All Questions
80 questions
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3
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110
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Is it possible to affect spin probability of an entangled particle?
Say I have entangled particles A and B. Assuming we are talking about photons, we have 50/% chance of particle A being spin up.
Is it possible to affect the probability of particle A to be spin up ...
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-4
votes
1
answer
184
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Bell's inequality in Claude Cohen-Tannoudji's Quantum Mechanics Volume III
Claude Cohen-Tannoudji et al, Quantum Mechanics Volume III, 2020 edition, Chapter XXI.F-3-b.$\alpha$ (F-7) on p.2210 states
$$\big\langle\hat A(a)\hat B(b) \big\rangle={\cal P_{++}+P_{--}-P_{+-}-P_{-+}...
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votes
2
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59
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Why we cannot transfer information using a spin entangled singlet? [duplicate]
In QM and QE effects an entangled particle pair is called also a singlet with some properties of the two particles like spin, non-locally correlated. However, there is no transfer of information ...
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3
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0
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93
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Constructing wavefunction for a mixed state
This question is somehow the reverse of another question.
If a quantum system $S$ is in a pure state, then we can find a wavefunction that describes $S$. This wavefunction is unique up to a phase ...
- 1,502
1
vote
0
answers
68
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Spin Correlation Analogy in Three Particle System
I have recently gone through the section on Bell's Inequality in "Modern Quantum Mechanics" by J.J. Sakurai and Jim Napolitano and I am curious as to weather or not there are analogous ...
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1
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57
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Can entanglement cause a chain reaction at distance after measure one particle? [duplicate]
if we have two electrons with their spins entangled like this:
And if we measure the spin on the left electron, then right electron spin also change like this:
So, my question is, do this spin ...
- 327
1
vote
1
answer
60
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When does multi-particle entanglement break? [closed]
Let's say we entangle four electrons based on their spin.
Now, let's say we measure the spin of electron A along the z-axis (up/down), and we observe a spin of 'up'. My understanding is that this ...
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2
votes
0
answers
98
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Entanglement measure for a continuous bipartite system
Consider a cavity-spin system and assume we want to quantify the correlation between them. The cavity system can be described using a bosonic mode whereas the spin system requires 2 bosons to describe ...
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2
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1
answer
179
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Does spin entanglement imply position entanglement?
My question is whether two electrons can be entangled only with respect to their spins but not with respect to some other observable, such as position.
I initially believed that spin-entanglement ...
- 1,073
0
votes
0
answers
57
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Total spin of Entangled Electrons
If, say, the z-spins of two electrons are maximally entangled (so that their composite state can be given by $|\Psi\rangle = \frac{1}{\sqrt{2}}(|u\rangle|d\rangle + |d\rangle|u\rangle)$, how do you ...
- 1,073
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0
answers
71
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Measurement of total angular momentum in quantum mechanics
Consider a system composed of two spin-1/2 particles. The total spin operator is defined as
$$
\mathbf{S} = \mathbf{S_1} + \mathbf{S_2}
$$
We can write a common eigenbasis of the operators $\mathbf{S^...
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votes
2
answers
172
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Creating entangled electron pairs using Stern-Gerlach apparatus?
The following is a drawing of the sequential Stern Gerlach experiment.
As we can detect the fraction of the electrons passing through each magnet, I suppose it is possible to detect how many ...
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2
votes
2
answers
112
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Does it matter when we measure the spin of the other entangled particle?
Let's say, we have 2 entangled particles: A and B which are a light-year away from each other.
We know if we measure the spin of particle A, we can be certain the spin of particle B will be in the ...
1
vote
1
answer
159
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A question regarding quantum entanglement
According to wiki,
Quantum entanglement is the physical phenomenon that occurs when a group of particles are generated, interact, or share spatial proximity in a way such that the quantum state of ...
2
votes
1
answer
299
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Measurement of the spin of the EPR pair in two orthogonal directions and how did Einstein tackle this?
Let us consider two types of measurement in the EPR experiment. In Bohm's description of this experiment, the state of the electron-positron (called the EPR pair) is given by
$$
|{\rm EPR} \rangle =\...
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1
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0
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52
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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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3
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148
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Isn't There A Much Simpler Way To Explain Why Quantum Entaglement At Distance Isn't Utile - That Spins Already Exist & React To Detection Predictably? [closed]
Tell me if this concept works for explaining in a much simpler way, why purposeful entanglement outcomes like FTL communication cannot work - and without relying on complex analyses of 'spooky action',...
1
vote
1
answer
198
views
Can one measure two components of spin exactly by measuring two components of entangled, say, electrons?
Of a single electron, two different components of spin can't be have simultaneously well defined values. But what if we entangle two of them and we measure, say, $(S_1)_z$ and $(S_2)_x$ simultaneously....
0
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2
answers
62
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If 2 electrons are entangled $[a] [b]$. if we throw a photon at $-[a]$ will it come out of $[b]-$? [closed]
i was confused as to what will happen when two electrons which are entangled and then if one is exposed to light. will the absorption and emission theory still work and if so how will it work in this ...
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1
answer
99
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Intuition for Results of a Measurement of Entangled Spins
Suppose I have a source that produces pairs of spin-1/2 spins in the entangled state
$$ \frac{1}{\sqrt{2}} \left( \mid\uparrow_z\uparrow_z\rangle + \mid\downarrow_z\downarrow_z\rangle \right). $$
The ...
- 1,576
2
votes
1
answer
611
views
Fermionic (or Bosonic) state vs Entangled state
One can see that the wavefunction for a system of two electrons (not very far apart) is one that cannot be written as a tensor product of individual states.
The same is true for a bosonic state.
For ...
- 1,461
1
vote
0
answers
115
views
Hamiltonian basis in multiple particle systems
I have a question about the choosing of basis that compose the Hamiltonian of multiple systems. For example if we have a system of N particles, that depend in our case of Pauli matrixes i will give ...
- 529
1
vote
0
answers
19
views
Do fullerenes have a better quantum yield than carbon quantum dot's of similar dimensions?
As a fullerene and a carbon quantum dot of similar dimensions would have the same surface area but different internal volume, as a Fullerene does not contain carbon within it's interior, but just the ...
- 471
0
votes
1
answer
175
views
Can two spin 0 particles be entangled?
Imagine that I have some kind of collision between two spin-0 particles that conserves the number of spin-0 particles, and I divide my lab in half with detectors on both sides called left L and right ...
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2
votes
1
answer
304
views
How do three electrons entangle with each other?
In a Quantum entity of two electrons entangled to each other, we know that when one electron's spin is known, then the other electron's spin is the opposite.
But, What's the case in a quantum entity ...
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1
vote
1
answer
161
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The Heisenberg model using the duality analysis
I would like to express the Heisenberg model using the duality analysis. It is shown here how to express the Ising model using Pauli matrices but I cannot get the relation $ \sigma _{i}^{z}= \prod_{...
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0
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0
answers
65
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Why can't we create energy/momentum by selectively measuring the spin of entangled particles?
Suppose we perform a Bell violation type experiment with spin, i.e. we have a stream of entangled, spin-anticorrelated particle pairs and we are to measure their spins along different axes.
Now for ...
1
vote
1
answer
116
views
Where, exactly, does the boundary lie between 'entangled' particles and merely 'interacting' or 'coupled' ones?
Have scientists discovered, yet, the precise amount of interaction needed to actually 'entangle' a pair or more of particles, beyond mere interaction or even coupling?
Is there a distinct difference, ...
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1
answer
21
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Does quantum entanglement, in and of itself, make the constituent particles more sensitive to external fields and forces?
I just watched PBS Space time episode called, 'Navigating with quantum entanglement'.
At about 10:25, host Matthew O'Dowd says that Peter Hore of Oxford University reviewed the evidence for quantum ...
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5
votes
3
answers
1k
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Can 2-qubit systems be represented in the Bloch sphere representation?
I am studying a system with 2 qubits, so I need, for a given state, a Bloch representation for each qubit.
I am having difficulties because I get results that do not have sense at all. For example if ...
- 529
1
vote
2
answers
736
views
Bell's theorem: why does "correlation = cosine" rules out hidden variables?
I was reading about entanglement and Bell's theorem and played around with the idea. Specifically the point that the correlation of the measurement of entangled electrons at arbitrary angles is given ...
- 29
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1
answer
79
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Measurements of spin correlations: Alice goes back to Bob and compares results
Take a fully entangled two-photon state, like
$|\Psi>=\frac{1}{\sqrt{2}}\big(|+->-|-+>\big)=\frac{i}{\sqrt{2}}\big( |yx> - |xy> \big)$
where one photon is sent to Bob and the other to ...
- 427
3
votes
0
answers
75
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Bounding the value of a function for separable spin states
Consider $N$ spin-1/2, for which we can define the collective spin operator $\vec{S}=\sum_i \frac{\vec{\sigma}^{(i)}}{2}$. My question is, what is the upper bound $U$ on
$$
f(\rho) = \text{Var}[ S_z ] ...
- 1,231
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2
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202
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Faster than light communication protocol via GHZ-state
I had a chat with a guy recently who claimed that multipartite entanglement might actually be used for superluminal communication, and his argument is very simple.
Suppose that 2 observers (Alice and ...
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4
votes
3
answers
847
views
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 ...
- 2,654
3
votes
1
answer
787
views
How to entangle two superconducting qubits?
I do understand qubits entanglement, and their spins but I do not understand that at circuit level how can we entangle 2 qubits? Is it related to 2 qubits gates?
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5
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5k
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Can quantum entanglement be used to coordinate actions at "FTL speeds" without breaking causality or actual faster-than-light communication?
I know there are a lot of similar question but I don't believe this to be a copy. I understand that if two people lived far away they could not transfer information through quantum entangled particles ...
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1
answer
67
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Quantum entanglement of a particle [duplicate]
I recently got to know that two particles are entangled and they share information instantly no matter how far they are kept from each other. It is also said that if one particle is kept at one side ...
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4
votes
1
answer
675
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Connection between spin angular momentum of a photon and circular polarization of light
Do photons have spin angular momentum only if they are part of a circularly-polarized beam?
I suspect that every photon always has spin angular momentum, but in most cases they have a superposition ...
0
votes
1
answer
160
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EPR Paradox clarification
Basically, the paradox works like this:
Two entangled particles (A and B) are a light year apart.
1. Measure the X spin of A
2. Measure the Z spin of B
You ...
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1
answer
404
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Is quantum entanglement contradictory with Schrödinger's cat?
As I understand, an electron spin is in a superposition state until the spin is observed. Due to the observation, it will either be spin up or spin down.
Due to quantum entanglement, if one electron ...
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1
answer
117
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Entanglement swapping proves there are no time paradoxes?
I am trying to make sense of the Delayed Choice Entanglement Swapping:
In the entanglement swapping procedure, two pairs of
entangled photons are produced, and one photon from each pair
is sent ...
- 395
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votes
0
answers
38
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Using entangled qubits to communicate [duplicate]
After reading up on quantum mechanics considering entangled qubits I was asking myself this simple question:
If one qubit (A) positioned on earth is entangled with another qubit (B) which is - say - ...
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5
votes
2
answers
561
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Entangled states and separable states
B
Two electrons in the same orbital is clearly an entangled quantum state since it is not a tensor product:
$$|\psi\rangle=\frac{1}{\sqrt{2}}(|\uparrow\rangle \otimes|\downarrow\rangle-|\downarrow\...
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1
vote
1
answer
270
views
Dirac Notation Tensor product
We can write a Singlet state of two $\frac{1}{2}$ spin particles like this:
$$|S\rangle = \frac{1}{\sqrt{2}}\left( |+ \rangle ⊗ |-\rangle - |-\rangle ⊗|+\rangle \right) $$
is this the same as ...
- 21
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votes
2
answers
301
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Entanglement, measurements and correlation
I am studying for my quantum mechanics exam and I came across the following question, I hope I reason correctly.
Consider two spin-$\frac{1}{2}$ systems one of which is measured by Alice and one ...
- 245
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2
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170
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Are all Bell tests valid only for linearly-polarized photons? [duplicate]
In https://arxiv.org/abs/1407.2605, it is argued that all photons are necessarily circularly polarized, and linearly-polarized photons must be a superposition of such circularly-polarized photons. ...
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1
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230
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Do particles remain entangled even after repeated measurements in different directions?
If we measure the spin of an electron along the $x$-direction and it turns out to be up then we know that upon measurement along the $y$-direction and in turn along the $x$-direction, the probability ...
user218113
6
votes
2
answers
846
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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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vote
3
answers
640
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Quantum entanglement, how do we know there was no spin? [duplicate]
Im not a scientist, so go easy on the explanation!
As I understand it we can create two entangled particles. The entangled particles have a spin property which is opposing. When we measure one of the ...
