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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If measurement causes entanglement with the observer, is any further measurement possible?

When an observer measures a system, the systems wavefunction collapses and they become entangled. Does this mean that any further measurement by the observer on the system is impossible, as the system ...
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Is non-unitarity from space-time expansion of the vacuum consistent with bulk/brane disentanglement?

We assume the vacuum is a manifold of highly entangled space-time quanta embedded in a higher dimensional bulk. The question asked here is how the Hilbert spaces for these domains may be related. If ...
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Is the NOON state entangled even if $N=1$?

Let $\mathcal{H}\cong\mathbb{C}^2$ be the Hilbert space of states of a single particle. Let $\{|\psi_A\rangle, \psi_B\rangle\}$ be a basis for $\mathcal{H}$. I now want to describe an ensemble of ...
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Can the dispersion relation of a spin lattice be modified by entanglement?

Suppose I have a cubic lattice of spins. Suppose I can prepare these spins in an uncorrelated state. No spin correlations. No entanglement. No nothing. Suppose I can measure the dispersion relation of ...
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Is spacetime in general relativity considered from the point of view of an unentangled observer? [closed]

Time from quantum entanglement: an experimental illustration https://arxiv.org/abs/1310.4691 "We implement this mechanism using an entangled state of the polarization of two photons, one of which ...
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Maximally entangled states of a qutrit system [closed]

How do I construct a qutrit system and then a maximally entangled state for a qutrit system?
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How can an entangled initial state lead to non-positive reduced dynamics?

According to https://arxiv.org/abs/1206.3794, the standard treatment models the time evolution of open quantum systems as follows: starting with a system in the initial state $\rho_0$ on the Hilbert ...
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Are Photons Entangled with Gravity? [closed]

Part 1: A photon's wavelength is red-shifted by expanding space. Since $E=hf$ the photon imparts some of it's energy to the curvature of space, slowing the expansion of space ever so slightly. Is that ...
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Calculation on the squashed entanglement

The definition of squashed entanglement:link I read a paper on this topic, and found a good result (Fig.2(a)) from it showing that the squashed entropy can indicate topological phase transition of SSH ...
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Where quantum physics and Neuropsychology overlap? [closed]

I am a current student looking toward a degree in Neuropsychology. I also have a fascination with quantum physics because there is something nagging me that looking outward at the universe and then ...
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What is the difference between coherence and entanglement? [duplicate]

I am confused to the two terms, my question is what is the difference between coherence and entanglement?
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The intepretation and maths behind the many-worlds interpretation of quantum mechanics

recently I started reading the book "Something deeply hidden" by Sean Caroll. In the book he talks about the many-worlds interpretation of quantum mechanics as a more elegant way of thinking ...
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From Entangled To Product

Start with an entangled pair of spin one-half particles: $|\psi\rangle = \frac{1}{\sqrt 2}(|\uparrow\rangle_z |\downarrow\rangle_z + |\downarrow\rangle_z |\uparrow\rangle_z)$ This cannot be factored ...
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SPDC One Arm vs Very Weak Coherent state

I know that SPDC(Spontaneous Parametric Down Conversion) is a method to generate heralded single photon source. So If we do homodyne tomography of single photon fock state, another arm of SPDC is used ...
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Calculating entanglement negativity without constructing density matrix

There are two procedures that I know of for finding the von Neumann entanglement entropy of a bipartite system split between $A$ and $B$ - a bad way and a better way. I have in mind computationally ...
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Does the collapse of an entangled photon triplet allow FTL communication? [duplicate]

I have read the assertion on other posts that if the entanglement of one photon of an entangled triplet is broken then the whole system collapses and the remaining two are no longer entangled, has ...
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How does a disentangler disentangle a short ranged entangled (SRE) states in multi-scale entanglement renormalization ansatz (MERA) circuit?

I encounter a problem in understanding the role of disentanglers in Multi-scale entanglement renormalization ansatz (MERA). I do understand that without disentanglers, some short ranged entangled (SRE)...
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Is there a physical intuition behind the disentanglement of Werner states and the sudden death of entanglement?

It is possible to write two-qubit Werner states in terms of a mixture between a Bell state and the maximally mixed state: \begin{equation} \rho_W = \lambda |\Psi_-\rangle \langle\Psi_-| + \frac{1-\...
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Is my simplistic understanding of a Qubit, Superposition and Entanglement correct? [closed]

I've gone through a number of Lenny Susskind's lectures on entanglement (both just for the joy of it and to better understand quantum mechanics) where he delves into the idea of a qubit. For the sake ...
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How are topological entanglement and quantum entanglement related?

There are recurring articles about the relation between quantum entanglement and topological entanglement: https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=topological+quantum+...
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Can speed be entangled?

When reading about quantum entanglement, I see that all variables that are discussed (spin for instance) have discrete states. Can a continuous variable such as speed be entangled? In the case of ...
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How to disentangle two entagled particles (photons) without distroying anyone?

Can a twin photon pair entagled in polarization get disentangled after one of the photons meets a PBS (polarization beam splitter). I suppose after the PBS the photon will go one way or another ...
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Why did Feynman say the double slit experiment "contains the only mystery" of quantum mechanics?

The double-slit experiment requires wave-like interference of possible paths according to the quantum action principle, but it doesn't require entanglement since it's a single-particle phenomenon. ...
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How is entanglement of non-spin variables incompatible with local realism?

It seems to me that spin/polarization entanglement is the only spooky one. That's because the entanglement of other observables would happen even in regular classical physics if we made a "...
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How can I actually get to the AKLT state from a product state in finite depth?

I'm currently learning about symmetry-protected topological phases in one dimension. The ground state of the AKLT model provides one such example. In particular, the AKLT state for any length $L$ ...
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Can we use entangled particles to find out what is going on inside a black hole?

If you throw one entangled particle inside a black hole can we know what is happening with it from looking into another entangled particle outside the black hole?
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FTL signaling by direction? [duplicate]

If the axis of a spin is determined by the first measurement, would using that to tell humans on Earth whether a planet light-years away is habitable be FTL communication? Depending on measurement ...
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Role of entanglement in quantum computing

I'm not a physicist and thus, I'm looking for a simple explanation on the role of entanglement in making quantum computers fast. I got a good analogy of a qubit: a coin tossed in the air can be ...
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What's wrong with my argument about entanglement entropy in QFT being time-independent?

Let's say we need to compute the entanglement entropy (EE) of a subsystem $A$ ($A=[0,L]$, $L>0$) in a 2D CFT. The density matrix of the total system (i.e., the real axis) is given by $$ \rho(t)=\...
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What is the precise statement of the rule "LOCC cannot create new entanglement"?

I think that the statement "LOCC cannot create new entanglement [between spatially separated regions $A$ and $B$]" is good enough when only two parties are involved in the process. However, ...
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What happen to a pair of entangled photons when they enter a polarizing beam splitter?

$\newcommand{\ket}[1]{\left|#1\right>}$ Suppose we have two entangled photons in the state of $(\ket{HH}+\ket{VV})/\sqrt{2}$ and one of them enters through the mode a of a polarizing beam splitter ...
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How does measurement work in this Multi qbit entangled system?

Given a 4 qbit entangled state: $C_1= 1/2 (|0000\rangle+|1100\rangle+|0011\rangle-|1111\rangle)$. Assuming we take one particle, and measure them it in the basis: $1/\sqrt2(|0\rangle+|1\rangle)$, $1/\...
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Can you create a pair of entangled particles with asymmetric probabilities of quantum states?

Is it possible to create two entangled particles emitted such that the "left" particle has a different liklihood of being measured in a certain state than the "right" particle?
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Calculating the probability of detection on two detectors in MZI using matrix operators

Given a pair of entangled particles: $$\frac 1{\sqrt2} \left[\frac{(\sqrt3H-\sqrt2V)}{\sqrt5}R + \frac{(\sqrt2H+\sqrt3V)}{\sqrt5}L\right]$$ If we measure a particle in $L/R$ basis the probability of ...
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Computation of two-site entanglement entropy of the critical transverse-field Ising chain

The paper Osborne and Nielsen, Phys. Rev. A 66, 032110 gives an exact solution for the two-site reduced density matrix for the transverse-field Ising model with periodic boundary conditions (Eq. 26): $...
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Where does the black hole firewall energy come from?

There are a couple threads on the firewall paradox on StackExchange, however I have not been able to find one that explains the physical origin of the firewall in black hole evaporation. There is an ...
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What Class of Interactions, excluding spontaneous pair creation, Entangle two particles [duplicate]

Un-Entangled particles have state vector: $|\psi\rangle = |p_1, p_2\rangle$, while entangled particles have a composite state vector $|\psi\rangle = 1√2(|p_1,p_2\rangle\pm |p^\prime_1,p^\prime_2\...
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Entanglement entropy of the infinite transverse field Ising chain at the critical point

Consider the $1$D transverse-field Ising model, $$H = -\sum_i \sigma_i^z \sigma_{i+1}^z-h\sum_i \sigma_{i}^x$$ at the critical point $h=1$ with periodic boundary conditions. Question: What is the (von ...
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How does the many-worlds interpretation solve spooky action at a distance?

If we take the classic example of two particles that are entangled with up spin and down spin, and we separate these particles a few light years apart and then observe them one after the other, they ...
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Mixed separable state with a continous probability distribution

A separable state is defined as follows: $\rho_{AB}$ = $\sum_{i} p_{i} \rho_{A}\otimes\rho_{B}$, where $\rho_{A,B}$ are pure states. Essentially it is a classical mixture of unentangled states. Such a ...
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Understanding Entanglement basis change in a practical sense

I am trying to understand the concept of basis change for a pair of entangled particles (in mathematical sense as well as what it means in case of detection on detectors in a Mach-Zehender ...
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Representation of an entangled state

I am reading the EPR paper. In the second section, they introduce a quantum wavefunction $\Psi\left(x_{1}, x_{2}\right)$ Let $a_{1}, a_{2}, a_{3}, \cdots$ be the eigenvalues of some physical quantity ...
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Quantum extremal surface and black hole evaporation inquiry

Recently there has been progress made in the black hole information paradox by using the tools of AdS-CFT correspondance. Specifically, the Page curve for an evaporating black hole has been ...
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2 qubit entanglement in a QC

To my knowledge, in a Quantum Computer it is possible to obtain any Bell state by applying a Hadamard gate onto a qubit state $|x\rangle$ followed by a CNOT gate onto some target qubit state $|y\...
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What is meant by the term "elements of physical reality" used by EPR paper?

I was thinking about the phenomenon of quantum entanglement. The widely accepted view of nature of reality is that of copenhagen interpretation(please correct me if I am wrong!) while Einstein pointed ...
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Intuition/Motivation behind mixed states

I have some confusion regarding pure, mixed and entangled states and I'm trying to gain some clarity on this. Set up, and my current understanding: One fundamental distinction I seem to have (please ...
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How does the proof for the area law for 1D systems work?

I am currently reading this paper in order to understand the proof of the area law for one dimensional, low energy systems such as 1D spin chains. The main area law theorem is given on page 13 and is ...
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Swap entanglement from harmonic oscillator chain

I am currently analyzing a system of coupled harmonic oscillators in a thermal state and have already asked a question related to this. I have calculated the entanglement entropy of a chain of two ...
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Can a pure un-entangled quantum state be correlated?

Is it possible for a joint probability distribution for two random variables $x$ and $y$ to be independent, i.e $P(x,y)$ = $P(x)P(y)$, but the mean values of distributions of $x$ and $y$ to be ...
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Quantum entanglement

Suppose we have two particles with entangled eigenfunctions. Let's say they are in two different regions of space, with different local Hamiltonians. Now we measure one of them, therefore the other. ...

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