All Questions
Tagged with quantum-mechanics quantum-entanglement
259 questions with no upvoted or accepted answers
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Orbits of maximally entangled mixed states
It is well known (Geometry of quantum states by Bengtsson and Życzkowski) that the set of $N$-dimensional density matrices is stratified by the adjoint action of $U(N)$, where each stratum corresponds ...
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9
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335
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Thermalization at quantum critical point and quantum many-body scars
Quantum scar states are a hot topic in condensed matter physics. Quantum scars are the eigenstates of a many-body system (e.g. a spin chain) that weakly violate the eigenstate thermalization ...
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8
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Time evolution keeps a certain product state always a product state. Is there a time-independent factorizable evolution for this state?
I am typically thinking of quantum spin chains in the following of some length $L$. I am OK without any locality in the assumptions on $H$.
I have a product state $|\psi\rangle$ and a potentially very ...
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8
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267
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Free Will Theorem question
The Kochen-Specker Theorem says, if I understand it correctly, that the results of spin measurements cannot be predetermined independent of measurement. They get to this conclusion by describing 33 ...
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6
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177
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A Venn diagram for: (non-)locality, (non-)realism, (non-)contextuality, (non-)signalling, (dis-)entanglement
I recently asked a (yet-unanswered) question about the relationship between state-dependence and violations of realism. The more I read on the subject, the more I find myself digging deeper in a ...
- 1,355
5
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1
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140
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How to measure the entanglement of three entangled qubits?
The entanglement of two qubits is calculated using "concurrence" and "negativity". Concurrence and negativity, however, are only used for "two" entangled qubits. Is there ...
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219
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Geometric measure of entanglement for fermions or bosons?
For a system consisting of multiple components, say, a spin chain consisting of $N\geq 3 $ spins, people sometimes use the so-called geometric measure of entanglement. It is related to the inner ...
- 1,197
4
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162
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Difficulties in proving the area-law conjecture in higher dimensions
A very famous and important open conjecture in condensed matter physics is the area law of entanglement entropy, which claims that in a locally-interacting quantum many body system, if the ground ...
- 1,025
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360
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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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253
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A question regarding Leonard Susskind's ER=EPR lecture
https://youtu.be/OBPpRqxY8Uw?t=1315 Right at this instance of the video Susskind starts talking about how space is actually connected by entanglement. (You should watch the video for a accurate ...
user212860
4
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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\...
- 10.3k
4
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328
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Definition of a gapless spin liquid
I understand the definition of a gapped spin liquid: it's a gapped, topologically ordered spin state - i.e. there does not exist a local unitary transformation that takes it to a product state in ...
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4
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1
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263
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Non-locality and topology
This is a purely speculative question:
Has there been any work that describes non-locality/entanglement in QM by using exotic topologies in configuration space?
The 'conceptual' picture that I have ...
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4
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2
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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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1
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258
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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 ...
3
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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
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Is there any notion of device independent test for correlation like quantum discord?
A Device independent test is a procedure used to characterise quantum resources with the minimal level of trust. If one wants to test correlations like entanglement in a device-independent way, we get ...
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Confusion about the tensor product structure of a multi-fermion Hilbert space
I often see people study entanglement in fermionic systems. The setup is often like this. Suppose we have a 1d lattice of $2L $ sites, which is divided into a left part and a right part, each with $L ...
- 2,165
3
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177
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Calculating entanglement negativity without constructing density matrix
There are two procedures that I know of for finding the von Neumann entanglement entropy of a bipartite system split between $A$ and $B$ - a bad way and a better way. I have in mind computationally ...
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3
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0
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83
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Macroscopic quantumness and decoherence
Can somebody suggest a "measure" that aims at mathematically defining a "macroscopic quantum state" which is compatible with decoherence or the thought "macroscopicity = ...
- 456
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52
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Can we place bounds on the minimum trace fidelity of concatenated quantum channels?
Say we have two quantum channels $\mathcal{E}_1$ and $\mathcal{E}_2$ with channel fidelities $\mathcal{F}_1$ and $\mathcal{F}_2$. Can we place any bounds on the fidelity of the channel $\mathcal{E}=\...
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3
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Device-independent QKD - truly achieving security?
The issue of security of QKD is, as commonly known, the practical implementation. I know that it is possible to hack commercial quantum crypto systems as it was done e.g. by the Quantum Hacking Lab. I ...
- 117
3
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264
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Projected Entangled Pair States (PEPS) and classical statistical physics
In the paper arXiv:quant-ph/0601075 the authors introduce an interesting correspondence between Projected Entangled Pair States (PEPS) and classical statistical physics. Basically, for any locally-...
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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
3
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49
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Entanglement and cosmological eras
I'm checking this paper: https://arxiv.org/abs/1405.7834
In the context of quantum cosmology, they assume that the universe evolves in an entangled state between the two epochs considered, i.e.$$\...
3
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0
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95
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Can we create "asymmetric" EPR pairs in experiments?
Asymmetric here is just for lack of a better word.
What I am looking for are experiments that can create a simple EPR pair but without 50% 50% chance of each state.
As I understand from standard ...
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3
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283
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$k$-local Hamiltonian with long range entangled ground states?
Is it possible, and if yes, is there a relatively simple example of a Hamiltonian that only has k-local terms but its ground state always has entanglement beyond $k$ sites?
For instance if $H = H_{...
- 2,542
3
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470
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Are there any physical realizations of a rebit (a qubit with real coefficients for probability amplitudes of a two-state system)?
While being introduced to the world of quantum computation and qubits, I’ve come across the term rebit, which I understand to mean a two-state quantum system that may be expressed as a real linear ...
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3
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What has quantum entanglement to do with navigating on the magnetic field?
In this video is simply explained how robins use the magnetic field to navigate. But the author is talking about quatum entangelment, but I don't see how this is related to navigating. Now there are ...
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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 ...
3
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2
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682
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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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3
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193
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Uncertainty Principle - measuring momentum on one entangled particle, position on the other
If two entangled particles are sent far apart and then at exactly the same time the position of one, and the momentum of the other, is measured, won't this mean that, because the corresponding values ...
- 123
3
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224
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Spin polarization of decay products
A relativistic moving particle, e.g. muon $\mu^+$, described by its four-momentum vector $p_\mu$, charge $e$ and with a given spin polarization, ${\bf S}=(S_x,S_y,S_z)$, decays into three particles, e....
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How can one trace out polaritonic degrees of freedom?
I have read the paper "Steady state entanglement between hybrid light-matter qubits", arXiv:0711.1830v2.
There, writers obtained density operator in matrix form after solving steady state equation (13)...
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2
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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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2
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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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2
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57
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Is it possible that a macroscopic object tends to a separable state without the need for objective collapse?
For a multi-particle system, superposition is in some sense equivalent to entanglement; with the Dirac field being treated as classical under second quantization, for example, we could at least argue ...
- 2,587
2
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95
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Computing Fubini-Study expectation values over $\mathbb{C}P^n$
In finite-dimensional textbook quantum mechanics, we postulate that states of our system are rays in a Hilbert space $\mathcal{H}$ with dimension $\dim{\mathcal{H}} = n+1$ where $n \in \mathbb{N}$, ...
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2
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Is it possible to conduct quantum teleportation with a larger than necessary e-dit and preserve entanglement?
Suppose you have an e-bit between qubits A and B. You want to transfer the state of qubit Q to B. You can utilize this e-bit to perform a quantum teleportation to achieve this.
Now instead suppose ...
- 1,805
2
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0
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46
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A Hamiltonian acts simply on each state in some subspace. Can it be identified with a single simple operator on the subspace?
This is a simplified version of a recent question I asked. My hope is that this simplified version will be easier to tackle. The motivation behind both of these questions is roughly to ask "Given ...
- 2,382
2
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121
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Obtaining a Matrix Product State (MPS) using Schmidt Decomposition for a Tripartite State
I understand that one method to derive an MPS representation of a quantum state involves applying the Schmidt decomposition $ N−1$ times. While I'm familiar with the diagrammatic notation, I wanted to ...
- 1,208
2
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80
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Entanglement in the Born-Oppenheimer approximation
Consider a scenario where we aim to model an atom as a system consisting of two components: the nucleus and the valence electron.
In this approach, we simulate the behavior of the electron within the ...
2
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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
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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
2
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Are von Neumann entropies of complementary but physically distinct subsystems in time-dependent settings identical?
We assume a quantum system AB with subsystems A and B and take the Schmidt decomposition of a state $\vert\psi_{AB}\rangle=\sum_i\lambda_i \vert a_i\rangle\vert b_i\rangle$ defined on the compsite ...
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2
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121
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How can two phonons be entangled when the atoms in the crystal are not?
It is my understanding that you can use phonons to make a gaussian packet, which would behave like a quantum particle. I also believe that you can make two such packets and entangle them, that is ...
2
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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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2
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63
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Can a phase factor entangle two states?
Does the following non-separable wavefunction represent an entangled state?
$\psi(x_1,x_2)$ = $\exp[i b x_{1}x_{2}]\phi(x_{1})\phi(x_{2})$
This state can not be factorized into functions of $x_{1}$ ...
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2
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0
answers
72
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Measurement operator in a Bell experiment
I'm trying to figure out why a Bell experiment gives rise to the payoff (measurement) operator used in this paper on quantum game theory.
Two players are each in control of one half of an entangled ...
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2
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0
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122
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Have objective collapse theories been ruled out by recent experiments?
Have objective collapse theories been ruled out by recent experiments, such as the entanglement of macroscopic objects? (vibrating drumheads)
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