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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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Which interactions produce entanglement?

I'm trying to better understand which kinds of interactions cause the entanglement. I do understand how it works in the cases of non-relativistic spin systems. For example, evolving a pure state in ...
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Say you run qubit $A = (|0⟩ + |1⟩) / √2$ and qubit $B = |0⟩$ through a CNOT gate. What is the state of qubit $B$ afterwards?

I am new to the weeds of quantum computing and this question is probably pretty elementary. Say you run qubit A = (|0⟩ + |1⟩) / √2 and qubit B = |0⟩ through a CNOT gate. What is the state of qubit B ...
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Have researchers managed to “reverse time”? If so, what does that mean for physics?

According to press releases, researchers have reversed time in a quantum computer and violated the second law of thermodynamics. What does that mean for physics? Will it allow time travel? Further ...
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Maximally entangled transpose trick [closed]

I have encountered this "mirror lemma" or "transpose trick" for maximally entangled states and want to prove it. It goes as follows: $(M^A \otimes I^B ) | \Phi \rangle^{AB}=(I^A \otimes (M^T)^B)| \...
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Basis ambiguity for bipartite entanglement

I am reading Decoherence, einselection, and the quantum origins of the classical from Wojciech Hubert Zurek And he says at some point that for a three partite quantum system there is no basis ...
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Are the electrons in the Delft University experiment really entangled?

I have been studying the Loophole-free Bell test at the Delft University. They were using the "Entanglement By Accident" method. When they trigger an electron, the electron falls back in one of two ...
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How to search Feynman diagrams for entanglement [closed]

This question is partly about physics, partly about love -- but I don't believe there's a Stack Exchange for the later :) My girlfriend is a PhD physicist who finds the notion of entanglement very ...
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Does a polarizer affect both entangled photons or only one?

Suppose you have two polarization-entangled photons A and B in the following state: \begin{equation} \Phi=\frac{1}{\sqrt{2}}\bigl(\left|H_{A},H_{B}\right\rangle + \left| V_{A},V_{B}\right\rangle\bigr)...
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Do Chern Insulators (QAHE) have topological order (long-range quantum entanglement)?

I know IQHE is a example having "invertible" topological order from Professor Wen's definition. And Topological Insulators is SRE because of necessary of underlying symmetry protection. After that, ...
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The maximally mixed state is at the center of all quantum states

The set of quantum states $\rho$ in $d$ dimensions is the set of positive semidefinite operators living in a Hilbert space of dimension $d$. Let us denote this set by $\text{Pos}(X)$ and note that ...
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Closest separable/ closest PPT state given an arbitrary state?

After searching for results, the closest result I found was https://aip.scitation.org/doi/pdf/10.1063/1.3591132?class=pdf. The summary of that paper is: Given an arbitrary seperable/PPT quantum state ...
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How does a photon released from particle A interacting with particle B, entangle them?

What does a photon carry from its parent particle that when it interacts with another particle, it entangles both the particles?
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Is it possible to reconduct “The delayed quantum eraser experiment” in home?

The delayed quantum eraser was conducted by Kim et al. in 1999 with the use of BBO crystals, polarizers, detectors and so on. I believe some of these equipments can be found online kinda cheaply, ...
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Does Quantum Entanglement happen between two wavefunctions? [closed]

Does the entanglement happen between two particles or two wavefunctions? If it's wavefunction then what happens to the two wavefuntions after getting entangled?
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What is the relationship between spin network spacetime and tensor network (entanglement) spacetime?

In 1971, Sir Roger Penrose, suggested a combinatorial construction of spacetime using the angular momentum of particles. This work led to and introduced the idea of spin networks which are ...
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Reality, locality, and universality in the EPR paradox

Apologies if this has been asked before. I did some searching but didn't see it anywhere asked quite like this. Thanks in advance for any insights. Caveat: I am an organic chemist and thus ...
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What is the difference between coherent states and the entangled states used in quantum illumination?

I've been reading about utilising quantum illumination systems to out perform classical microwave radars, but I've gotten confused over a point they're making regarding the distinction between the ...
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Better Entanglement Explanation Required [duplicate]

It seems to me I have not read or understood a better explanation for quantum entanglement than Einstein's left and right glove explanation. If some Aspect (pun intended) of an entangled quantum ...
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What happens when measuring a subsystem of a product bipartite state?

I have an horrible doubt, as I found that in my books the question is never directly addressed. How is collapse defined to work for measures of composite system? I always assumed that when we ...
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Does something prevent superposition at our scale?

I often encounter the argument that quantum mechanics reduces to classical mechanics at sufficiently big scales, as soon as h becomes sufficiently small respect to the actions involved. I clearly ...
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Why does $\max\{S(\rho_A)-\sum_i p_i S(\rho_A^i)\}$ measure classical correlations, and not full (classical + quantum) correlation?

Let a quantum system be given which has two subsystems $A$ and $B$ so that the Hilbert space decomposes $\mathscr{H}\simeq \mathscr{H}_A\otimes \mathscr{H}_B$. If the state of the system is $\rho$, ...
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Do qubits need to be in superposition to be entangled? [closed]

Do qubits need to be in superposition to be entangled? Put another way, can qubits be entangled but not in superposition?
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Description of quantum state of entangled photons after polarizer

I'm wondering if anyone can help me understand how a polarizer changes the quantum state of two polarization-entangled photons. I haven't found a clear description in the literature. Suppose you have ...
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Holograms and Entanglement: Does recording an entangled signal in a hologram “observe” the entangled state?

Suppose I have two photons $A$ and $B$, which are entangled. As I have had it explained to me, entanglement works more or less as follows: I observe or measure $A$, and find $A=S_a$, where $S_a$ ...
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Fidelity and approximately unentangled states

It is well known that fidelity distributes over product states, i.e. $$F(\rho \otimes \rho, \tau \otimes \tau) = F(\rho, \tau) \cdot F(\rho, \tau),$$ and more generally, $$F(\rho^{\otimes N}, \tau^{...
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So is quantum entanglement actually FTL “communication” or is it mundane pre-determination?

I have to say right off the bat, I'm a little frustrated that there seem to be very contradictory answers about this, at least to a layman like me. If two particles are entangled and you separate them ...
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How unique is the Schmidt decomposition?

I read the Schmidt basis is unique "up to a phase" or as stated here in answer given by Norbert Schuch "modulo degeneracies". If I choose a Bell state $|\psi\rangle = \frac{1}{\sqrt{2}}\left(|0\...
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Can global unitaries create entanglement?

So I am reading this paper, which states multiple times that: Indeed, global (and thus entangling) unitary operations are capable of extracting more work than local operations from a set of ...
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Possible to write $N$-partite W state as sum of less than $N$ product states?

The $N$-qubit W state is $|W\rangle = \sum_{n=1}^N |n\rangle$ where $$|n\rangle \equiv |0\rangle_1\cdots|0\rangle_{n-1}|1\rangle_n |0\rangle_{n+1} \cdots |0\rangle_{N}$$ is the product state with the ...
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Definition of Entanglement

The definition of quantum entanglement, found on the internet and the literature is: On a bipartite system $\mathcal{H}_A \otimes \mathcal{H}_B$, let $\rho$ be a mixed state. It is said to be ...
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Entanglement in quantum physics [duplicate]

Mathematically what is the difference between pure separable state and entangled state ? Can anyone explain with equations?
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Is a mixture of Bell states entangled?

Setup: I have a two qubit system $\mathbb{C}^2 \otimes \mathbb{C}^2$ an we consider the Bell states $\newcommand{\ket}[1]{\left|#1\right>}$ $\newcommand{\bra}[1]{\left<#1\right|}$ $\...
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Entanglement, measurements and correlation

I am studying for my quantum mechanics exam and I came across the following question, I hope I reason correctly. Consider two spin-$\frac{1}{2}$ systems one of which is measured by Alice and one ...
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Annihilation Operator on Entangled State - How Does This Not Break No-Communication Theorem?

If I have the state $$|\psi\rangle=\frac{1}{\sqrt{2}}\left(|2\rangle_{A}|0\rangle_{B}+|0\rangle_{A}|2\rangle_{B}\right)$$ and apply the annihilation operator to B $$a_{B}|\psi\rangle=|\phi\rangle=\...
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Constructing a maximally entangled basis of a bipartite system $\mathcal{H}=\mathbb{C}^d\otimes\mathbb{C}^d$, starting from the reference state?

$\newcommand{\Ket}[1]{\left|#1\right>}$ Given $\Ket{\phi}=\frac{1}{\sqrt{d}}\Sigma_{i=1}^d\Ket{i}_A\otimes\Ket{i}_B$, where $d=\textrm{dim}(\mathcal{H}_A)=\textrm{dim}(\mathcal{H}_B)$, how does ...
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What is the reason for the reported opposite-of-QM handedness of photons in a Bell test with circularly polarized photons?

In https://file.scirp.org/pdf/OPJ_2016111414355792.pdf, a Bell test using circularly-polarized photons is reported. The data shows that coincidences happen in the reverse handedness predicted by ...
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Are all Bell tests valid only for linearly-polarized photons? [duplicate]

In https://arxiv.org/abs/1407.2605, it is argued that all photons are necessarily circularly polarized, and linearly-polarized photons must be a superposition of such circularly-polarized photons. ...
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Will more than one composite boson can stay in the same energy state if constituent fermions has moderate entanglement?

Let say we consider two distinguishable fermions(bi-fermions) in compact form. The case when both fermions are existing as free fermions, they will obey Pauli exclusion principle. In other case if ...
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Could spooky action at a distance be explained by particles staying linked in another dimension? [closed]

The math suggests that there are 11 or more "dimensions", most of which we know very little about. What if entangled particles are linked in one of those "other dimensions" and when we separate them ...
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Is it possible to entangle particles for their path?

In the figure below, you have a source of entangled particles that then sends these particles in opposite directions. Each stream of particles heads towards a double slit with a detector screen ...
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Example of time-dependent factorization of a Hilbert space

In these notes on entanglement in QFT it is pointed out that in the Heisenberg picture the factorization of a Hilbert space is time-dependent (pages 18 and 19): In the Schrödinger picture, it is ...
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can entangled photons produce an interference pattern?

Suppose you have a laser that emits entangled photons in opposite directions towards a double-slit screen setup as shown below. The photons are entangled such that if a photon on the left travels ...
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What is the criteria for two fermions to behave as composite boson? [duplicate]

Other than that overall spin for pair of fermions is integral, what are properties needed to make composite boson. In some articles they say fermionshave to be in compact form , so what happens when ...
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How to make two fermions entangled with each other?

How to make two fermions entangled with each other. When we say two particles have to interact with each other to get entangled, what kind of interaction is that?
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Entanglement invariant under local basis change?

Today in the lecture the professor said that if we have an entangled state between two systems A and B $$\mid \psi_{AB} \rangle = \frac{1}{\sqrt2}(\mid 00\rangle+\mid 11\rangle)$$ There is no local ...
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How does entanglement create empty space?

It is claimed that empty space is full of quantum fields which their waves perfectly cancel out each other, but this can only happens when all the waves are entangled. Is this claim valid because I ...
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Will entangeld particles stay entangled?

I want to consider an experiment: We create two entangled particles a and b such that the spin of each is opposite to the other. We then measure spin in z direction (1). So we get a is up and b is ...
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What is the variance of |S| in Bell's inequality (CHSH inequality)

Sorry that this isn't a quick question but I didn't know how to make it shorter. I am struggling with this for quite a long time and I would appreciate every help that I can get. I could not find a ...
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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?