What is Entangled particles: Definition and 62 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. cianfa72

    I EPR in Bohm formulation

    Hi, I was reading about the EPR paradox in Bohm simplified formulation. From my understanding the paradox is that Bob is actually able to get a value for the positron's spin along both the ##z## and ##x## axes. Since electron and positron are entangled, he get the value of spin along ##z##...
  2. H

    A Is entanglement also for time-like intervals?

    Hi Pfs, I read that entanglement is related to a set of paticles that interacted in the past. When Bob and Alice share two entangled particles there is a space like interval between their measurements. I wonder if there is also time like entanglements. suppose that Bob receive his particle a t...
  3. H

    A Entangled particles in curved spacetime

    i do not know if the question about entangled particles has found mainstream answers; Suppose that pairs of maximally entangled particles are shared by Bob and Alice in a time independant gravitational field. Bob measures the spin in the direction of far fixed stars. There is a direction in...
  4. BillTre

    B Understanding Entangled Particles in Different Time Frames

    Here is something I don't understand which I expect someone here can explain. If one member of an entangled pair goes on a trip at relativistic speeds, there will be two different frames of observation, with two different elapsed times. The time frames can get off-set by years, over a long...
  5. nomadreid

    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...
  6. Jerome71

    B Do entangled particles always react in an anti-symmetric way?

    I am a non scientist and nonetheless a have the following question: once particles are (intrinsically) entangled do they always react in the opposite way/ anti-symetric to each other? Source https://www.physicsforums.com/forums/quantum-physics.62/post-thread
  7. C

    I Are entangled particles really separated?

    I would greatly appreciate answers or perspective on these questions, thank you in advance! Since entangled photons are traveling at the speed of light relative to our spacetime, my understanding is that they experience no time or distance from the moment they are generated to the moment they...
  8. R

    I After entanglement swapping, how can we verify the new entanglement?

    Suppose photons 1 and 2 have entangled spins. And, so do 3 and 4. A Bell-state measurement (BSM) is performed on photons 2 and 3 to cause entanglement swapping so 1 and 4 are now entangled. This is done many times creating many 1 and 4 pairs. Suppose the original 1s and 2s were prepared such...
  9. H

    I Information diagram for entangled particles?

    Hi Pf I read that when two particles are maximally entangled , all the information is in the correlations between the particles. If we need 1 bit to describe one particles. Are two bits in the correlations? three particles may be entangled and then we need to considere 3 bits. As entanglement...
  10. R

    I Measuring Entangled Particles in two different Basis

    Consider two entangled spin half particles given by the generic form of Bell Equation in Z-axis: ##\psi = (a\uparrow \uparrow + b\downarrow \downarrow)## where ##a^2+b^2=1## In a (2D) planer rotated (by an angle ##\theta##) direction the new equation can be given by: ##|\psi \rangle =...
  11. R

    I Measurement of an entangled Particles in two Different Bases

    Consider two entangled spin half particles given by the generic form of Bell Equation in Z-axis: ##\psi = (a\uparrow \uparrow + b\downarrow \downarrow)## where ##a^2+b^2=1## In a (2D) planer rotated (by an angle ##\theta##) direction the new equation can be given by: ##|\psi \rangle =...
  12. CarawayBlossom

    I Measurement of Entangled Particles causes up or down spin?

    In reading around, it seems that in the case of entangled particles, it is the measurement of one of the particles that causes the other one to be it's opposite spin and that there's some means of info transfer going on caused by the measurement. I'm not understanding why it would not be that...
  13. Souma

    I The "size" of entangled particles

    Hello everyone, I hope you are having a nice day, I was reading [The principles of Quantum Mechanics by P.A.M Dirac], and I was attracted to the definition of size. The book says: "If the object under observation is such that the unavoidable limiting disturbance is negligible, then the object...
  14. jackmiller2003

    I Time Delay Information Transfer with Entangled Particles

    Hello, my name is Jack and I'm a year 11 student in Australia. After listening to, and reading some information regarding quantum entanglement, I'm still a little unsure about the solution to a thought experiment: Let's say that I create a situation in which multiple pairs of particles are...
  15. B

    B Can quantum entangled particles be split and sent into a black hole

    I am new here so apologies in advance. When a virtual particle and anti particle appear at the event horizon of a black hole, before they destroy each other, they are split with one being sucked into the black hole and the other becoming exhaust. Are these the same particles as the quantum...
  16. F

    I Space and time dependence of entangled particles

    It seems that the entanglement of two particles does not change with time and can cross long distanced as long an neither particle decoheres with the environment. This makes me wonder if the wave function for that entanglement can have any time or space dependence? I only did a brief search for...
  17. Quarky nerd

    B Entangled electrons in the Stern-Gerlach experiment?

    I have always been interested in entagled electrons. so I thought about the stern gerlack experiment and simly wondered what would happen to entagled electrons in such an experiment. (although the prefix says high school i am able to appreciate more complicated answers).
  18. Danny Boy

    A Equation in a paper about Dicke states

    Can anyone with basic knowledge of Dicke States assist with explaining how we arrive at equation (4) in the paper 'Entanglement detection in the vicinity of arbitrary Dicke states': <Moderator's note: link fixed> $$\langle J^2_{x} \rangle_{\mu} = \sum_{i_1,i_2} \langle J_{xi_{_1}} \rangle_{\mu}...
  19. L

    I Generalizing the Bell Inequality for Arbitrary Measurement Axes

    EDIT: I realize now that I have fundamentally misunderstood a crucial aspect of deriving the Bell inequality for this case which is the existence of the third axis. The setup of the problem did state that the axes were chosen at random. Therefore I can't just look at the possibility of choosing...
  20. X

    B Measuring entangled particles along several angles

    Heya, I was wondering what happens when you measure the spin of particles along several different angles on an entangled pair. As far as I understand Bell's theorem, it basically says that if you first measure particle A in some angle, and then measure a particle B at some some angle, you'll...
  21. N

    B Entanglement: How Does it Work and Its Implications in Everyday Life?

    I don't have much of a background in quantum physics so be patient with my questions please. Basically I want to know how does entanglement actually work? Is information being transferred faster than we can detect it or is there some invisible link between particles that causes the phenomenon we...
  22. T

    B Understanding Quantum Entanglement: Debunking Common Misconceptions

    I have some questions about quantum entanglement 1 - Why is this phenomenon considered so surprising, if it just states conservation of quantities (e.g. spin, momentum) and we are already accostumed with conservation of quantities in classical physics 2 - Suppose we have, say, a hydrogen...
  23. L

    B Entangled particles and lorentz contraction?

    can lorentz contraction be measured via quantum entanglement with one of the entangled particles moving near the speed of light? would the particle in motion be affected by lorentz contraction? if so, would the particle at rest follow suit and appear affected?
  24. M

    I Energy associated with entangled particles

    Assuming two particles are entangled, is there a quantifiable energy associated with separation distance? Rephrasing the question: If two entangled particles are distance x1 apart and another pair of identical entangled particles are distance x2 apart, is there a difference in the energy...
  25. A. Neumaier

    I Entangled particles in the thermal interpretation

    Ah. It was skepticism about publishing, not about giving a comprehensive account! Yes, I am preparing a book on quantum mechanics, which will contain an account of the thermal interpretation - but primarily to macroscopic, nonrelativistic reality, where it is obvious that it gives the correct...
  26. K

    I What's so strange about entangled particles?

    In a thought experiment, there is a spin-0 source emitting particles. (I suppose you already know what is that experiment.) Two observers in opposite sides along the same axis measure opposite spin components. If one observer measure, say, spin up, then the second observer will certainly measure...
  27. entropy1

    I The mechanism of entangled particles

    First let me ask this: Consider a pair of entangled photons fired at a respective detector after passing respective polarisation filters. If a photon passes a polarisation filter, is it in a superposition of having passed and not having passed? Is the measuring device (that detects the...
  28. F

    B Entangled Particles: Understanding the Interaction

    Pardon me for asking a very simple question, but this is something that I'm confused about. If we have a pair of entangled particles, and we measure the state of one of the particles, then the state of the other particle becomes fixed instantaneously. But what if we have three entangled...
  29. entropy1

    B Number of entangled particles in nature

    I understand that we can create entangled particles in the lab. But how many (non-locally) entangled particles (such as photons/polarisation or electrons/spin) exist in free nature?
  30. F

    I Do entangled particles exist in superposition?

    Perhaps someone can show me some simple math showing two states in superposition and entanglement so I can see how entanglement relates to superposition. Thanks.
  31. sciencejournalist00

    The phase between 2 waves describing 2 entangled particles?

    Each particle has a wave associated to it according to the principle of wave-particle duality. Between two waves there is a phase difference. What is this phase difference in the case of entangled particles? 0 degrees? 90 degrees? 180 degrees? Somewhere in between?
  32. newjerseyrunner

    Time frame of entangled particles

    I was thinking about entanglement today and found myself struggling to envision entanglement and relativity working together. If I send entangled electrons out in opposite directions, their states are not determined until one of them is observed. I know that. What if one of them passes...
  33. TotalNovice

    B Speed of interaction between entangled particles

    When the spin of an entanglement particle is established, it is said that the corresponding spin of its entangled twin is revealed immediately, (via wave collapse?), and that this interaction can occur across a substantial distance. It has also been said that this immediate interaction can occur...
  34. R

    Using GHz multi-particle entanglement for FTL communication

    Although there are numerous questions about entanglement and FTL communication, I can't find anything directly related to this. I proposed a method for FTL communication via GHZ multi-particle entanglement at...
  35. X

    Do entangled particles decay simultaneously?

    I apologise in advance for my rather minimal knowledge of physics. Please assume that anything I write below is just my current understanding, and may very well be incorrect... Elementary particles decay into other elementary particles. When a a subatomic particle decays into a pair of other...
  36. K

    Entangled Particles and the Hadron Collider?

    If one was to entangle two particles and either send the two particles at each other, or send one of the entangled particles and observe the other; what do you think might happen? If there are any problems with getting an entangled particle into the Hadron Collider, please say so.
  37. R

    Two entangled particles One on Earth, the other on a space ship

    First off I'd like to say my understanding of physics isn't very good, so I'm sorry if this question is a non sequitur. Suppose we have two entangled particles, one on Earth, and the other on a ship that is traveling away from Earth at near to light speed. According to quantum...
  38. Ookke

    Multiple entangled particles and no-communication

    Does the quantum no-communication theorem apply to multiple particle entanglement as well? To illustrate the idea, suppose we had particles A, B and C with entangled spin. We measure A and B locally using, say, detector angle 0 for particle A and angle 45 for particle B. Particle C is measured...
  39. L

    Entangled Particles: Can They Be Distinguished?

    Dear Learned Physicists, can anyone educate me on: is it possible to distinguish entangled particles before and after interaction in theory? what I'm trying to find out is, if two entangled particles can be separated and one interacted with to produce a result at the other end, such...
  40. M

    Entangled particles inside a black-hole

    Would it be possible to study black-holes just by entangling particles and sending some of them right into a black-hole?
  41. craigi

    Measurement of entangled particles causes dechorence at a distance?

    If we entangle particles and separate them by a large distance, can the action of measuring one cause decoherence at the other's location? If yes then does this violate relativisitic causality? Could we not use this process to transmit information instantaneously? If no then why not? Is there...
  42. F

    Can you ever have an odd number of entangled particles?

    This should be a very easy question to answer, but oddly enough I'm not certain what that answer is. So I thought that someone here could help me. It's easy enough to visualize and understand a pair of entangled particles, and how measuring the state of one instantly effects the state of the...
  43. L

    Understanding Entangled Particles: Is it Possible?

    I am trying to understand how it may be possible to maintain two particles in an entangled superposition. Is this possible or has this already been done?
  44. K

    Manipulating spin of entangled particles

    Hello all! I'm trying to wrap my head around this quantum entanglement thingy. As far as I have understood, if you have two entangled particles, say two electrones, it is impossible to predict their spins since they are thought to be in superposition of both up and down. But if you measure...
  45. S

    Why do entangled particles have to be in an indeterminate state?

    Why do (we assume that) entangled particles (say photons/electrons) have to be in an indeterminate state?
  46. P

    Proof of Entangled Particles being Indetermined

    I'm sure this has been asked a million times, but how do we know that entangled particles don't "choose" their states when they're separated? If possible, choose the most basic example. Thanks!
  47. D

    Entangled particles and relativity

    What happens when you take two entangled particles and put one on a spaceship that moves close to the speed of light? Now you measure one of the two particles. The state of the other should become decided at the exact moment you do the measurement. But there is no absolute simultaneity if you...
  48. M

    Simultaneously Measuring Entangled Particles

    Let's say you have entangled particles emitted from a source. What happens if you measure the spin of both entangled particles simultaneously with respect to the reference frame of the source? Would the spins measured still be opposite (anti-correlated)? Would you even get a definite...
  49. S

    How are entangled particles separated/distinguished?

    Entangled particles are used in various experiments for example: DCQE (delayed choice quantum eraser) Bells tests Mach Zehnder (modified) DCES (delayed choice entanglement swapping - Ma) Now, one out of a trillion photons, on average, gets entangled after passing through the SPDC (?)...
  50. W

    How are entangled particles captured and contained?

    I'll admit I have a very small understanding of this phenomena in general. I read once that entangled particles are emitted as pi mesons decay. What I am unclear about is how do scientists know when to be ready to capture them and how do they contain them? Any help would be much appreciated.