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15 votes
8 answers
2k views

Given entanglement, why is it permissible to consider the quantum state of subsystems?

Quantum entanglement is the norm, is it not? All that exists in reality is the wave function of the whole universe, true? So how come we can blithely talk about the quantum state of subsystems if ...
confused's user avatar
  • 151
14 votes
1 answer
1k views

Backward causality: A question/extension to Ma et al.'s "Experimental delayed-choice entanglement swapping"

In a philosophically rather interesting experiment, Ma et al. show that backward causality exists in quantum physics. An Ars Technnica-article gives a less technical account. From Ars Technica: ...
tom4everitt's user avatar
1 vote
2 answers
2k views

Would Quantum entanglement theoretically allow prediction of the future?

This article describes how a choice made by the recipient of an entangled photon can affect measurements taken on that photon's "partner" before the decision was made. So let's say there are two ...
StriplingWarrior's user avatar
13 votes
3 answers
5k views

Entanglement spectrum

What does it mean by the entanglement spectrum of a quantum system? A brief introduction and a few key references would be appreciated.
leongz's user avatar
  • 4,044
7 votes
4 answers
1k views

Does this Zeilinger group result provide experimental proof of backward-in-time causation?

Does this recent Zeilinger group delayed choice entanglement experiment imply backward-in-time influences? http://arxiv.org/abs/1203.4834 From the abstract: "This can also be viewed as “quantum ...
Jim Graber's user avatar
  • 5,807
15 votes
1 answer
2k views

Entanglement in time

Quantum entanglement links particles through time, according to this study that received some publicity last year: New Type Of Entanglement Allows 'Teleportation in Time,' Say Physicists at The ...
Giulio Prisco's user avatar
1 vote
1 answer
1k views

Entanglement and conservation

Is the following assertion sufficiently unique to merit a paper? Every absolute conservation law implies a corresponding form of entanglement, not just spin (angular momentum). Linear momentum ...
Terry Bollinger's user avatar
5 votes
2 answers
600 views

Quantum entanglement of spin along multiple orthogonal axes

Picture an entangled pair of spin 1/2-spin particles with total spin 0. In the diagram, particle 1 of the pair is moving to the left (-y), and particle 2 to the right (+y). If a z-oriented SG$^*$ is ...
Sergio's user avatar
  • 2,695
1 vote
4 answers
231 views

Is a macroscopic pair where I observe one of them quantum entanglement?

Of what little I know/understand about quantum entanglement can somebody confirm if the following experiment is a good analogy to quantum entanglement of pair of particles? PS: please don't laugh as ...
Shreedhar Kotekar's user avatar
12 votes
4 answers
2k views

Energy Measurements in a Two Fermion Double Well System

This question is related but my question here is much more elementary than discussions of the Pauli principle across the universe. There has been a fair amount of discussion around at the moment on ...
twistor59's user avatar
  • 17k
3 votes
4 answers
4k views

Entanglement and the double slit experiment

Is the double slit experiment an example of entanglement when it seems as if the photon is going through both slits? Or put another way, is it at this stage when we attempt measurement we see a photon ...
Paul Merrifield's user avatar
-1 votes
2 answers
192 views

Are all classically impossible quantum possibilities entangled?

Any entangled state represents a quantum possibility that is classically impossible. Is the converse true? That is, are all states that are quantum mechanically possible but classically impossible ...
Jim Graber's user avatar
  • 5,807
0 votes
1 answer
455 views

Relativity and Entanglement

Say we have two particles which are entangled so that they have opposite spins. If one is up, the other is down. They are sent off to two spatially separated observers A and B. Both observers can ...
user7348's user avatar
  • 1,114
6 votes
2 answers
2k views

Can cellular automata be reconcilied with quantum mechanics?

CAs are deterministic representations of the universe, which, according to the Bell's inequality are not entirely accurate. Cells interact "locally" (only with the closest neighbours), while quantum ...
federicot's user avatar
  • 163
1 vote
1 answer
234 views

How Is Entanglement Created Among Qubits?

How are qubits entangled? I understand the basics of entanglement but what I do not get is how it occurs in nature or in the lab. What causes entanglement to occur or what is done to the particle to ...
11D Reality Hacker's user avatar
6 votes
1 answer
393 views

Optimality of the CHSH strategy

The maximum achievable probability of the Clauser-Horne-Shimony-Holt game is $\cos^2(\pi/8)\approx85.355\%,$ which can be proved with Tsirelson's inequality. But I don't imagine that this remained ...
Charles's user avatar
  • 766
9 votes
2 answers
1k views

Quantum Computing, Qubit Creation/Entanglement

I am currently a high school student researching quantum computing. I was referred to this site by Google and a friend. Currently I am researching the qubit part of quantum computing. My question is ...
11D Reality Hacker's user avatar
3 votes
0 answers
226 views

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)...
kate's user avatar
  • 31
1 vote
1 answer
251 views

Number of conditions for a two-particle state to be decomposable

Suppose we have a general two-particle state $ \Phi (x_1, x_2 ) = \sum_{n_1,n_2} \phi_{n_1,n_2}(x_1,x_2)|n_1,n_2> $, where $n_1$ can be any of $n$ possible states, and $n_2$ can be any of $m$ ...
Raphael R.'s user avatar
5 votes
2 answers
3k views

The mathematics of entanglement

I've finally managed to get a grasp on the Bell test experiments and all that they imply about our reality. Now I'm curious about the mathematical derivation which allowed Schrodinger to predict the ...
Gearoid Murphy's user avatar
4 votes
2 answers
816 views

Has anyone actually "seen" entanglement? [duplicate]

I want to know if the following has been done experimentally; after the spin (or any other characteristic with a probability of 50%) of 2 entangled particles has been measured, we change the spin of ...
user6328's user avatar
4 votes
2 answers
10k views

What is "quantum locking"?

I've always assumed that "quantum locking" was a term invented by the writers of Dr Who, but this video suggests otherwise. What is quantum locking? Is it real?
orome's user avatar
  • 5,169
100 votes
0 answers
4k views

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 ...
David Bar Moshe's user avatar
8 votes
1 answer
163 views

Sub and super multiplicativity of norms for understanding non-locality

In relation to various problems in understanding entanglement and non-locality, I have come across the following mathematical problem. It is most concise by far to state in its most mathematical form ...
user avatar
2 votes
1 answer
919 views

Spin up, spin down and superposition

I'm just starting to study quantum mechanics. Please explain the error in this thinking: You set up decay of two $\pi$ mesons and get $2\mathrm{e}^-$ on Mars and $2\mathrm{e}^+$ on Earth. On Earth ...
William Entriken's user avatar
5 votes
2 answers
750 views

Tracking photon color in Bell experiments

In parametric down-conversion, it is said that a driving photon is converted into two entangled photons whose frequencies add up to the driving frequency. Yet in discussions about entanglement ...
Marty Green's user avatar
  • 4,219
4 votes
2 answers
554 views

Coincidence detectors in Bell tests: How close is close enough?

When is a coincidence a coincidence? We know that to identify entangled photons, the electronics is set to look for simultaneous clicks at opposite detectors. The size of the window is to some degree ...
Marty Green's user avatar
  • 4,219
24 votes
8 answers
5k views

Quantum entanglement vs classical analogy

Consider that we have two balls, one white and one black, and two distant observers A and B with closed eyes. We give the first ball to the observer A and the second ball to the observer B. The ...
kuzand's user avatar
  • 2,176
5 votes
1 answer
2k views

Constructing a maximally entangled qutrit state from $n$ Bell states

I've read that maximally entangled qubit states are a good "unit" of bipartite entanglement since it is possible to create any other entangled state from them using local operations and classical ...
John Schanck's user avatar
12 votes
2 answers
2k views

What is quantum discord?

What is quantum discord? I stumbled upon this term on Quantum Computing: The power of discord, but have never heard of it before. Can you give a bit more mathematical explanation of the term here?
asmaier's user avatar
  • 10k
1 vote
2 answers
481 views

Can two entanglement particles satisfy at same time two different wave functions?

The wave function determines the probabilities of an outcome, and the wave function is parameterized "shaped" depending on the measurement set. i.e. two different measurement set would lead to two ...
HDE's user avatar
  • 2,937
17 votes
2 answers
6k views

Why is it valid to take the partial trace to describe a subsystem?

In derivations of decoherence, there eventually comes a point when we are asked to take the partial trace over the environment. Why should this be valid for an entangled system? Why should taking the ...
Gunther Grossmann's user avatar
9 votes
2 answers
1k views

Has quantum entanglement been demonstrated to be able to take place over infinite distances?

In my poor understanding of quantum physics, quantum entanglement means that certain properties of one of two 'entangled' quantum particles can lead to change over infinitely large distances when the ...
Decent Dabbler's user avatar
4 votes
1 answer
6k views

Quantum Entanglement: how to generate 2 entangled particles?

I get quantum entanglement but I don't quite get how one would go about generating two complementary particles that are entangled (a photon and its entangled sibling, an electron and its entangled ...
JohnIdol's user avatar
  • 187
9 votes
2 answers
806 views

Can observations of entangled particles affect their unobserved counterparts?

There are two experiments that are often used to explain Quantum Mechanics: the two-slit experiment and the EPR paradox. I am curious what would happen if you combined them. Imagine an experiment ...
Nick Retallack's user avatar
6 votes
1 answer
723 views

Expansion of multi-particle state vector as a sum of n-entangled states

Physically, quantum entanglement is ranged from full long-range entanglement (Bose-Einstein condensate), described by a basis of states that look like this: $$ |\Psi\rangle = |\phi_{i_{0} i_{1} ... ...
lurscher's user avatar
  • 14.8k
2 votes
3 answers
669 views

Is it possible to bind an entangled electron to the outer shell of an atom or just a proton?

Say we start with an entangled electron--positron pair and we separate them. I want to take the entangled electron and bind it to a proton or the outer shell of an atom. Is it possible to do this ...
QEntanglement's user avatar
6 votes
4 answers
381 views

Justification of ignoring large set of entanglements

If we can think about the universe as a wave function then many particles should be entangled with many other particles in the universe. The obvious question arises why we don't see those ...
user avatar
3 votes
3 answers
1k views

Local measurement of entangled particle pairs and interpretation of state

This just started to bother me after reading yet another entangled particle question, so I hate to ask one myself, but... If we have two entangled particles and take a measurement of one, we know, ...
Mitchell's user avatar
  • 2,108
4 votes
3 answers
719 views

Entangled particles

So we have two particles (A and B) that are entangled. From what I understand, entanglement isn't destroyed, it is only obscured by subsequent interactions with the environment. Particle A goes ...
David's user avatar
  • 51
3 votes
1 answer
3k views

How does the notion of weak measurement resolve Hardy's paradox?

How the notion of weak measurement resolves Hardy's paradox?
Eelvex's user avatar
  • 1,260
8 votes
2 answers
3k views

Unsolved entanglement problems

What are the current most important theoretical problems on quantum entanglement? What is that we don't yet understand about how it works? (Not considering interpretation etc problems)
15 votes
4 answers
5k views

What's the difference between an entangled state, a superposed state and a cat state?

1) Can a state be entangled without also being a superposition? (Please give an example.) 2) Can a state be a superposition without being entangled? (Again, an example please.) 3) And what about ...
Jim Graber's user avatar
  • 5,807
3 votes
2 answers
1k views

What is Timelike Quantum Entanglement?

I came across a New Study at http://arxiv.org/abs/1101.2565. Which talks about Timelike quantum entanglement. What does that mean? Comment added by L.Motl: The same preprint has been discussed in ...
Kiran Kumar's user avatar
9 votes
3 answers
4k views

What is quantum entanglement? [closed]

What is quantum entanglement? Please be pedagogical. Edit: I have updated my background under my profile.
Amir Rezaei's user avatar
6 votes
1 answer
5k views

What are the benefits of quantum information "teleportation"?

I read occasionally popular science articles and from time to time encounter issues about quantum information teleportation. (this one for example http://www.physorg.com/news193551675.html) So far I ...
Andrey Tatarinov's user avatar
5 votes
2 answers
820 views

What is time teleportation?

I read this article about time teleportation. Can someone explain the concept better?
Amir Rezaei's user avatar
5 votes
1 answer
401 views

Detecting coherence

Is there a way to know if a particle is acting as a wave or a particle? Alternatively, if an entangled particle was already measured? A - Yes So any experiment over an entangled particle that let ...
user avatar
124 votes
10 answers
23k views

Why is quantum entanglement considered to be an active link between particles?

From everything I've read about quantum mechanics and quantum entanglement phenomena, it's not obvious to me why quantum entanglement is considered to be an active link. That is, it's stated every ...
Andrey Tatarinov's user avatar
8 votes
2 answers
879 views

How do electrons interact if one of them had just exited the two slits of the double-slit experiment?

Consider the following experiment: a double-slit set-up for firing electrons one at a time. Let's now add a second electron (orange), which is fired parallel to the first one, but in the opposite ...
Roman Starkov's user avatar