Entangled particles for communication

[Dror]

By saying that entangled pairs are no longer entangled at encoding/decoding (Or any other means of disruption whatsoever), as a lot of you are saying at different threads, you are bassicaly saying that entanglment is nothing more then two particles born with the same properties. And, by that you are dismissing the whole notion of entanglment.

That kind of contradicts the whole meaning of 'entanglment'.

It makes an entangled pair nothing more then 2 particles with the same properties somewhere in the universe. (Sorry for repeating my self)

 

A question: Does interfering with the first entangled particle causes the same interference in the second? Or for argument's sake, the implication of intereference on the first particle doesn't have to be the same as the implication on the second particle. As long as there is SOME implication on the second.

 

If not, then entanglment is dismissed.

 

If it is, then communication between entangled particles is possible with a very very clever protocol, The way I see it. (At no time at all, ofcourse).

 

I'm no expert and would like to hear what you think.

[swansont]

It's not two particles with the same properties, it's two particles with a specific relation between certain properties (they could be the same, but they could be different) and you don't know what these properties are until you measure them.

 

There are people looking at "adiabatic measurements" designed not to destroy the entanglement, but as far as I know these haven't been successfully carried out. I suspect that all this will do is redefine the basis set for measurement, and while there would be a change in the second particle's state, the information about which basis set to now use will still have to be transmitted clasically.

[Dror]

Exactly what goverens the nature of an entanglment? Is it analogous to the observed path of a particle? As in, probabilistic? If you repeat the creation of 2 entangled particles twice, will the entanglment be different each time?

 

And, (I'm probably wrong and that's why I'm asking) it isn't clear to me why the mesurment will destroy the entanglment.. At mesurment the uncertinty law will come into action, but I have a feeling that with good accurate framework for the protocol it is possible to yield a change of 0 probability at rest, and results in recodnision of intentional change at the recieving end.

 

It depends if it's possible to say "This and that will not accure without further intervension, even after I messed around with the particle once".

 

And further, if the reciving end program knows the nature of the intervension meant to convey 'change' to it, it might just have a pretty good idea of the nature of the entanglment with repect to the other particle.

I admit this is a bit vague..

 

Correct me if I'm wrong.

[Severian]

This comes about in quantum mechanics because of the uncertainty relation. That is, some properties of particles cannot be assigned simultaneously. The classic (no pun intended) example is position and momentum. If one measures the position of a particle, the act of measurement has fixed its position, and while position is defined, momentum is not defined.

 

In the case of entanglement in the EPR 'paradox', which is what you are refering to, the two electrons are produced together in such a way that their spin is undefined. Since the spin of an electron (when defined) can be up or down only, it forms a nice binary switch for sending messages.

 

However, while their individual spins are not defined, their sum is, so if electron A were spin up then electron B would be spin down (note this is not yet the case since neither has defined spin). It is only when spin of one of them is measured that a definite value of spin is taken, and since we know the relation between their spins, knowing one tells us the other.

 

Although this is most definitely non-local (measuring the spin of the particle at one space-time point instantly 'changes' the value at another) it cannot be used to send signals faster than light. This is for two reasons: 1. we have no control over whether electron A chooses spin up or down - it is random; 2. we cannot measure the spin of B before the signal is sent, so cannot detect any 'change' in its behaviour.

 

This is why the EPR 'paradox' is not a paradox at all. We are still constrained by having information flow as sublight speeds.

[Dror]

If it's possible to check one of the spins without chaging it, then this information COULD be transmitted to the reciever by classical means, when the transmiter and reciver are close, then the reciever can safly fly off 100 light years away, and recive data instantly from the transmiter.

 

AGAIN, correct me...

[5614]

I know that currently there must be a classical means of transporting the data, ie. with a laser, so it's not instant... however if you can read the spin without breaking the entaglement then I'm not 100% sure, although I think that you'd still need that classical channel of communications.

[blike]

(measuring the spin of the particle at one space-time point instantly 'changes' the value at another) it cannot be used to send signals faster than light.

So measuring one collapses the wavefunction of both particles spin? If we measure the spin of one, then we instantly know the spin of the other, right? Does the same hold true for measuring position?

[5614]

If we measure the spin of one, then we instantly know the spin of the other, right? Does the same hold true for measuring position?

 

yes for the first question, no it doesn't for the second.

 

The position are unrelated to each other... and besides uncertainty says you cannot know that, or is that why you asked?

[swansont]

So measuring one collapses the wavefunction of both particles spin? If we measure the spin of one, then we instantly know the spin of the other, right? Does the same hold true for measuring position?

 

I'm not sure how you'd entangle a particle's position.

[blike]

If the entangled electrons are created in a manner that sends them in opposite directions, then measuring the position of one would reveal the position of the other one.

 

<--e1-- ( source ) --e2-->

 

uncertainty says you cannot know that

Uncertainty doesn't say we can't know the position by itself.

[swansont]

If it's possible to check one of the spins without chaging it' date=' then this information COULD be transmitted to the reciever by classical means, when the transmiter and reciver are close, then the reciever can safly fly off 100 light years away, and recive data instantly from the transmiter.

 

AGAIN, correct me... [/quote']

 

But nothing in all that travels faster than c. It will take you >100 years to get the information.

 

Once you measure the spin of A, you know the spin of B. That breaks the entanglement. Remeasuring the spin of A then does nothing.

[swansont]

This comes about in quantum mechanics because of the uncertainty relation. That is' date=' some properties of particles cannot be assigned simultaneously. The classic (no pun intended) example is position and momentum. If one measures the position of a particle, the act of measurement has fixed its position, and while position is defined, momentum is not defined.

 

In the case of entanglement in the EPR 'paradox', which is what you are refering to, the two electrons are produced together in such a way that their spin is undefined. Since the spin of an electron (when defined) can be up or down only, it forms a nice binary switch for sending messages.

 

However, while their individual spins are not defined, their sum is, so if electron A were spin up then electron B would be spin down (note this is not yet the case since neither has defined spin). It is only when spin of one of them is measured that a definite value of spin is taken, and since we know the relation between their spins, knowing one tells us the other.

 

Although this is most definitely non-local (measuring the spin of the particle at one space-time point instantly 'changes' the value at another) it cannot be used to send signals faster than light. This is for two reasons: 1. we have no control over whether electron A chooses spin up or down - it is random; 2. we cannot measure the spin of B before the signal is sent, so cannot detect any 'change' in its behaviour.

 

This is why the EPR 'paradox' is not a paradox at all. We are still constrained by having information flow as sublight speeds.[/quote']

 

But the entanglement of the spins isn't really an example of the HUP, it's an example of the conservation of angular momentum. Entanglement stems from such conservation laws and the fact that particles can be in undetermined states/superpositions, which is related to the question of the existence of hidden variables (whether the electron in question had that spin orientation all the time, and you just measured it, or was it really undetermined until you measured it)

[Dror]

Once you measure the spin of A, you know the spin of B. That breaks the entanglement. Remeasuring the spin of A then does nothing.

 

I don't understand why that breaks the entanglment.

[swansont]

If the entangled electrons are created in a manner that sends them in opposite directions' date=' then measuring the position of one would reveal the position of the other one.

 

<--e1-- ( source ) --e2-->

[/quote']

 

Just from classical conservation of momentum that's true. Entanglement carries with it certain QM implications from the superposition of states and, of course, the quantization of parameters. I don't think this case qualifies as entanglement.

[swansont]

I don't understand why that breaks the entanglment.

 

Measurement collapses the wave function - it's no longer in a superposition of two states.

[Dror]

Measurement collapses the wave function - it's no longer in a superposition of two states.

 

This means that at the first cohesive interference to either of the particles (regardless of if it teaches anyone anything about their state) the entanglment is destroyed.

Kind of useless then, and makes you wonder how did they even prove the entanglment in the first place..

Cohesion in reality really collapses ALL of the wavefunctions.. Just the cosmic background radiation is sufficient for that..

 

The question is, if after the collapse, a new wavefunction is born..? Saying that there isn't one just cause we don't know it is kind of baseless, and the uncertinty principle only strenghes that point in a wierd way.

[blike]

Just from classical conservation of momentum that's true. Entanglement carries with it certain QM implications from the superposition of states and, of course, the quantization of parameters. I don't think this case qualifies as entanglement.

I ask because Einsten proposed a thought experiment (commonly called the EPR Paradox) in which photon's positions were entangled.

 

This link implies that photons can be spatially entangled. "Since spatially entangled photons are generated whenever individual photons pass through an optical beamsplitter, the practical study of entanglement with photons is connected to the realisation of devices for generating single photons on demand, henceforth referred to as single-photon sources."

 

However, it also throws my whole understanding off a bit. I was under the impression that, when presented with a beam splitter, a single photon has a 50/50 chance of passing through or reflecting. In it's unmeasured wavefunction state (quantum state? I don't know the terminology), the photon's wavefunction shows an equal probability of finding the photon in either path. If mirrors are set up to reconverge the paths, the wavefunction actually interfere with itself, producing a typical wave interference pattern on a photographic plate (after successive photons are subjected to the same setup).

 

That's really a side point, but I'm confused again. Ah well.

[swansont]

I ask because Einsten proposed a thought experiment (commonly called the EPR Paradox) in which photon's positions were entangled.

 

This link implies that photons can be spatially entangled. "Since spatially entangled photons are generated whenever individual photons pass through an optical beamsplitter' date=' the practical study of entanglement with photons is connected to the realisation of devices for generating single photons on demand, henceforth referred to as single-photon sources."

 

However, it also throws my whole understanding off a bit. I was under the impression that, when presented with a beam splitter, a single photon has a 50/50 chance of passing through or reflecting. In it's unmeasured wavefunction state (quantum state? I don't know the terminology), the photon's wavefunction shows an equal probability of finding the photon in either path. If mirrors are set up to reconverge the paths, the wavefunction actually interfere with itself, producing a typical wave interference pattern on a photographic plate (after successive photons are subjected to the same setup).

 

That's really a side point, but I'm confused again. Ah well.[/quote']

 

I thought we were discussing electrons; that's what my post was based on anyway.

 

What I think the link is referring to is that if you send a photon through a polarizer at 45 degrees, you put it in a superposition of the two possible polarization states - vertical and horizontal. So I think the spatial refers to the two dimensions of the polarization, not the location of the photon.

[swansont]

Measurement collapses the wave function - it's no longer in a superposition of two states.

 

This means that at the first cohesive interference to either of the particles (regardless of if it teaches anyone anything about their state) the entanglment is destroyed.

Kind of useless then, and makes you wonder how did they even prove the entanglment in the first place..

Cohesion in reality really collapses ALL of the wavefunctions.. Just the cosmic background radiation is sufficient for that..

 

The question is, if after the collapse, a new wavefunction is born..? Saying that there isn't one just cause we don't know it is kind of baseless, and the uncertinty principle only strenghes that point in a wierd way.

 

AFAIK That's one reason why in teleportation experiments you never realize the 100% fidelity that quantum theory predicts. You teleport many states, but not all of your EPR pairs have avoided interaction that disrupts them. But all you have to do to demonstrate it is beat the 50% fidelity of classical theory, and Jeff Kimble's group at CalTech did that.

[Dror]

Where could I find info about "adiabatic measurements"?

[swansont]

Where could I find info about "adiabatic measurements"?

 

Sorry, I don't recall where I read about the group that was investigating this.

[5614]

How exactly do you measure the angular momentum of an electron?

 

(as in what method is currently used?)

[swansont]

How exactly do you measure the angular momentum of an electron?

 

(as in what method is currently used?)

 

 

Nobody really does it anymore' date=' directly, because it's well established.

 

The original experiment was the Stern-Gerlach experiment. Since the electron has spin, it's affected by magnetic fields, so S-G passed some Silver atoms through an inhomogeneous magnetic field. The deflection was quantized - either up by a set amount or down by a set amount - rather than continuous, which showed that the spin of the orbital electron was spatially quantized; the direction of the field defined direction, and then the electrons were either spin 'up' or "down" with respect to that quantization axis.

 

Atomic clocks do measurements of it, after a fashion, since many of them rely on interactions between the spin up and down state within an atomic orbital. The two states have different energies, known as the hyperfine splitting, in atoms where the nucleus also has spin, and hence a magnetic field.

 

This hyperfine splitting shows up in many atomic spectra, as well as the spin-orbit interaction between the spin and the orbital angular momentum, known as the fine structure. So a lot of the measurements, or confirmations, are spectroscopy experiments.

 

 

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[Sayonara]

Dror, please stop using the code tags in place of the quote tags.

 

It creates semantically incorrect HTML, and forces the page to expand beyond the right margin of visitors' web browsers.

[Kygron]

Is there a difference between being in a superpostion of states and being in an unknown but definite state? If measuring forces a random state, how do we know that that state didn't exist in the first place? This is mostly about entangles pairs, but I suppose it could apply to single particles as well.