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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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Symmetric Quantum Discord

Let's define the symmetric quantum discord in the following way: $$D(A:B):=\frac{1}{2}(D(A|B)+D(B|A))$$ where $D(A|B)$ is the usual quantum discord given as $$D(A|B) = I(A:B) - \mathcal{J}(A|B)$$ ...
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Is causality a universal law? [on hold]

Sorry, I'm not very confident on my understanding of anything at a quantum level and superpositions have me wondering....
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A unitary that induces Entanglement Breaking Channel for any ancillary state

Let's say there is a unitary operator $M:\mathcal{H}_1\otimes\mathcal{H}_2\to\mathcal{H}_A\otimes\mathcal{H}_B$ such that the channel $\Phi$ given as $$\Phi(\rho)=\text{Tr}_BM(\rho\otimes\phi)M^\...
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Why is the entropy of a subsystem density matrix equal to that of the other subsystem?

If we have some system $\rho_{AB}$ we can find the entanglement entropy as $S(\rho_A)=S(\text{Tr}_B(\rho_{AB}))=S(\text{Tr}_A(\rho_{AB}))$ But why are the entropies of the subsystems $\rho_A$ and $\...
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Example of mixd entangled state [duplicate]

I'm looking for specific examples of all combinations of pure, mixed, separable and entangled states, but I would prefer if they all have some physical significance. For pure states, the separable ...
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If photons have no mass, why do they gain mass in photon entanglement?

Photons are massless particles. However, this article states that photons can gain mass when they become entangled. How can this happen? From the article: Physicists create new form of light ...
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Is this entanglement of formation identity true?

Let a tripartite system be given with pure state $\rho_{ABC}$. My adviser said that the entanglement of formation satisfies the identity $${E_F}_{A(BC)}^2={E_F}_{AB}^2+{E_F}_{AC}^2$$ Where ${E_F}_{...
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Entangled partical amount for binary communication [on hold]

i now understand that the measurement of one particle only reveals the state of the other and breaks the wave function. so my Q has changed to - is it possible to flip the spin of an entangled ...
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Is long range entanglement detectable with k-local operators?

Given a state $\psi \in H_1\otimes H_2\otimes ... H_n$, and there is long range entanglement, is it possible to certify this by only using k-local operators where $k < n$? To make it concrete but ...
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Conservation of quantum information in mixed states quantum secret sharing (QSS) schemes?

Consider a $((3,5))$ pure state quantum secret sharing (QSS) scheme. For instance this paper: arXiv:quant-ph/9901025. If I divide any 5 shares to two sets then allways one of those two set are ...
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How do we know Entanglement is real? [closed]

I am aware of the Bell experiment that is supposed to demonstrate that entanglement exists. The only differences between photons, that I am aware of, are frequency and energy. So, in the Bell ...
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Wave function collapse of entangled photons?

If we have two entangled photons then they are described by the same wave function. But if I measured the position of the signal photon and it collapses into a position, would the idler photons ...
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Path entanglement

Two particles can be polarization entangled. When one measures the first one to be horizontal. the other one is perpendicular. But how are two particles path - entangled?
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Gear Theory — Entanglement — Spooky action at a distance explained? [duplicate]

Nothing can travel faster than the speed of light. Not even an electrical signal. Entanglement seems to violate this. But, does it? Imagine 100 gears lined up in sequence all the way across a ...
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In $e^+e^-\to e^+e^-$ scattering is the final outgoing state with hard and soft particles together known?

In $e^+e^-\to e^+e^-$ scattering there is a production of low energy photons (soft photons). This seems to be connected with infrared divergences. One way to deal with this seems to be the so-called "...
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Tensor Network from Lattice

I read an article about tensor networks and they seem very interesting and a promising approach to studying the relations between entanglement, gravity and quantum information. How does this network ...
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How to bound the dimension of infinite dimensional Hilbert space?

Lets say I have a density matrix $\rho$ encoded into a physical system. Lets say I can have access to as many copy of $\rho$ as I want. I perform hmodyne detection on the copies in phase and out of ...
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No-S-$B^+$ Theorem like Results

Classification of multipartite quantum state is an interesting topic in quantum information and there have been many accomplishments in the field. For example, according to the result of Thapliyal, ...
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Does quantum forbid duplication or copy of states?

Suppose a beam of particles or a single particle, or a single particle. Is it possible to make a duplication of the beam or a single particle without collapse the states of the original particles? I ...
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What is some good reference for tensor network?

I am a physicist who is now doing some research in quantum information, which might involve tensor network.
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What's the tensor network representation for local ground state?

It's well known that many topological phases can be represented using matrix product state and PEPS. Example, toric code, $H=-\sum_{v}A_v-\sum_p B_p $. My question is: What's the tensor network ...
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Using entanglement for communication based on the double slit experiment [duplicate]

When we perform a double slit experiment we receive interference pattern. when we measure in a specific slit the exact location of the photon (or electron) the interference pattern disappears and we ...
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Recognizing Entanglement from the Density Matrix in 2-qubit case

We know that the density matrix for a 2-qubit system can be written in the Pauli representation as : $$\rho = \frac{1}{4}\sum_{ij}t_{ij}\sigma_i\otimes\sigma_j$$ where $\sigma_i$ are the Pauli ...
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State of Matrix Product States

What is a good summary of the results about the correspondence between matrix product states (MPS) or projected entangled pair states (PEPS) and the ground states of local Hamiltonians? Specifically, ...
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1answer
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Differentiating between Tensor Networks

I am trying to study tensor networks and their application to quantum phase transitions. However, I had a question concerning the connection between the projected entangled-pair states (PEPS) and the ...
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Question about general direction of development of application of Tensor Networks to AdS/CFT

I am quite interested in the area of application of Tensor Networks(TNs) to AdS/CFT correspondence. I would like to clarify certain points in order to get better general picture. What is the main ...
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Relationship between the 'relevant states' and the states that can be generated by a quantum computer?

In the huge quantum state space, for example n-qubit space has a state number as $2^{2^n}$, usually we are interested in two different kinds of 'physical' states. Set 1: The relevant state which is ...
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Simple analytic examples of Multi-scale Entanglement Renormalization Ansatz (MERA)

I want to understand Multi-scale Entanglement Renormalization Ansatz (MERA) with very elementary examples. So far I could find references which are mostly based on numerics. It would be a great help ...
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1answer
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Is ER= EPR a Hidden Variable Theory & If NOT Why?

John Bell showed that entanglement cannot be a local hidden variable theory in 1964. Leonard Susskind and Juan Malcadena have proposed that non-traversable wormholes, Einstein-Rosen bridges (ER) are ...
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Do particles remain entangled even after repeated measurements in different directions?

We know that if we measure the spin of electron along X and it turns out to be up and then we measure along Y and then again along X, the probability of spin being up along X is 50% and down is 50% ...
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1answer
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A telegram from Mars

Say, people fly to Mars, get in trouble, and want to send us a SOS signal. The average distance is 225 Gm, so they must wait 25 minutes to hear our response at the speed of light. This may just be too ...
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1answer
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Coincidence count probability in the entanglement-enabled delayed choice experiment

I'm trying to understand the entanglement-enabled delayed choice experiment and I'm kind of stuck at the term "coincidence count probability" which I can't seem to find definition of. Their full set ...
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What are non-local hidden variables?

It is said that Bell's Inequality basically denies all possible local hidden variables theories as solutions to entanglement but what does a non-local hidden variable theory mean and how does it get ...
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Is the eavesdropper detected by chance in QKD?

So I was starting to familiarise myself with QKD when I came across this video on YouTube breaking down the process (I believe this is the E91 protocol but I could be mistaken) using an animation that ...
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Electron-Photon entanglement

Are the electron and photon emerging from a Compton scatter entangled? If so, what experiment can be done on one that will give information about the other?
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Is Time emergent from quantum entanglement?

This paper shows that 2 photons in a toy universe (isolated box) are seen as static to an outside observer unless interacted with/entangled with. It states that it shows that time emerges from ...
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Can superconductivity be understood to be a result of quantum entanglement?

I've been trying to find out what causes superconductivity. I have seen a lot about what characterizes it (e.g the Meissner effect). But I haven't found much on the root cause. The closest I have ...
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Under what conditions does a beam splitter entangle two input photons?

There is a dispute on PhysicsForums related to what are the conditions necessary for two photons to be entangled by a beam splitter. Lots of references given by the forum users but they never arrive ...
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Shoot entanglet photons into black hole [duplicate]

What would happen if you shoot entanglet photons into a black hole, one falling in and another to a detector. Seems, doing so would in principle allow measurements of the horizon and what's inside. ...
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Conditions for a separable state to be a CPB-state

A bipartite quantum state $\rho$ is called a CPB(Complete Product Basis)-state if its eigenvectors can be expressed as product state (a pure state of the form of $|\phi\rangle \otimes |\psi\rangle$). (...
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Are there Bell test experiments that don't require coincidence monitor?

CHSH (2-channel) for instance doesn't count for a case where all 4 channels didn't detect anything (simultaneously) at some point in time. As I understand, such all-absence shouldn't happen/matter at ...
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Does time emerge from entanglement?

I stumbled upon the hypothesis that time emerges from quantum mechanics as long as the observer is part of the system. That is, the evolution of an entangled state can serve as a clock for an observer ...
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How do we know that entanglement allows measurement to instantly change the other particle's state? [duplicate]

I have never found experimental evidence that measuring one entangled particle causes the state of the other entangled particle to change, rather than just being revealed. Using the spin up spin down ...
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Can Matrix Product States be used to describe ergodic states?

The eigenstates in the ergodic phase of the disordered interacting quantum many-body system follow a volume law of entanglement. Since MPS follows area law, can we use any MPS based algorithm (ITEBD ...
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BB84 protocol vs. E91 protocol

What is the pros and cons of E91 protocol and BB84 protocol? Which one is faster than the other, and which one is more safe?
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1answer
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Are photons in laser entangled?

When energy is pumped into the atom, photon is being emitted which would then get absorbed by another already excited atom causing the atom to emits 2 similar photon. Soon more photons are produced in ...
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Can you entangle 2 different kind of particles? [duplicate]

Usually we see people attempt to entangle photon with another photon, electron with another electron and atom with another similar atom, I'm wondering could 2 different particle, atom or molecules be ...
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How do we know that two quantum states are entangled?

When two particles are entangled, is there a way of determining that a particle is entangled if one does not have access to both particles?
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Measuring electron spin with multiple Stern–Gerlach apparatus at an angle

To separate the two spins of electron of a particular direction, we use Stern-Gerlach apparatuses to apply a non-uniform Magnetic field. Suppose we have two identical electrons and we know their spin ...
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Why does my thought experiment involving entanglement appear to violate the speed of light?

I have a thought experiment that "seems" to allow messages passed at faster than the speed of light. Since that is impossible I'd like to learn what is wrong with my experiment/reasoning. Start with ...