What is Entanglement: Definition and 868 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.
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...
Hi
I hope that you will humour me as I try, in layman’s terms to explain a question I have.
I recently watched BBC4’s excellent program about quantum entanglement and it got me thinking.
I am aware that there are currently experiments ongoing to prove that quantum entanglement is real and...
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)...
Afshar's experiment (https://en.wikipedia.org/wiki/Afshar_experiment) sent a light beam through a double-slit to get interference, put wires in the places where there were dark fringes, then refocused the light with a lens get "image" the two slits. I'm wondering what entanglements would break...
Reading about the HOM effect, two photons come to a half-silvered mirror from opposite sides but they both end up going the same way - one always reflects while the other transmits. Which path they take is random but they both take the same path. Is the path they end up on initially a...
Dear community,
I frequently have problems imagining the EPR paradoxon, hence how I can imagine entangled states.
Let's say we have the Bell state of the basis /0> and /1>: Ψ = 1/√2 (\00>+\11>), where the first element of the ket belong to particle/qubit 1 and the second element to...
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...
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...
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...
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...
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...
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?
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...
I should first acknowledge 2 important points. I don't read papers on QFT, and therefore barely know how to spell it. And second, although I read many papers on entanglement (theory and experiment) I don't know if I have ever seen much reference to anything I might label QFT (that being...
1. An elementary particle like a photon or electron can be measured in 2 possible states - spin up or down in electron, vertical or horizontal in photons. We'll call those states state 1 or 2, and the measuring device state A or B.
If for example, we measure a photon with a polarizer in State...
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...
Why can't absolute time be used to describe events? Previously I tried to describe entanglement collapse on this forum in terms of absolute time, but I was told more or less this was not valid. I don't understand why.
If the proper time that we use is based on the fact that the speed of light...
As far as I can tell nobody actually knows how quantum entanglement really works, and yet everyone assumes that quantum states are teleported faster than light. But what if there isn't any actual teleportation, because both sides still share the same position? Maybe in a dimension we cannot...
This thread is to look at the notion of wave function collapse and relativity of simultaneity. The other thread I started on QFT has helped to clarify a lot, so hopefully this one can do the same.
I may have this all wrong, but I will outline my question and hopefully someone can point out...
A photo:
https://petapixel.com/assets/uploads/2019/07/quantum.jpg
The popular press version (with above photo):
https://petapixel.com/2019/07/13/this-is-the-first-ever-photo-of-quantum-entanglement/
The full paper in Science Magazine:
https://advances.sciencemag.org/content/5/7/eaaw2563
Is Quantum information theory the only theory that handles multi partite entanglement? I have been digging through the net but the only papers I have come across are quantum information theory based.
Though I don't have an issue with QIT, I would prefer to study examinations done with...
The problem with this is that only one push's worth of energy was expended. One push's worth of input cannot produce two push's worth of output, for this would violate the law that says you can't get more movement out of something than the amount of force you exert onto it (to put is very...
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...
Suppose I "produce" positron-electron pairs one at a time (by produce I mean in plain language the inverse of annihilation). Next I measure the vector of the magnetic moment of each positron. Is each pair that is produced entangled? Without measuring the electrons, can I now be much more certain...
Which of these premises is impossible or incorrect according to our current understanding of quantum entanglement?
Given 2 entangled particles, p1 and p2:
Observing paired particle p1 induces a change in spin on paired particle p2.
There a way of detecting a change in spin on particle p2...
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...
I'm happy to admit that I am struggling to digest this recent arXiv paper - https://arxiv.org/abs/1904.11974 - but I am interested in whether its reasoning and provisional conclusion seems insightful or merely interesting.
My summary of their work is that the researchers consider colour...
I'm finishing my master studies in Theoretical Physics and my thesis is being about entanglement measures. I've read several papers in which all kind of properties that one would desire to have in a good entanglement measure are exhibit. One of these properties that one would consider...
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...
I should clarify, I am a linux administrator by trade and I have no physics or scientific background outside of working at a medical institute, So if my ideas are off and seem wild and misguided, please forgive my ignorance, and while you are at it be grateful for a fresh perspective. (joke) So...
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...
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...
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...
Hello all, I have read that these two properties are complimentary, similar, at least in a sense, to uncertainty between momentum and position. John Cramer used what he referred to as a Sagnac Source to vary the balance between entanglement and coherence. What, physically, has to be done to...
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...
I am re-reading paper(s) by Huw Price and Ken Wharton that present an interesting explanation of entanglement. I will see if I can paste a link, otherwise one can find them on arXiv: one is for example, “Disentangling the Quantum World”. The idea is that there could be retro-casual communication...
I have a question about quantum entanglement, I see saying that it would not be possible to communicate faster than light using this technique, but for example, and I understand that it is not the information that travels, however if for example we use the interval of the measurement and read as...
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...
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...
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:
\begin{equation}...
Hi,
Suppose I had a pair of protons that are entangled, and one or both of those protons decayed( https://en.wikipedia.org/wiki/Proton_decay ) over time, does that mean the entanglement between the protons does not exist anymore, and would it be fair to say that the entanglement decayed over time?
Homework Statement
Suppose two polarization-entangled photons A and B in the following Bell state:
\begin{equation}
\Phi=\frac{1}{\sqrt{2}}\bigl(\left|H_{A},H_{B}\right\rangle + \left| V_{A},V_{B}\right\rangle\bigr)
\end{equation}
1. What is the state if the photon A passes through a...
"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...
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...
OK, going to ask a question that I sort of know is going to be shot down but at the moment I can't make sense of this.
If I send a machine/robot with a particle that is quantum entangled with another particle that is left on earth. When one particle is blue the other is red. The machine also...
Many people talking about there are similarities and common positions in quantum entaglement and superposition with entropy. I need to know about this phenomenon
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...