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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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Is photon's direction entangled?

Consider a free electron, with photon, that runs to electron under some angle(as everybody says). Compton scattering is happening, and electron instantly reemits photon in different angle. First, ...
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Classical and quantum correlations in bipartite system

I would like to know how to answer following questions: Is there classical/quantum correlations in given bipartite pure/mixed state? I have gathered several definitions. Some of them (it seems) ...
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Alice's Caterpillar measuring the quantum state of her entangled particle collapses her connection to Bob? [closed]

I understand that eventually in this discussion there will be some unexplained universal law that we will all have to accept as a God's law and build every physics proof upon that. But I want to know ...
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Quantum mechanics or creationism [closed]

I read this article recently: https://arxiv.org/abs/1802.00227, where author claimed that quantum mechanics is a sort of creationism! He argued in three different cases to show quantum mechanics does ...
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Is entanglement *not* intrinsic to state, but dependent on division into subsystems? (Susskind QM)

I'm working through Susskind's "Quantum Mechanics" book (TTM series), which I quite like. Background In Lecture 7 (Chapter 7), he studies a 2-spin system. A single spin has eigenvectors: $$|u\...
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Why can't the Schrödinger equation be solved exactly for multi-electron atoms? Does some solution exist even in principle? [duplicate]

It is common knowledge that the Schrödinger equation can be solved exactly only for the simplest of systems - such the so-called toy models (particle in a box, etc), and the Hydrogen atoms; and not ...
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When does the exchange of energy take place in the Quantum Eraser experiment?

I found this PBS video regarding the delayed choice / quantum eraser experiment, which, like all the others I've seen, states that the interference pattern generated by the photons that pass through a ...
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Entanglement fidelity for a single qubit?

So I have been working out problems from the book "Quantum Computing Explained" by David McMahon. On page 166 he defines "Entanglement Fidelity" and says the following: Consider a density operator for ...
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Is entanglement an indeterminate property until measured?

This question is related to Physics SE Objective or Subjective, but is not the same. There was roughly a 10:1 imbalance of answers (weighted by reputation points) to the effect that entanglement is ...
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In what sense is it 'more complicated to navigate through a Hilbert space to find more complex states'?

In the lecture titled Entanglement and Complexity: Gravity and Quantum Mechanics, Professor Leonard Susskind implies (at time 17:49) that the complexity of a quantum state impacts how 'complicated' it ...
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Squeezed state in Fock basis

I'm learning quantum optics, and I'm starting to manage boson algebra. In particular, as a pure exercise, I would like to express a squeezed vacuum state in the Fock basis, which, according to ...
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How to write a wave function for two entangled photons in a (J,P)=($0$,+) state?

I'm studying some experiments using pairs of atomic photons, in particular those photons that are consequence of the $(J=0)\rightarrow(J=1)\rightarrow(J=0)$ cascade of calcium. All of them start ...
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CNOT-Gate acting on entangled qubit state with more than two qubits

I asked this already in reddit but got now answers, so I hope here is the right place. Question in short: Is there a general form of the CNOT Gate, so I can apply it to an arbitrary entangled state ...
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if observing entangled particles causes the entanglement to break, how do we know that they were ever entangled?

I have a working understanding of physics, and am working through quantum entanglement. I read that when we observe one entangled particle, we then know the spin of the other. I also know that when ...
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Is Time emergent from quantum entanglement?

This paper shows that 2 photons in a toy universe (isolated box) are seen as static to an outside observer unless interacted with/entangled with. It states that it shows that time emerges from ...
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What's the fuss over quantum entanglement about? Is it because people have misunderstood? [closed]

The first video I've seen about quantum entanglement claimed that changing A's spin will immediately take effect on B's spin as well. The more I read into this topic the less it seems that this is ...
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Is quantum entanglement “observer dependent”?

See also here. According to the answers of the mainstream entanglement should be objective. Once more this question asked in a more mathematical way... Any vector $|{\psi}>$ of norm 1 in a Hilbert ...
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Questions on Bell's theorem and Disproving the hidden variable theory

I'm not a physicist (computer science background), but always was fascinated with the concept of entanglement. As many, the first intuition I had was - well, the two particles share some knowledge ...
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The term “Coherence” is overloaded!

I have troubles with modern terminology in the field of quantum information technologies. There are a lot of new terms that everyone is using and no-one takes time to explain, even though modern ...
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A hollow black hole & quantum entanglement?

Let us consider the following thought experiment. we create an entangled pair of particles and we separate them. next, we would place 1 particle inside a massive hollow sphere and the other particle ...
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Reconcile a pair of two-qubit boundary-state separability probability analyses

It is now clearly well-established--though formalized proofs are still largely lacking—that the probability, with respect to Hilbert-Schmidt measure, that a generic two-qubit state is separable/...
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Peeking inside Schroedinger's Black Box (not his cat box)

While exploring variations on Shroedinger's Cat, I came up with the following puzzle: Define a Schroedinger's Black Box (SBB) as a volume whose contents have no causal connection to an observer A. ...
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Do time crystals require quantum entanglement?

In this video (at this time in the video) the vlogger seems to make a statement that the only way time crystals can be achieved is if some of the atoms (at a periodic distance from each other) are ...
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Are *all* quantum states indeterminate if they are outside the observer's light cone?

Is the state of a particle arriving from outside an observer's light cone merely unknown, or is it quantum mechanically indeterminate? I guess the question needs a background in the form of a ...
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Is the common interpretation of quantum entanglement just a perversion? [duplicate]

The idea of spooky action at a distance seems to imply that the information was always present at each end, but the common interpretation is that information can be transmitted instantaneously. Is ...
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Are the phases of entangled photons correlated?

I know the polarization of entangled photons are correlated, but are the phases of the wavelengths correlated as well?
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Current quantum theory does not offer a dynamical mechanism to account for energy transfer in position-measurement, WHY?

In Quantum Mechanics (vol. 1) by Tannoudji et al., there are two pages discussing "perturbation created by a position measurement" (p.278-279). They consider a particle in an infinitely deep potential ...
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Simulating the evolution of many-boson states

My task is to simulate the scheme presented in this paper: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.77.062316. In this question: Creating an operator for a polarizing beam splitter, I ...
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What exactly are Joint measurements?

So far, I understand what is LOCC. In the literature there is also another kind of measurement usually referred as joint measurements. I wonder if someone can explain the exact meaning of this joint ...
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Covariance matrix and displacement after homodyne detection on three-mode state

I'm provided with the covariance matrix of a three-mode cluster state (a state which is obtained by entangling three momentum squeezed states). I want to see what happens to this state when I measure ...
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1answer
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Physical meaning of parametric down conversion of photon

I would like to understand what is the physical processes occurring in the particle picture when parametric down conversion of photon happens. The description in Wikipedia states: "A nonlinear ...
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Entanglement entropy calculation

I have been trying to understand this paper: Susskind. “Black Hole Entropy in Canonical Quantum Gravity and Superstring Theory.” [1402.1128] Long Short-Term Memory Based Recurrent Neural Network ...
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3answers
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Is BEC the same as entanglement?

I understand that Bose Einstein condensate is: A Bose–Einstein condensate (BEC) is a state of matter of a dilute gas of bosons cooled to temperatures very close to absolute zero. Under such ...
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Quantum Computing, and if a Q-bit can be entangled and in superposition at the same time

I was wondering if a Q-bits can be in both entangled and in superposition at the same time?
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If quantum entanglement is not an active link between particles, then how did China transmit data via Earth-to-space quantum entanglement? [duplicate]

So based on this thread : Why is quantum entanglement considered to be an active link between particles? which was asked 7 years ago, the answer says "Entanglement is being presented as an "...
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Can someone explain in a simple manner that How can we use Quantum entanglement to transfer data instantly when we seperate photons? [duplicate]

So I'm an IT engineer and i just watched a video about Quantum entanglement, that somehow when we split the entangled photons, any change happening to one of them will be reflected to the other one ...
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Why can't we increase quantum entanglement via LOCC?

Why must every measure of entanglement satisfy the crucial property that its expected value is non-increasing under LOCC?
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How does a unitary operator acts on an entangled $|\psi\rangle$? [duplicate]

If there is: $$ U = \sum_{x,y \in \mathbb{Z}_2} |x \dot{+} y\rangle |y\rangle \langle x| \langle y| $$ acting over $|\psi\rangle$ which can be $|00\rangle, |01\rangle, |10\rangle, |11\rangle$, does ...
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How does the trace acts over one of the two entangled qubits? [duplicate]

If $|\psi\rangle$ is entangled, i.e, is in the subspace spanned by the states $|00\rangle, |01\rangle, |10\rangle, |11\rangle$ how does the partial trace operates on the density operator $|\psi\rangle\...
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Can we use entangled black holes to form wormholes?

If any object can be made black hole by crushing it to its Schwarzschild radius, can we do the same process in two different points in the space keeping the quantum states of both the objects ...
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Negative of Quantum Gates and Entangled States

I'm well aware that using this quantum circuit: $\hskip2in$ $\hskip2in$Quantum circuit used in the generation of EPR pairs. It is possible to create entangled state of two qubits (EPR pairs), in ...
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How many parametric down converters are needed to entangle four photons?

In quantum optics, how many parametric down converters are needed to entangle four photons?
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Never ending quantum superposition? [closed]

So I am wondering over the Copenhagen Interpretation how can a decoherence or measurement ever happen. If in the beginning of the universe everything was in a state of coherence being in ...
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Wigner Function for an Entangled Composite System

How is it possible to compute the Wigner function for a composite system that is prepared in an entangled state? In particular, consider the state $|ψ_{AB}\rangle=\frac{1}{\sqrt{2}}(|0_A\rangle|1_B\...
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A telegram from Mars

Say, people fly to Mars, get in trouble, and want to send us a SOS signal. The average distance is 225 Gm, so they must wait 25 minutes to hear our response at the speed of light. This may just be too ...
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Entangled qubits measurement

I have two entangled qubits. I measure one of the qubits and get a 1. Am I correct in saying that the second qubit instantaneously collapsed to a correlated state, even before I measured it? Or does ...
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Is there a way to determine if one qubit of an EPR pair has collapsed after the other was measured?

My friend on the other side of the universe and I share the respective qubits of 2 EPR pairs. My friend measures one of the qubits, so one of my qubits will collapse. Must I measure this collapsed ...
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Why does quantum entanglement break?

Why does quantum entanglement break when the corresponding particles are intefered with? Is there possibly any way to actualy use it, such as in interstellar communication?
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How do we know quantum entanglement really happens?

Quantum entanglement says that we can entangle two particles with an opposite property, separate them, and that they exist in an indeterminate state until we observe one of them. eg. If we create a ...
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What do physicists mean when they say QM proves randomness?

Some physicists like Michio Kaku has said that the physics has proven randomness. Even some of my friends(they're not physicists) cite the Delayed Choice Quantum Eraser experiment and entanglement to ...