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Sustained, high-fidelity quantum teleportation

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Re: Sustained, high-fidelity quantum teleportation

#111

Post by stelfer »

Rooster61 wrote: post #108
tsimionescu wrote: post #106

Earlier quoted context omitted.

We take a process that produces entangled pairs, and send the 2 particles to different places. As long as we don't measure their state (whatever that means - TBD :) ), they remain entangled.

But that begs the question of how we know that we have produced an entangled pair. Do we not need a metric to use to make sure they are entangled?

The act of measurement on the second particle destroys the entagled state of the pair. That effect can be measured on the first particle's side (which may be far away... on the other side of the galaxy maybe).

Re: Sustained, high-fidelity quantum teleportation

#112

Post by rapht »

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 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 measuring in fact means looking at some Mx(Z(S,t)) where you actually never know S but only some kind of end product Z that is believed to reflect S but is itself the result of an unknown operation on S that QM people call "collapse".

So you seek Mx(S) but in fact spend your time looking at Mx(Z(S)) and draw conclusions on S... but I have yet to hear anyone explain to me, physically what is Z, how it works, etc. Lots of statistics, but no real understanding of that "collapse" process.

Re: Sustained, high-fidelity quantum teleportation

#113

Post by jiofih »

fastball wrote: post #98
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?

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

Re: Sustained, high-fidelity quantum teleportation

#114

Post by pontus »

davidhyde wrote: post #102
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…

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 in one box and the other in another box. Send one of the boxes far away. When you look down at the box that you kept, there is no way of knowing if it contains a left handed or a right handed glove; it's a 50/50 shot either way. Similarly you have no idea what the other box contains. You then decide to open your box and find a right handed glove. You then immediately know that the other box contains a left handed glove. In some sense this feels similar to what we see in entanglement but I don't think most people would claim that you opening your box somehow compelled the other glove to pick left/right. They were just always that way, you just didn't know which glove was where.

The claim, however, is that in QM it's not like this. Instead, your act of measuring your system actually does compell the other system to change.

For a long time there were a lot of heated arguments around all of this (most prominently between Einstein and Bohr) trying to figure out if the state of either box was truly undecided until you opened it or if there could have been some type of "hidden variable" that we had yet not discovered that nonetheless dictated what the state was (i.e. could it be more like the glove example or was it truly a new "spooky action at a distance"?)

For a long time physicists believed that this was an unanswerable question and should be relegated to philosophy. It wasn't until Bell discovered his inequality that this was dispelled. He designed an experiment that could be conducted to tell the two stories apart. When it was carried out, it was determined that nature is not like the glove example but rather consistent with the truly quantum story around entanglement. In other words, your measurement of your system actually does compelled the other system to change.

Re: Sustained, high-fidelity quantum teleportation

#115

Post by tsimionescu »

whatshisface wrote: post #67
nynx wrote: post #64

Earlier quoted context omitted.

I remember reading a paper somewhere (can't find it now) on how it should be possible to compress N bits of data into log2(N) qubits. This would be utterly transformative since you could then compress basically any amount of data into just a few qubits (e.g. 1e15 bits would fit in ~50 qubits).

You're not going to get that data back out, though. When you measure 50 qbits, you get 50 bits, and you destroy the state irrecoverably.

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?

Re: Sustained, high-fidelity quantum teleportation

#116

Post by jaggirs »

Rooster61 wrote: post #108
tsimionescu wrote: post #106

Earlier quoted context omitted.

We take a process that produces entangled pairs, and send the 2 particles to different places. As long as we don't measure their state (whatever that means - TBD :) ), they remain entangled.

But that begs the question of how we know that we have produced an entangled pair. Do we not need a metric to use to make sure they are entangled?

Your question boils down to whether or not the quantum circuit that entangles the qubits works correctly. You can simply test the reliability of the quantum circuit by running it multiple times and checking that the outcomes behave as you expect. For example, if your circuit is supposed to generate the entagled pair [00 or 11 with equal probability] you would expect measurements to either be 00 or 11 and never 01 or 10.

Re: Sustained, high-fidelity quantum teleportation

#117

Post by magicalhippo »

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?

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 receiving end, they measure some random combination of "spin up" and "spin down". Without anything else, this information is for all intents and purposes noise.

What you can do however, is to send a message using regular means with what you measured: "up, down, down, up, down". Ok, at least now they can check that what they got was the exact opposite. However that still doesn't tell them anything.

So instead what you do is that you change the measurement settings, and send via regular means not just your measurement results but also your measurement settings. So you'll send "H left, V down, V up, H left, H right". The recipient will then take the measurement settings (H or V) and measure the entangled particles in the same way you did, and then note down that they get the opposite.

Note now that you suddenly got a way to communicate some actual information. By agreeing in advance that a Vertical measurement means 0 and a Horizontal measurement means 1, you can send information to the recipient.

However also note that you had to make a measurement of your particles and then send the results using regular means, limited by the speed of light. So why bother with this complicated setup? Why not just send the data without all this entangled stuff?

And indeed, for just sending plain messages it makes no sense to use entangled pairs.

However as I noted in my other post, the inability to clone entangled states means an eavesdropper can be detected using the entangled setup.

Re: Sustained, high-fidelity quantum teleportation

#118

Post by tsimionescu »

ajuc wrote: post #56
up6w6 wrote: post #53

Quantum teleportation cant transport data faster than light, but something very similar which looks more useful in terms of daily needs is superdense coding [0]. Basically you could encode two bits as one qubit, which looks like some type of "physical compression" and the algorithm is almost the same as teleportation. [0]: https://en.wikipedia.org/wiki/Superdense_coding

Isn't that the same as just using 4 voltages (or any other analog property) instead of 2 to encode 2 bits instead of 1? Qbits are analog so it's no surprise it can be done with them as well. Flash memory already does that in classical chips. There's nothing preventing you from going further - 8 voltages for 3 bits etc.

The problem with analog encodings is that it often doesn't scale at all - the energy required to measure your analog state precisely enough to send large numbers of bits becomes too impractical.

Re: Sustained, high-fidelity quantum teleportation

#119

Post by pontus »

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…

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 do. My personal favorite is the many world's approach that in many ways is simpler than traditional QM because it says that there's no such thing as a measurement. Instead, when you think you're measuring something what you're really doing is entangling yourself with the system you're measuring which means that your state is no longer separate from the state of the system. There's a part of you that sees each outcome.

This is actually already how microscopic systems work: if two particles collide and get entangled, the state of each particle sort of splits in two. The only thing that MWI says is that this dynamics also applies to macroscopic objects.

Re: Sustained, high-fidelity quantum teleportation

#120

Post by jiofih »

phyalow wrote: post #89
c22 wrote: post #47

Earlier quoted context omitted.

If you're entangled before you're destroyed have you really been destroyed?

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

Not the same as entanglement means you can’t have both parts at the same time, you just need to decide which one you’re gonna “use” while the other is destroyed.
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