Questions tagged [quantum-entanglement]

Quantum entanglement is the mechanism by which quantum correlations between two sub-systems survive even after being physically separated from an interaction region. The correlations could in principle survive without neither time nor space constraint.

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Why would the classical (local) correlation in Bell's experiment rule out the cosine of the angle?

This question expands on a specific detail of this previous question: Why would classical correlation in Bell's experiment be a linear function of angle? I would have commented on/answered that ...
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Is entanglement a bulk property or surface property?

The degree to measure entanglement in a many-body system is entanglement entropy. There are different measures for entanglement entropy, for example, Von Neumann entanglement entropy and Renyi ...
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How can you (computationally) calculate the halfchain entanglement entropy of a spinchain?

I am simulating a (small) spinchain with exact diagonalization and dynamics. I would like to track the entanglement entropy of half the chain with the other part of the chain. I have the vectors of ...
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Are entanglement witness operators together with delayed entanglement swapping a violation of the non-communication theorem?

An entanglement witness operator lets you measure a system (let us say two particles) and determine if it was entangled or not. A delayed entanglement swapping allows to delay the decision to entangle ...
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Entanglement and cosmological eras

I'm checking this paper: https://arxiv.org/abs/1405.7834 In the context of quantum cosmology, they assume that the universe evolves in an entangled state between the two epochs considered, i.e.$$\...
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Generalised basis independent relationship between Von Neumann Entropy and Purity of a quantum state [duplicate]

I have been trying to find a generalized relationship between Von Neumann Entropy and Purity of a quantum state? In this post there is a discussion of a specific case for a qubit and a more general ...
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What's quantum about quantum discord?

Quantum discord is a quantity that relates two subsystems of a quantum state. It reduces to the entanglement entropy for pure states, but it differs for mixed states; separable states have zero ...
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Are the two interference patterns at the output of an interferometer quantum entangled?

When an interferometer creates an interference pattern a second interference pattern is always created. The two patterns are opposites of each other. Knowing one pattern lets me accurately predict the ...
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Schmidt decomposition for Matrix Product State in canonical form

During a lecture on comp. Quantum Phyiscs the concept of Matrix Product States (MPS) (with open boundry conditions) was introduced in the form $$\vert\psi\rangle = \sum_{i_1,\dots,i_n}\psi_{i_1\dots ...
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Plus and minus in the four bell states [closed]

I'm currently working on a project about Bell's inequality and have done an experiment using photons to debunk the hidden variable theory. However, I'm having trouble understanding the four Bell ...
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In Bell's inequalities, how is realism violated, and how can one preserve locality?

I have heard the violation of Bell's inequalities asserts either realism or locality needs to be abandoned. I understand the notion of locality being violated but can someone explain how realism is ...
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Is there a class of Bell inequalities that are violated by certain mixtures of known orthogonal entangled states?

Is there a class of Bell inequalities that are violated by certain mixtures of known orthogonal entangled states? There exist a multitude of classes of Bell inequalities that can be used in ...
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Do distillable states necessarily violate the CHSH inequality?

Let $\rho$ be a bipartite quantum state. If $\rho$ is distillable will it necessarily violate the CHSH inequality ($|\langle A_0(B_0+B_1)+A_1(B_0-B_1)\rangle| \leq 2$)?
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What is the effect of magnetic fields applied to an entangled pair of particles?

Just curious...if we can create a pair of entangled particles and position one of them in a strong magnetic field would it have as a consequence that we can't change the spin of the other because that ...
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Would 'seeding' two individual black holes with entangled particles make those two black holes become entangled?

As I understand it from watching some Susskind lectures, if you create two black holes from the same 'pool' of entangled particles, the result can be described by ER and EPR to be a pair of entangled ...
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If all energy in the universe originally occupied the same point in space [closed]

How can it not be entangled? What function of physics can allow everything that comes after to be completely separate? Do we have any known laws of physics that can make such a definition? Thank you ...
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Do we know when a particle has a tangled pair?

By looking at a particle alone, can we tell if it has a tangled pair somewhere in the universe? Moreover when measuring the spin of a electron, we can't predict it, unless we know the spin value of ...
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Find the normalized state and function such that the equation holds for arbitrary unitary matrix [closed]

I have been recently puzzled with a problem I do not know how to solve. Here is the setting and some of my thoughts on the problem. Given: Let us denote the set of all unitary $d \times d$ matrices ...
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Delayed choice quantum eraser for communication [closed]

I know entanglement communication is kind of the "perpetum mobile of quantum mechanics" but I still can't give it up. Here's a setup: Make 10000 entangled photon pairs at T0 and let them loop in ...
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An electron in time

I've read many times now that when one measures the spin of an entangled electron (for example) the state of its partner is known instantaneously and this is true across any theoretical distance. ...
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Faster than light communication via entanglement? [closed]

Let's say site S and site S' have a very large number N of pairs of entangled particles: $(p_1 p_1') (p_2 p_2') \ldots (p_N p_N')$. so particles $p_1 p_2 \ldots p_N$ are located at site S and ...
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Is the speed of quantum entanglement interaction greather than $c$? [duplicate]

How much time should not elapse between measurements of e- and e+ spin computed at a distance ot 1km to not detect effects of the entanglement? There should be a finite speed? Maybe greater than $c$?...
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Are there any natural phenomena that use quantum entanglement

I am aware that there are a growing number of natural phenomena that can now be understood as using quantum effects - from photosynthesis to migration navigation of some birds. Is there any known ...
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Prove that any state can be generated from a maximally entangled state via LOCC

$\newcommand{\ket}[1]{|#1\rangle}$ I have proved that in a Hilbert space $\mathcal{H}=\mathbb{C}^2\otimes\mathbb{C}^2$, starting from the Bell state $\ket{\Phi^+}=\frac{\ket{00}+\ket{11}}{\sqrt{2}}$ ...
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Can this setup produce a no interference pattern from entangled photons from a BBO crystal?

Related question from physics exchange where the pictures were taken: From the delayed choice eraser paper from Kim et al. 1999 My question is probably quite simple. I want to know if changing the ...
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EPR related question

Suppose two entangled electrons are emitted from a source. After traveling a long distance in opposite directions, each electron passes through separate but otherwise identical double slits. Will both ...
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How does the superposition of states come up in Bohmian Mechanics?

In Bohmian Mechanics, it assumes a universal wave field in which particle's motion can be calculated using Newton's law of motion: \begin{equation} m\frac{d^{2}x}{dt^{2}} = - \nabla(V+U) \end{...
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Entanglement between what?

According to the standard definition of "Entropy of Entanglement" https://en.wikipedia.org/wiki/Entropy_of_entanglement one starts from the density matrix of a pure state $$ \rho=|\psi\rangle\...
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Are individual socks from a pair a good metaphor for entangled particles? [duplicate]

I’m trying to come up with a good way to explain quantum entanglement to myself, I came up with a metaphor. Say that one person takes a pair of socks and two boxes. They put one sock in one box, and ...
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Is it possible to classify “branches” in the MWI of QM? [closed]

In Everett's Many Worlds Interpretation of QM, what would be classified as a "world" or a "branch"? Would the "reality" of what I'm consciously experiencing be distinguishable from a separate "...
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What is actually being entangled?

I read that even observation can result in entanglement so I assume it is true for any interaction but I like to know what physical properties can be entangled beside spin states? Can we entangle ...
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Why does a flat entanglement spectrum contradict to the holographic CFT vacuum?

As stated in the introduction of 1806.05007 (https://arxiv.org/pdf/1806.05007.pdf), a certain tensor network called the holographic code has a flat entanglement spectrum (ES), i.e. the reduced density ...
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Do quantum logic gates have propagation delay like classic logic gates?

Classic logic gates that are usually constructed using conventional semiconductor technology are subject to propagation delay, which means it takes some time for the output of a gate to reflect on its ...
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Does an electron spin change cause an entangled electron to change simultaneously with reference to us? [duplicate]

If two electrons are entangled, and suddenly the spin of one of them changes, does this affect the other electron to change spin simultaneously WITH REFERENCE TO US?
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Connection between spin angular momentum of a photon and circular polarization of light

Do photons have spin angular momentum only if they are part of a circularly-polarized beam? I suspect that every photon always has spin angular momentum, but in most cases they have a superposition ...
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Implications of MIP*=RE for physics?

Background Earlier this month (Jan 2020) a pre-print was posted to the arXiv claiming to have proved the equivalence of the complexity classes $\mathrm{MIP}^{*}$ and $\mathrm{RE}$ (see below for ...
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How do tensor products and direct sums fit into quantum mechanics?

I understand that at times tensor products or direct sums are taken between Hilbert spaces in quantum mechanics. I don't, however, know when this can be done or when it should be done. I would like ...
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Do humans entangle particles, or are they already entangled?

I recently visited our city hall and there was an exhibition which featured one of our local physicists, a John Stewart Bell. Although it was presented well the technical information was very basic. I ...
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Will an entangled system give different measurement then just correlation? [duplicate]

I have read this question: Correlation vs. entanglement for composite quantum system Entangled states can produce nonclassical correlations, but this is not necessarily the case. So far so good. ...
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Can a quantum computing advantage exist in a deterministic universe?

Is a deterministic universe with nonlocal hidden variables incompatible with the quantum superpositioning of states that is required for quantum computing?
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Is it possible in principle, to store (retrieve) information into (from ) a region of spacetime?

Following Van Raamsdonk, the emergence of spacetime is related to the entangling of field theory degrees of freedom. When we observe correlations in the electromagnetic and other fields, they are a ...
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Existence of ER bridge among entangled pairs

Recently I have read ER=EPR paper written by Leonard Susskind. It suggests "where there is the entanglement, and there exist spacetime geometrical connection between such pairs". But how one can ...
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Are entangled particles indistinguishable?

I have read this question: https://physics.stackexchange.com/a/462620/132371 where John Rennie says: When the two particles become entangled the means the total wavefunction Ψ can no longer be ...
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Thermalization in non-disordered systems

The eigenstate thermalization hypothesis explains the mechanism of the thermalization of generic many-body quantum systems. The presence of disorder, on the other hand, provides an elegant example of ...
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What happens to entangled particles when one of them is measured?

Imagine two entangled photons came into existence and one of them is being measured. We have altered one of the quantum states of the photon and caused its wavefunction to collapse and then we ...
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How practical is it to isolate two entangled qubits?

Starting with two qubits $|00>$, I can obtain a bell state by applying a Hadamard gate and then a CNOT gate [ref]. The two qubits in a bell state are entangled. It is a common part of giving the ...
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How to detect the entanglement of two qubits by means of a statistical tool?

Is there any robust statistical approach to help us in determining the entanglement of the two qubits based on the results of their so many times measurements? After having the set of measurements ...
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Does quantum entanglement imply a universal “now”? [duplicate]

If a particle on earth is entangled with a partner particle in Andromeda, and the wave function collapses here on earth, the wave function would, as I understand it, collapse in Andromeda too. Does ...
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Is it possible to have 2 entangled photons and only let one out of a box? [closed]

I need to know soon for an important project
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Can decoherence provide a loophole for FTL communication in a system of multiple entangled particles?

I am a layperson when it comes to the study of quantum mechanics and do not quite have the expertise to properly evaluate the questions I am going to post here. But here it goes. Suppose three ...

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