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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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$k$-local Hamiltonian with long range entangled ground states?

Is it possible, and if yes, is there a relatively simple example of a Hamiltonian that only has k-local terms but its ground state always has entanglement beyond $k$ sites? For instance if $H = H_{...
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How does measuring the location of one entangled photon affect its pair?

If you have two entangled photons that are moving in opposite directions and you measure the location of one of them, what happens to the wave function of the location of the other photon?
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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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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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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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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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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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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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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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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 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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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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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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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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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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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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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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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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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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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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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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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 ...
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can particles that have two entangled properties remain entangled if only 1 of the properties is measured at a time?

Let's suppose you have two entangled particles that each have two entangled properties. The first property can be either A or B and the second can be either 0 or 1. According to this video, ...
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can entangled particles return to superposition?

Let's suppose we have two entangled particles that can either have the values of 1 or 0 when measured. Before measurement, both particles are in a superposition of both 1 and 0 and have a 50/50 ...
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Relation between entanglement and average number of modes that are taken by constituent particles of coboson

Let we have composite boson made of pair of fermions, and fermions are entangled with each other. The state of composite boson can be written as $ \sum_m \sqrt{\lambda_m} a^{\dagger}_m b^{\dagger}_m |...
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How does N00N state interferometry give a cosine dependence on $N\phi$

$\newcommand{\ket}[1]{\left|#1\right>}$ A Mach-zahnder interferometer with $\ket{\alpha}$ input in port A and $\ket{0}$ in port B gives an intensity difference in the two detectors after the second ...
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How does the Many-Worlds Interpretation of Quantum Physics relate to Consciousness? [closed]

According to the Many-Worlds Interpretation of Quantum Mechanics, each quantum event causes the world to "branch" into a superposition of outcomes. How come "my" consciousness only chooses one of ...
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Is quantum entanglement the “architecture” for a quantum chip?

I am trying to understand the role of entanglement in quantum computing. Is it a method used to link/entangle states of qubits so each state is dependent on it's entangled partner? And If ...
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Quantum mechanics, information and reality [closed]

Call me old fashioned but I'm rather fond on thinking of the electron as a physical thing with slots for information called mass, energy and velocity and charge. (And other ones called baron number ...
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How is the quantum internet better than the standard one? [closed]

First, I'm not even remotely a physicist, but a novelist who is trying to incorporate the quantum internet into a book, so I would love you forever if any math used to explain is relatively simple. I'...
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Something strange in the quantum Ghost imaging experiment

In his paper, Strekalov, D. V., Sergienko, A. V., Klyshko, D. N., & Shih, Y. H. (1995). Observation of two-photon “ghost” interference and diffraction. Physical review letters, 74(18), 3600., they ...
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Quantum entanglement with different particles

I ve been searching for some techniques that are used to create quantum entangled pairs. I can see that almost all of them are related with photons and rarely electrons. Are there other methods that ...
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If the dimension of a space is prime, are quantum states in it guaranteed to be entangled?

A rather obvious question perhaps but if I have a Hilbert space of dimension $d$ and $d$ is prime, I cannot possibly write my state as $$\rho = \sum_i p_i\rho_a\otimes\rho_b$$ simply because the ...
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Is the photon pair generated from the electron-positron annihilation entangled?

Is the photon pair generated from the electron-positron annihilation entangled? And would they work as a source of entangled photons suitable for experiments in quantum optics?
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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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Can Information created by entangled particles be received faster than light by an observer?

I am the captain of a spaceship at the moon of Tellus, and my colleague Trillian is the captain of another spaceship at Proxima Centauri. I need to deliver a package to Trillian, who is going to meet ...
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Quantum Entanglement between unlike particles

Can quantum entanglement occur between two unlike particles, like one photon and one electron? Or one proton and one electron?
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Is this news article about quantum radar legit? [duplicate]

From https://www.digitaltrends.com/cool-tech/quantum-radar-stealth-aircraft/ In principle, a quantum radar functions like a regular radar — only that instead of sending out a single beam of ...
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Negativity for a diagonal reduced density matrix

Suppose one has a tripartite system A,B,C with density matrix $\rho$ , and with reduced density matrix $\rho_{BC}=\text{Tr}_A\ \rho$. Suppose $\rho_{BC}$ is a diagonal matrix. As the partial ...
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A question regarding Leonard Susskind's ER=EPR lecture

https://youtu.be/OBPpRqxY8Uw?t=1315 Right at this instance of the video Susskind starts talking about how space is actually connected by entanglement. (You should watch the video for a accurate ...
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How does measurement-affected spin of an entangled particle not communicate information ftl?

Am I not understanding this video correctly: https://www.youtube.com/watch?v=tafGL02EUOA&t=5m20s Since up/down or left/right orientation of the particle depends on the physicist's choice at ...
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Is the spin in quantum entanglement set at the moment the particles are separated, instead of when measured? [closed]

Is the spin in quantum entanglement set the moment the particles are separated, instead of transmitted instantaneously to its twin particle at the time of measurement? Wouldn't this make more sense ...
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Entanglement in atoms, nuclei and quantized fields

Are the particles that are bounded by chemical bonds or nuclei joined by nuclear forces entangled or are they pure states? In addition to that, are the subatomic particles in the atoms and nuclei, ...
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Why does E91 protocol use the CHSH inequality instead of QBER?

I am now studying about E91 protocol. In the process, I saw that E91 protocol utilizes the CHSH inequality instead of QBER. Other protocol like BB84 protocol uses QBER for the rate of the existence of ...
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How do we know quantum entanglement works no matter the distance?

It is said quantum entanglement works regardless of distance. 2 particles can be entangled and information is shared instantaneously, even if they are lightyears away from each other. But how do we ...