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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Quantum effects on phonon transport?

Phonons are the quanta of vibrations in a crystal lattice. As the name suggest, phonons are already a quantum phenomena but when studying heat transfer the phonons are mostly very classical (wave ...
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Karen Barad's theory of agential realism - does it stand on solid scientific ground or is it quantum quackery? [closed]

Before you vote to close this question as off-topic, please note that while my interest in the topic lies on the metaphysical guesswork, my question here is specifically about Karen Barad's scientific ...
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Tandem double slit experiment (similar to quantum eraser in setup)

Say you have two double slit experiments, DSA and DSB. Each has a photon source and the work is expected producing interference patterns that would can be observed by only looking at DSA or DSB they ...
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Holograhpic Dual of Replica Manifold

Following https://arxiv.org/abs/1501.05315, the conformal map $w=z^\alpha$ can be interpreted as a conical defect geometry on the bulk side. In particular, we can obtain the replica manifold by the ...
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Can one measure two components of spin exactly by measuring two components of entangled, say, electrons?

Of a single electron, two different components of spin can't be have simultaneously well defined values. But what if we entangle two of them and we measure, say, $(S_1)_z$ and $(S_2)_x$ simultaneously....
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Does changing the state of one entangled particle, change the state of the other? [duplicate]

I have just understood what quantum superposition truly means in terms of the bra-ket vectors and the vector space. From my understanding, two entangled particles affect the probability of each ...
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QBism and causes of EPR correlation

I wish to clarify my understanding of the QBist (eg Fuchs et al), or Quantum Bayesianism, interpretation of quantum mechanics and in particular its approach to the EPR problem. Suppose we consider a ...
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Entanglement across a bi-partition of a many particle system

Alice and Bob each have two qubits labelled $A', A$ and $B, B'$ respectively. The density operator of the total system is $\rho_{A'ABB' } \in \mathcal{L}(\mathcal{H}^2 \otimes \mathcal{H}^2 \otimes \...
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Are Coherent Superpositions of Energy Forbidden in Page-Wootters Construction?

Let me start with a summary of the Page--Wootters (PW) construction to set up convention. My description follows that of Section II.A in this paper. In the PW construction, one considers a Hilbert ...
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Technical meaning of "no faster-than-light communication"?

I often read sentences like "relativity forbid faster-than-light communication" or "quantum entanglement cannot be used to convey information faster than light", but it seems to me ...
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Entropy Calculation from Regularized Hamiltonian

I refer to Srednicki's 'Entropy and Area' paper here.I want to derive the expression for entropy (20), given by $$S_{l}(n,N) = \xi_{l}(n)[-\log\xi_{l}(n) + 1], \xi_{l}(n) = \frac{n(n+1)(2n+1)^{2}}{64l^...
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Can Quantum Entanglement Be Achieved Between a Particle and Existing Particles That Are Quantum Entangled?

As the above title mentions, is it theoretically possible for a particle to achieve quantum entanglement with an existing set of particles that are already entangled? I did try searching for a ...
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How is spooky action at a distance measured?

My recent background reading: https://www.science.org/content/article/more-evidence-support-quantum-theory-s-spooky-action-distance I have been trying to understand how experiments actually measure ...
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Photon-wise justification of chosen compensation crystal length in SPDC

I know a compensation crystal in SPDC schemas serves the temporal walk-off erasment purpose. However, I am not sure why half the length of the original crystal is opted for. I read an article that ...
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Specifications of Bell's inequality experiments conducted untill now

Has any Bell's inequality experiment, conducted until now, which has satisfied the following criteria? Having more than three measuring orientations. In the typical experimental setup I have seen ...
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Will entanglement survive interaction with classical environment?

Suppose I meet with my friend on the New Year's and we entangle two qubits in a Bell state. The qubits are physically realized as some solid state devices that can be transported easily. I take a ...
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Has the possibility of time travel been proven or demonstrated? [duplicate]

This study by Lesovik et al., 2019 seems to claim they have either sent information or an electron back in time. What is the current status of the idea of time travel (at least on the scale of ...
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Why do they say there is no signal in the Cramer-Herbert nonlocal quantum communication scheme?

I refer to the paper "An Inquiry into the Possibility of Nonlocal Quantum Communications " , by John Cramer and Nick Herbert. Section 6, a wedge - modified path entangled EPR experiment ...
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Coupled and uncoupled qubits - Hilbert space representation

Suppose I have two situations: one where two qubits, $q_A$ and $q_B$, exist independently (on separate sides of the quantum chip, maybe), and one where they exist with some coupling between them. And ...
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If 2 electrons are entangled $[a] [b]$. if we throw a photon at $-[a]$ will it come out of $[b]-$? [closed]

i was confused as to what will happen when two electrons which are entangled and then if one is exposed to light. will the absorption and emission theory still work and if so how will it work in this ...
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How do I prove that a reduced density matrix has properties of a density matrix?

The properties of a density matrix are defined as follows: $(1) \ \ \mathrm{Tr}\rho = 1 $ $(2) \ \ \rho^\dagger = \rho $ $(3) \ \ \rho \ge 0 $ $(4) \ \ \mathrm{Tr}\rho^2 \le 1 $ A reduced density ...
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Intuition for Results of a Measurement of Entangled Spins

Suppose I have a source that produces pairs of spin-1/2 spins in the entangled state $$ \frac{1}{\sqrt{2}} \left( \mid\uparrow_z\uparrow_z\rangle + \mid\downarrow_z\downarrow_z\rangle \right). $$ The ...
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Is central charge related to symmetry?

I am currently reading the paper "Theory of finite-entanglement scaling at one-dimensional quantum critical points" by Pollmann et. al. and I am trying to understand the central charge in ...
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Is a highly entangled quantum system synonymous with a strongly correlated system?

Is a highly entangled quantum system synonymous with a strongly correlated system? From wikipedia a key characteristic of a strongly correlated system is that "the behavior of their electrons or ...
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When we talk about entangling properties do we really mean we are entangling states, since properties are a classical description?

I am attempting to be more concise in my own explanations and have been advised that properties is the incorrect term to refer to when preparing quantum states. Is it more accurate to refer to a ...
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If you measure one "share" of an entangled pair, will the resulting pair be a product state?

If you do a partial measurement on one "share" of en entangled pair, will the resulting pair no longer be entangled, i.e will be a product state?
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Why aren't states with 3 basis vectors considered entanglements in two qubit system?

I am going to take out normalization factors for simplicity. $$|00⟩+|11⟩$$ $$|00⟩−|11⟩$$ $$|10⟩+|01⟩$$ $$|10⟩−|01⟩$$ I can see why these states are entangled but I don't see why the following states ...
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Quantum Mechanics: Interference and entanglement with the Hong-Ou-Mandel (HOM) effect

My most sincere apologies if this question seems jumbled/incoherent. New here and very confused. Context: From what I understand, if a photon is incident on an input port of an ideal 50/50 beam-...
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Can local vacuum disentanglement and emergent time offer a new interpretation for properties of particles, modelled as black holes?

The notion that spacetime may emerge from entanglement between factors comprising a Hilbert space decomposition of the vacuum has been suggested by many (for example, M. Van Raamsdonk “Building up ...
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Which Hamiltonians turn disentangled states into entangled ones?

I have been solving example oral-exam questions about quantum mechanics, and I am stuck on the last question: "Describe the spin space for $N$ particles of spin 1/2. Give an example of an ...
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Lower bound on the spectral gap in finite size critical systems with locality

Local quantum systems tuned to criticality are gapless in the thermodynamic limit. The rate at which the ground state spectral gap approaches zero as the system size $L \rightarrow \infty$ carries ...
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How to compute the Schmidt decomposition of a bipartite pure state?

I'm trying to work out the entropy of entanglement of my state but I'm struggling to put it into a Schmidt decomposition, i.e. in the form: $\sum_i \alpha_i |u_i \rangle |v_i \rangle$. Currently I ...
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Trouble understanding the math of the preferred basis problem

I've been trying to understand the preferred-basis problem in QM, specifally in the Everettian intepretation. To quote an answer to another question which discusses this: In my opinion, the situation ...
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Entanglement in coupled harmonic oscillator system

I am considering the problem of two coupled harmonic oscillators. Ignoring factors of two, the Hamiltonian for this system is $$H=p_1^2+p_2^2 +k x_1^2+kx_2^2 +k(x_2-x_1)^2$$ One can do a nice ...
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Why does the signal detector D0 in the delayed choice experiment always show a blur?

As explained in many answers such as https://physics.stackexchange.com/a/516512/64051, the delayed choice quantum eraser cannot be used to send information back in time because the detector D0 always ...
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What if the twins in the twin "paradox" in special relativity are two entangled quantum systems? [duplicate]

We consider a theoretical twin paradox experiment in which the "twins" are in fact two entangled quantum systems (electrons, protons , atoms whatever may be the case ). Accordind to a &...
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Why isn't this a way to transfer information faster than light using quantum entanglement?

Suppose Alice and Bob have entangled particles A and B that are very far apart. Alice uses Particle A to send information through its spin along the x axis, and Bob uses Particle B to receive this ...
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How to measure the entanglement of three entangled qubits?

The entanglement of two qubits is calculated using "concurrence" and "negativity". Concurrence and negativity, however, are only used for "two" entangled qubits. Is there ...
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Thermalization at quantum critical point and quantum many-body scars

Quantum scar states are a hot topic in condensed matter physics. Quantum scars are the eigenstates of a many-body system (e.g. a spin chain) that weakly violate the eigenstate thermalization ...
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How to correctly understand the monogamy of entanglement?

If the degree of entanglement between qubits A and B is 100% (total entanglement), then neither of them can entangle with qubit C. However, if the entanglement between A and B is partial (say 80%), ...
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Mutual information of a tensor network

Suppose I've got a tensor network (TN) representing some bipartite quantum state, $|\Psi\rangle$. Using the Schmidt decomposition, I can write $|\Psi\rangle = \sum_{k=1}^r \sqrt{\lambda_k}|\chi_k\...
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How to transform any pure two-mode Gaussian state into "canonical two-mode squeezed state form"?

I'm new to quantum optics and am going through a review paper by Braunstein & Loock. There, after calculating the partial von Neumann entropy for a two-mode squeezed vacuum $$\hat{S}(\zeta)|00\...
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If ER = EPR conjecture is true would it allow faster than light (FTL) communication?

If entanglement is really carried through by wormholes can't we for instance use that wormhole to send information through it? If we understand that maybe we can scale up the wormhole to allow people ...
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How can a basis change make a state suddenly separable?

I am working through my quantum optics textbook by Grynberg, Aspect and Fabre, and this concept has tripped me up a little. (13) is an inseparable state, whereas (15) IS separable - but they are the ...
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Is there a numerical measure of entanglement? [duplicate]

Does there exist a numerical measure of entanglement? For example, if you consider two qubits, they can be in a product state (with no entanglement) like $\frac1{\sqrt2}(\lvert\uparrow\uparrow⟩+\lvert\...
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Does a subsystem being mixed imply the state is entangled?

If a pure state, $\rho_{AB}$, has subsystems described by mixed density matrices, the overall state is entangled (as far as I understand). Can you conclude the same with an initially mixed bipartite ...
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Concurrence between 2 qubits of tripartite system

If we consider a tripartite system (say of 3 qubits) described by density matrix $\rho_{ABC}$, does the concurrence $C(\rho_{BC})$ still accurately measure the entanglement between subsystems B and C? ...
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A notational confusion in a Bell-type inequality

In the tripartite Bell-type inequality know as Svetlichny inequality, given in this (freely available) article. The quantity $M_{ijk} = Tr [\rho(\sigma_i \otimes \sigma_j \otimes \sigma_k)]$, $i,j,k\...
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Fermionic (or Bosonic) state vs Entangled state

One can see that the wavefunction for a system of two electrons (not very far apart) is one that cannot be written as a tensor product of individual states. The same is true for a bosonic state. For ...
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Can a two-qubit state be unentangled via unitary operations?

There is an operator that takes a state for example of the type $$|\psi \rangle=\frac{1}{\sqrt2}( |0\rangle | 1\rangle+|1\rangle | 0\rangle)$$ Into a not entangled type,and i guess that for unitary ...
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