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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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Theoretical approach of the quantum CNOT gate

Let's $ |\psi \rangle $ be $ \sqrt{1} \begin{pmatrix} \frac{2}{8} \\\ \frac{6}{8} \end{pmatrix} $, $ |\phi \rangle $ $ = \sqrt{1} \begin{pmatrix} \frac{3}{4} \\\ \frac{1}{4} \end{pmatrix} $, and $ |...
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Quantum Entanglement, connected or just in tune? [duplicate]

I read a few of the questions suggested to be similar, but I didn't see what I am looking for. Please feel free to point me in the right direction if I just missed it... (there are quite a few). ...
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What are the “parts” of separable quantum states?

Of course I could only speak of whole states and banish the word "parts". Mathematics enables some quantum states to be separated into partial states. When the whole state has finite dimensions, the ...
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What does it mean to prepare two particles in Bell state?

My lecture note on Quantum Teleportation says "The way quantum teleportation works is first two spin-1/2 particles are prepared in the Bell state, where A and B correspond to who received the ...
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279 views

Quantum computing: CNOT gate

A CNOT gate can be written as follow: $$\begin{pmatrix}1 & 0 & 0 & 0 \\\ 0 & 1 & 0 & 0 \\\ 0 & 0 & 0 & 1 \\\ 0 & 0 & 1 & 0 \end{pmatrix}$$ So using ...
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How does entanglement work for states where number of particles is undefined

Entanglement and separability are often described in terms of tensor products of particle states. For example the Alice/Bob two qubit example. Many states do not possess a definite number of ...
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174 views

Understanding the GHZ game

I am trying to understand the basics of the GHZ nonlocal game in quantum information theory. I have searched for all the info i could find, thought i understood, but still face some basic ...
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1answer
103 views

Double-slit experiment on cats makes inconsistency?

According to Wave-Particle Duality, in a normal double-slit experiment, the quantum object behaves as wave and many stripes are displayed on the screen. But if we measure which way the quantum object ...
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How can one understand entanglement when there is only a single particle?

As per my knowledge, at least 2 particles are needed to show entanglement in the context of some property. But nowadays, mode entanglement is a well known phenomena which says that a single particle (...
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0answers
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Using entangled particles to transfer energy or a force?

My question is a two part question: When two particles that have mass are entangled (e.g. two electrons) are their Higgs field(s)/mass also entangled? If mass is an entanglable property of particles ...
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0answers
128 views

Physical intuition of partial trace in a GHZ state [closed]

I'm having trouble understanding the physical information one might extract from partial traces, by interpreting the partial trace of a qubit as if the qubit was averaged out of the system. For ...
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Can we always find a Hilbert space corresponding to a region of spacetime?

In defining entanglement entropy in field theories we take a region A in spacetime. Now if $\mathcal{H}$ is the Hilbert space of the field theory we assume that we can decompose this Hilbert space ...
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Using QED to calculate the the success probability of sending a pair of entangled electrons

Some procedures of quantum information technologies, such as quantum teleportation, require two parties to share a pair of entangled particles, i.e., each party has access to one of the particles, and ...
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2answers
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$mn$-dim Hilbert space vs product of $m$ and $n$ Hilbert spaces

Assume one has a Hilbert space $\mathcal{H}_K$ of dimension $K=mn$. Associated to it, there exists a set of all possible quantum transformations $\mathcal{E}$. Of those, the most important are, of ...
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1answer
97 views

Quantum Entanglement - assumptions about entanglement across universe

Can entanglement occur in one to many relationship, or many to many? If so, are there entangled particles occurring across the universe that have persisted since the big bang, or does entanglement ...
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2answers
78 views

EPR-Bohm paradox and Robertson uncertainty relation [closed]

Consider Bohm's version of the EPR paradox, where spin-1/2 are used. The bipartite system is in the state $$ \vert \Psi\rangle = \dfrac{1}{\sqrt{2}} \left( \vert \uparrow_x \rangle_A \vert \...
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Connection between entanglement, reversibility, and gravitational waves

Imagine a movie of the collision between two nuclei, resulting in multiple fragments. Now imagine the movie running backwards: it looks very implausible. The laws of physics, being time-symmetric, ...
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1answer
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Question about how the optimal rate of two-qubit pure state interconversion is calculated

In this paper [0], the optimal rate of interconversion between pure states is calculated on the bottom of page 7. Basically, it is argued that since the entanglement cost and distillable entanglement ...
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1answer
211 views

How does a reflected photon keep its entanglement [duplicate]

I'm curious how a photon from an entangled pair, travelling through say a delayed choice quantum eraser experiment, can bounce around between mirrors but remain entangled with its partner photon, when ...
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2answers
160 views

Question about Bell's Theorem

I'm working through trying to understand this, but the math of the original paper, and subsequent actual academic papers are a bit beyond me. So I've turned to simplified explanations, all of which ...
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1answer
69 views

Why don't quantum gates get entangled with the qubits they operate on?

I am currently taking a first course in quantum information theory and having trouble understanding how quantum gates are possible in practice. How does a unitary quantum gate evolve the qubit without ...
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Stability of the interference pattern of a subsystem in an entangled state when the other subsystem is being measured?

I have a double slit apparatus (in fact, any apparatus with some observables that can give an interference pattern will do), which constantly firing electrons towards the screen, thus the computer ...
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1answer
367 views

Quantum entanglement used for FTL communication? [duplicate]

I'm aware of many ideas people have come up with for this question. I understand why they don't work. However I was thinking about another method involving an array of multiple entangled particles. ...
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1answer
68 views

Would it be possible to quantum entangle two large objects?

I'm new to quantum mechanics and I was wondering is there a limit on the size and material of objects that can be entangled? And how would one go about entangling these objects? I read that photons ...
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1answer
56 views

Singularity and entanglement on surface of event horizon

An entangled pair of particles is created with one particle embedded onto the surface of a singularity. The other particle at some stated epsilon distance above the event horizon of the singularity. ...
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Where do the coefficients come from when measure a qumode in a entangled multi-mode system?

After measuring the third qumode of $\sum_i b_i \int |e_i\rangle |p\rangle_{p,R} |\gamma\lambda_i p\rangle_{q,S} dp$ in the $q$ quadrature, we get $\sum_i b_i/\lambda_i|e_i\rangle|q_S/\gamma \...
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How is quantum entanglement consistent with the relativity of time?

It is well known that relativity predicts time moves slowly near massive objects e.g.. time moves slowly for clocks on earth as compared to clocks on GPS satellites by about 40000 nanoseconds. ...
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1answer
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Can we prepare a superposition of two many body state efficiently using quantum circuit?

Let's say we have two quantum many body states $|\psi_1\rangle$ and $|\psi_2\rangle$(or equivalently, two quantum circuit $U_1$ and $U_2$ ), also an ancilla qubit $\alpha|0\rangle+\beta|1\rangle$. The ...
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3answers
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Is it possible to know the efficiency of a particle detector without assuming the truth of Theory (e.g.,, Quantum Theory)

I reorganized the question to clarify exactly what it is that I'm asking. Suppose an experiment is performed where a particle detector records 50 particles per second, on average. Absent any other ...
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1answer
108 views

What are the quiescent states of the surface code?

Notation $$X \equiv \sigma_x \qquad Z \equiv \sigma_z$$ Introduction The surface code is a quantum error correction code. In the surface code, we have a two-dimensional grid of qubits each coupled ...
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1answer
115 views

What is the state of a single electron in an entanglement?

If we consider a bipartie system that is entangled, $$|\psi^{AB}\rangle=\frac{|{\uparrow_z\downarrow_z}\rangle-|{\downarrow_z\uparrow_z}\rangle}{\sqrt{2}} $$ And we want to know the probability ...
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219 views

Is quantum teleportation really teleportation? [closed]

I don't understand how quantum teleportation is considered teleportation if no information is transfered (in q-bits). Wouldn't it be less misleading calling it "quantum encrypting" or something ...
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1answer
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How viable is using Quantum Entanglement for long distance communication? [duplicate]

In fiction, (notably Ernest Cline's novel Armada and Charles Stross's books Singularity Sky and Iron Sunrise) Quantum Entanglement is used for long distance communication. In reality, given our ...
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1answer
260 views

Why we should be disturbed by Bell's theorem [closed]

Mermin (1985) quotes an unnamed physicist as saying: Anybody who's not bothered by Bell's theorem has to have rocks in his head. This seemed like a pretty dire indictment, so I decided I had ...
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will two entangled particles still remain entangled if one of the pair if sent inside of a black hole? [duplicate]

Imagine that i had two entangled photons . I kept a photon beside me without knowing its spin and sent the other photon inside a black hole . Then i measure the spin of the photon beside me . Is is ...
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2answers
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Measuring two components at once using Entanglement

Heisenberg's uncertainty principle states that it is impossible to measure two properties of a particle (like $S_z$ and $S_x$) with certainty at the same time. Consider the following experiment (from ...
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What are the benefits of quantum teleportation of continuous variables over discrete variables? [closed]

I have been learning about quantum teleportation in general and I was wondering what was the motivation behind achieving quantum teleportation of continuous variables. Edit: I'm aware that it is ...
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2answers
980 views

How are entangled states created?

I understand that when I have two separate states that their combination state increases the Hilbert Space to $|\psi_1\rangle \otimes |\psi_2\rangle$ For example, looking at a simple example where ...
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1answer
130 views

Wheeler's delayed-choice experiment as a time machine?

Hi everyone this is my first post so please be nice :) Okay so imagine this setup: Let's say that we do the classic double-slit experiment with a twist. We all know how the double slit experiment ...
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1answer
69 views

Wave function collapse, EPR paradox and information transfer

For a classical formulation of the EPR paradox, two particles are produced, with total momentum zero and separated by a long distance. So say we measure the momentum of one particle first, and measure ...
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1answer
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What does passing one qubit that is entangled with another through a CNOT gate do?

I am watching this video in which a programming language for quantum computing is introduced. There is code like this: ...
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95 views

What's the tensor network representation for local ground state?

It's well known that many topological phases can be represented using matrix product state and PEPS. Example, toric code, $H=-\sum_{v}A_v-\sum_p B_p $. My question is: What's the tensor network ...
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1answer
75 views

Toy model for entangling interaction?

Most of the time entanglement is described using some instantaneous classical measurement from an apparatus, but it is possible to describe it between two quantum systems, with two systems $O$ (for ...
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1answer
614 views

Quantum Entanglement - faster than light communication [duplicate]

very new here - first question in fact. So, don't be fooled by the title of my question. I understand that the Bell Paradox - and non-locality - prevents information being exchanged in a faster than ...
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1answer
36 views

Physical system for exchanging entanglement between types of particles

What is an example of physical system in which entanglement is transferred from one pair of particles to a different pair of a completely different type of particle? For instance, if I have two ...
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1answer
93 views

How can two photons with different frequencies created by parametric down conversion be indistinguishable? [closed]

We have two photons created by the same four wave mixing process. How are they indistinguishable if they have different wavelengths? I know that as they are created at the same moment by the same ...
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2answers
80 views

Efficient quantum circuit to prepare a set off entangled state

If I try to prepare an entangled state on $N$ pairs of qubits ($2N$ qubits) as $$|\psi\rangle=\sum_{i=1}^{2^N}w_i|ii\rangle , $$ with $w_i$ real factors, is there an efficient quantum circuit to ...
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7answers
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How can quantum entanglement be explained in layman terms?

I was just wondering if one entangled particle was able to influence its partner, because they both remain connected through the electromagnetic field. e.g via the same field line, so that either ...
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Teleportation and boosted observer, special relativity

According to relativistic quantum information, entanglement, purity of states are not Lorentz invariant. Here I check such a problem. If we have two coordinate systems $R_A,R_B$ with relative ...
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0answers
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Why does a photon crossing a beamsplitter end up in a coherent superposition? [duplicate]

A single photon, passing through a beamsplitter (BS), ends up in the state $\frac{1}{\sqrt2}(a_1^\dagger + a_2^\dagger)\lvert\operatorname{vac}\rangle$, which is a coherent superposition of the two ...