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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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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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Beyond Born Oppenheimer Approximation

My general question is about the conical intersections that are seen in the energy landscape of some systems. Why are the dynamics of such systems of special interest? Could you give some examples ...
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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 ...
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Can a single-qubit state be nontrivially extended to a non-pure state?

Consider a generic single-qubit state $$\rho=\lambda_1\lvert \lambda_1\rangle\!\langle \lambda_1\rvert+\lambda_2\lvert \lambda_2\rangle\!\langle \lambda_2\rvert\in\mathcal H_S.$$ I am interested in ...
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Lists of universal quantum information processing protocols

Many quantum information processing tasks require the knowledge of the quantum states being processed. On the other hand, the solutions of some tasks are universal in the sense that they do not rely ...
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Left and Right Eigenvectors of Transfer Matrix in Matrix Product States (MPS)

Let $\lvert{\psi}\rangle=\sum_{i_1i_2...i_n}Tr(A^{[1]}_{i_1}A^{[2]}_{i_2}...A^{[n]}_{i_n})\lvert{i_1 i_2...i_n}\rangle$ be a MPS, where $i_k=1,2...d$ and $A^{[k]}_{i_k}$ are $D\times D$ matrices ...
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Question about the shift of patterns in the Delayed Choice Quantum Eraser

In the double-slit Delayed Choice Quantum Eraser experiment by Scully with the installation in the figure below, as in Wikipedia, when a photon hits D4 or D3 the intricated photon hits D0 with a ...
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Entangled state density matrix: Defintion and example

I was looking at this question but still don't fully understand the distinction between classically correlated mixed states and entangled mixed states. I understand that a pure state is considered ...
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Can 2D graph state be represented by branching MERA?

A 2D graph state is a highly entangled state to support general measurement based quantum computation. But its state complexity is relative low. Branching MERA represents also a set of low complexity ...
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Cosmic entanglement background?

Could we measure the entanglement (if any!) of the cosmic microwave (neutrino, graviton,...) background radiation? Related to this: does the LCDM model of Cosmology predict something about cosmic ...
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Quantum entanglement in QFT [duplicate]

What is the explanation of the quantum entanglement in the quantum field theory framework?
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POVM measurement evolution of a quantum system [closed]

We know that during measurement quantum system evolves as: $$ |\psi_f\rangle = \frac{M_r |\psi_i\rangle}{\sqrt{\langle\psi_i|M_r^\dagger M_r|\psi_i\rangle}} $$ where $M_r$ is the measurement operator ...
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Can a single measurement on an arbitrary qubit determine if it had been entangled or not?

I have a single isolated qubit. Can I perform any measurement that will tell me if it had been entangled with another qubit or not?
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Entangled photons

How do you identify correlated events between a pair of entangled photons if these photons arrive at different times at their respective detectors?
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Maximal violation of the Wigner inequality?

In my atomic physics lecture we derived the Wigner inequality: $p_{++}(\alpha,\beta)+p_{++}(\beta,\gamma)-p_{++}(\alpha,\gamma)$ $\geq0$ where $p_{++}(\alpha,\beta)$ for example is the probability ...
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Are there any physical realizations of a rebit (a qubit with real coefficients for probability amplitudes of a two-state system)?

While being introduced to the world of quantum computation and qubits, I’ve come across the term rebit, which I understand to mean a two-state quantum system that may be expressed as a real linear ...
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Why is entanglement the only type of quantum correlation that can exist between subsystems of pure state?

I read that if we have composite system having a pure state, then only quantum correlation its subsystems can have between them is entanglement. Why is that so?
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Alternative Delayed Choice / Quantum Erasure Experimental Configuration

In every experimental setup of the Delayed Choice and Quantum Erasure experiment that I've seen, the choice of what complementary property to measure or whether to include the second beam splitter ...
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what kind of information is shared between entangled particles [closed]

when two entangled particles are separate by distance . what exactly happens to them , that influence us to consider they connected each other?
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Is uncertainty and correlations actualy the same thing?

In this paper on page 2 it is said that The entropy $S(\rho_A)$ measures the amount of correlation (classical and/or quantum) between $A$ with the external world. Now this is confusing me a little ...
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Reduced density matrix of two spins

I am reading this (https://arxiv.org/abs/1209.0062) article about constructing order parameters from reduced density matrix. The author is discussing long-range order by taking antiferromagnetic spin ...
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How to write reduced density matrix of a single site

To clear my concepts about density matrices, I am trying to solve the following system as an example. The density matrix is defined as $$\rho=|\psi\rangle \langle \psi |$$ where $|\psi\rangle$ is ...
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Characterization of unitary qudit gates modulo (same) local unitary gates

I would like to know if there is any way to characterize unitary gates acting on two systems of the same dimension (for example, let us consider the finite Hilbert space $\mathcal H = \mathbb C^d$) in ...
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General properties on reduced density matrix with assumption on the global density matrix

Let's consider $\mathcal{H_1} \otimes \mathcal{H_2}$ the space of the problem. I call $\rho$ a density matrix of the full space and : $\rho_1=Tr_2(\rho)$ the reduced density matrix in $\mathcal{H_1}$...
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Decoherence: faster than light?

The answer to this question suggest that one can solve the measurement problem by decoherence. If I understand it correctly the decoherence appears when the quantum state interacts with the ...
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Given density matrices of the subsystems is it in general possible to recover density matrix of the whole system?

Let us consider bipartite system in an entangled mixed state. Since its density matrix can always be diagonalized we can write it in following ways: $$\rho_{AB} = \sum_{j} p_{j} | \psi_j\rangle \...
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What are the possible reasons for transformations failing in SLOCC?

I apologize beforehand if this is a really noobie question. But I'm trying to understand one aspect of SLOCC. In a SLOCC scenario, we want to transform state $A$ to $B$ using only LOCC operations. ...
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Non-zero expectation values of entangled states

I am trying to understand the relations/properties for non-zero expectation values of entangled states. More explicitly, I was previously told (unfortunately without source), that for maximally ...
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The non-locality of Quantum Mechanics

I often heard physicists mentioning the EPR paradox or the quantum entanglement when talking about the non-locality of quantum mechanics, but wasn't quantum mechanics already non-local from the ...
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Composite behavior of composite bosons with the help of entanglement

Are there any good books related to composite bosons using quantum information approach or that can help to understand Bose Einstein condensation of composite bosons?
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Quantum-entangled macroscopic objects

As a thought experiment, what would happen if you had two macroscopic metal cubes, 1 kg each, and they are entangled atom-for-atom? Each atom in cube A would be entangled to its corresponding atom in ...
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Does entangled states must become non entangled states after the measurement?

In tensor notation. A state vector $|uv\rangle$ is a tensor product(non entangled states) if and only if there is $A\in E_1(u)$ and $B\in E_2(v)$ such that $A\otimes B$. So by postulate of quantum ...
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Justification for “which path” inference in Hardy's paradox?

I'm going over Hardy's paradox again recently and whenever I try to think about it I always get confused with the classical part of the analysis. Just as a brief summary, the setup of the paradox is ...
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Setting spin direction of particles and relation to quantum entanglement

Based on my understanding of the Stern–Gerlach experiment, a particle's spin direction can be measured and will keep that value for future measurements in the same direction. However, if a measurement ...
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Density matrix for system and surroundings

In my QM lecture it was claimed that if you have a system with degrees freedom $\vec{s}$ and its surroundings which have degrees of freedom $\vec{u}$ then every density matrix for the combined system ...
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Classical correlations in bipartite entangled mixed state

I have recently asked somewhat related question and got very illuminating answer. After some thinking however I have realized that (at least) one more point is unclear to me: How can we check ...
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Is photon's direction entangled?

Consider a free electron, with photon, that runs to electron under some angle(as everybody says). Compton scattering is happening, and electron instantly reemits photon in different angle. First, ...
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Classical and quantum correlations in bipartite system

I would like to know how to answer following questions: Is there classical/quantum correlations in given bipartite pure/mixed state? I have gathered several definitions. Some of them (it seems) ...
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Alice's Caterpillar measuring the quantum state of her entangled particle collapses her connection to Bob? [closed]

I understand that eventually in this discussion there will be some unexplained universal law that we will all have to accept as a God's law and build every physics proof upon that. But I want to know ...