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Sustained, high-fidelity quantum teleportation

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Re: Sustained, high-fidelity quantum teleportation

#71

Post by spurgu »

abdullahkhalids wrote: post #29
junon wrote: post #20

Earlier quoted context omitted.

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

Physical qubits are much more sensitive to transmission noise than physical bits. If you want to transmit qubits, teleportation allows you to do it with higher fidelity than by physical transmission. Why do you want to transmit qubits? For various emerging quantum information technologies, each with different potential economic impact.

This simple explanation clicked for me, thank you!

Re: Sustained, high-fidelity quantum teleportation

#72

Post by pontus »

drdeca wrote: post #66
sebmellen wrote: post #42

Earlier quoted context omitted.

Very interesting. If I understand correctly, does this mean cloning is a functionally impossible task? I've always been entertained by the paradoxes where someone is teleported ala Michael Crichton's Timeline , and is then (due to some glitch) "duplicated", leading to interesting quandaries about "who is who". If Penrose is right about consciousness [0], this means all these fantastical paradoxes are just fantasy, ri…

Indeed there is a “no cloning theorem”. Iirc it essentially comes from the fact that time evolution is linear, and a function that sends stateOfIntetest \otimes constantState to stateOfInterest \otimes stateOfInterest for all values of stateOfInterest would be quadratic?

Yes, it's a direct result from the linearity of time evolution in QM. Like you say, if state1 x raw_material -> state1 x state1 (i.e. we are able to take some raw material and clone state1) and also state2 x raw_material -> state2 x state2, then linearity forces us to also have (state1 + state2) x raw_material -> state1 x state1 + state2 x state2. This is not the same as (state1 + state2) x (state1 + state2) which is what we'd want in order to clone arbitrary states.

Re: Sustained, high-fidelity quantum teleportation

#73

Post by vez- »

tomrod wrote: post #57
pontus wrote: post #44

Earlier quoted context omitted.

Correct, cloning in even the simplest sense of a single particle state (let alone a whole human) is impossible in quantum mechanics.

It sounds like state send is partial using the "phone" analogy. Can there be triplicates of entanglement?

Yup https://en.wikipedia.org/wiki/W_state

Re: Sustained, high-fidelity quantum teleportation

#74

Post by pontus »

tomrod wrote: post #57
pontus wrote: post #44

Earlier quoted context omitted.

Correct, cloning in even the simplest sense of a single particle state (let alone a whole human) is impossible in quantum mechanics.

It sounds like state send is partial using the "phone" analogy. Can there be triplicates of entanglement?

There are limits to how much three systems can be entangled with each other. It turns out that one system cannot be maximally entangled with two different systems. In fact, these types of arguments are often used in studying information paradoxes in black holes.

Re: Sustained, high-fidelity quantum teleportation

#75

Post by pontus »

edge17 wrote: post #51
pontus wrote: post #38

Earlier quoted context omitted.

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…

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…

The receiver does not know that a message has been sent until the first person contacts them classically. It's a common mistake to think that quantum teleportation is a new way of sending information. It's really a way to use classical communication in order to leverage entanglement to bypass various limitations of quantum mechanics.

So, the two people communicating would e.g. start out together and create a pair of entangled systems A and B. The person in possession of system B would then travel far away. The person in possession of system A then decides that they want to teleport a new system C to the person far away. They do this by placing system C next to system A and then performing a measurement on the combined system A+C causing these two states to become entangled. We now have an implicit entanglement between system C and system B that is far away. The person in possession of system A+C now picks up the phone and calls the other person to tell them what the outcome of their measurement on A+C was. The person far away then uses this information to determine a way to manipulate their state B in a certain way (the particular way in which they need to do this depends on the outcome of the measurement of A+C). Once that manipulation is complete, the system they have in their possession (B) is now in the quantum state that C was originally in. The system C, unfortunately has been destroyed in the process.

Re: Sustained, high-fidelity quantum teleportation

#76

Post by motoboi »

edge17 wrote: post #51
pontus wrote: post #38

Earlier quoted context omitted.

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…

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): you DESTROY the first box. Was the cat dead or alive before you destroy it?

If it was dead, you kill the cloned cat. If it was alive, you let the cloned cat live.

So now, congratulations, you have teleported the cat just as it really was when you first cloned it.

Obviously this is a gross approximation, but the central idea is that quantum teleportation let you clone and transport a particle, but you have to find a way to capture it's state and send it encoded in light or whatever method you prefer (fax?).

UPDATE: The cat belonged to Schrödinger, actually.

Re: Sustained, high-fidelity quantum teleportation

#77

Post by rodiger »

lemonspat wrote: post #63
magicalhippo wrote: post #16

Earlier quoted context omitted.

> 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 commun…

So you have to measure every time you want to transmit?

There is no "transmission." When you measure you learn the state of one of the particles, and you can use that information to deduce the state of the other (which until measurement is indeterminate). You can see that this information is useless unless communicated classically.

Re: Sustained, high-fidelity quantum teleportation

#78

Post by rodiger »

mhh__ wrote: post #32
kazinator wrote: post #25

Earlier quoted context omitted.

> Yes and no. Could a QM answer be otherwise?

The idea that QM means anything can happen is a popular misconception, at a very basic level the probaility P(X) of X happening can quite happily be zero.

I think this was more a joke about superpositions than "anything can happen"

Re: Sustained, high-fidelity quantum teleportation

#79

Post by ChrisLomont »

junon wrote: post #20
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…

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

Superdense coding allows sending two bits of classical information by only sending one qubit physically. Ideally this will double transfer rates. And in any case it provides another "modality" for data communication, which may provide tradeoffs that have benefits in other directions.

I'm pretty sure (haven't been in the field a while) that superdense coding the densest coding that one can gain using quantum entanglement.

https://en.wikipedia.org/wiki/Superdense_coding

Re: Sustained, high-fidelity quantum teleportation

#80

Post by throwaway888abc »

pontus wrote: post #75
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…

The receiver does not know that a message has been sent until the first person contacts them classically. It's a common mistake to think that quantum teleportation is a new way of sending information. It's really a way to use classical communication in order to leverage entanglement to bypass various limitations of quantum mechanics. So, the two people communicating would e.g. start out together and create a pair of…

Thanks for all your explanations here
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