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Sustained, high-fidelity quantum teleportation

news.fnal.gov

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Re: Sustained, high-fidelity quantum teleportation

#11

Post by pontus »

Mizza wrote: post #8
pontus wrote: post #7

Earlier quoted context omitted.

No, while entanglement acts instantaneously across a large distance, there's no way for that "signal" to carry any information. In order to complete the teleportation the two parties must somehow communicate in order to convey an additional piece of information. This communication would be classical and slower than light.

Any chance that communication could happen _before_ the measurement? Entangle some matter, give half of it to the space team and then have inter-stellar walkie talkies?

In some sense the two systems that are entangled are connected instantaneously. The problem is that the information doesn't reside in either system but rather in them as a whole. So, if you were in space with one part of your "walkie talkies" you wouldn't be able to make sense of the information unless you have access to the other system. This access would usually occur through some slower than light channel.

Imagine that you have 2 coins and give one to someone on earth to flip and one to someone on the moon to flip. The outcomes in either location is random 50/50, but interestingly they are perfectly correlated (HH, TT). When you flip a coin you don't have the ability to pick it's outcome. The only thing you can pick is whether or not to flip it at all. So, imagine you want to communicate one bit of information from earth to the moon and decide that a 1 will be encoded as "flip the coin" and 0 as "don't flip the coin". When you're standing on the moon and want to reveal the information you flip the coin and see e.g. H. There are two ways this could have happened: either the earth coin had already been flipped and showed H or the earth coin had not yet been flipped and you just randomly got a H. In other words, the outcome by the flip is useless by itself.

Re: Sustained, high-fidelity quantum teleportation

#12

Post by nabla9 »

context: Using term teleportation makes sense in the quantum realm.

No-cloning theorem in quantum physics says that it's impossible to do exact copy of unknown quantum state. But it turns out that you can teleport it when the original state is destroyed.

You can't have exact `cp` command for the quantum state, only `mv`. If this makes you think linear logic, you are right.

Re: Sustained, high-fidelity quantum teleportation

#13

Post by lemonspat »

magicalhippo wrote: post #9
Mizza wrote: post #8

Earlier quoted context omitted.

Any chance that communication could happen _before_ the measurement? Entangle some matter, give half of it to the space team and then have inter-stellar walkie talkies?

The whole point is the other party needs to know the result of your measurement, and you don't know that before you measured (otherwise the particles wouldn't be entangled).

I think OP is asking, can you entangle, measure, discuss, and then go travel to another galaxy with real time communications?

Edit: or can the communication only happen once and then you need to remeasure over classic communication?

Re: Sustained, high-fidelity quantum teleportation

#14

Post by cambalache »

Mizza wrote: post #8
pontus wrote: post #7

Earlier quoted context omitted.

No, while entanglement acts instantaneously across a large distance, there's no way for that "signal" to carry any information. In order to complete the teleportation the two parties must somehow communicate in order to convey an additional piece of information. This communication would be classical and slower than light.

Any chance that communication could happen _before_ the measurement? Entangle some matter, give half of it to the space team and then have inter-stellar walkie talkies?

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 coins" , one to each general, the coins will arrive at both sites at the exact start of the battle.Both will show the same face when "measured". The generals have agreed previously that if they turn out "heads" they will both attack from the flank, in the other case, they will do a front-attack.

Of course you dont need a quantum system for this, you could have agreed on other stuff (like it if it is raining that day at certain place or sending a framed coin by regular mail) but I think the quantum solution is the more elegant, assuming no 3rd party snooping.

Re: Sustained, high-fidelity quantum teleportation

#15

Post by djxfade »

magicalhippo wrote: post #9
Mizza wrote: post #8

Earlier quoted context omitted.

Any chance that communication could happen _before_ the measurement? Entangle some matter, give half of it to the space team and then have inter-stellar walkie talkies?

The whole point is the other party needs to know the result of your measurement, and you don't know that before you measured (otherwise the particles wouldn't be entangled).

So what can this really be useful for?

Re: Sustained, high-fidelity quantum teleportation

#16

Post by magicalhippo »

lemonspat wrote: post #13
magicalhippo wrote: post #9

Earlier quoted context omitted.

The whole point is the other party needs to know the result of your measurement, and you don't know that before you measured (otherwise the particles wouldn't be entangled).

I think OP is asking, can you entangle, measure, discuss, and then go travel to another galaxy with real time communications? Edit: or can the communication only happen once and then you need to remeasure over classic communication?

> travel to another galaxy with real time communications?

As I said, no. How would you relay your measurement results in real-time? Without those measurement results, the receiver would in essence just hear white noise.

That's the perplexing part of entanglement. Somehow it feels like information is instantaneously transferred from A to B, yet the information content is somehow zero so can't be used for proper communication.

Re: Sustained, high-fidelity quantum teleportation

#17

Post by boie0025 »

pontus wrote: post #7
sreejithr wrote: post #3

Does this mean information can be transmitted faster than light speed?

No, while entanglement acts instantaneously across a large distance, there's no way for that "signal" to carry any information. In order to complete the teleportation the two parties must somehow communicate in order to convey an additional piece of information. This communication would be classical and slower than light.

This reveals my severe ignorance about quantum mechanics; but I've always wondered if entanglement could be used to transmit binary data by way of timing and presence or lack of presence of a transmission. So maybe every 1ms is a position, and either something is sent or not.

Re: Sustained, high-fidelity quantum teleportation

#19

Post by magicalhippo »

djxfade wrote: post #15
magicalhippo wrote: post #9

Earlier quoted context omitted.

The whole point is the other party needs to know the result of your measurement, and you don't know that before you measured (otherwise the particles wouldn't be entangled).

So what can this really be useful for?

I'm no expert, but AFAIK one thing is secure communication, in the sense that the recipient can detect if anyone is eavesdropping.

As I've understood it, to "listen in" the eavesdropper has to destroy the entangled state by measuring it, and there's no way to perfectly clone the entangled state before doing that.

The recipient can compare the entangled data with the measurement results (sent via classical means) and detect statistical inconsistencies if there is an eavesdropper.

edit: I see their page[1] mentions quantum metrology, which I found reference to in a page[2] describing work to improve GPS and similar detection using quantum entanglement. Not sure if it's directly related but seems like there should be room for some interesting work using this quantum network in this area.

[1]: https://ieqnet.fnal.gov/

[2]: https://news.engineering.arizona.edu/news/quantum-entangleme...

Re: Sustained, high-fidelity quantum teleportation

#20

Post by junon »

pontus wrote: post #5
sabellito wrote: post #2

I'm confused, as I'm an enthusiast but definitely no physicist. The article states that quantum information is being teleported via entanglement. However, it was my understanding that one cannot transfer information in such a way as the "information" is only revealed when interacting with the particle. Could someone perhaps clarify what's going on?

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…

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