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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What is the physical basis behind the independence assumption in Bell's theorem?

Bell's theorem rests on two main assumptions: locality and independence. Locality has its basis in relativity and it seem to be a property of all known physical theories accepted today (General ...
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Are Cooper pairs (of electrons) entangled?

I have read this question: Does a normal torch emit entangled photons? where Quantum Journalist says: Although all the particles of a BEC are in a single quantum state, they are not ...
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Doesn't the Schrödinger's cat inside the box cause the probability wave function to collapse long before a human opens the box?

My point is that there is no superposition of dead and alive. The cat will cause the probability function to collapse long before we open the box. What am I missing here? Isn't the cat capable of ...
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How common is entanglement?

In scientifically educated layperson terms (someone who remembers their college math and physics classes from a STEM related degree but never pursued physics specifically), how common is quantum ...
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Using entangled qubits to communicate [duplicate]

After reading up on quantum mechanics considering entangled qubits I was asking myself this simple question: If one qubit (A) positioned on earth is entangled with another qubit (B) which is - say - ...
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Unit of measurement for entanglement between two particles?

Do any theoretical frameworks employ a unit of measurement to quantify the "strength" of the entanglement between two particles? For example, when the particles are unentangled, the unit is 0, but ...
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If an anticorrelated entangled particle's state is “flipped” via some interaction, will the resulting particle be correlated or anticorrelated?

I vaguely remember learning in college that this had been done experimentally, but can't recall enough detail to find more information. Suppose you create two particles with an entangled property, e....
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Quantum Key Distribution via Modified Double Slit Experiment

Here's my reasoning: Setup 1: Take a traditional double slit experiment and turn on the photon source. Interference fringes should appear on a detector screen placed opposite the photon emitter, ...
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Why are entanglement and purity non-linear functions of $\rho$?

Any linear function of the density matrix can be related to a proper observable, but is it not the case of entanglement and purity?
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Effect of different reference frame on quantum entanglement measurement

According to what I read about entanglement, when you measure the spin of one of the entangled particles all the possibilities collapse to one value immediately and the other particle will give the ...
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Is there a way to measure the degree of superposition of a quantum state?

I am wondering if there is a way to calculate the amount of superposition that a quantum state is in. For example, if I have a $2$-qubit quantum system, with basis $\mathcal{B} = \{|00\rangle, |01\...
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Is it correct, according to quantum mechanics, to say that an observer can get entangled with the measured system?

Entanglement applies to any physical system, observers included. However, what one typically means by the observer being entangled with the measured system seems not to comply with the requirements ...
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What is the simplest circuit on a quantum computer that “proves” superposition/entanglement

I am a Physics Teacher at pre university level. Tunneling, wavefunctions and uncertainty are on the syllabus on a very vague way. Quantum superposition and entanglement are not but tie in well and I ...
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Is it possible that the speed of sound in some strange quantum material is faster than the speed of light in vacuum [closed]

If we believe the spacetime structure(including the limit of speed) could emerge from some vacuum structure(such as string-net condensation),then, is it possible that the speed of mode of excitation ...
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5 qubit quantum error correction property [closed]

Having $$\eqalign{ |v_0⟩&=|00000⟩+|11000⟩+|01100⟩+|00110⟩+ {}\cr &\quad |00011⟩+|10001⟩−|10010⟩-|10100⟩− {}\cr &\quad |01001⟩−|01010⟩−|00101⟩−|11110⟩− {}\cr &\quad |...
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Entangled states and separable states

B Two electrons in the same orbital is clearly an entangled quantum state since it is not a tensor product: $$|\psi\rangle=\frac{1}{\sqrt{2}}(|\uparrow\rangle \otimes|\downarrow\rangle-|\downarrow\...
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Understanding Bell's inequality

I'm struggling to understand Mermin's article "Is the moon there when nobody looks?". In this article, a Bell experiment with three measurement settings is discussed. Is it possible to reduce the ...
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Entanglement of Werner States

Let the Werner state $$\rho_W = W\mid\Psi^-\rangle\langle\Psi^-\mid + \frac{1-W}{4}\mathbb{I},\ W\in [0,1],$$ where $|\Psi^-\rangle=(|01\rangle-|10\rangle)/\sqrt{2}$. I have repeatedly heard that ...
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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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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 vectors of the eigenbasis ...
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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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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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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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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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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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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 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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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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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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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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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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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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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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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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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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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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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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If quantum entanglement isn't a physical thing how does it makes quantum teleportation work? [duplicate]

I read about superposition and correlation between entangled particles and there is no physical communication between them, but obviously quantum teleportation uses this correlation to send something ...
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How do I apply no signaling condition for the given Quantum Adder problem

The Unitary transformation for the Imperfect Quantum Adder problem is such: Both the states belong to two Hilbert Spaces and we are trying to add both states to one system using Ancillary qubit. I ...
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How does weak measurement affect quantum state?

I'm trying to understand how to describe the quantum state after weak measurement using these two toy examples. Hopefully, these simple examples and your answer will help others who want to learn ...