Sustained, high-fidelity quantum teleportation • Hacker News Board

Re: Sustained, high-fidelity quantum teleportation

#151

Post by BirderO44 » Sat, Dec 19, 2020, 2:03 AM UTC

rd11235 wrote: post #93

This article says this result may transform communication, but seems to make no mention of why . What improvements does this have over classical communication?

I think the possibilities are immense but too much of the reporting etc. focuses on FTL.

Wouldn’t it be nice to have a cell phone with perfect reception, anywhere? Someone at the bottom of the ocean talking to some at the top of Everest, on the other side of the planet? On the moon? Mars?

Let’s talk quantum internet. No longer would we have to rely on internet providers. No more Comcast, no more AT&T. Devices can just connect to other devices without someone in the middle. Governments could no longer block sites or suppress ideas.

Re: Sustained, high-fidelity quantum teleportation

#152

Post by Buttons840 » Sat, Dec 19, 2020, 2:42 AM UTC

Kranar wrote: post #145

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…

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 t…

Thanks for the explanation!

How can we know anything about quantum entanglement when measuring the entangled particles, or any effects they have, cause the whole thing to collapse?

Re: Sustained, high-fidelity quantum teleportation

#153

Post by rocqua » Sat, Dec 19, 2020, 3:04 AM UTC

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…

So, once the recipient gets the classic message, they can instantly set their local system to the same state as was measured by the sender's system?

Can this be used to 'pause' a system, by delaying the transmission of the classic message? Or do you get the state the original system would have had, had you not measured it?

Re: Sustained, high-fidelity quantum teleportation

#154

Post by pluto9 » Sat, Dec 19, 2020, 3:25 AM UTC

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…

> Now, an interesting side effect of the quantum version is that the first measurement of the original system is necessarily destructive. As such, it's not like you'll end up with two copies of the same thing (which is what would happen in the classical version) so there's no discussion necessary around the distinction between teleportation and cloning.

Is it possible to have three entangled systems? It seems that would allow sending the state information from one to the other two, thus destroying the original but leaving two "copies".

Apologies if the answer to this is obvious, I'm a total layman when it comes to this stuff.

Re: Sustained, high-fidelity quantum teleportation

#155

Post by mathgenius » Sat, Dec 19, 2020, 3:28 AM UTC

Rooster61 wrote: post #97

So, something that always bothered me, and my admittedly ignorant understanding of QM as a non-physicist. I understand that the quantum state of a particle can be teleported, and that classical information must be passed to observe the particle and collapse the state correctly. My question is, if we can't observe the particle, how do we know that that particle is entangled with the original one?

Given an unknown state, it's not possible to know if the parts are entangled. There is no experiment to determine this. The "facts" of quantum have this weird disconnect with reality.

If you have access to many copies of the same state, then you can do tomography to learn if the state is entangled or not. But this is a different situation to what you are asking in your question. Actually I'm not entirely sure what your question is asking, but I won't delete this comment just yet.

Re: Sustained, high-fidelity quantum teleportation

#156

Post by mathgenius » Sat, Dec 19, 2020, 3:31 AM UTC

sreejithr wrote: post #3

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

Correlations are "transmitted" faster than light speed. You could call this information. Quantum information theory is way more complicated (and many open questions) than Shannon information theory.

Re: Sustained, high-fidelity quantum teleportation

#157

Post by danielheath » Sat, Dec 19, 2020, 4:29 AM UTC

rapht wrote: post #112

pontus wrote: post #75

Earlier quoted context omitted.

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 the explanations. I have to say I am always at a loss when quantum physicists start talking about "measurement". In the classical world, measuring means looking at a particular variable x in a system S at time t, S(t) and via some process specific to x (which we want to measure), Mx, obtain the value of Mx(S(t)). In QM by contrast, it seems that measurement itself has an action upon the system so that…

I find it easier if I consider momentum from photons bouncing.

You measure the colour of an object by bouncing light off it and seeing what comes back. The objects state is modified when the light hits it, since it imparts momentum.

Re: Sustained, high-fidelity quantum teleportation

#158

Post by roywiggins » Sat, Dec 19, 2020, 5:26 AM UTC

8note wrote: post #82

junon wrote: post #20

Earlier quoted context omitted.

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

Supposing we get enough qubits to support teleporting you, we can regularly entangle qubits and ship them around the world, then when you want to travel, we can teleport you, and the trip time will be how long it takes to send the message. This cuts your trip time down from hours long flights to seconds long trips

It might be easier just to destructively scan and reconstruct you classically and ignore the quantum stuff. If the person who steps out of the booth still thinks they're "you", problem solved, and without dealing with quantum states. How much do you care that your cells are all quantum-mechanically identical post-teleport? Not much!

Re: Sustained, high-fidelity quantum teleportation

#159

Post by roywiggins » Sat, Dec 19, 2020, 5:29 AM UTC

Buttons840 wrote: post #152

Kranar wrote: post #145

Earlier quoted context omitted.

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 t…

Thanks for the explanation! How can we know anything about quantum entanglement when measuring the entangled particles, or any effects they have, cause the whole thing to collapse?

One straightforward way is you perform the same experiment over and over, the quantum weirdness will show up in the statistics. Eg you might test if two measurements are the same more often than they should be by the laws of classical probability.

Re: Sustained, high-fidelity quantum teleportation

#160

Post by roywiggins » Sat, Dec 19, 2020, 5:34 AM UTC

bluesign wrote: post #110

Poc wrote: post #99

Earlier quoted context omitted.

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…

“ So there exist situation where we can use entanglement to achieve better results, for example in "non-local games", where players sharing entangled state can win with probability 1, when "classical player" with probability Do you have an example for this? This is really interesting

It's called "pseudo-telepathy." The wikipedia page has an example of a game:

https://en.m.wikipedia.org/wiki/Quantum_pseudo-telepathy