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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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Do qubits need to be in superposition to be entangled? [closed]

Do qubits need to be in superposition to be entangled? Put another way, can qubits be entangled but not in superposition?
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Have researchers managed to “reverse time”? If so, what does that mean for physics?

According to press releases, researchers have reversed time in a quantum computer and violated the second law of thermodynamics. What does that mean for physics? Will it allow time travel? Further ...
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What Really is Quantum Entanglement? [duplicate]

So this is one of those topics that pop science likes to latch onto, but what I want to know is what quantum entanglement really is. Ive taken my upper devision QM class (at least the first semester) ...
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Which interactions produce entanglement?

I'm trying to better understand which kinds of interactions cause the entanglement. I do understand how it works in the cases of non-relativistic spin systems. For example, evolving a pure state in ...
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When two particles are maximally entangled, can we decompose the combined system arbitrarily?

This will sound like a philosophical question (i.e. a question with no actual theoretical content), but I dont intend it as such. Admittedly, my question is based on a very vague intuition. Also note,...
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1answer
123 views

Entanglement in a uniform magnetic field

Suppose we throw only one particle (1) with spin-1/2 into a uniform magnetic field while letting free the other entangled particle (2). The state of the particle-2 will not be affected and this is a ...
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1answer
132 views

How can eigenvectors of a Hermitian matrix be entangled?

You have a tensor product space $H_1 \otimes H_2$. Any vector $w$ in this space has a Schmidt decomposition: $$ \mathbf{w} = \sum_{i} \alpha_i \mathbf{u_i}\otimes\mathbf{v_i} $$ Vector $w$ is not ...
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How to produce a given entangled state of two quantum bits?

I was watching Leonard Susskind's video series on quantum entanglement, where he looks at the spins of two electrons. In particular, there are entangled states of the form $$\alpha\left|\uparrow \...
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Say you run qubit $A = (|0⟩ + |1⟩) / √2$ and qubit $B = |0⟩$ through a CNOT gate. What is the state of qubit $B$ afterwards?

I am new to the weeds of quantum computing and this question is probably pretty elementary. Say you run qubit A = (|0⟩ + |1⟩) / √2 and qubit B = |0⟩ through a CNOT gate. What is the state of qubit B ...
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Do Chern Insulators (QAHE) have topological order (long-range quantum entanglement)?

I know IQHE is a example having "invertible" topological order from Professor Wen's definition. And Topological Insulators is SRE because of necessary of underlying symmetry protection. After that, ...
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What is the Reduced Density Matrix?

The difference between pure and mixed states is the difference in their density matrix structure. For density matrix $\rho$ of mixed state the trace of $\rho^{2}$ should be less than 1. For pure ...
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Implication of non-positive tripartite information

Hayden et al 2011 showed that tripartite information is non-positive given Ryu-Takanayagi formula. (For definition of tripartite information, see for instance section 4.4 of this paper) Is there ...
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Basis ambiguity for bipartite entanglement

I am reading Decoherence, einselection, and the quantum origins of the classical from Wojciech Hubert Zurek And he says at some point that for a three partite quantum system there is no basis ...
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Interpreting the Quantum Fisher Information

I recently saw this article in Nature Physics regarding Quantum Fisher Information (QFI): http://www.nature.com/nphys/journal/v12/n8/full/nphys3700.html Frankly I have not been following Quantum ...
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1answer
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Does a polarizer affect both entangled photons or only one?

Suppose you have two polarization-entangled photons A and B in the following state: \begin{equation} \Phi=\frac{1}{\sqrt{2}}\bigl(\left|H_{A},H_{B}\right\rangle + \left| V_{A},V_{B}\right\rangle\bigr)...
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Are the electrons in the Delft University experiment really entangled?

I have been studying the Loophole-free Bell test at the Delft University. They were using the "Entanglement By Accident" method. When they trigger an electron, the electron falls back in one of two ...
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129 views

Properties of controlled z-rotations

Given n qubit gate of the form c-z-z-z... (shorthand for c-z between qubit 1 and 2 followed by c-z between 1 and 3 and so on up to n qubits) it seems to be possible to find local unitaries which will ...
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How to search Feynman diagrams for entanglement [closed]

This question is partly about physics, partly about love -- but I don't believe there's a Stack Exchange for the later :) My girlfriend is a PhD physicist who finds the notion of entanglement very ...
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1answer
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The maximally mixed state is at the center of all quantum states

The set of quantum states $\rho$ in $d$ dimensions is the set of positive semidefinite operators living in a Hilbert space of dimension $d$. Let us denote this set by $\text{Pos}(X)$ and note that ...
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Differentiating between Tensor Networks

I am trying to study tensor networks and their application to quantum phase transitions. However, I had a question concerning the connection between the projected entangled-pair states (PEPS) and the ...
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Closest separable/ closest PPT state given an arbitrary state?

After searching for results, the closest result I found was https://aip.scitation.org/doi/pdf/10.1063/1.3591132?class=pdf. The summary of that paper is: Given an arbitrary seperable/PPT quantum state ...
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How does gravity affect entanglement?

Hypothetically an entangled pair of atoms are placed inside two similar satellites orbiting at the same altitude, one of the satellite will then slow its speed and descend towards the surface of Earth....
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How does a photon released from particle A interacting with particle B, entangle them?

What does a photon carry from its parent particle that when it interacts with another particle, it entangles both the particles?
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1answer
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Mathematical formulation of partial wave function collapse

If given a wave function which is an element of a Hilbert space $\mathcal{H}$ which is a tensor product of two other Hilbert spaces $\mathcal{H}_1\otimes\mathcal{H}_2$, how do you formulate ...
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Is it possible to reconduct “The delayed quantum eraser experiment” in home?

The delayed quantum eraser was conducted by Kim et al. in 1999 with the use of BBO crystals, polarizers, detectors and so on. I believe some of these equipments can be found online kinda cheaply, ...
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Does Quantum Entanglement happen between two wavefunctions? [closed]

Does the entanglement happen between two particles or two wavefunctions? If it's wavefunction then what happens to the two wavefuntions after getting entangled?
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Why does $\max\{S(\rho_A)-\sum_i p_i S(\rho_A^i)\}$ measure classical correlations, and not full (classical + quantum) correlation?

Let a quantum system be given which has two subsystems $A$ and $B$ so that the Hilbert space decomposes $\mathscr{H}\simeq \mathscr{H}_A\otimes \mathscr{H}_B$. If the state of the system is $\rho$, ...
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What is the relationship between spin network spacetime and tensor network (entanglement) spacetime?

In 1971, Sir Roger Penrose, suggested a combinatorial construction of spacetime using the angular momentum of particles. This work led to and introduced the idea of spin networks which are ...
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What is the variance of |S| in Bell's inequality (CHSH inequality)

Sorry that this isn't a quick question but I didn't know how to make it shorter. I am struggling with this for quite a long time and I would appreciate every help that I can get. I could not find a ...
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1answer
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Reality, locality, and universality in the EPR paradox

Apologies if this has been asked before. I did some searching but didn't see it anywhere asked quite like this. Thanks in advance for any insights. Caveat: I am an organic chemist and thus ...
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Better Entanglement Explanation Required [duplicate]

It seems to me I have not read or understood a better explanation for quantum entanglement than Einstein's left and right glove explanation. If some Aspect (pun intended) of an entangled quantum ...
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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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Can PEPS explain the holographic principle in quantum gravity?

Condensed matter physicists have shown using quantum information that in many condensed matter systems, entanglement entropy only scales as the area of the boundary, and not the volume. This is the ...
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What is the difference between coherent states and the entangled states used in quantum illumination?

I've been reading about utilising quantum illumination systems to out perform classical microwave radars, but I've gotten confused over a point they're making regarding the distinction between the ...
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What happens when measuring a subsystem of a product bipartite state?

I have an horrible doubt, as I found that in my books the question is never directly addressed. How is collapse defined to work for measures of composite system? I always assumed that when we ...
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Methods to distinguish between pure/mixed states and entangled/separable states

I'm a little confused about how we can distinguish between pure/mixed states and entangled/separable states and I would really appreciate some help! I understand a density operator $\rho$ represents ...
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Does something prevent superposition at our scale?

I often encounter the argument that quantum mechanics reduces to classical mechanics at sufficiently big scales, as soon as h becomes sufficiently small respect to the actions involved. I clearly ...
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Why does the separability of the hydrogen wavefunction vary under different reference frames?

I know that two particles are entangled when their Hamiltonian contains an interaction part, but it seems to me that for the ground state of the hydrogen atom $\frac{1}{\sqrt{\pi}}\left(\frac{1}{a_0}\...
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Holograms and Entanglement: Does recording an entangled signal in a hologram “observe” the entangled state?

Suppose I have two photons $A$ and $B$, which are entangled. As I have had it explained to me, entanglement works more or less as follows: I observe or measure $A$, and find $A=S_a$, where $S_a$ ...
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Description of quantum state of entangled photons after polarizer

I'm wondering if anyone can help me understand how a polarizer changes the quantum state of two polarization-entangled photons. I haven't found a clear description in the literature. Suppose you have ...
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So is quantum entanglement actually FTL “communication” or is it mundane pre-determination?

I have to say right off the bat, I'm a little frustrated that there seem to be very contradictory answers about this, at least to a layman like me. If two particles are entangled and you separate them ...
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Fidelity and approximately unentangled states

It is well known that fidelity distributes over product states, i.e. $$F(\rho \otimes \rho, \tau \otimes \tau) = F(\rho, \tau) \cdot F(\rho, \tau),$$ and more generally, $$F(\rho^{\otimes N}, \tau^{...
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How unique is the Schmidt decomposition?

I read the Schmidt basis is unique "up to a phase" or as stated here in answer given by Norbert Schuch "modulo degeneracies". If I choose a Bell state $|\psi\rangle = \frac{1}{\sqrt{2}}\left(|0\...
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Definition of Entanglement

The definition of quantum entanglement, found on the internet and the literature is: On a bipartite system $\mathcal{H}_A \otimes \mathcal{H}_B$, let $\rho$ be a mixed state. It is said to be ...
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Does the Hong-ou-Mandel effect happen within a delay equal to the coherence time of the two photons?

I read that photon bunching happens for thermal light within the coherence time of the thermal light. That is, if the time for detection is greater than the coherence time of the thermal light, no ...
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Possible to write $N$-partite W state as sum of less than $N$ product states?

The $N$-qubit W state is $|W\rangle = \sum_{n=1}^N |n\rangle$ where $$|n\rangle \equiv |0\rangle_1\cdots|0\rangle_{n-1}|1\rangle_n |0\rangle_{n+1} \cdots |0\rangle_{N}$$ is the product state with the ...
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Can global unitaries create entanglement?

So I am reading this paper, which states multiple times that: Indeed, global (and thus entangling) unitary operations are capable of extracting more work than local operations from a set of ...
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1answer
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Is a mixture of Bell states entangled?

Setup: I have a two qubit system $\mathbb{C}^2 \otimes \mathbb{C}^2$ an we consider the Bell states $\newcommand{\ket}[1]{\left|#1\right>}$ $\newcommand{\bra}[1]{\left<#1\right|}$ $\...
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1answer
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Entanglement in quantum physics [duplicate]

Mathematically what is the difference between pure separable state and entangled state ? Can anyone explain with equations?
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Quantum entanglement and the big bang

Prior to the Big Bang all matter was compressed into a point of high density. Why isn't all matter already entangled?