# Entanglement Definition and 112 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.

View More On Wikipedia.org
1. ### 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...
2. ### 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?
3. ### Quantum Entanglement & Spooky Action at a Distance!

Quantum Entanglement & Spooky Action at a Distance! Bell's theorem.
4. ### 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?
5. ### I Issues on notation and concept of entanglement

As a follow up from my other thread, where I consider popular media describe entanglement sort of as: and I think this may be wrong. As a follow up question I want to put forward this: A singlet state of entangled particles is notated in a superposition of product states as: ##|up, down...
6. ### I Is this popular description of entanglement correct?

In some popularized discussions of entanglement, you often hear that: This seems to me not necessarily the case. In this formulation, particle A is viewed through measurement outcome and particle B through ontology. If the measurement basisses of Alice and Bob are parallel, Alice's outcome...
7. ### I Entangled particles in black hole decay?

First, I was not sure whether this should go into the Relativity or the Quantum Physics rubric, but since the central question is about entanglement, I opted for the Quantum. I do not have the necessary sophistication to follow string theory arguments, and even most explanations in...
8. ### I Consistency of model with QM

I started another thread on this but it went off into other topics. Hoping to focus on the math here, specifically whether or not the model presented in here is consistent with QM. Let's measure the polarization at the same angle ##\alpha = \beta = \pi/3## (##\varphi_1=0...
9. ### I Mixed states from entangled state

I'm an undergrad in physics, and have been asking myself the following question recently. Suppose you have a pure quantum state p (von neumann entropy=0), made of 2 sub-states p1 and p2 that are entangled. Because they are entangled, p \neq p1 x p2. Hence the entanglement entropy of p (=0) is...
10. ### I Understanding Coherence vs. Entanglement in Quantum Eraser

As I understand it, either coherence or entanglement can be in an experiment or some of each, varying continuously between the two. Because of this, coincidence detection is needed to pick out interference patterns among all the data. Coherence would refer to quantum waves of photons taking...
11. ### A Entanglement Witnesses close to GHZ states

Consider page 2 of Toth's paper Entanglement detection in the stabilizer formalism (2005) . To detect entanglement close to GHZ states, they construct entanglement witnesses of the form $$\mathcal{W} := c_0 I - \tilde{S}_{k}^{(GHZ_N)} - \tilde{S}_{l}^{(GHZ_N)},$$ where...
12. ### I Why does "control" mean "coherency" in Wigner's friend paradox?

There we read: "note that if Wigner did not know this phase due to the lack of control of it, he would describe the “spin + friend’s laboratory” in an incoherent mixture of the two possibilities". Why is this the case? Given that the author has propoede neither a citation nor a proof for this...
13. ### A Non-entangling environmental noise in QM/QFT

Does non-entangling environmental noise actually exist? The existence of such a thing seems to be fundamentally inconsistent with the properties of quantum entanglement. It seems to me that if non-entangling environmental noise does exist, then standard QM - or QFT for that matter -...
14. ### A QFT topics for entanglement entropy

I want to know what are the QFT topics that I need to understand in order to proceed in reading papers on entanglement entropy such as, Entanglement Entropy and Quantum Field Theory Entanglement entropy in free quantum field theory Entanglement entropy: holography and renormalization group An...
15. ### I Entanglement setup that shouldn't allow for the transmission of info

Suppose we produce a polarisation-entangled photon pair ##A_1## and ##A_2##. Then we entangle another pair ##B_1## and ##B_2##. Now suppose that these photons will not interact with anything, sending ##A_1## and ##B_1## to Alice and ##A_2## and ##B_2## to Bob. Alice measures the polarisations...
16. ### B Entanglement Query

1) Entanglement is about the behaviour of a particle can be determined by measurement of the behaviour of another particle. At some website, it explains that the reason behind is due to conservation of momentum. When a particle is split into two particles (say particle A and particle B)...
17. ### I Entanglement after measurement

Three related questions: (a) In a pair of entangled particles, after one is measured/observed/determined/collapsed, my understanding is that the measurement breaks the entanglement so that after the measurement, unless something happens to re-entangle them, they are no longer entangled. Correct...
18. ### I Entanglement/correlations in time

From what I know about (bi-partite) entanglement, we write the combined Hilbert space as a tensor-product of Hilbert spaces for a particle at ##A## and a particle at ##B##, ie ##\mathcal{H} = \mathcal{H} ^{A} \otimes \mathcal{H} ^{B}##. If the particles share a non-separable state, they are...
19. ### B Can entangled particle pairs be measured at the same time?

Has there been an experiment where 2 particles that are entangled are measured at the same time? If so what was the result? Can any observer occupy the same frame of reference down to an electron? Don't we all exist at different times based on our frame of reference so none of us can share the...
20. ### I (In)dependence in entanglement experiments

If an entanglement experiment, whereby an entangled pair of particles is measured at both ends, is independent of the next entanglement experiment with another pair of entangled particles, how can there be a correlation? It seems that each independent run does not influence the next run, but...
21. ### I Do spin operators 'appear' to commute for an entangled spin state?

Let's consider Bohm's paradox (explaining as follows). A zero spin particle converts into two half-spin particles which move in the opposite directions. The parent particle had no angular momentum, so total spin of two particles is 0 implying they are in the singlet state. Suppose we measured Sz...
22. ### A How does this study affect the holographic principle?

In a recent study (https://phys.org/news/2018-08-flaw-emergent-gravity.html) it has been discovered an important flaw in Emergent/Entropic Gravity because it has been discovered that holographic screens cannot behave according to thermodynamics... But then, doesn't this also invalidate...
23. ### A What is a 'Quantum Coherence'?

I have recently been reading some stuff on quantum information in the physics literature which refers to 'a mechanism by which a measurement in A determines quantum coherences in B', where A and B are subsystems of a larger system. I am aware of the meaning of the terms 'decoherence' and...
24. ### Quantum Entanglement textbook/notes

Hi everyone, Could anyone recommend a good QM textbook (undergrad-ish level) or some lecture notes that treat entanglement from the ground-up? Most of the stuff I have seen online on entanglement seem to fly pretty quickly into information-theory or abstract group-theory type stuff, which I am...
25. ### I Continuous variable entanglement

Hi all, I have learned the very basics of entanglement (discrete, 2 particle systems) and was hoping that someone can recommend introductory (undergrad-level) material for continuous-variable, 2 particle entanglement. Stuff I have found online so far (like this...
26. ### I Franson interferometer: Entanglement?

Hi. As far as I understand the Franson interferometer, the photons are in an entangled state like $$\left|\Psi\right\rangle=\frac{1}{\sqrt{2}}\left(\left|\text{short}\right\rangle\left|\text{short}\right\rangle+\left|\text{long}\right\rangle\left|\text{long}\right\rangle\right)$$ if the setup...
27. ### Work Check: Simple entanglement problem

Homework Statement Could someone assist me in skimming through my work for this problem? Many thanks! I attached an image of the problem below. Also, I only need help for the first part (part a), cheers. Homework Equations General entangled state vector of a two-particle system: |\psi...
28. ### I Question about noise in entanglement experiments

Hi everyone While learning about quantum mechanics, I became curious about the real-life experimental data. Wikipedia says that entanglement experiments require coincidence counters, because the majority of the signal received by detectors is noise. It further says, that coincidence counters...
29. ### B Quantum entanglement phenomenon

Hi there, Question from a biologist with very poor background in physics, but willing to understand quantum physics. I think quantum entanglement shocks everyone, even if it has been proven right. I would love to know if there is any hypothesis or crazy theory out there to explain why or how...
30. ### I Phase shift after double-slit for entangled photons

Hi all, I'm trying to understand how to describe the quantum state of entangled photons, including their phase, if one of them encounters a double-slit. Here's a simple example: Suppose you have two polarization-entangled photons A and B in the following Bell state: ...
31. ### Quantum state of entangled photons

Homework Statement Suppose two polarization-entangled photons A and B in the following Bell state: $$\Phi=\frac{1}{\sqrt{2}}\bigl(\left|H_{A},H_{B}\right\rangle + \left| V_{A},V_{B}\right\rangle\bigr)$$ 1. What is the state if the photon A passes through a...
32. ### Quantum Entangled Living Bacteria

"Schrödinger's Bacterium" Could Be a Quantum Biology Milestone I can't believe I'm only seeing this article now. Achieving quantum mechanical effects with large systems, especially complicated ones such as bacteria - let alone one in vivo - has been a longstanding goal in experimental QM. To...
33. ### B Are Quantum Entities in Macro Sized Objects Entangled?

Hi. This is my first posting on the Physics Forum so please forgive any issues as a result. I am a (reasonably educated) lay person with a strong physics interest with extensive readings -- so please be patient with my questions. :-> My questions and interest in these issues are sincere. I...
34. ### I Does quantum entanglement depend on the chosen basis?

Hi, In this presentation about quantum optics it is mentioned that the same quantum state |Ψ> has different expressions in different mode bases : factorized state or entangled state. This presentation is related to this video : In some way entanglement isn't intrinsic. It depend on the...
35. ### I ER=EPR baby steps: causation or not?

The questions concern the extension of the holographic principle to the identification of a wormhole between two black holes with negative cosmological constant and an entangled pair on its boundary, included in the conjecture known as EPR=ER ( Maldacena, Susskind). I refer to...
36. ### A Causal nonseparability

Are timelike entanglement and experiments demonstrating causal non-separability by quantum superposition of causal orders an indication that causal principles may not be applicable to quantum mechanics?
37. ### A Novel entangled photon source with heralding

Just saw this article from a highly respected research team, and thought some might enjoy seeing how the state of the art continues to develop rapidly: 12-photon entanglement and scalable scattershot boson sampling with optimal entangled-photon pairs from parametric down-conversion Han-Sen...
38. ### I Validity of a paper on the standard conclusions from Bell, etc

In most popular explanations of entanglement, the quantum information of an entangled two-particle system changes without regard to the distance between the two particles. The following paper seems (to my unprofessional eye) to be questioning this interpretation...
39. ### I Do macro objects get entangled?

If we consider the Unitary evolution of the wavefunction, and interpret measurements as becoming in superposition, taking it that the measurement device gets in a superposition of spin up and spin down, do two measurement devices that each measure one particle of an entangled pair become...
40. ### B Library of Babel quantum state

Hello! If the Library of Babel has 10^(2,000,000) books, does anyone think that it is possible to create a quantum state (with a quantum computer) that represents this Library? I think that in a classical way it is impossible, but in a quantum way? I find it quite interesting! What about you? :)
41. ### B Quantum entanglement and special relativity

There are a pair of entangled particles moving in opposite directions. A measurement is done on particle A, the wavefunction collapses randomly, you observe either spin up or spin down, A does an action at a distance on B, particle B instantly collapses to the opposite spin state, a measurement...
42. ### B Entanglement and pair-production and annihilation

I am confused about entanglement, but I am not a physicist. The concept sounds cool and I want to understand in a way so that it is familiar with what I already know. I want to know if I am interpreting this right: 1) If we have a photon that produces a pair of electron and positron, the...
43. ### I Entangled Clocks?

I just overheard an engineer saying "There are no two clocks in the world that tell identical time". She was describing a time syncing mechanism to another engineer, but it made me think... In theory, can something large enough to be used as a clock become fully entangled with a copy of...
44. ### I QM Assumptions Regarding Entanglement Properties

In a nutshell I think that in local realistic theories it is assumed that: Each entangled object has definite properties at all times, even when not observed. I know the assumption is proved to be incorrect but is that an assumption actually made in such theories? But what assumptions about...
45. ### B Information transfer using entanglement?

I split this off a separate thread in response to a post of @Nugatory . The matter was about the (im)possibility of transfering information using entanglement. This is a basic thread, so I keep it simple: There are two particles/detectors, A and B. The particles are in the singlet state. My...
46. ### I Does spatial entanglement reduce total classical entropy?

In classical mechanics, if a system consisting of one particle suddenly became two particles, the entropy of the system would increase because the number of spatial degrees of freedom would double. But, in QM, I believe, when one particle decays into two particles, the two new particles would be...
47. ### I Quantum entangled image sensor for space camera-telescope

Could this be a possibility at some point? Since entanglement is not affected by distance, could we send cameras out to extremely distant places and get instantaneous signals? Only the image sensor would have to be entangled. It would still take the same amount of time as usual to get the camera...
48. ### I Two particle interference

If one beam from SPDC is sent to a double-slit or similar experiment, it can form interference if the which-way information is erased in the other beam. To detect it, photons have to be counted in coincidence because there will be both interference and "anti-interference" that add up to a...
49. ### Insights Entanglement Entropy – Part 2: Quantum Field Theory - Comments

ShayanJ submitted a new PF Insights post Entanglement Entropy – Part 2: Quantum Field Theory Continue reading the Original PF Insights Post.
50. ### I Satellite based entanglement over 1200 km

This weeks Science frontpage article: Source: http://science.sciencemag.org/content/356/6343/1140