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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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quantum superdense coding -How many qubits are needed?

This is an extract from Wikipedia about superdense coding: "In quantum information theory, superdense coding is a quantum communication protocol to transmit two classical bits of information (i.e., ...
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Quantum entanglement, how do we know there was no spin? [duplicate]

Im not a scientist, so go easy on the explanation! As I understand it we can create two entangled particles. The entangled particles have a spin property which is opposing. When we measure one of the ...
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What are some useful resources for simple two-body quantum simulation?

Are there available, commercial or free, numerical packages for simulating quantum dynamics that have built-in tools especially for quantum mechanics? For the purpose of this question, I want to ...
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A question on the no-cloning theorem

When reading papers on the no-cloning theorem, I understand that one is searching for a unitary matrix $U$, such that for any state $\vert \phi_A \rangle$ in a Hilbert state $H_A$, we have that $U\...
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Von Neumann entropy in the algebraic approach

In the algebraic approach to QM a quantum system has associated to it one $\ast$ algebra $\mathfrak{A}$ generated by its observables. A state is a positive, normalized linear functional $\omega : \...
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How Forces are Represented from Quantum Computation or Entanglement Perspective?

In the fields of Quantum Computation and Entanglement there is content on the structure of quantum systems and artificial quantum systems like qubits and quantum cellular automata. I have even seen ...
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Entanglement in double slit experiment

Suppose we send 2 entangled photons through the 2 slit, does measuring the position of one of the photons affect the other particle's wave function or does the other particle still behave as a wave ?
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If quantum cloning was possible would entanglement enable faster than light communication?

Assume quantum mechanics allowed you to clone particles. How could you use quantum entanglement to communicate faster than the speed of light? In the 1980s a scientist proposed using quantum cloning ...
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Finding all the entangled states and the measurements for which they violate Bell's CHSH inequality

I have heard that finding all the entangled states that violate Bell's CHSH inequality, and finding the measurements for which they do so is an NP hard problem. Is that true?
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Cosmic entanglement

Is there a way to measure the entanglement of CMB photons? Could it be possible to measure such a thing and/or entanglement of photons from stars and/or cosmic neutrinos/gravitational waves and/or any ...
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Does the physical distance between qubits influence the speed of quantum computers?

I've heard lots of talk over the past decade about the potential of quantum computing. Most of what I've read about the topic (mostly articles targeted at the layman) focuses on the exponential ...
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What are unital maps?

In quantum information, one defines an unital map as the one that preserves the identity operator $\epsilon (I) = I$. A popular example is the so-called amplitude damping channel. My question is, ...
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Deriving Reduced Density Matrix for Electron Z Coordinate Position from State Vector of Electron Z-Spin Entangled with Z Momentum

Suppose I have a composite state vector that is a product state of z spin and z coordinate momentum of an electron (tensor product of state vectors for z spin and z momentum). Then, at $t_0$, the z ...
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How would a quantum radar detect the state/information change of a remote, unobserved particle?

The Department of National Defence is investing into a technology they are coining quantum radar. The premise is that they would create an entangled pair of photons, and shoot one out into the air ...
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What is the difference between no-disturbance principle and non-contextuality?

The no-disturbance (ND) principle states that, for any three observables A, B, and C such that A and B are compatible, and A and C are compatible, the probabilities of outcomes of A do not depend on ...
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What are the range of values of the negativity in quantum mechanics?

The negativity of a quantum state, given by $N(\rho)=\frac{||\rho^{\Gamma_A}||-1}{2}$ where $\rho^{\Gamma_A}$ is the partial transpose with respect to subsystem $A$ of the density matrix of a ...
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Quantum entanglement of photons proximity

When scientists entangle photons or atoms or other objects, is it only their state that gets entangled? What about their proximity in space? If you move one sideways will you see the other one move ...
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Heisenberg's uncertainty principle and measurements

While studying quantum key distribution, I came across this postulate which states that given two entangled particles A and B at Alice and Bob sites respectively, if Alice measures A to be $\uparrow$ ...
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Quantum entanglement and 'perfect' synchronized measurements [closed]

Given two entangled particles A and B barely separated by a very tiny small distance such as the communication delay between both observers Alice and Bob willing to measure the state of A and B ...
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What is the formal definition of a system?

According to quantum mechanics, bosons can occupy the same quantum state in the same system but fermions cannot because the wave function for the interchange of two identical fermions would be ...
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Are identical particles always entangled even when not interacting?

Aren't the states of two identical particles always entangled even if they are not interacting? The states of two identical particles are either symmetric or antisymmetric i.e., cannot be written as ...
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Time Evolution of a Maximally Entangled State [duplicate]

My question is basically this: Does a maximally entangled state stay maximally entangled under the time evolution? Assume our Hilbert space is $\mathcal{H}_A \otimes \mathcal{H}_B$. At the time $t=...
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Entanglement of formation of the mixture of maximally entangled states

Suppose we have two spin-$S$ systems. Let $\left| \psi_{a,b} \right\rangle = \frac{1}{\sqrt{2}}(\left| a,b \right\rangle+\left| b,a \right\rangle)$ be the maximally entangled state. ($a\neq b$ and $-S\...
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High-dimensionality of multi-electron wavefunctions

My understanding is that the main difficulties in understanding superconductivity and topological insulators is related to the fact that free electrons are highly entangled and models that simplify ...
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Heisenberg picture and expectation value with position operator

I'm struggling with the use of the Hesenberg picture in Quantum Optics, where we use the position operator of a mode $\hat{x}_1$ defined as $\hat{x}_1=\frac{1}{2}(\hat{a}_1+\hat{a}_1^\dagger)$. ...
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Entangled photons entering a polarizing Beam splitter

I understand what happens to a polarization beam splitter (PBS) when a simple input is given, what happens when an entangled pair of photons is used as inputs? For example, for the state $|HH\rangle +...
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Do polarization measurements of entangled photons in any basis always produce correlated outcomes?

I am looking to confirm or clarify my understanding of a measurement on an polarization entangled pair of photons. I am considering an entangled pair in the state |ϕ+⟩ =1√2(|HH⟩+|VV⟩) (positively ...
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Entangled photon absorption redshift/blueshift scattering

I have read these questions and answers and none answered my question. What happens when one of the photons in entangled state gets destroyed? Does measuring destroy entanglement Information and ...
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Entangled photon triplet

I have read these questions and answers, and it made me curious How does measurement affect multi-particle entangled particles? How do particles become entangled? What is quantum entanglement? Two ...
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Status of entanglement/black hole entropy in quantum field theory

It used to be said that in QFT, black hole and entanglement entropy is overcounted because entropy turns out to depend on volume, even if we somehow eliminate infinities. Does this assessment still ...
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How are joint measurements of quantum states realized experimentally?

I have a question regarding the experimental implementation of joint measurements of quantum states. While I understand for example the teleportation protocol using Bell states, something that still ...
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Quantum solution to the two-army problem?

The two-army problem can be described as follows: Two blue armes is separated by the white army, they can win the battle only if they attack together. However, the messenger who send the attack ...
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Superposition of polarized photon

I understand that a $+45^\circ$ polarized photon is a superposition of $0^\circ$ (vertical) and $90^\circ$ (horizontal): $|\!+\!45\rangle = 0.707 (|H\rangle + |V\rangle)$. So measuring the ...
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How to properly calculate off-diagonal terms in covariance matrix for entangled Gaussian state?

I would like to ask how to properly calculate the off-diagonal terms in covariance matrix for the entangled Gaussian state? E.g. from https://arxiv.org/abs/0810.0534v1 we have a coherent Gaussian ...
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Unitary transformation from pure unentangled state to pure entangled state

I believe that for any state $|\Psi\rangle$, that is unentangled, and for any state $|\Phi\rangle$, of the same dimension that is entangled, there is a unitary operator $\hat{U}$ such that $\hat{U}|\...
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Can urea crystals be used to generate pairs of entangled photons?

According to this link https://patents.justia.com/patent/4639923, urea crystals can be used as optical parametric oscillators. The parametric oscillator uses a nonlinear crystal to convert the pump ...
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Entanglement and the Interaction Hamiltonian

I understand that entanglement is irreversible, due to loss of information. I also believe that, at least sometimes, it can be produced using an appropriate "interaction hamiltonian", $\hat{H}_{int}$, ...
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Understanding entanglement entropy in the transverse field Ising model

I'm working on a project studying the transverse field Ising model in 1D with periodic boundary conditions, given by $$H = - J \sum_{i=1}^{N} \sigma^z_i \sigma^z_{i+1} - h \sum_{i=1}^N \sigma^x_i$$ I ...
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Wigner function in quantum teleportation

I'm starting to manipulate Wigner function and I'm studying the Wigner function approach in Quantum Teleportation. My reference for this is page 29-30 of Braunstein and Van Loock "Quantum ...
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How to describe time dependence piece of a stationary state with average phase

I am reading this paper on the synchronization of atomic clocks via entanglement (https://arxiv.org/pdf/quant-ph/0004105.pdf) and can't figure out how they are using $\Omega = (E_{1} - E_{o})/\hbar$ ...
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What is the minimum number of separable pure states needed to decompose arbitrary separable states?

Consider a separable state $\rho$ living in a tensor product space $\mathcal H\otimes\mathcal H'$, with $\mathcal H$ and $\mathcal H'$ of dimensions $D$ and $D'$, respectively. If $\rho$ is separable, ...
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Why in the definition of GHZ state only the $M>2$ case is considered?

I have a basic question about the definition of the Greenberger–Horne–Zeilinger state , which is defined as the maximally entangled state. Why is it only considered applicable for subsystems $M > 2$...
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How do entangled matter condensates fall?

Is there any experimental measurement of how highly entangled matter states fall (or highly entangled photons curve) in a gravity well? I've found some preliminary experimental setups in this paper ...
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Can scatter interference patterns from entangled pairs carry information?

I am having problems wrap around why you can't send information faster than light with a similar setup to the quantum eraser experiment. ( https://arxiv.org/abs/quant-ph/9903047v1 ) What I mean is, ...
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If a state can be efficiently represented by a Projected Entangled Pair State (PEPS), can we prepare it physically?

If we can use PEPS (Projected Entangled Pair State) to represent a many body quantum state, can we generate it by a quantum computer? As far as I can understand, PEPS is dual to a quantum computer ...
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Question on Eigenstate thermalization hypothesis

My understanding of a system that satisfies the eigenstate thermalization hypothesis (ETH) is the following. If we consider a local operator, $O_i$ we can relate its expectation value with respect to ...
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What is some good reference for tensor network?

I am a physicist who is now doing some research in quantum information, which might involve tensor network.
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Entangling two quantum systems that are separated by a distance

If two quantum systems are entangled their measurements become correlated, even if they are separated by a distance. But what if one has two different quantum systems at hand, initially un-entangled ...
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Can the entanglement entropy of a ground state scale extensively?

To my knowledge, entanglement entropy (EE) for the ground state of a many-body system scales subextensively, that is: \begin{equation}\lim_{\text{Vol}_A \to \infty} \frac{S_A}{\text{Vol}_A}\to 0.\end{...
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Does time emerge from entanglement?

I stumbled upon the hypothesis that time emerges from quantum mechanics as long as the observer is part of the system. That is, the evolution of an entangled state can serve as a clock for an observer ...