge of why quantum entanglement occurs?
  34. Delta2

    I Entanglement and FTL signaling in professional scientific literature

    According to professional scientific literature and to our best understanding, are there any suggestions that entanglement might imply some sort of faster than light signaling between the entangled particles? I know that according to relativity nothing can travel faster than light, but what...
  35. C

    What is the mechanism behind Quantum Entanglement?

    What is the mechanism behind Quantum Entanglement? Why do only subatomic particles exhibit Quantum Entanglement?
  36. A

    I What happens to the other particle in an entanglement state?

    if I have two particles in an entangled state, I make them travel in different directions, and I measure the state of only one of them then I know the outcome of the measurement of the other. But when I take a measurement on the first particle, what happens to the second? Does it undergo a...
  37. A

    I Entanglement in NRQM and QFT

    What is entanglement in QM and QFT? I understood that it only corresponds to the concept of linear combination of states with multiple particles. Seeing lectures on YB it seems to me that it is something much deeper than that. What did I miss? How is it treated in QFT? I am studying NRQM from...
  38. VictorMedvil

    I How Do You Break Quantum Entanglement Bonds?

    I was doing some research into quantum entanglement but it is never well described how you break the bonds once they are formed does anyone have any expertise in this area on how to break quantum entanglement bonds? The best that I can understand is the bond is broken when interaction with the...
  39. F

    B Quantum entanglement and energy conservation

    As my current studies have proven conservation of energy is a universal law. How is it possible for two entangled particles to be equally or similarly affected when a force or energy is applied to a single member of the entangled pair? The production of such a pair would be invaluable to...
  40. entropy1

    I Is the moment of the measurement well defined?

    Suppose Alice and Bob do an experiment with an entangled pair of particles, for instance electron spin with SG magnets. Now suppose Alice her SGM is stationary while Bob his SGM is switching fast between parallel to Alice and perpendicular to Alice. So there are two possibilities: correlation...
  41. F

    I Determination of entanglement by observing only one photon

    Hi everyone, background for my question is here: and https://www.researchgate.net/publication/45424433_Direct_generation_of_photon_triplets_using_cascaded_photon-pair_sources: My question is whether it's possible to determine if two photons are entangled without using a coincidence counter...
  42. Arquimedes

    I What is the physical meaning of concurence (quantum information)?

    Hello, I am currently studying about entanglement on spin-1/2 chains and I was able to find some information about the mathematical point of view of concurrence but I can't understand the physical meaning of it . Can somebody help me, please?
  43. W

    Is quantum entanglement life?

    hello: I don’t know where to post this and I think this is as good a place as any here. question: does quantum entanglement explain how we are? I am having trouble Even trying to express the question above. But, I just viewed PBS space-time series 6 episode eight: “how do quantum states...
  44. Quantum Entanglement & Spooky Action at a Distance!

    Quantum Entanglement & Spooky Action at a Distance!

    Quantum Entanglement & Spooky Action at a Distance! Bell's theorem.
  45. entropy1

    I Inference about entanglement ontology

    Suppose particles P1 and P2 are spin entangled in singlet state, then, if someone claims that IF particle P1 is found to be in spin-up state when measured, that THEN particle P2 is in spin-down state, does that follow from the minimal formalism, or is it just an assumption?
  46. L

    B Quantum Entanglement information transmission idea to knock down....

    I work in IT and am a layman in the quantum world. I have obviously misunderstood something in my amateur reading of quantum, but if someone could explain my mistake in the above scenario it might be very insightful for me! Forgive me if the terminology is not correct - or if indeed lay folks'...
  47. S

    I Does entanglement interfere with causality?

    Suppose someone entangled 2 particles many years ago and kept particle E here on Earth and sent particle S a light year away from Earth. So the observation of particle E on Earth would fix the state of particle S a light year away. But they did it in such a way that observing particle E would...
  48. stephen8686

    I Entanglement, Mixed or Pure State?

    I have a source that says when two particles are entangled, we must describe them using the density operator because it is a mixed state. But I have another source that says that the singlet state of two spins is an entangled state, but that has a wavefunction. So could someone explain what I am...
  49. S

    I Macroscopic quantum entanglement?

    Very confused about this article and the experiment it's based on. I'm not very knowledgeable on this, but I'm very confused on what's happening here. It seems extremely weird to me
  50. S

    B Confused about Entanglement

    Most of my questions are from these articles: https://www.google.com/amp/s/www.sciencealert.com/if-you-thought-quantum-mechanics-was-weird-wait-til-you-check-out-entangled-time/amp https://www.scientificamerican.com/article/how-einstein-revealed-the-universe-s-strange-nonlocality/...