All Questions
Tagged with quantum-mechanics quantum-entanglement
1,354 questions
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What is the outcome of Type I SPDC if the pump beam is circularly polarized?
I'm thinking about entangled photon pairs generation, specifically about Type-I SPDC where you use a pair of non-linear crystals such as BBO with their optical axes crossed and then the pump beam is ...
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Where is the raw entanglement data for John Clauser's experiment that won the Nobel prize of 2022? [closed]
Other than the delft experiment (which uses post hoc filtering) there are few experiments which publish the data from bells CHSH experiments.
Can anyone explain to me why I should accept the ...
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Entanglement collapse and relative order of events
According to QM (and many confirming experiments) outcomes (e.g. spin) of entangled particles measurement are non-locally correlated (i.e. can’t be pre-configured for such correlations).
It seems that ...
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What are the implications of rejecting the local tomography assumption?
Recently I presented on the paper by Renou, et. al. in Nature (Quantum theory based on real numbers can be experimentally falsified) developing an experimental technique for rejecting real ...
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Single particle measurement in entangled states [closed]
There is a system with two particles in a state given by the wave function $F(x_1,x_2)$. What does it mean to make a measurement on one of the particles? This is not defined among the axioms of ...
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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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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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Role of entanglement in quantum computing
I'm not a physicist and thus, I'm looking for a simple explanation on the role of entanglement in making quantum computers fast. I got a good analogy of a qubit: a coin tossed in the air can be ...
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Decomposing density matrix on non-orthogonal and incomplete set of states
I've recently come across the following conclusion in a quantum optics paper. The paper studies the dissipative dynamics of a two-qubit system and presents the following approximation for the steady-...
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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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Does the no signalling theorem in quantum mechanics beg the question?
I had always thought similarly and then came across a paper here that argues this.
The abstract is as follows:
Many authors state that quantum nonlocality could not involve any controllable ...
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How do I prove that a reduced density matrix has properties of a density matrix?
The properties of a density matrix are defined as follows:
$(1) \ \ \mathrm{Tr}\rho = 1 $
$(2) \ \ \rho^\dagger = \rho $
$(3) \ \ \rho \ge 0 $
$(4) \ \ \mathrm{Tr}\rho^2 \le 1 $
A reduced density ...
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Is the spatial spread of the wave function over large distances the rule or the exception in quantum mechanics?
In quantum mechanics, I've read that wave functions for individual particles technically extend over all space, though the probability density tends to decrease with distance. This suggests that a ...
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What is the difference between the EPR paradox and Bell's inequalities?
I am a newbie here, hope I will be able to get accustomed on this forum.
I am trying to understand what quantum entanglement is. Obviously, for this it is very useful to understand Bell's theorem. ...
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How to maximally entangle two qubits? [duplicate]
I'm looking for a transformation that entangles two qubits $\mid{\psi}_1\rangle = \alpha_1 {\mid}0\rangle + \beta_1 {\mid}1\rangle$ and $\mid{\psi}_2\rangle = \alpha_2 {\mid}0\rangle + \beta_2 {\mid}...
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How entanglement harvesting works?
Recently, I learned about entanglement harvesting from this video from the Institute of Quantum Information (Waterloo). So as I understand it you have two particles, let's consider two spins 1/2 ...
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Under what conditions does a beam splitter entangle two input photons?
There is a dispute on PhysicsForums related to what are the conditions necessary for two photons to be entangled by a beam splitter. Lots of references given by the forum users but they never arrive ...
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What is the unbalanced Mach-Zehnder interferometer?
I read about unbalanced Mach-Zehnder interferometers in investigating the time-energy entanglement. I wanted to know what is it. and what is its purpose?
Thank you.
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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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Where does the concept of quantum entanglement originate?
A video I watched from PBS spacetime talked about how the pilot wave theory provides an explanation for why quantum entanglement occurs, where it is a result of having global hidden variables. I just ...
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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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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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Electron "clouds" in an atom
I have a very naïve understanding of atomic structure and quantum mechanics. But from what I have heard about the double slit experiment with an electron, we can say that if we shoot photons on the ...
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Interference pattern with known photon path
I read about the Delayed-choice quantum eraser experiment and came up with an idea. As we know, if we can determine which slit the photon passed through, we cannot observe an interference pattern. But ...
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Can somebody explain to me one of the Bell experiments?
I can't understand the experiment in this paper.
In this experiment, two electrons at different points $A$ and $B$ are entangled with one photon each. Then, the photons are entangled in point $C$ ...
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Understanding how entanglement physically arises in nature
My background is in CS and I have just started to learn Quantum Computing. From what I have understood: Given a set of $N$ qbits that are in some superposition state, can entangle these qbits with ...
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Is electron correlation in Quantum Chemistry a consequence of many-body entanglement?
Electron correlation is largely defined in Quantum Chemistry as the set of properties that the celebrated Hartree Fock Approximation cannot model accurately. One popular example is the phenomenon ...
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How can non-realism alone explain quantum entanglement?
I’ve seen similar questions on this site but haven’t found a clear answer for this question. For clarity, I’ll define how my layman brain is interpreting locality and realism…
Locality: Information ...
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Do quantum entangled particle pairs actually interact? [duplicate]
I believe this question could help simplify the accepted understanding of quantum entanglement for myself and others with little to no real physics background.
So my understanding of one way to create ...
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Is it possible to detect entanglement by observing only one of the entangled particles?
For example, suppose an entangled electron and anti-electron have been produced in an inertial center-of-mass frame. If we have just a single detector and it observes a positron, then it's probably ...
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Another Entanglement Question - Can you tell if the wave function of an entangled particle is collapsed?
Apologies for the additional question on a topic that seems to be queried relatively frequently in this forum -
I was unable to find an explicit answer to this specific question in searching physics ...
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Are quantum measurements on two different particles always commutative?
If we have some quantum system of two entangled particles -- let's call them $a$ and $b$. Let $A$ and $B$ be some arbitrary observables we can measure on the two particles, respectively.
Is it ...
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Are states of entanglement particles defined at creation? [closed]
Are states of entanglement particles defined at their creation?
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Is quantum entanglement independent of measurement timing?
If I have the entangled state
$\vert \uparrow \downarrow \rangle + \vert \downarrow \uparrow\rangle$, and then
measure the spin of one particle and find it to be $\uparrow$, will the spin of the ...
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Is the electron pair in an orbital entangled?
In the video "Bird navigation: The Quantum Around You. Ep1", Dr. Andrea Morello seems to imply that the electron pairs in atomic orbitals are entangled and will exhibit properties of ...
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Why does the interference patterns in delayed choice quantum eraser cancel out? Why/how does it have a phase shift?
Talking about this experiment https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser#The_experiment_of_Kim_et_al._.282000.29
I am assuming that anyone able to answer my question is already ...
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What is the difference between no-disturbance principle and non-contextuality?
The no-disturbance (ND) principle states that, for any three observables A, B, and C such that A and B are compatible, and A and C are compatible, the probabilities of outcomes of A do not depend on ...
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Density matrices and locality
I'm having trouble understanding the physical aspect of entangled sub-systems, suppose I have a global entangled state:
\begin{equation}
|\psi\rangle = \frac{1}{\sqrt{2}}\left(|0\rangle_{A} \otimes |1\...
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Coincidence count probability in the entanglement-enabled delayed choice experiment
I'm trying to understand the entanglement-enabled delayed choice experiment and I'm kind of stuck at the term "coincidence count probability" which I can't seem to find definition of.
Their full set ...
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How is quantum entanglement detected experimentally?
What type of detector was used to detect entanglement of different systems, for example, photons?
Are there new detectors being made now?
What technique is used for this detection?
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Are electrons in higher energy orbits less entangled with the nucleus?
I understand that an electron in orbit is technically entangled with the nucleus. Is an electron with higher energy (further away from the nucleus) less entangled with the nucleus? Does a free ...
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Why use 2 nonlinear crystals in conjunction for SPDC?
I've seen a few articles (ex: 1, 2) that use either two BBO crystals consecutively or a BBO and a KDP crystal to create entangled photon pairs from Spontaneous Parametric Down-Conversion (SPDC). And ...
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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
$$\big\langle\hat A(a)\hat B(b) \big\rangle={\cal P_{++}+P_{--}-P_{+-}-P_{-+}...
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Are all instances of quantum non-locality problem artifacts of the use of classical concepts in quantum physics?
Consider experiments involving entangled spins, say two-spin 1/2 particles in the singlet state:
$$\left|\psi\right> =\frac{1}{\sqrt{2}}\left[\left|\uparrow\downarrow\right> - \left|\downarrow\...
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A question about Quantum entanglement, 45° Light Polarization, and faster than light communications
I recently read "Atomic Adventures" by James Mahaffey. In the book (Chapter 10) he mentioned a theoretical way of faster-than-light communication. My brain instantly searched for flaws in ...
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Entanglement distance and quantum mechanics length scales
In this answer https://physics.stackexchange.com/a/9253/256311, the author said
Usually, we think of quantum mechanics to govern the microscopic world, involving length scales of under a micrometer, ...
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Entangled Photons passing different polarizers
Consider two entangled photons leaving a source. Both are polarized at angle 0. One travels to the left and the other to the right. The one on the left encounters a polarizer at 30 degrees and the one ...
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How can quantum entanglement not be non-local?
I know this kind of question has been brought up many times.I have read many posts here regarding this but I still have a problem with a certain aspect of it so please bear with me.
Lets consider the ...
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Is there quantum randomness that significantly affects our macro world?
I was reading about the MWI and how there would be vastly different versions of "you" in the different branches.
I'm just wondering, though, which quantum events lead to such different ...
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Is two degree quantum entanglement possible? And by two degree of entanglement I mean one in space and other in time? [closed]
Whenever we try to entangle two particles, the entanglement lasts for a very short period. If we observe our present universe we can see that all the matters present in our universe are finely ...
