Unconditional quantum teleportation?

[Farsight]

Just for the record, since he's posting in this thread...

For the record, xtifr's comment above was not inaccurate. I don't routinely refer to luminaries such as Wheeler as cranks. But I do routinely refer to what Einstein said. I post on JREF because it has a burden of cuckoo-in-the-nest naysayer quacks who peddle pseudoscience and try to spoil any sensible physics discussions.

Thanks much! This is a very good way to look at the issue.

My pleasure Crossbow. Also have a look at work by Jeff Lundeen et al. See for example his semi-technical explanation:

"We have developed a methodology for measuring the wavefunction directly, by repeating many weak measurements on a group of systems that have been prepared with identical wavefunctions. By repeating the measurements, the knowledge of the wavefunction accumulates to the point where high precision can be restored. So what does this mean? We hope that the scientific community can now improve upon the Copenhagen Interpretation, and redefine the wavefunction so that it is no longer just a mathematical tool, but rather something that can be directly measured in the laboratory."

 

[Crossbow]

Well…whether or not the big ‘E’ was wrong…he apparently was not exactly right. Given his achievements (and the complexity of the subject) he can hardly be faulted for the occasional error.

Thing is…this effect is certainly one of the stranger features of QM. Is there something decidedly revolutionary going on here? This experiment (…and, presumably…others before it) seems to confirm some variety of instantaneous communication / information transfer.

Is this an accurate conclusion…or merely an error of measurement / interpretation?

If it is an accurate conclusion…how does this occur?

Thanks much.

I think these issues were the ones the Einstein was struggling with.

His natural inclination was that there is always a way to define a specific thing precisely, the trick is to figure it out. However, in quantum mechanics it can be quite difficult, if not impossible, to figure out what a specific thing is doing.

 

[Dancing David]

What are the odds that…if you were around a 100 years from now…you would disagree with that conclusion?

Apparently they’re planning upping the ante. A kilometer or more.

What use is it? Is that why scientists do theoretical work? ...and because the question " how does that happen " may be rather significant in this particular case.

Um, not much, it take special conditions and entangled particles, so to communicate occurs like this:
-entangle particles
-maintain in indeterminate state
-separate by distance
-changing state of one particle changes the state of other

This is not really teleportation now is it.

So there are two practical barriers to any information sending

--maintain in indeterminate state (this may or may not get easier, but it is real challenge)

-the nature of entanglement means you need a way to know when the state of your 'receiver' particle has changed, without changing it by checking. A truly very large barrier.

Still not teleportation, more like flipping a switch and it changing the position of another switch, except any time you look at the switch it is set and no longer usable to transmit, and you have a problem because of that. What is you stop the entanglement by checking the receiver? then the sender can'ts end you information.

this is not exactly an issue solvable by technology, it is the nature of entanglement.


It means theoretically (and teher are some major practical abrriers)

 

[Dancing David]

That seems rather dubious given that entanglement based technology is already in existence.

ETA: See, for example, this press report: http://www.economist.com/blogs/babbage/2012/02/quantum-cryptography

And so how do they know the state of a particle changing without actually changing it and breaking the entanglement?

How exactly does that work, other than you pointing to a blog post?

What technology is actually doing this at this time...


are they using the QM to transmit data, or to see if someone snooped?

 

[annnnoid]

Um, not much, it take special conditions and entangled particles, so to communicate occurs like this:
-entangle particles
-maintain in indeterminate state
-separate by distance
-changing state of one particle changes the state of other

This is not really teleportation now is it.

So there are two practical barriers to any information sending

--maintain in indeterminate state (this may or may not get easier, but it is real challenge)

-the nature of entanglement means you need a way to know when the state of your 'receiver' particle has changed, without changing it by checking. A truly very large barrier.

Still not teleportation, more like flipping a switch and it changing the position of another switch, except any time you look at the switch it is set and no longer usable to transmit, and you have a problem because of that. What is you stop the entanglement by checking the receiver? then the sender can'ts end you information.

this is not exactly an issue solvable by technology, it is the nature of entanglement.

It means theoretically (and teher are some major practical abrriers)

I didn’t say it was ‘teleportation’. I didn’t say it was anything. I was actually asking if it was something. According to the links Reality Check (and others) provided…it appears to be something

According to the original links I included…what they are describing is a situation where ‘information’ relating to one situation is transferred instantaneously through no discernable means to another situation some distance away (a distance they are currently in the process of expanding to something like a kilometer). I suppose we can leave aside, for now, the problems inherent in measuring the temporal and physical properties of events on such a scale (unless they question the conclusions).

I am asking if this is this what is happening?

If so…what explains it? Presumably…’information’ cannot travel instantaneously nor can it travel through no discernable means. There are laws of physics that proscribe such activity are there not?

 

[Dancing David]

I didn’t say it was ‘teleportation’. I didn’t say it was anything. I was actually asking if it was something. According to the links Reality Check (and others) provided…it appears to be something

According to the original links I included…what they are describing is a situation where ‘information’ relating to one situation is transferred instantaneously through no discernable means to another situation some distance away (a distance they are currently in the process of expanding to something like a kilometer). I suppose we can leave aside, for now, the problems inherent in measuring the temporal and physical properties of events on such a scale (unless they question the conclusions).

I am asking if this is this what is happening?

If so…what explains it? Presumably…’information’ cannot travel instantaneously nor can it travel through no discernable means. There are laws of physics that proscribe such activity are there not?

we don't know?

It does appear to be violating the SoL for sure.

However there are many theories about how entanglement works, I am not sure any of them have one a following across the board.

My own, untrained take comes from reading Guth's Inflationary Universe. In the introduction or early chapters he talks about how certain field 'klang' or fall out during the inflationary epoch and later.

They are fields or events or quasi particles that span the universe at that time or portions of it. So as the universe continues to expand they are there in the back ground. And it may be that they under pin the potential for entanglement.

 

[dlorde]

According to the original links I included…what they are describing is a situation where ‘information’ relating to one situation is transferred instantaneously through no discernable means to another situation some distance away ...

I am asking if this is this what is happening?

If so…what explains it? Presumably…’information’ cannot travel instantaneously nor can it travel through no discernable means. There are laws of physics that proscribe such activity are there not?

As I understand it (if I go wrong, please put me right, I want to know), you can't send information FTL. The apparent setting of a remote particle's state according to a measurement on its local partner is not strictly considered a transfer of information because it's a one-time, single bit affair. You don't know the initial state of either particle and can't set or select it in advance, and finding out the state of one breaks the entanglement; so while you might call it a form of communication, it can't transfer any usable information and doesn't threaten causality.

However, it really depends on which interpretation of QM you prefer. The 'many worlds' interpretation says that upon entanglement the universe is in a superposed state of the two possibilities, e.g. A- local particle spin up, remote spin particle down, and B - local particle spin down, remote particle spin up. When you measure the local particle, you join the superposition, one you seeing A and the other you seeing B. This way, everything is deterministic and there's no spooky action-at-a-distance - each of you just discovers which universe you're in.

It works, but it feels terribly wrong. I tend to work around my disbelief and think of it as saying reality may not actually consist of a many worlds multiverse, but it behaves exactly as if it does

 

[edd]

It's a weird business, this quantum teleportation. You have to transmit classical information (subject to classical speed limits) to make it work, but that classical information doesn't carry the information alone to complete the teleportation. You have to have the entangled particles do their bit too.

This is one of those things where the universe makes me want to back off slowly, but it's kind of hard to back off slowly from a universe you're in.

 

[sol invictus]

For the fun of it, let me try to make an analogy to quantum teleportation using green and red balls.

Suppose Alice has a device that creates pairs of balls. It always creates two of the same color, but they might both be red or green. The balls emerge from the machine inside an opaque bag made of clingy plastic wrap.

Alice carefully wraps the plastic around each ball and then tears the bag in two (so she doesn't see the color of either ball), and mails one (still wrapped) to Bob. Now Alice and Bob each have a ball. If either opens the wrap, they instantly know what color the other's ball is. That's an entangled pair in my analogy, and sharing such a pair is a prerequisite to quantum teleportation.

Later, Alice acquires another wrapped ball from somewhere else. She doesn't know what color this new ball is, and she doesn't want to find out, but she does want to somehow "teleport" its color to Bob. She puts both of the balls into a special device that compares their color. The device doesn't tell her the color, it just says if two balls put into it have the same color or not.

If the result is that the balls are the same color, then Alice knows Bob's ball is the same color as the new one she got, and she calls him on the phone to tell him so (that's classical information being transmitted, another key part of the quantum teleportation protocol). If instead the result is that the balls are a different color, she calls Bob to tell him that instead. Fortunately Bob also has a special device. You put a ball in, and (without revealing the color before or after) it switches the color from whatever it was to the other color.

So, after receiving Alice's call and either doing nothing or switching the color of his ball using his device, Bob now knows for sure he has a ball of the same color as the one Alice got, even though neither he nor Alice knows the color of any of these balls.

That's quantum teleportation.

(ETA - all analogies are imperfect by definition, so you might be wondering what the main differences are here. The biggest is that here, the balls all have a definite color, and it's just that Alice and Bob don't happen to know what it is. Instead, in QM the entangled states they share and the state to be teleported don't have a definite "color" - that's what violations of Bell's inequalities proved. Another, more minor difference is related - there are more possible QM states than simply "red" and "blue"; instead, the balls can be in an arbitrary linear combination of the two.)

 

[edd]

This is the problem I'm having that's making me want to back off slowly. In your analogy, you're basically using a shared secret to make a one-time pad to encrypt your one classical bit, and you send your one classical bit to get the one bit back out at the end. Fine - a bit for a bit.

In quantum teleportation you have a shared quantum 'secret' and your qubit - your linear combination of red and green, and you send 2 classical bits to get your one qubit out at the other end. 2 classical bits don't seem to be 'enough' for your qubit...

 

[GnaGnaMan]

And so how do they know the state of a particle changing without actually changing it and breaking the entanglement?

How exactly does that work, other than you pointing to a blog post?

What technology is actually doing this at this time...

It is physically impossible to measure the state of a particle without disturbing it. This is not a technological limitation but fundamentally impossible in the same way that it is impossible to create energy/mass out of nothing.

It is unclear what these questions have to do with anything.

are they using the QM to transmit data, or to see if someone snooped?

QM is used to securely distribute a cryptographic key.

By the by: Do you know what functional fixedness is?

 

[Dancing David]

It is physically impossible to measure the state of a particle without disturbing it. This is not a technological limitation but fundamentally impossible in the same way that it is impossible to create energy/mass out of nothing.

It is unclear what these questions have to do with anything.


QM is used to securely distribute a cryptographic key.

By the by: Do you know what functional fixedness is?

No but I can learn, however it sure looks like they are using QM to make sure that the data was not intercepted, did I misread that?

So not exactly data transmission, it just uses QM to know if someone has snooped the data stream? So they can make sure that the transmission of keys is secure, but not transmit the keys through QM.

I am often way wrong, so I stand to be corrected.

 

[sol invictus]

This is the problem I'm having that's making me want to back off slowly. In your analogy, you're basically using a shared secret to make a one-time pad to encrypt your one classical bit, and you send your one classical bit to get the one bit back out at the end. Fine - a bit for a bit.

In quantum teleportation you have a shared quantum 'secret' and your qubit - your linear combination of red and green, and you send 2 classical bits to get your one qubit out at the other end. 2 classical bits don't seem to be 'enough' for your qubit...

It's not merely 2 cbits (classical bits) that buys you your qbit. It's 2 cbits + 1 ebit (entangled qbit). See this https://en.wikipedia.org/wiki/Bennett's_laws .

But you're right that there is something really weird about the fact that a continuous parameter (describing the unknown state Alice receives) can be teleported to Bob by some experiment Alice does that merely yields four possible outcomes. I don't think there's a way to extend my analogy to capture that.

 

[Senex]

Unconditional quantum teleportation my ass. What if a fly lands on you while in the teleporter? It would be like in the movies. You'd be part fly part man. Quantum teleportation should just be left to us magicians. Scientists can't be trusted.

 

[GnaGnaMan]

Well…whether or not the big ‘E’ was wrong…he apparently was not exactly right. Given his achievements (and the complexity of the subject) he can hardly be faulted for the occasional error.

Can anyone be faulted for the occasional error? No? So why mention it in his case but not in others?

This experiment (…and, presumably…others before it) seems to confirm some variety of instantaneous communication / information transfer.

That probably hinges on the precise definitions of information and communication.

Suppose someone in space, where there is no friction to complicate matters, throws a ball. Because of conservation of momentum you know that the momentum of ball and thrower must be equal but opposite.
Therefor, when you measure the momentum of the ball you immediately know the momentum the thrower has (or had, because it doesn't tell you anything about what happened to the thrower since then.)

Entanglement is exactly like that. This is not an analogy. This is basically how spontaneous parametric down-conversion creates entangled photons.
You can't use entanglement for communication in the same way you can't use astronaut and ball for communication.

Where it gets weird is that in the quantum world, momentum is indeterminate until it is measured. The momentum is randomly determined at the moment of measurement. But because of the conservation law, this also determines the momentum of the other particle.
Intuitively you'd think that the random result would have to be transmitted to the other particle but there is nothing in the theory that says that. It is just our everyday intuition. However, our everyday world arises from the quantum world. Insisting on everyday intuitions here is the tail wagging the dog.
Moreover you run into big problems with relativity. It's not just a speed limit being broken. Causality goes out the window. Instantaneous communication implies simultaneity and even absolute time.

I'm not a physicist but IMHO interpreting entanglement in terms of communication is a complete non-starter.

 

[GnaGnaMan]

No but I can learn, however it sure looks like they are using QM to make sure that the data was not intercepted, did I misread that?

So not exactly data transmission, it just uses QM to know if someone has snooped the data stream? So they can make sure that the transmission of keys is secure, but not transmit the keys through QM.

For cryptography, you want a random key. QM offers many sources of true randomness.
When entangled particles are measured in the same way at different locations, then the results will be random but complementary.

I think you could say that the key is generated in both locations at once.

If someone intercepts and measures one of the entangled particles, then there won't be matching keys generated. If the person prepares a particle with the correct state and sends it on, then this will be detected via Bell's inequalities. That's a bit of a long story.


Functional fixedness is a psychological bias. We tend to think of tools as for one specific purpose and find it difficult to conceive of other uses.
You originally doubted the usefulness of entanglement. However, your posts since then suggest that you did not consider entanglement in any other context but for communicating with (Which is a shame since that is the one thing it certainly can't be used for, according to current theory). I speculate that this may have been an example of aforementioned bias.

 

[PhantomWolf]

The issue is that quantum entanglement will not lead to anything really useful. It usually is only over limited distances, very particular conditions. And as communication it has extreme limitations, in that codes would have to be arranged in advance and the the entangled bit moved to location.

It depends if there is a practical limitation to the distance between the particles, and how fast and easy it becomes to change and read their states.

If we become able to change and read their states in a millisecond, and have no limit to the distance, then it could be very useful. Imagine having real time communications with a team of explorers on the Martian surface, or with a satellite around Neptune.

True it might not be possible, but each step is another one toward understanding the possible uses of this "spooky action".

 

[GnaGnaMan]

It depends if there is a practical limitation to the distance between the particles, and how fast and easy it becomes to change and read their states.

If we become able to change and read their states in a millisecond, and have no limit to the distance, then it could be very useful. Imagine having real time communications with a team of explorers on the Martian surface, or with a satellite around Neptune.

True it might not be possible, but each step is another one toward understanding the possible uses of this "spooky action".

http://en.wikipedia.org/wiki/No-communication_theorem

 

[PhantomWolf]

http://en.wikipedia.org/wiki/No-communication_theorem

I'm aware of this theory, I'm also aware that it's based on assumptions that might or might not be true.

 

[GnaGnaMan]

I'm aware of this theory, I'm also aware that it's based on assumptions that might or might not be true.

Could be true, could be false... Might as well toss a coin. Why do people bother with that whole science stuff anyway?

PS:
Relation between theory and theorem?
"Unlikely, sir. They spell and pronounce their names differently." -Waylon Smithers