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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Are their known paradoxes regarding a detector that is entangled with a particle it detects?

Is it possible to design a system where a detector is entangled with the particle it detects, and to create a paradox from this? Upon observance of a state by the detector, a correlation between the ...
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Entanglement and the relativity of simultaneity: an experiment [duplicate]

Setting of the experiment: Triplets Alice, Bob, and Carla have plenty of entangled electrons when they depart from Earth. Alice stays behind, Bob goes to Zeta Reticuli, and Carla goes to Andromeda. ...
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In Quantum imaging with undetected photons, why is the idler photon sent through a second non-linear crystal if it is then discarded?

I've been reading up on quantum ghost imaging and other similar approaches. In particular, I'm reading a paper Quantum Imaging with Undetected Photons by Gabriela B. Lemos et al and trying to ...
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If the twins with time dilation can see each other's clock or talk using an entanglement device, how does a twin think his clock is faster? [duplicate]

We have a case with two twins traveling away from each other, and each twin thinks the other person's clock is slower. And one answer has an explanation: it is what each twin "perceives", ...
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Entanglement entropy of Werner state [closed]

Assume that we have a mixture of an entangled state $|\psi^{-}\rangle=\dfrac{1}{\sqrt{2}} (|01\rangle - |10\rangle)$ and a completely mixed state $ \mathbb{I}_4/4: $ $\hat{\rho}=\dfrac{N}{4}\mathbb{I}...
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Is the existence of absolutely separable quantum states possible in nature? [closed]

Can quantum systems be in an absolutely separable state, without any relationship? Or does this approximation and some measure of entanglement (for example, through gravitational interaction) still ...
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Bell State Measurement Algorithm

I'm relatively new to quantum computation and am taking a course in it. I was wondering if it is possible to code an algorithm which would be able to take an input of a 2 qubit state and perform a ...
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What, if anything, is wrong with this proposal for faster-than-light communication using entanglement swapping? [closed]

What, if anything, is wrong with this proposed mechanism for faster-than-light communication? Here are pastes from the proposal: Spooky Communication at a Distance Faster-Than-Light Communication via ...
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Does entanglement swapping happen instantaneously?

Is entanglement swapping instantaneous? Suppose photons 1 and 2 are entangled. And, 3 and 4 are entangled. A Bell-state measurement (BSM) is performed on photons 2 and 3 to cause entanglement ...
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Can an entangled pair of qubits be created across a distance by using purely classical communication?

Can an entangled pair of qubits be created across a distance by using purely classical communication, i.e. without using quantum communication?
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Why isn't the final screen in the double slit experiment a 'detector' which would collapse the wave function retroactively?

I'm sorry if this is a stupid question but I'm someone who is not from the physics world trying to learn about this for the first time (I'm reading Daniel Golding's Quantum Physics for Beginners and ...
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Entanglement cycles [closed]

Absolute amateur here with a question that was probably answered eighty years ago somewhere. Suppose we have three particles of a type such that if two are entangled, then if either particle is ...
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Is there a superposition of two density matrices?

Say I have two density matrices $\rho_0 = \sum_{i,j} \alpha_{i,j} |i\rangle \langle j|$ and $\rho_1 = \sum_{i,j} \beta_{i,j} |i\rangle \langle j|$. What is the density matrix $\rho$ corresponding to ...
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Do we have entanglement measure in continuous variables (Not witness)?

I have been searching for entanglement measures for continuous variable systems like we have entanglement entropy for discrete variable systems. My searches have ended in entanglement witnesses. I was ...
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How to imagine wavefunction branching?

This is a question particularly geared toward the Many Worlds interpretation, but I think it could be translated to other approaches as well. I am not sure I understand exactly what sort of events ...
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How do I show the separability of this density matrix?

I am stuck since a longer time regarding this exercise where I need to work with a density matrix of the given form $$\displaystyle \rho_{AB}(X)= \frac{1}{N+\text{tr}X^2}\left( {\begin{array}{cc} ...
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Do bipartite unitary operators always have a “non-entangling” basis?

Let's say I have a unitary operator $U: A \otimes B \to A \otimes B$. Will there exist a basis for $A \otimes B$ consisting of non-entangled vectors which do not become entangled under $U$? If not, ...
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How can information be communicated with entanglement? [closed]

Forgive my overly simplistic view of this. But I have been wondering this for years. Any insight would be greatly appreciated. So imagine you have two entangled particles, and they are separated and ...
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Easiest way to see that superposition is necessary?

I have been discussing QM with a friend and wanted to explain why holding on to realism means we must accept non-locality. Essentially I got hung up on explaining Bell's Theorem in simple words. I ...
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Is High-Fidelity Quantum-Entanglement Data-Transfer Real?

I was discussing quantum entanglement with a friend and explaining that faster than the speed of light data transfer isn't a reality with our current understanding of physics. He brought up quantum ...
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Can two quantum systems ever NOT interact?

Given that two quantum systems will always be connected by fields, is it really possible for two quantum systems to remain completely unentangled? Even if the entanglement is destroyed by observation, ...
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Does 100% Visibility of coincidences in CHSH-bell measurement mean quantum entanglement?

In the coincidence detection ++, +-, -+, -- from both alice and bob's detectors, if the visibilty of coincidence is over 70.7%, does it mean quantum entanglement?
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Quantum memory time capacity

let's suppose that we have a quantum repeater which of course the main goal of it is to assure the quantum entanglement swapping, in this repeater we have 4 modes A,B,C,D my question is what is the ...
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Entanglement entropy inequalities for a sum of wavefunctions

I am learning about entropy in quantum mechanics, and I am trying to develop some tools and intuition. One tool that I have found very helpful has been $$\sum_{i=1}^k \lambda_i \mathbb{S}[\rho_i] \...
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What quantum gates are needed to get the state $|01\rangle+|10\rangle$ from $|00\rangle$?

I was wondering if I start with two qubits in the state $$|00\rangle$$ If it's possible to apply gates to get it to the state $$\frac{|01\rangle + |10\rangle}{\sqrt{2}}$$ I have tried applying the ...
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Can Compton scattering generate entangled photons and electrons?

Since the Compton scattering process is where a high energy photon scatters off an electron, would it be a plausible result for the energies of the scattered photon and the scattered electron to be ...
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How can anything be unentangled?

If the universe is made of fields that exist everywhere, won’t every point in the universe become entangled with those around it, expanding out at the speed of light? How could two systems nearby one ...
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Bose-Einstein condensate: anti-Helmholtz coils and temperature dependence, if one is observed

To get a Bose-Einstein condensate anti-Helmholtz coils are used to hold the BEC together. I am looking for a relationship between the strength of the magnetic field of the coils and the temperature at ...
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Lindblad Master equation for an entangled system and environment?

I am currently trying to work out the math involving the time derivative of the density operator of an entangled system. (The total density operator, $\rho_T,$ is pure, but the 2 reduced density ...
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The bi-local term in Newtonian gravitational potential energy

I'm working my way through the paper Observable quantum entanglement due to gravity Equation 2 is a Taylor expansion of Newtonian gravity potential. They say the second term is bi-local and so cannot ...
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The emergence of space-time from entangled states

I recently read an article by Yasunori Nomura (https://arxiv.org/abs/1711.05263), in which he says that space-time is an emerging phenomenon. At the same time, space-time disappears when the ...
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What happens to spin after measurement, does it evolve and randomise like position evolves after measurement?

When a particle's position is measured, if one considers the wavefunction to collapse then one can assume it collapses into a delta function peaked at the measured position, and then after some finite ...
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Is entanglement a non-unitary transformation?

My question is motivated by the facts that measurement is a non-unitary transformation, and entanglement is necessary for measurement. To clarify: I mean to say the transformation associated with the ...
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Calculating the reduced density matrix of two separate experiments

Consider setting up two experiments $\alpha $ and $\beta$. In experimental setup $\alpha$ we can prepare $n$ possible pure states $\{| \psi_1 \rangle, | \psi_2 \rangle, ... | \psi_n \rangle\}$ with ...
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What is quantum local “unrealism”?

Bell's theorem is often interpreted to have destroyed local realism. This means something like the properties of the particle do not exist until they are being measured. I am not sure what does this ...
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Finding the trace of a system explicitly

Consider that we are working with a joint system composed of system A with basis $|\alpha_j\rangle$ and system B with basis $|\beta_j\rangle$. In my notes the density operator is denoted as follows: $$...
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Understanding the density operator in quantum mechanics for a joint system

Consider that we are working with a joint system composed of system A with basis $|\alpha_j\rangle$ and system B with basis $|\beta_j\rangle$, we may write a general density matrix for the joint ...
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Daniel Harlow vs. Don Page: average entropy of a subsystem

Consider a bipartite system $\mathcal{H}_{\mathcal{A B}}=\mathcal{H}_{A} \otimes \mathcal{H}_{B}$. Let's take $|A|\leq|B|$, where by $|\cdot|$ I mean the dimenstionality of the indicated system. Also, ...
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Continuous variable Fourier gate and teleportation circuit

I am studying the teleporting circuit of continuous variable showed by Weedbrook, pag. 37. Fig. (164); in that, they propose an input state to the circuit as $$ \left|\psi\right>\left|0\right>_{...
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Are quantum fluctuations observable?

I know I am asking out of my limits, but i felt relaxed thinking about this. Now I read somewhere about quantum fluctuations (that energy can be generated in randomly at two positions where total sum ...
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Calculating Entanglement Spectrum from reduced density matrix

Im reading about Entanglement Spectrum in very nice webiste here. It says: After a system is partitioned into subsystems {A} and {B}, a state of the entire system may now be expressed as a matrix with ...
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What causes quantum entanglement to end?

I seems strange to me that I could not easily find information on "when quantum entanglement ends?", "What causes quantum entanglement to end?". Google search finds explanations of ...
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Can the narrative of spooky action at a distance be derived from the mathematics of quantum mechanics?

The usual popular (and also not so popular) science lore about entanglement goes about like this: Two particles enter into contact (or a single particle decays into two new particles) and hence get ...
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Expected value in EPR

I was trying to read this paper http://philsci-archive.pitt.edu/3714/1/epr-bell-iep-preprint.pdf and I would like to prove the Equations (1), (2) but I am not so good for quantum mechanics so I would ...
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Schrödinger's Killer App: What are the possible outcomes of the quantum pocket watches?

I'm a theroretical computer scientist trying to get an intuition for quantum computing, and I'm currently working through "Schrödinger's Killer App"[1] by the late Jonathan P. Dowling. ...
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EPR paradox and quantum entanglement

I have just been watching about EPR experiment on Leonard Susskind videos called entanglements and at the end of the 7 lecture he said that for say two particles of opposite spin up/down and at long ...
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Topology of Helium 3A and 3B

The question concerns the topology and dimensions of Helium 3A and 3B A. The Helium 3A phase shows the same low energy excitations as those of a 2 spatial dimensional chiral p-wave superconductor --- ...
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What mathematical property must the Hamiltonian have for the system to evolve to an entangled state? [closed]

For a closed system composed of two subsystems in a pure, non-entangled state, what mathematical property must the Hamiltonian have for the system to evolve to an entangled state? What property must ...
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Measuring one particle on this entangled state

If we have a normalized entagled state: $$ |a\rangle=N (|1\rangle|2\rangle+|1\rangle|-2\rangle+|1\rangle|2\rangle-|-1\rangle|2\rangle) $$ Where $|2\rangle, |-2\rangle$ describes particles A and $|1\...
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What if we test three photons instead of two in Bell's paradox?

I am not a physicist and do not know anything about quantum mechanics (except that it can be formulated using Hilbert spaces), but watching a vulgarization video about Bell's paradox, I had a question ...

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