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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Entangled Material

Recently I saw the case of creating two entangled electrons with opposite spins from the Higgs boson (which has no spin). The electron spins are always opposite to equal zero as a sum - conserving the ...
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Short term entanglement states range of entanglement

After reading the question and answers in Definition of short range entanglement I wonder why the definition of a short-range entangled state is given in terms of its possible deformations - A SRE ...
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Spontaneous parametric down conversion resource

I have recently begun working on a project where I will be modeling some experiment that uses SPDC to generate entanglement. However, I have not formally studied SPDC in any capacity. Is there any ...
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Holographic Entanglement Entropy for a mathematician

I'm not sure if this is an appropriate question for this site, but figured I'd give it a shot. I'm a mathematician (with an extremely limited background in physics) and have come across some ideas ...
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What is an “optimal ensemble” used to demonstrate quantum steering?

I am reading Wiseman et al. (2007) and Jones et al. (2007) papers on quantum steering. They relied on the existence of "optimal ensembles" for a Local Hidden State theory. They make use of ...
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Is partial trace method valid for highly entangled state?

Suppose there are four wires of qubits and the state vector is highly entangled. I want to know the density matrix (or state vector) of wire-$1$ only. In this case, is it available to use partial ...
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Delayed measurement of entangled states

If we have two entangled particles $A$ and $B$ and we separate them (supose that after entanglement, both travel at same speed in lab reference frame in opposite directions), then we measure a ...
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What is an example of a system with non-vanishing topological entanglement entropy at finite temperatures?

In this paper: https://doi.org/10.1088/1367-2630/14/3/033044 it is show that for Kitaev toric code looses topological entanglement entropy over long times if it is thermally opened. What is an example ...
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Can separating the plates of a charged capacitor create quantum entanglement?

If you take the case of a parallel plate capacitor with air as a dielectric, it's my understanding that separating the plates increases the voltage between the plates proportional to the distance and ...
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Quantum Entanglement and transfer of time information, is it possible? [duplicate]

If two particles are quantum entangled...let’s call them particle A and particle B. You measure the state of particle A. At this point, can you know the exact time at which the particle B goes from ...
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Confusion regarding bra-ket notation and proof of a ket equation

My question is two-part. First, imagine a bipartite quantum state $|\Phi \rangle_{AB}$, made of $2n$-qubits, shared between Alice and Bob (with $n$-qubits each). Alice performs some unitary operation $...
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Why the intrinsic entanglement in a topologically ordered state lowers the entanglement entropy?

The entanglement entropy of the topologically ordered state exhibits the universal additive constant to the area law: $$S \sim \alpha L^{d-1} - \gamma $$ The topologically ordered states are long-...
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What's the definition of the EPR limit?

I am reading this paper, nature.com/articles/s41567-018-0347-x, which is about the generation of high-dimensional entanglement using two non-commuting variables, frequency-bin and time-bin. On page 7 ...
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What exactly is the difference between entanglement and correlations?

Often when describing entanglement in an informal way, we talk about perfect correlation or anticorrelation of measurements of faraway particles, but I don't see how such correlation is particular to ...
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Problem regarding quantum entanglement and special relativity

So this question is about the strange phenomenon of quantum entanglement, or "spooky action at a distance", as Einstein called it. In particular, it's about the simplest conceivable case of ...
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How to check if a qubit in a circuit is equal to another qubit?

If we have a set of qubits q1 ,q2 and q3 for example, $$ \vert{q_1}\rangle = a \vert{0}\rangle +b \vert{1}\rangle\\ \vert{q_2}\rangle=\vert{0}\rangle\\ \vert{q_3}\rangle=\vert{0}\rangle $$ the state ...
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A simple question about hyper entanglement

Do we call hyper entanglement as a high-dimensional entangled state? Hyper entanglement is defined such as $$|ψ\rangle = \frac{1}{\sqrt{2}}(|HH\rangle_{ab}+|VV\rangle_{ab})\otimes\frac{1}{\sqrt{2}}(|...
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Why does the entanglement of quantum fields depend on their distance?

When watching Seans Carrol's "A Brief History of Quantum Mechanics", he mentioned around the 50th minute (the video I linked to starts at that point) that [about quantum fields in vacuum] .....
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Hyper entanglement vs Maximally entangled bipartite state

Let's suppose we have hyper entanglement of two photons such as $$|ψ\rangle = \frac{1}{\sqrt{2}}(|HH\rangle_{ab}+|VV\rangle_{ab})\otimes\frac{1}{\sqrt{2}}(|SS\rangle_{ab}+|FF\rangle_{ab})$$ where $|H\...
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Are equal-time commutators well-defined in QFT?

I'm working through some QFT course notes and I just want to check that my understanding of equal-time commutators is correct. I don't have much to go on -- the notes rather cryptically insert the ...
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Entanglement and Mixed States

The Wikipedia page for "Density Matrix" (https://en.wikipedia.org/wiki/Density_matrix) takes each of a pair of entangled photons as an example of a mixed state: A radioactive decay can emit ...
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Wave function collapse for experimental quantum state preparation

Has the John von Neumann projection, or the wave function collapse, been used in the experimental preparation of quantum states? As an illustration, for a pair of EPR-type entangled 2-level atoms $$|\...
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Does the spin measurement of entanglement need to be simultaneous?

We know that after measuring the spin direction of two entangle particles the entanglement breaks down due to decoherence and this measurement can only be done once. So if that's the case then it ...
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How many time can we measure the same entangled pair?

We know that the entanglement state is a very fragile state and it can be disturbed by very minimum interaction with environment, so how many times can we measure(spin direction) the same entangled ...
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How do scientists produce entangled photons?

In many experiments like the Delayed Choice Quantum Eraser Experiment, scientists use entangled photons. How do scientists make/obtain them? What is the realistic level of control scientists have ...
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Particle between entangled particles? [closed]

Is a particle in between an entangled pair of two other particles affected if a measurenent is done on one of the entangled particles? In other words. Do we know nothing happens in the space between ...
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Compatible Observables in QM vs OQS

In elementary QM courses we always consider that components of momentum vector form a complete set of commuting observables. I am confused whether this is an input to our theory or whether we somehow ...
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Can an electron be entangled with a photon? [duplicate]

Is it possible to quantum entangle the spin of an electron with the polarization of a photon? How could this be done?
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Can a photon be entangled with an electron?

In QFT is it possible to quantum entangle the polarization of a photon with the spin of an electron? How could this be done? Would it be possible, for example, to use the flipping of the electron spin ...
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Why can’t an elementary particle be forced to have a particular outcome in an entangled pair?

I read in a blog Quantum Entanglement: Slower Than Light that one can not force a particle from an EPR pair to have a not statistical outcome for the entanglement parameter. I can not understand why? ...
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Conditional density matrix

Let there be a composite system $\mathcal{H}=\mathcal{H}_{A}\otimes\mathcal{H}_{B}$, where $\mathcal{H}_{A}$ and $\mathcal{H}_{B}$ are Hilbert spaces of two subsystems of $A$ and $B$. Suppose the ...
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Why can any pure bipartite state be written in the Schmidt decomposition as $|\Psi\rangle=\cos\theta|00\rangle+\sin\theta|11\rangle$?

The beginning of this paper (pg.no. 1) on generalised Schmidt decomposition of three qubit states mentions the following: The Schmidt decomposition allows one to write any pure state of a ...
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Is quantum entanglement maintained after measurement?

Say there's a positron and an electron which are occupying quantum states and are entangled. There's two observers, Alice and Bob. Assume Alice measured spin of positron along $z-$axis and obtained $+...
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Von Neumann Entropy of $n$ qubits

Why is the maximum entropy of $n$ qubits (in $\log_2$ units) equal to $n$? How does one calculate $\operatorname{Tr}(\rho \operatorname{log} \rho)$ (since $\log \rho$ must be expanded). What is even ...
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Optimizing MERA disentanglers to represent a specific state

I've read multiple papers (e.g. https://journals.aps.org/prb/abstract/10.1103/PhysRevB.79.144108) on the multiscale entanglement renormalization ansatz (MERA) where algorithms are given for how to ...
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Can quantum particles really communicate in the past?

The quantum entanglement and delayed choice quantum eraser variation to the double slit experiment has given very strange results. So if a particle is detected then the pattern is changed. Does the ...
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Applying a rotation on an entangled state

I want to understand an experiment but I am struggling with the (basic) math/braket notation. In the experiment two ions are entangled and separated into two wells $A$ and $B$. The spin state of the ...
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Entanglement thought experiment [closed]

Consider two drummers: W and E. They meet in the middle of the country and synchronize. W strikes his drum on the up-beat, E on the down-beat. They remain forever permanently synchronized, each ...
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Are all GHZ-states isotropic?

Are Greenberger-Horne-Zeilinger (GHZ) states defined by: $$ |GHZ_{d}^{M}\rangle = \dfrac{1}{\sqrt{d}} \sum_{i=0}^{d-1} |i^{ (1)}\rangle \otimes \cdot \cdot \cdot \otimes |{i}^{ (M) }\rangle $$ by ...
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What does multipartite entanglement mean? [closed]

From Wikipedia: In the case of systems composed of m > 2 subsystems, the classification of quantum-entangled states is richer than in the bipartite case. Indeed, in multipartite entanglement apart ...
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Infinities in the diagonal matrix while calculating Von Neumann entropy

Consider the example case:$$|\phi\rangle=\frac{1}{\sqrt{2}}(|01\rangle-i|10\rangle)$$ From this we can easily calculate the density matrix:$$\rho=\frac{1}{2}\begin{bmatrix}0&0&0&0\\0&1&...
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How does a general $n$-qubit entangled state deal with monogamy of entanglement?

Now since only two electrons (or any quantum object) can be maximally entangled due to the monogamy of entanglement how does the general $n$-qubit states which has more than two qubits make use of ...
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Quantum computer measurement

Hello Stackexchange physics, This question is mostly a request for a source of information on quantum measurement and decoherence that is consistent and independent of the particular context. When I ...
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Entangled photons and CCD

Its a question about entangled pairs and double slit experiment. I have seen and read many such on the stackexchange but I can not really understand what is happening! So please don't stop the ...
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How do entangled states increase computational space for quantum computers compared when only using product states? [duplicate]

My question is: n-qubit product state. $$|\psi\rangle=(a_1|0\rangle+a_2|1\rangle)\otimes(b_1|0\rangle+b_2|1\rangle)\otimes \cdots$$ n-qubit Entangled state $$|\psi\rangle=(a_1|000\cdots000\rangle)+...
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How is the orientation of quantum spin testing of entangled particles made the same between two distant tests?

Quantum entanglement says that two spatially-separated particles tested for quantum spin on the same orientation will be guaranteed to produce opposite results, but testing in a different orientation ...
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How Do I Calculate the Score of a Bell's Game

I am reading "Quantum Chance" by Nicolas Gisin. I would like to know how to calculate the Bell Game outcome given in the following section of the book. BACKGROUND The Bell Game begins with a pair ...
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Quantum Fisher Information vs Classical Fisher Information

I am currently studying about Fisher Information and have a rather simple doubt which I can't figure out. Is it possible to have Quantum Fisher Information (QFI) more than Classical Fisher ...
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Why use the entanglement in Conway's free will theorem?

In Conway's Strong Free Will Theorem paper, the proof consists of two parts. In the first part, they proved the Specker-Kochen theorem. In the second part, they constructed two experimenters that are ...
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How are two-qubit states experimentally projected into the Bell basis?

To perform Quantum Teleportation, we need state of two entangle qubits in the Bell basis $$\left(\frac{|00\rangle + \rangle11\rangle}{\sqrt{2}}, \frac{|00\rangle - |11\rangle}{\sqrt{2}}, \frac{|01\...

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