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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Entanglement of Werner States

Let the Werner state $$\rho_W = W\mid\Psi^-\rangle\langle\Psi^-\mid + \frac{1-W}{4}\mathbb{I},\ W\in [0,1].$$ I have repeatedly heard that such a state is separable if $W\leq\frac{1}{3}$ and entangled ...
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Entanglement and coherence

I have come across a wonderful review of entanglement by Chris Drost in his answer to this post. One part that left me puzzled was: (This post is merely an attempt to understand a portion of Chris' ...
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Does non-orthogonality of joint states follow from non-orthogonality of ALL marginals?

Suppose we have a composite system $AB$ and we are considering two joint (pure) states $$ \psi,\psi^\prime \in \mathcal{H}_{AB} . $$ Let's define the reduced or marginal states of $A$ and $B$ in the ...
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A possible way to test whether integrated information theory causes collapse?

One criticism, of the hypothesis that consciousness causes collapse, is that 'consciousness' has not been well defined. Perhaps, in order to test whether consciousness causes collapse, it is best to ...
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How to find optical toy models of entangled quantum mechanical systems?

I recently read Arnold Neumaier's lectures on uncovering classical aspects of quantum mechanics: Classical and quantum field aspects of light Optical models for quantum mechanics I can't find the ...
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What can an entanglement witness tell me, exactly?

Let's say I encounter particles and I want to know, with certainty, if they are entangled. Can entanglement witness help me determine this? More specifically: 1. Can I prepare a witness at any time ...
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Can I raise a quantum mechanical operator to another quantum mechanical operator?

The most complicated power operation for operators that I have seen is an operator (or a sum of operators) raised to a number. How can I handle an operator raised to another operator? Is this even ...
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Dirac Notation Tensor product

We can write a Singlet state of two $\frac{1}{2}$ spin particles like this: $$|S\rangle = \frac{1}{\sqrt{2}}\left( |+ \rangle ⊗ |-\rangle - |-\rangle ⊗|+\rangle \right) $$ is this the same as ...
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Measuring a quantum state for entanglement

I am new to the concept of quantum entanglement and measurement. The scarce knowledge about them that I have is As soon as you measure, the system collapses into one of the eigenbasis in which you ...
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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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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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Test the non-signaling principle

Has the non-signaling principle in quantum mechanics been tested experimentally?
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Is there a maximum quantum advantage in sensing?

This is a rather conceptual question. Quantum sensing takes advantage of entanglement (and other quantum properties such as squeezing) to get variances that scale much better than the ones one can ...
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What makes particles lose entanglement or do they ever lose it?

Once we generate a pair of entangled electrons or photons, will they ever lose their entanglement? If yes, then what causes them to lose entanglement? If measuring them causes them to lose ...
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How can work be generated from quantum correlations?

There are several articles (like this or this one) concerning extracting work from (quantum and/or classical) correlations. Nevertheless, these articles appear to be quite dense and without showcasing ...
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Quantum Entanglement and Faster than light Communication

Suppose, Alice has a Billion of photons in random states, while Bob has their entangled pairs. Suppose, Alice can measure the spin of the photons in X axis, while Bob can measure their spin in an axis ...
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Does preventing/delaying a quantum jump in one part of an entangled system affect the other part?

This question is related to the new experimental work related to catching and reversing a quantum jump in mid-flight. The relevant work can be found here. Assume that we have two entangled quantum ...
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Are there theories or experiments involving multi variable particles?

Every hidden variable/entanglement experiment I’ve ever heard described involves ONLY one variable of either polarization or spin. Therefore Venn calculations backed by diagrams highlight the ...
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How do you know when two objects are entangled? [duplicate]

I’m not asking how would you entangle two objects. I want to know how would you know they are entangled?
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Difference between two definition expression of quantum Fisher information metric

The symmetric logarithmic derivative $L_i$ is defined as the operator solutions to equations $$\frac{1}{2}\left(L_i\rho_{\lambda}+\rho_{\lambda}L_i\right)=\partial_i\rho_{\lambda}$$ The quantum Fisher ...
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What happens to physical qubits when they decohere during quantum error correction?

Say you have a scaled up system with 98% fidelity so 2% decoheres. These are physical qubits doing error correction on information encoded on logical qubits. 98% of these entangled qubits will be ...
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Why does the effect of measuring an entangled particle is not apparent when examining only one particle? [closed]

As far as I understand, the Bell test experiments show that observations (measurements) of entangled particles have an effect on one another. This effect is apparent when examining the results of ...
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Why is quantum entanglement surprising? [duplicate]

What about quantum entanglement is suprising, and what makes it a strictly quantum effect?. Suppose we have an particle of spin 0 which decays into two other particles, each with either spin up or ...
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Can entanglement be explained as a consequence of conservation laws?

This article at NewScientist magazine (subscription required) describes entangling photons by passing them through a half silvered mirror. http://www.newscientist.com/article/mg21929282.100-quantum-...
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Example of experimental or observational proof of quantum indeterminacy before measurement

How do we know that certain properties are indeterminate or undefined until they are measured? Take the example of quantum entanglement, saying that if you have two entangled particles, such as the ...
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Alignment of axes of measuring devices in a quantum entanglement experiment

Concerning quantum entanglement Wikipedia says: Measurements of physical properties such as position, momentum, spin, and polarization, performed on entangled particles are found to be ...
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Measuring non-commuting properties on entangled particles

Suppose we start with an entangled quantum state such that two particle spins are always perfectly correlated so that $S_{ax} = S_{bx}$ and $S_{az} = S_{bz}$. Suppose I measure $S_{ax}$ = +1 and $S_{...
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Is there a classical correspondence of the entanglement entropy in isolated quantum systems?

For an isolated quantum system, one can study the time evolution of entanglement entropy after a quantum quench (always a pure state), which has a rich behaviour in various different models. However, ...
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Pulling apart the atoms of a topological insulator

Consider a topological insulator. In order to destroy a topological phase, the band gap of the bulk system should close at some point (passing thru a conducting state), but if the atoms that make up ...
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Why are quantum-entangled particles nonlocal (spooky)? [duplicate]

Please explain why the following explanation of entanglement is wrong: Consider the two electrons in a hydrogen atom. Their spins are fermion-entangled (in a covalent bond): if one is up, the other ...
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Under what conditions does a beam splitter entangle two input photons?

There is a dispute on PhysicsForums related to what are the conditions necessary for two photons to be entangled by a beam splitter. Lots of references given by the forum users but they never arrive ...
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Separability of density operators on tensor product spaces

Consider a composite system $\mathcal{H}=\mathcal{H}_{A}\otimes\mathcal{H}_{B}$ where $\mathcal{H}_{A}$ and $\mathcal{H}_{B}$ are Hilbert spaces of constituent components (say two qubits). Let $\rho_{...
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Wave function collapse of entangled photons?

If we have two entangled photons then they are described by the same wave function. But if I measured the position of the signal photon and it collapses into a position, would the idler photons ...
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Difference between an entangled pair of photon and two magnets in a box? [duplicate]

I try to understand quantum entanglement and especially what it’s called « Action at a distance » from my understanding, if you have a pair of entangled photon, after measuring the polarization of ...
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Interpreting the Quantum Fisher Information

I recently saw this article in Nature Physics regarding Quantum Fisher Information (QFI): http://www.nature.com/nphys/journal/v12/n8/full/nphys3700.html Frankly I have not been following Quantum ...
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Is the Schmidt basis the one minimizing entanglement?

I know, that for a compound system $ |\psi \rangle_{AB} $ we can find the Schmidt basis, which is an unique one. Is it at the same time the basis, in which the two subsystems are minimally entangled? ...
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Entanglement Swapping of Werner states

I am provided with two Werner states \begin{equation*} \rho=F|\phi^+><\phi^+|+\frac{1-F}{3}(|\phi^-><\phi^-|+|\psi^+><\psi^+|+|\psi^+><\psi^+|) \end{equation*} or ...
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Can post-selecting on the screen in the Delayed Choice Quantum Eraser experiment be used to predict the quantum-eraser measurement results?

I'm curious about QM and spent the last 3 days thinking about Delayed Choice Quantum Eraser (DCQE) experiment, but I couldn't solve this issue: Assume we do the DCQE experiment so that for the whole ...
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Quantum eraser without the quantum eraser

I've recently watched PBS SpaceTime's video about the quantum eraser. In it, Matt describes a kind of crystal that splits particles into pairs and entangles them. The entangled particles are then ...
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Coincidence count probability in the entanglement-enabled delayed choice experiment

I'm trying to understand the entanglement-enabled delayed choice experiment and I'm kind of stuck at the term "coincidence count probability" which I can't seem to find definition of. Their full set ...
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How many tensor product terms are necessary to express a separable state? [duplicate]

Wikipedia (https://en.wikipedia.org/wiki/Separable_state) defines a separable state, as a state $\rho$ which can be written as: $ \rho = \sum_{k=1}^l p_k \rho_1^k \otimes \rho_2^k $ where $\sum_{k=1}...
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What's the meaning of distant measuring?

this is the context: Einstein, Podolsky, and Rosen concluded that there are two possibilities in the case of the spinning particles. Either (a) there is some mysterious, nonlocal interaction that ...
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Projection of Quantum State onto Bell State

I am very interested by this paper on entanglement swapping and timelike entanglement. The one thing I get really tripped up with is the whole idea of a projection onto a bell basis. I understand ...
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Is the Proof of Gisin's Theorem Wrong?

I have been trying to understand Gisin's paper, Bell's inequality holds for all non-product states. As I've worked through the proof he provides for his theorem, I've run into two things that simply ...
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Can Quantum Entangled particles convey information faster than speed of light? [duplicate]

Can Quantum Entangled particles convey information faster than speed of light. If they don't what was wrong with John Bells experiment?
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Superoperator cannot increase relative entropy

So I have to show that a superoperator $\$$ cannot increase relative entropy using the monotonicity of relative entropy: $$S(\rho_A || \sigma_A) \leq S(\rho_{AB} || \sigma_{AB}).$$ What I have to ...
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How to identify whether a quantum state is entangled or separable if you can measure it only in one basis?

Suppose you have two qubits and don't know whether they're in an entangled state $ \frac{1}{\sqrt{2}}\left(|0,0\rangle + |1,1\rangle\right)$ or in an equal mixture of states $ |0,0\rangle $ and $ |1,1\...
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Recognizing Entanglement from the Density Matrix in 2-qubit case

We know that the density matrix for a 2-qubit system can be written in the Pauli representation as : $$\rho = \frac{1}{4}\sum_{ij}t_{ij}\sigma_i\otimes\sigma_j$$ where $\sigma_i$ are the Pauli ...
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EPR paradox: instantaneous vs very fast?

An EPR quantum experiment can be explained by instantaneous collapse of the wave function regardless of the distance separating a pair of entangled particles. But do we have the certainty that the ...
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Representation of Translation Invariant Matrix Product States

Consider a matrix product state on $\mathbb{C}^{d N}$: $$ \Psi = \sum_{\sigma_1,...\sigma_N} A_1(\sigma_1) ... A_N(\sigma_N) |\sigma_1 ... \sigma_N \rangle \quad \quad (\text{OBC MPS}) $$ with some ...