Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Sustained, high-fidelity quantum teleportation

news.fnal.gov

Post reply on HN
Print view View original

161–169 of 169 posts

Re: Sustained, high-fidelity quantum teleportation

#161

Post by fastball »

jiofih wrote: post #113
fastball wrote: post #98

Earlier quoted context omitted.

We had digital data storage before we had the internet. Back then, if you wanted to share digital information with someone else, you had to save that information to a hard disk (or similar) and physically transport it to their location. Then we invented the internet and now you can transmit digital information between digital computers over wires / EM signals. Quantum teleportation / entanglement does the same but fo…

If sending a pair of bits over the wire accurately describes this, then what is the achievement of transmission over 44km? Wouldn’t it simply be limited to our existing fiber coverage? Feels like something is missing here

Sorry, I was mixing terms. You need to send classical information over the wire, but you're also sending photonic qubits thru fiber, just not as a communication method. The fiber is how you get the entangled qubit to the other side of the network (entangled qubits are generated together). But again, this is not how you are communicating information. Technically this doesn't need to happen over fiber, you just need to somehow have entangled qubits in two separate locations, and obviously if we're talking about entangled photons, fiber is probably the best way to separate them.

Information is communicated once the entangled photon has reached the other side of the network, at which point a measurement at one end of the network is made and communicated (classically). Once again though, the purpose of the classical bits is not actually to send information, but rather because any measurement of one part of the entangled system results in its collapse (Copenhagen interpretation), and thus a collapse of the entangled quantum system as a whole. So if you have a quantum computer doing some calculation with qubits and those qubits have been entangled with others at the other side of the network, you can perform measurements on the qubits at one end and this will collapse the qubits at the other end. This would not be possible to do as you'd expect from classic digital stuff, where you can just send the info over the wire.

The distance is limited because of fidelity loss in fiber - you actually need your single photonic qubit (entangled with another qubit at the source) to make it from one end of the network to the other. In order to extend the range you can use quantum repeaters, which are effectively just a sequence of entanglements.

As an aside, higher quality fiber made in microgravity (ZBLAN) could also help bring these transmission distances up.

Re: Sustained, high-fidelity quantum teleportation

#162

Post by tsimionescu »

whatshisface wrote: post #131
tsimionescu wrote: post #115

Earlier quoted context omitted.

Per the wikipedia page, the protocol provides both encoding and decoding, and there have been experimental realizations that successfully retrieved the original message. Is the article inaccurate?

The parent comment was talking about data compression, the article is talking about copying a quantum state. Data compression will not work because you can't read a quantum state into classical information.

The article on superdense coding that started this mini thread [0] talks about a protocol for sending 2 bits of information as 1 qubit, and recovering the 2 bits on the receiver side.

More specifically, it seems that actually there are 2 shared qubits - one half of an entangled pair ahead of time; and one additional qubit when the message is decided. While 0 information can be transmitted using the entangled pair, entangled qubit + 1 qubit leads to 2 classical bits of information.

[0] https://en.wikipedia.org/wiki/Superdense_coding

Re: Sustained, high-fidelity quantum teleportation

#163

Post by Poc »

cambalache wrote: post #142
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…

> 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…

Ah yes, you're right, I hadn't seen it from that angle.

Re: Sustained, high-fidelity quantum teleportation

#164

Post by magicalhippo »

lemonspat wrote: post #125
magicalhippo wrote: post #117

Earlier quoted context omitted.

No, you have to transmit every time you want to transmit. When measuring one particle of an entangled pair and you get say "spin up", you know immediately that if someone measures the other particle (with the same measurement settings) they'll get "spin down", and vice versa. The chance that you get "spin up" or "spin down" is 50/50 and, as far as we know, cannot be affected or determined in advance. So, on the recei…

That was a very clear explanation, thank you. So all my grand ideas about how quantum communication might work are now sadly dead

Yeah, it seems we're doomed to pay the price of time.

Re: Sustained, high-fidelity quantum teleportation

#165

Post by [deleted] »

pontus wrote: post #119
rapht wrote: post #112

Earlier quoted context omitted.

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…

You've hit the nail on the head. This is what's called the measurement problem in quantum mechanics and it's arguably the biggest open question in foundational quantum theory. Nobody knows what a measurement actually is nor does anyone know what happens during a measurement. There are some modified versions of QM that tries to place this on a more rigorous footing, but none of them have convinced everyone that they d…

[deleted]

Re: Sustained, high-fidelity quantum teleportation

#166

Post by rkagerer »

pontus wrote: post #126
davidhyde wrote: post #123

Earlier quoted context omitted.

The glove explanation is excellent, thank you. I don’t think I’ll ever be able to bend my mind enough to leave Einstein’s spooky action camp. For now I’m just going to start believing that we live in a simulation and that entangled things are just structures that share memory ;)

There is a fairly recent development in theoretical physics called ER=EPR that attempts to clarify entanglement by conjecturing that two entangled particles are equivalent to two particles that have a worm-hole connecting them. To me, this is a very elegant way of addressing this weird action at a distance. https://en.wikipedia.org/wiki/ER%3DEPR

Thanks! Complete layman here, but intuitively I always just kind of figured there was some more proximate connection through a higher dimension - kind of like we're 2D ants unaware of the shorter route through the folded paper.

Re: Sustained, high-fidelity quantum teleportation

#167

Post by dkersten »

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…

Thanks for your fantastic explanations!

Re: Sustained, high-fidelity quantum teleportation

#168

Post by moron4hire »

roywiggins wrote: post #10
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?

You can't use entanglement (on its own) to communicate at all . https://en.wikipedia.org/wiki/No-communication_theorem

What's funny to me is that this topic comes up A LOT. Like, people hear about entanglement, and then decide they are smarter than those dumb physicists and have figured out FTL communication in 15 minutes.

Why is that? This keeps happening. Quantum entanglement is not a new concept. It's almost certainly older than 99.9% of the people on this site. Yet the myth that it could enable FTL communcation continues to persist.

Re: Sustained, high-fidelity quantum teleportation

#169

Post by whearyou »

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…

Sounds like it’s utility is like compression of the data describing system B?
Post reply on HN