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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)?

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 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?

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?