What is Quantum entanglement: Definition and 297 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. FreeThinking

    Dr. Eddie Boyes' Youtube videos on Quantum Entanglement

    FYI & FWIW: I just found what I think is an excellent series of Youtube videos on Quantum Entanglement by Dr. Eddie Boyes for those of us who have never quite understood it. I'm only about half-way through, but I have already found it valuable. Just wanted to share my "discovery". I tried...
  2. S

    B Entangled quants inside a particle accelarator

    Due to the energie that is put in when entangling quants my question arises in the outcome. If two quants entangled with each other are accelarated in an particle accelarator and hit each other, they should (as i understand it) cancel each others spin out and therefore cancel the energie they...
  3. M

    I Is entanglement a consequence of the singularity of the early universe?

    Does it make sense to explain entanglement as a consequence of the ‘singularity' of the early universe? If proto matter/energy was in some mathematically definable ’small’ space, perhaps information was likewise compressed, and entanglement is a consequence of this early cosmological locality...
  4. J

    A In the depicted setup, what would happen? (entangled photons split into 2 different paths)

    Please consider the depicted setup. We assume that the source is producing only H/V polarization entangled photon pairs (which is separately confirmed prior to the experiment). Scenario 1) First suppose that the depicted 45 Degree Linear Polarizer A is absent. Photon B's polarization is in H/V...
  5. Pixelex

    B Quantum Entanglement in Practice

    How is quantum entanglement done in practice for different particles with different properties eg. spin, polarization, etc.? Why is entanglement useful for quantum circuits? *I am curious about how they are actually entangled and used. I don't know how is entanglement applied in practice so I...
  6. A

    I Measuring electron spin and measuring entangled electron spin

    Hi people, Lets assume, we have a stern gerlach setup and we are going to measure an atom's last orbit electron's spin with 60 degree from vertical axis. Therefore, in this case, our outcomes would be 3/4 for spin up, 1/4 for spin down. Let's assume, we have the same setup but we are going to...
  7. A

    I Delayed Choice Quantum Eraser with altered beam splitter

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  8. J

    I Can anyone explain me this quantum entanglement experiment

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  9. jaketodd

    I Entanglement may reveal more about black holes? ...Even micro ones!

    Could an entangled particle (or larger entangled object), sent into a black hole, reveal anything new about black holes, with the connected entangled partner outside the black hole? Can entanglement escape the singularity and communicate with its partner? I've heard the singularity is a rip in...
  10. jaketodd

    I What is the limit in size that two particles/objects can be entangled?

    What is the limit in size that two particles/objects can be entangled? With the double slit experiment, I know that there are matter waves, of large size - not just individual photons. So can a matter wave be entangled with another one, or even just a single particle with the matter wave...
  11. 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...
  12. 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...
  13. 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...
  14. 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...
  15. S

    Quantum entanglement and the Schrödinger experiment

    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...
  16. physicsclaus

    How to calculate dark count from data collected?

    Hello everyone, I am trying to measure the dark count from a measurement a SPDC source. Although I collected data from the signal generator, I do not know how to obtain dark count rates per second. I only know the following definition Dark counts refer to the tiny amount of DC current in the...
  17. H

    I Bob and Alice, Wigner and his friend

    Hi Pfs, I wrote in another threas that when a source emits maximally entangled pairs of photons with nul global momentum and null global angular momentum, there is no local properies for the photons shared by Alice and Bob. i said that the the source only emits correlations. it has no sense to...
  18. 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?
  19. 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...
  20. 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?
  21. C

    What is the mechanism behind Quantum Entanglement?

    What is the mechanism behind Quantum Entanglement? Why do only subatomic particles exhibit Quantum Entanglement?
  22. 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...
  23. F

    The investigation into the dynamics of entangled particles.

    Hey I'm Fraser, I am a Strathclyde university graduate with a master's in electrical and mechanical engineering. I am interested in furthering my knowledge on quantum entanglement in relation to general dynamics. I am interested in researching its capabilities in ICBs. I hope I can find...
  24. 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...
  25. 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...
  26. L

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

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  27. 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
  28. E

    B Quantum entanglement and communications

    Could quantum entanglement be used to send faster-than-light communications across vast distances in space, in the distant future? For example, if humans establish permanent colonies on Mars, it would take some time to send communications between Earth and Mars. Could a future technology...
  29. S

    I When does quantum entanglement occur?

    If I understand the idea of EPR correctly, the quantum entanglement occurs between two particles for which the total momentum is known (therefore, knowing the momentum of one particle, one can recalculate the other, and this contradicts the uncertainty principle). Then the question arises...
  30. L

    B Why isn’t quantum entanglement just correlation?

    They say spin up and spin down is correlated at at any distance and that it can’t be explained by basic logic. say I rip a photo in two, shuffle them and put them in two boxes and send them light years away. No matter which box I open, I can’t know which half I have but when I open it the photo...
  31. D

    A Max Fidelity at $$F=|\langle\psi|\phi\rangle|^2$$

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  32. D

    A Fidelity for quantum state at t=0

    fidelity for pure state with respect to t=0 is 1. My teacher told me this. But I am not getting this. This is my detailed question the initial state(t=0)##|\psi\rangle=|\alpha\rangle|0\rangle## the final state (t) ##|\chi\rangle= |i\alpha\sin(t)\rangle|cos(t)\alpha\rangle## Fidelity between the...
  33. edukso

    B Which book is best for learning about quantum entanglement?

    Hello! I would be really thankful if you could recommend me a good book for an introduction to quantum entanglement . Thanks!
  34. Orenshved

    What's needed to use quantum entanglement for FTL data transfer?

    Hey all, I need help with the book I'm currently writing. What would it take (even theoretically) to use quantum entanglement for FTL data transfer? From what I understand, the state of entangled particles can not be changed without breaking the entanglement. Do you think this would ever be...
  35. L

    B How do you tell when particles are or aren't entangled?

    I'm trying to get my head around the principles of quantum entanglement (would love to get some schooling on the issue but I don't have that kind of money) and trying to discern the actual science from the horrible misconceptions is difficult. I know you can't transmit information faster than...
  36. RockSteadyUK

    I Quantum entanglement. -- Spin up, spin down, isn't it the same?

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  37. K

    B Question about quantum entanglement

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  38. M

    I Quantum Entanglement with Public Variables

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  39. J

    B Is a Quantum Entanglement Camera the Key to Seeing Into the Future of Earth?

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  40. P

    B Quantum Entanglement Communication Radio

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  41. F

    B Quantum Entanglement spin measurement

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  42. S

    B Double Slit Paradox: A Wave-Particle Mystery

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  43. K

    A Quantum entanglement and quantum foundation

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  44. Quantum Entanglement: Spooky Action at a Distance

    Quantum Entanglement: Spooky Action at a Distance

    Fermilab's Don Lincoln describes Quantum Entanglement, and how it is different from classical statistics and hidden variables scenario.
  45. T

    B Layman's question re quantum entanglement

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  46. pixel

    I Quantum Entanglement: What I Know

    What I know of this only comes from popular presentations of the subject. So let's say there are two particles, A and B, known to have opposite values of a particular property such as spin. We don't know which particle has which spin until we measure the spin of one of the particles, say A. Then...
  47. Quantum Alchemy

    I How does Classical Physics explain Quantum Entanglement?

    As a Computer Programmer, it's hard to wrap my head around Quantum Entanglement and non locality being explained in the context of Classical Physics. In other words, if the universe at it's core is physical where does Quantum Entanglement fit within a physical picture of reality? There's been...
  48. Meruem

    B Is quantum entanglement a four dimensional force?

    Why two bodies are related with each other despite the distance between them. It is unbelievable that entanglement exist regardless of any distance. Why on universe are so deeply connected with each other where distance does not play any role for the phenomena?
  49. G

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    I am building an SF universe. While it is very similar to ours, but casual FTL exists. I don't think Lorentz transformations shouldn't work at all, but rather extra dimensions can somehow circumvent them. One kind of FTL is direct communication through entanglement. There can be two version of...