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Comment by junon | Hacker News Reader

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 teleported it. Now, with the discovery of quantum mechanics, this process no longer works because there is no way to measure the complete state of a quantum system. For example, you could measure the position of each particle to arbitrary accuracy or you can measure the speed of every particle to arbitrary position, but you can't do both (Heisenberg's uncertainty principle). So, it would seem like one could not construct a perfect replica of a quantum system in a new location by measuring its state in the original position.

The cleverness of quantum teleportation is that you use entanglement to sort of short circuit this limitation. You let entanglement do the heavy lifting to sort of "copy" the state from one location to another and then perform a measurement in the original location to uncover just enough information so that the person in the second location can manipulate its system to reconstruct the original state.

It's sort of like reconstructing the state without actually knowing what that state is.

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. Classically you'd be cloning the system but quantum mechanically you'd really truly be teleporting it (in fact, there's a result in quantum mechanics called "the no cloning theorem" that proves that cloning in QM is impossible).

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 this in terms of the right analogy?

edit: Thank btw, this comment was fascinating to me.

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.

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 velocity of the pendulums the “system”? Because this is a classical system you can measure the position and velocity of the one pendulum and know that the other pendulum is at the exact same position and velocity. They are “coherent” in a way. However, in a quantum system, the medium (say a photon of light) is so fragile that measuring it removes its coherence to its entangled twin. This decoherence does not destroy the photon but the future information it carries. It now carries new information unrelated to the originally entangled photon. Kind of like having to stop a pendulum to figure out it’s position and velocity. You haven’t destroyed the pendulum but you have destroyed the potential for it to give you information about the other pendulum in the future. Following on from this analogy, if you crashed pendulum c into your one pendulum and destructively measured the resulting position and velocity you could send this information to the second pendulum to get that pendulum to set another pendulum in motion that would have an identical future to pendulum c before its system was destroyed. Thus, no information is really flowing between the two entangled photons because they are just “vibrating” identically until one is disturbed.

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.

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.

your act of measuring your system actually does compell the other system to change

Is it “to change” or “to define”? I’m a layman QM theorist, but I think this inaccuracy appears often in popsci media (if it is one). There was no info to change in the first place, there was only an undetermined coherence of either outcome. Is that correct?

Why should we replace our current networks with quantum networks? And can one particle be entangled at the same time with more than one other particle?

Thanks for your fantastic explanations!

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.

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.

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.

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.

Yes. This is the key point, I think, and it didn't seem well-communicated in the article.

Thank You.

That destroy my hopes of having (Close to ) Zero Latency Communication with Quantum Teleportation / Entanglement. We are still bound by the speed of light!

Yes, all of our physics only works if we assume that there is a maximum physical speed, which only massless particles like light can even reach. QM is perfectly consistent with this well-confirmed observation.

If classical communication is still needed to this degree, what value does this approach bring vs classical communication?

The dependency on classical communication would imply that it's not lower latency or higher throughput, and will remain subject to signal loss or degradation.

It's the only way to reliably communicate a quantum state. Want to network quantum computers? This is how you do it.

Got it, thanks for the explanation.

Also, not to get ahead of ourselves (understanding this is research), but what is the use/benefit of this method? We can already send and receive information over great distances with and without wires at seemingly high speeds. Is this a new level of speed? Are there some previous limitations of distances that are now surmountable? Is the power or cost envelope required somehow reduced in some obvious way (not today, but in some future commercial implementation)?

I think you missed his remarks:

> 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.

I meant more like, wireless and fiber are both classical ways to send data but each clearly has a benefit. In the same vein, does this new method have some clear benefit?

Maybe it may be useful for achieving truly one-way communication? Could it also be a stepping stone for transmitting state-heavy data? For example a human being with a consciousness :).

I know this is probably a stupid question, but would a text message be considered classical in terms of communication?

Yes

Thanks for all your explanations here

Sounds like it’s utility is like compression of the data describing system B?

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.

A Heisenberg cat is one you know how fast it can run, but you have no idea where the damn critter is?

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, right?

[0]: https://bigthink.com/paul-ratner/why-a-genius-scientist-thin...

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

Though cloning a human may be possible if the brain does not rely on quantum state. Obligatory "warm and wet" objection.

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.

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

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.

Hey yeah I remember reading Penrose's "The Emperor's New Mind" in maybe 1998?, an interesting take on the nature of consciousness... fascinating stuff.

I don't think you would need exact duplication for these situations to arise.

it would seem like one could not construct a perfect replica of a quantum system

It was always confusing why this replica has to be perfect. Even two “you” separated by a nanosecond in time are not perfect replicas at all. Why not to allow some impreciseness and teleport “quantumly uncertain” bodies instead of perfect copies? You can’t even know what’s perfect or not by the same principle. Moreover, maybe our structure allows very imperfect copying while retaining the same functionality after few minutes of teleache.

So we'd still be limited by the speed of classical communication in sending the object's state to the new location? I hope I'm understanding this correctly.

Yes, to achieve entanglement in the first place you have to either do something to two objects at a distance and that action can’t exceed the speed of light (and often is light) or you have to entangle two objects and then move them to their destination.

Teleportation is achieved by doing a specific action on one and then communicating to the other to do a specific action to the other.

In no case can information be transmitted faster than light.

Yes, that's correct

It sounds like start trek’s transporter beams may be more accurately described than we thought.

That's exactly what I was thinking! Running far too much into fandon, this holds also for the fictional 'pattern buffer'. It has to examine the original AND communicate that state back to the duplicate quick enough or the 'clone' will be inaccurate, maybe even enough to kill someone a la Star Trek: The Motion Picture.

They repeatedly treat the pattern like data.

Riker splits, Tuvix!, the one where the other doctor ages and gets de-aged in the transporter, the one where Scotty is stored in it for decades, etc.

It's also where the meme comes from how Star Trek transporers are actually suicide booths [0].

[0] https://arstechnica.com/gaming/2017/09/is-beaming-down-in-st...

> 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.

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?

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.

Does this mean quantum gizmos will ‘teleport’ the data over space instead of using wires? Like, will a quantum processor pull the data in the quantum ram using teleportation?

Well, not that short range! We are talking about where you have to transmit the qubits at least a few dozen meters.

So perhaps distributed quantum computers spread across a university, city or country.

Also, remember, you still need wired or wireless transmission of bits to do the teleportation protocol.

It means quantum “networking” will be transmitted over classical communications channels.

This simple explanation clicked for me, thank you!

If you want to transmit 1 quantum bit (qubit), then you need to transmit 2 classical bits. Why is this useful? Because otherwise you have to carry the qubit over by hand. It's really just "quantum ethernet" not "quantum teleportation".

There are actually other uses, too, about error tolerance, allowing you to quality-control some steps of the computation and repeat them if necessary without risking damaging the results of other steps.

You still have to carry the qubit over by hand, but you can do it asynchronously to the change

I think that undersells the advantage. You have to carry a qubit over, but it could be done even before know what you will need to send.

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!

The point is actually in even transferring an arbitrary quantum state from one quantum particle to another, and given an entangled pair existing in advance. Transferring a quantum state from one particle to another isn't an easy or obvious task, because you can't measure it or you would collapse it.

Note that "classical" is just stating that a classical channel is good enough to serve that purpose. A channel that preserves quantum state can of course be used, it's just that that isn't required for this to work.

All classical phenomena are quantum, we just use the word "classical" to describe subsets of quantum phenomena acting qualitatively in ways that are consistent with macroscopic phenomena.

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

It clearly makes no practical sense at the moment outside of research.

There are quantum communication links for satellites that guarantee that the data streams are not tampered with.

https://directory.eoportal.org/web/eoportal/satellite-missio...

https://spectrum.ieee.org/tech-talk/computing/networks/quant...

Less "tampered with" (because they can be trivially blocked or corrupted) and more "uninspected."

The point is to test physics models.

Suppose Alice and Bob have a pair of entangled particles. But Bob wants to give his part of the pair to Charlie.

Quantum teleportation lets him do that over the phone.

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