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Sustained, high-fidelity quantum teleportation

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Re: Sustained, high-fidelity quantum teleportation

#141

Post by TuringTest »

motoboi wrote: post #76
edge17 wrote: post #51

Earlier quoted context omitted.

Maybe I'm dense, but I still don't understand. The cloning explanation made sense to me, but to the original question - how does the recipient know the message is done being sent without the sender picking up the phone and calling the recipient...? i.e. Is there some equivalent of a termination code/header sort of thing that the recipient is looking for in the 'bit stream' or whatever? Or am I not even thinking about…

When you entangle the particles, you have a copy of it. You then send _the clone particle_ over a mean (like fiber optics, if its a photon). Please note that you send the actual cloned particle, not information about it. Think about a cloned Heisenberg cat. You have to send the actual box with the cat inside. So now you have two copies of the same particle in two different locations. Now the tricky (and useless part)…

A Heisenberg cat is one you know how fast it can run, but you have no idea where the damn critter is?

Re: Sustained, high-fidelity quantum teleportation

#142

Post by cambalache »

Poc wrote: post #99
bluesign wrote: post #54

Earlier quoted context omitted.

Sorry but this example doesn't make sense, how it is faster than light?

It is not faster than light. I think what he wanted to show was a situation, in which entanglement and quantum mechanics, are superior to classical physics. i.e someone using quantum mechanics would have an advantage over someone who don't. However, as he said, in this situation the two generals could have agreed on something else, like if it was raining or not. In fact, something they could have done is to flip a co…

> In fact, something they could have done is to flip a coin, and split with a copy of the result, which they only look at when they launch the attack. It would have the same effect.

Yes but the difference is that in this case, the plan already exists from the moment the coin is flipped until the result is revealed to the generals. In the QM case no one (not even God) knows what the result will be until it is actually measured,but once that happens the result is known "instantly" at both places (assuming both coins arrive at the same time), so in that sense is a FTL communication (of course it is not real communication)

Re: Sustained, high-fidelity quantum teleportation

#143

Post by wruza »

pontus wrote: post #38
junon wrote: post #20

Earlier quoted context omitted.

This makes zero sense. If the information must be conveyed classically anyway, what's the point?

Great question! This gets to the heart of why quantum teleportation has any value at all. So, before QM there was already a sense in which you could teleport an object: simply measure its state perfectly and send that information to another location and have them reconstruct that state particle by particle. In principle the new system would be indistinguishable from the original system and you could claim that you've…

it would seem like one could not construct a perfect replica of a quantum system

It was always confusing why this replica has to be perfect. Even two “you” separated by a nanosecond in time are not perfect replicas at all. Why not to allow some impreciseness and teleport “quantumly uncertain” bodies instead of perfect copies? You can’t even know what’s perfect or not by the same principle. Moreover, maybe our structure allows very imperfect copying while retaining the same functionality after few minutes of teleache.

Re: Sustained, high-fidelity quantum teleportation

#144

Post by cambalache »

bluesign wrote: post #54
cambalache wrote: post #14

Earlier quoted context omitted.

No communication is possible BUT, I have this toy scenario where a "quasi-communication" may be possible in FTL speed. "2 generals command 2 armies hundreds of km apart. They want to attack a common enemy, there are 2 options, A) And all-front attack. B) From the flanks. The generals want the plans to remain uncertain until the last second before the attack. So from an intermediate point, they send two "entangled coi…

Sorry but this example doesn't make sense, how it is faster than light?

The "communication" of the final decision is FTL. If you choose a non-QM method, like checking it if rained in some place or using the pic of a flipped coin, that final decision can only be "communicated" at a sub-luminar speed.

Re: Sustained, high-fidelity quantum teleportation

#145

Post by Kranar »

Buttons840 wrote: post #136
pontus wrote: post #5

Earlier quoted context omitted.

You entangle two systems but in order to actually complete the teleportation you need to measure one system and then convey the outcome of that measurement to the other party. This information is needed by the second party in order for them to be able to correctly collapse the state of their system into one that is identical to the original system being teleported. The information that the first party must convey to…

ELI5: I have a red ball and a blue ball. I randomly place each ball into one of two boxes. I give you one box, and keep the other box for myself, then wave my magic wand and transport you 5,000 light years away. You look inside your box and see a red ball. In that moment you learn that I have a blue ball in my box. You gained that information in an instant, about a box 5,000 light years away. (This is an explanation…

While what you say is true, it doesn't have anything to do with quantum mechanics. I'm sure the ancient Greeks would have had no problem understanding that if you put a red ball in one box and a blue ball in another box then if you open a box and see a blue ball you'd know that the red ball is in the other box.

In quantum mechanics, entanglement is more about the fact that until a measurement is performed on one of the balls, both balls are simultaneously blue and red and will behave as if it was in a superposition of both colors up until a measurement is performed. By extension, any other property that depends on the color of those balls will also be entangled with the balls and behave in a superposition of whatever properties are entangled.

For example if a washing machine washes clothes with hot water if the blue ball is in box A, and washed with cold water if the blue ball is in box B, then that washing machine will be washing clothes with hot and cold water until a measurement is performed on any of the balls or the washing machine itself. The clothes being washed will simultaneously be expanding (from hot water) and contracting (from cold water).

Only once a measurement is performed on any part of the entangled system will every property of the system collapse into a definite state of red or blue, hot or cold, expanded or contracted.

Re: Sustained, high-fidelity quantum teleportation

#146

Post by wruza »

pontus wrote: post #114
davidhyde wrote: post #102

Earlier quoted context omitted.

Thank you for all your comments, very illuminating! Is the following classical analogy flawed? Say you have two pendulums and you set them in motion together so that they swing in perfect synchrony. Then you move the one (still swinging) pendulum to another location without disturbing it. Would it be reasonable to say that the physical pendulums are the “medium” and evolving information about the exact position and v…

It sounds like there's some stuff in your analogy that is similar to the QM situation. I would caution against placing too much emphases on these analogies though since a very important aspect of all of this is not just that the two systems are correlated but rather that they are entangled. There's a classic analogy to this when we talk about entanglement: imagine taking a pair of gloves and mixing them up. Put one i…

your act of measuring your system actually does compell the other system to change

Is it “to change” or “to define”? I’m a layman QM theorist, but I think this inaccuracy appears often in popsci media (if it is one). There was no info to change in the first place, there was only an undetermined coherence of either outcome. Is that correct?

Re: Sustained, high-fidelity quantum teleportation

#147

Post by russdill »

chiefofgxbxl wrote: post #137
Buttons840 wrote: post #136

Earlier quoted context omitted.

ELI5: I have a red ball and a blue ball. I randomly place each ball into one of two boxes. I give you one box, and keep the other box for myself, then wave my magic wand and transport you 5,000 light years away. You look inside your box and see a red ball. In that moment you learn that I have a blue ball in my box. You gained that information in an instant, about a box 5,000 light years away. (This is an explanation…

(My understanding; I only took one course in quantum mechanics) Arguably the information that you "gained" was predetermined when you decided which colored ball to place in which box. You didn't gain any knowledge that you couldn't have known before you were teleported 5,000 ly away. What's also more difficult is that the balls wouldn't have definitive colors, but exist in a probabilistic state / superposition. But y…

This big missing piece here is that it's not a simple red/blue property. Spin is the easiest to understand. When you measure, the spin will either measure up, or down. The key though is you can measure spin against any angle. If you measure the spin of the two particles at the same angle, you will get opposite answers. And you can choose what angle to measure at* long after the particles have separated.

* There is one theory of QM that removes the possibility of choice from the universe.

Re: Sustained, high-fidelity quantum teleportation

#148

Post by cambalache »

_underfl0w_ wrote: post #88
bluesign wrote: post #54

Earlier quoted context omitted.

Sorry but this example doesn't make sense, how it is faster than light?

I think key is that both generals have already agreed on a predetermined "meaning" of each coin flip outcome, rather than having to communicate it on a different, slower medium after the coin has been flipped. So really it just front-loads that portion (I.e. removes it from consideration as part of the proposed solution) instead of allowing it to slow down solving the overall problem. It requires precomputation/agree…

Yes of course that is why it is not real communication, but in a sense is better than a classical solution because nobody actually knows the final decision until the last second. Of course you could get philosophical and say that an enemy can intercept the quantum coin and measure it, thwarting the plan, so in that sense is the same thing that sending just a pic, but then you will be pulled by the same kind of discussion the physicists has spent the last 100 years doing. Check for example the correspondence between Einstein and Born https://www.amazon.com/Born-Einstein-Letters-1916-1955-Frien...

Re: Sustained, high-fidelity quantum teleportation

#149

Post by altvali »

pontus wrote: post #114
davidhyde wrote: post #102

Earlier quoted context omitted.

Thank you for all your comments, very illuminating! Is the following classical analogy flawed? Say you have two pendulums and you set them in motion together so that they swing in perfect synchrony. Then you move the one (still swinging) pendulum to another location without disturbing it. Would it be reasonable to say that the physical pendulums are the “medium” and evolving information about the exact position and v…

It sounds like there's some stuff in your analogy that is similar to the QM situation. I would caution against placing too much emphases on these analogies though since a very important aspect of all of this is not just that the two systems are correlated but rather that they are entangled. There's a classic analogy to this when we talk about entanglement: imagine taking a pair of gloves and mixing them up. Put one i…

Why should we replace our current networks with quantum networks? And can one particle be entangled at the same time with more than one other particle?
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