# Tag Info

## New answers tagged entanglement

0

You will break an entanglement once you change the spin of one component independently.

11

Entanglement is simply a particular kind of quantum multiparticle state: it happens to be the "most common" kind of state in the sense that if you choose a random quantum superposition from a multiparticle state space, it will almost surelybe (in the measure-theoretic sense) entangled, so it's a little curious why entanglement takes some effort to observe in ...

1

The underlying framework of physics is quantum mechanical, this is the current level of physics development. Classical theories emerge from this underlying framework in a consistent and understood manner. Quantum mechanics has differential equations that have to be solved for specific situations . The square of the solutions of these equations give the ...

1

Consider a two-state system. Particle A can either be in state $|\!\uparrow\rangle$ or state $|\!\downarrow\rangle$. If there are two independent particles in the system, then each particle can either be $|\!\uparrow\rangle$ or $|\!\downarrow\rangle$ so we label the overall system states $|\!\uparrow\uparrow\rangle$, $|\!\uparrow\downarrow\rangle$, ...

1

There is no precise answer to the cause of entanglement is known (I did not come across any such answer). The superposition principle and the tensor product structure of Hilbert space of combined system leads to the possibility of entangled state. It is well understood in the case of pure state, but for the mixed state case it is quite involved.

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I.The cause of the quantum entanglement is that the wave-function (w.f.) of the involved particles doesn't have the form of the w.f. of independent particles. So, let's begin by defining what is the w.f. of independent particles. The w.f. of a single particle, let's call it A, can look like |A> = \frac {\lvert x_A \rangle + \lvert y_A \rangle.} ...

1

First of all I recommend you to read the answer of Hypnosifl to the question "Entangled photons never show interference in the total pattern without coincidence count" implies FTL. Next, what flip you want to transmit? Flipping the spin of a particle is a local, unitary operation, not even a measurement. You pass your (spin endowed) through a ...

4

As Wolphram already stated, no information can be communicated via local measurements on entangled systems. We are so certain that this is true that this fact is considered as one of the so called No-go theorems, called the no-communication theorem. The reason this is true is any measurement of one part of an entangled system is completely random. So what ...

6

There is no way to send information using entangled particles. For instance , in your exacple, Huston is unable to know if you have already made a measurement. All it can do is make it own measurement whenever he wants, and then conclulde what your measurement will be or was if you already measured it.

1

I understand that initially the photons have both the same polarization, but no fixed polarization relative to the filters. If so, as the relative polarization is random, there should be full symmetry between passing the filters and being stopped by them. Now, to your questions: 1) For the polarization singlet, (whose state is {|x>|x> + |y>|y>}/sqrt2) ) ...

1

As in Holger Felder's Answer, to make an entangled state, all known ewxperimental techniques are local insofar that entangled states must be 'created' by a single interaction: you need to produce a pure quantum state e.g. of two photons with opposite spin in a given direction, so that the pure quantum state is nonfactorisable (i.e. can't be written in the ...

0

To make two photons entangled they had to be produced together. For example with down-conversion in non-linear crystals or from quantum dots which emit pairs of photons. This is needed for quantum cryptography. On the other side the photons from a laser beam if one let them through a polarisator are entangled in their frequency and in their field direction. ...

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See this article: Časlav Brukner, Markus Aspelmeyer, and Anton Zeilinger, "Complementarity and Information in "Delayed-choice for entanglement swapping", arxiv.org/abs/quant-ph/0405036v1 . You can find it in Internet, in the arXiv quant-ph. By the way, it is BETTER to read the article than the issue in Wikipedia about entanglement swapping. The treatment in ...

2

Atoms can interact in a variety of ways, it varies from experiment to experiment. In principle two qubits with spins 1/2 could interact via magnetic force. In this case they would have to be extremely close. This experiment would also be extremely hard. An easier way would be to use ensembles of atoms. Atomic beams can be prepared where atoms are held in ...

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Here is a suggestion of how to produce the state in your second formula. Choose to make holes in a screen, NOT at the two points A and B of which I spoke in my first answer, but at two other points, e.g. on top of the upper cone, and on the bottom of the lower cone. Then, pass the upper beam through a beam splitter $BS_1$. Block the transmitted wave, let ...

1

Spontaneous Parametric Down Conversion is a process that produces pairs of photons. The process goes like this: a strong beam of ultra-violet (UV) photons, is sent upon a down-conversion crystal (placed inside your black-box). Inside the crystal, the UV photon is SPLIT into two photons, named (I don't know why) SIGNAL and IDLER. In general the two photons ...

-2

I try to answer your questions, one by one. First of all please don't use the words "information passes". We use to ask whether some sort of "signals" pas, because have NO IDEA what passes there. John Bell said that "something is moving there faster than light". But, actually we DON'T KNOW if it is so. NEITHER do we know whether the properties of the two ...

0

It is not proven that the properties are aren't predetermined, yet Bell's theorem shows us that who believes in determinism must abandon locality, and that who believes in locality must abandon determinism. Most physicists choose to abandon determinism, since locality, as tested time and time again by relativity, seems to describe the world well. ...

2

What makes you think that information can be transferred in this way in the first place? I cannot tell you why your method won't work if I don't know what method you have in mind. In any case, I can take a guess as to what you're referring to. Suppose we have a pair of electrons in the state (up to normalization) \$\left| +\right> \left| -\right> ...

0

Entanglement results when the system under consideration belongs to one quantum mechanical solution of the specific problem. The inflationary period as introduced at the beginning of the Big Bang model is a quantum mechanical solution to the boundary problem of the very early universe. At those sizes and energy densities it is assumed that everything is ...

0

EPR might seems a thought experiment, but it has been translated into an actual physic experiment, especially by physicist Alain Aspect. You say quantum physic gives bizarre conclusion, that it is in an uncomfortable position, or that "philosophy of quantum mechanic" is at doubt. That's especially why it is important to really perform the "disturbing" ...

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Even for things that seem very clear from the theory, you will want to check them. You asked I mean, what were they expecting ----- were they expecting the states of the particles to be not in co-relation? How would they explain for that? Well, of course they were expecting the entanglement. But finding that this is NOT there, would have been a huge ...

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What you seem to not get from the paper is that the EPR-thought experiment actually made a prediction: It predicted that there are correlations within quantum systems that are stronger than in any possible classical system or any local hidden variable theory. The "spooky action at a distance" is just failing classical intuition. Don't read too much into it ...

3

And you cannot communicate information through entanglement anyway. So my question again : why the experiments? I think you will find that the technology is important in communication. Quantum cryptography is a way of sending secure messages with, for example, entangled photons used to send a key. If photons are intercepted between the sender and the ...

2

A thought experiment is not really an experiment, but an idea. Science requires people to be able to test ideas with reproducible experiments. I can't reproduce a thought "experiment". A reproducible physical experiment produces physical observations. See "Empirical research".

1

Yes. A polarizing filter absorbs the wave polarized orthogonally to the polarization direction of the polarizer. Assume that the particles are photons in the polarization singlet, |Singlet> = (1/sqrt(2)) {|x>|x> + |y>|y>}. For simplicity let x be the direction of polarization transmitted by the filter for one of the particles. You see that in the ...

2

Yes, as it turns out there are distinct types of multipartite entanglement not witnessed by the spectra of reduced states. The simplest example is probably the one from Bennett et al., "Exact and asymptotic measures of multipartite pure-state entanglement", Phys. Rev. A 63, 012307 (2000). For the case of three 4-dimensional systems, consider the states ...

1

To answer your question shortly: No, they are not the same phenomenon. First of all, it is much easier to think of quantum states as vectors (in something called the Hilbert space, but simply put they obey linearity), and not as particles or waves. Superposition Let's start with a single particle qubit (since you're talking about quantum information) ...

0

First of all the words "exchange of information" are not so good. An entanglement is a CONSTRAINT on two particles or more. For instance the famous PHOTON SINGLET is described by the state (1) |Ψ> = [!/sqrt(2)] {|u>|u> + |v>|v>} where u is whatever direction in space that we want to choose, and v is perpendicular to x in the polarization plane (which is ...

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