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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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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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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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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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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 ...
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Is entanglement *not* intrinsic to state, but dependent on division into subsystems? (Susskind QM)

I'm working through Susskind's "Quantum Mechanics" book (TTM series), which I quite like. Background In Lecture 7 (Chapter 7), he studies a 2-spin system. A single spin has eigenvectors: $$|u\...
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Why can't the Schrödinger equation be solved exactly for multi-electron atoms? Does some solution exist even in principle? [duplicate]

NOT a duplictae, see EDIT below It is common knowledge that the Schrödinger equation can be solved exactly only for the simplest of systems - such the so-called toy models (particle in a box, etc), ...
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When does the exchange of energy take place in the Quantum Eraser experiment?

I found this PBS video regarding the delayed choice / quantum eraser experiment, which, like all the others I've seen, states that the interference pattern generated by the photons that pass through a ...
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Entanglement fidelity for a single qubit?

So I have been working out problems from the book "Quantum Computing Explained" by David McMahon. On page 166 he defines "Entanglement Fidelity" and says the following: Consider a density operator for ...
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Is entanglement an indeterminate property until measured?

This question is related to Physics SE Objective or Subjective, but is not the same. There was roughly a 10:1 imbalance of answers (weighted by reputation points) to the effect that entanglement is ...
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In what sense is it 'more complicated to navigate through a Hilbert space to find more complex states'?

In the lecture titled Entanglement and Complexity: Gravity and Quantum Mechanics, Professor Leonard Susskind implies (at time 17:49) that the complexity of a quantum state impacts how 'complicated' it ...
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Squeezed state in Fock basis

I'm learning quantum optics, and I'm starting to manage boson algebra. In particular, as a pure exercise, I would like to express a squeezed vacuum state in the Fock basis, which, according to ...
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How to write a wave function for two entangled photons in a (J,P)=($0$,+) state?

I'm studying some experiments using pairs of atomic photons, in particular those photons that are consequence of the $(J=0)\rightarrow(J=1)\rightarrow(J=0)$ cascade of calcium. All of them start ...
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CNOT-Gate acting on entangled qubit state with more than two qubits

I asked this already in reddit but got now answers, so I hope here is the right place. Question in short: Is there a general form of the CNOT Gate, so I can apply it to an arbitrary entangled state ...
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if observing entangled particles causes the entanglement to break, how do we know that they were ever entangled?

I have a working understanding of physics, and am working through quantum entanglement. I read that when we observe one entangled particle, we then know the spin of the other. I also know that when ...
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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 ...