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lota-putty

1. Size of the Particle teleported?

2. Displacement distance & velocity?

3. Energy consumed?

Edit:

4. Is it really a teleportation?

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1. None (the size of a photon).

2. Short & speed of light.

3. Irrelevant, since this is about whether it was actually doable, not about doing it efficiently.

4. The output particle is impossible to distinguish from the input one, but it didn't travel from input to output, so... how else would you define "real" teleportation?

  how else would you define 
  "real" teleportation?

As something other than such a ridiculous game of 3 card monte. That's all this has ever been.

Put a red marble and a blue marble in a sock, and then close your eyes and withdraw one marble from the sock. Place it in a mitten, and send the mitten to a friend.

Then we claim that the person who looks at their marble first has defined the reality of the other by default, and that qualifies as teleportation?

Teleportation has been demonstrated as a thought experiment countless times, and this doesn't pass the smell test. The concept of teleportation is instantaneous transportation of physical objects with mass and volume across arbitrarily large distances, seemingly as if stopping time and stepping through a door that connected point A and point B, in order to skip over the effort of transiting real physical distance.

This concept may be unrealistic, but we don't squat on antiquated terms like luminiferous ether when we conceptualize quantum foam.

There's this academic inertia to opportunistically squat on a word with a loaded meaning, all to gain favor and generate buzz and hype.

People desperately want recognition for a contrivance by leaning on a word. They know that using the word "teleportation" to describe something less exciting will provoke magical thinking among outsiders.

Since this process cannot produce effects faster than light, it's not even a slight improvement beyond radio or lasers or any form of radiant communication.

People don't refer to these efforts appropriately. The objective of these experiments is tamper evidence for quantum communication channels, and nothing more.

«Austrian and Chinese scientists have succeeded in teleporting three-dimensional quantum states for the first time»

So, no particles were moved, it's just cloning the state of one particle to another. It's also important, but it's not something like the "beam me up" type of teleportation.

But aren't elementary particles of the same kind only distinguishable by their state?

This is exactly what "quantum teleportation" means and it has never meant anything else. "Beam me up" is pure fantasy.

This reminds me of the ansible machine in Orson Scott cards Enders Game.

I highly recommend discussion of the development of the ansible in Ursula K. Le Guin's The Disposessed as well, although she makes it pretty clear that there's a different theoretical basis for it ("simultaneity theory") than the one Card uses.

Does it address what would happen if I, relative to your reference frame, got in my spaceship, accelerated to .5c and tried to talk to you over my ansible? From my perspective, you're traveling slowly in time, but from your perspective, I'm traveling slowly in time, so who's voice would sound slowed down?

I don't remember if the interface was realtime or async off the top of my head, but it would strike me as uncharacteristic of her (and strange in the context of the plot) to not take that into account.

I'm not even sure if the ansible interface is actually described anywhere in that book....

It's transferred, not cloned (which implies a copy). If you could clone quantum states; then you'd get FTL communication.

That's interesting! Can you please elaborate a small bit?

Well, we'll see how small the elaboration is... :)

A brief review of how quantum teleportation works. (I have a diagram in quirk that might help with the explanation

https://algassert.com/quirk#circuit=%7B%22cols%22:%5B%5B1,%2...)

You start off with two qubits in a known state, say |0>. You then shift one of the qubits (using a Hadamard gate) into a equally mixed state of

qubit_1 = k(|0>+|1>)

and then use a controlled NOT gate to put the entire system in the state of

qubit_2 = k(|00>+|11>)

(where k = 1/sqrt(2)).

So you then ship one of the qubits off to your friend in Antarctica (say, qubit_1). You then pull out qubit_3. Now, let's say it's also in some known state |0> to begin with, and then you apply a bunch of gate operations to it, resulting in it being in the following state:

qubit_3 = a|0> + b|1>

Where a and b are two complex numbers whose magnitude sums to one.

You use another controlled not gate, this time, qubit_3 will act on qubit_2. This now has entangled qubit_3 into the qubit_1 and qubit_2 system. Finally, you apply another Hadamard gate to qubit_3. The system is now in the following state

  |Psi> =  |00>(a|0>+b|1>) + |01>(a|1>+b|0>)
          +|10>(a|0>-b|1>) + |11>(a|1>-b|0>)

Note, that we've written the above state in a way that separates out the two qubits you control, and the qubit that your friend controls.

Now, you measure the two qubits that you control. Doing this causes the full state to collapse to one of the four above states. If you measure |11> then your friends particle is in the following state:

  |Psi'> = a|1>-b|0>

So you call up your friend and you say to him "Oh hey friend, I just measured it, and if you want the original state, you want to do a simple phase shift (Z gate) and swap the probabilities (X (NOT) gate)."

The important thing to note here, is that from your perspective, the collapse happens immediately. The submitting of the classical information is just to 'patch up' the quantum state that got slightly out of whack.

Now, suppose your friend has the ability to clone qubits. You and your friend agree ahead of time when you'll perform the measurement.

Once the measurement is performed, your friend makes a large number of clones of the resulting state. He can then extract statistically the values of a and b (values of which, you had control over).

So not only would we be able to do FTL communication, we'd basically also have bandwidth limited only by the number of copies your friend could make.

Thanks!

I was about ask: Why is cloning a problem, but not a downconverter crystal (specifically a Type-0 SPDC [0]) that creates two photons with the same polarization as the input?

Then I realized: the two output photons are also entangled so you can only make 1 independent measurement of the pair.

[0] https://en.wikipedia.org/wiki/Spontaneous_parametric_down-co...

Hey! That's pretty cool! I wasn't familiar with that (and probably would have been stumped by the question :) ). Thanks for the solution! Also; thanks for a great example of something that looks like cloning (and could be mistaken as such) but isn't. I'll need to keep it in mind when explaining things in future.

By the way; the math reason for the impossibility of cloning states boils down to the fact that Quantum State evolution is described by a unitary operator. The linear algebra[1] shows that this means that you can't have a process that takes a state

  Q(|psi> x |r>) => |psi> x |psi>

For a general |psi>. So it'd be very surprising to discover a process that pulls this off. :)

That said; I find it very surprising that causality gets preserved in regular quantum mechanics by a completely unexpected mechanism, namely unitary state evolution. Regular QM doesn't know about special relativity, or its needs, and by pure accident it seems to prevent a giant loophole that would play havoc with causality.

[1] https://en.wikipedia.org/wiki/No-cloning_theorem#Theorem_and...

Technically in Star Trek I believe transporters recreated you at the destination, and no particles were moved either.

That would explain Star Trek teleportation between two teleporter stations - where the necessary hardware exists to reconstitute matter - but how did teleportation to/from random planet surfaces work? Why have teleporter suites in ships when they can apparently beam people directly from a planet’s surface to sick-bay, for example.

That is the definition of teleportation in every scifi setting I know of

At least in some mass transfer occurs.

Also, think about the way teleportation in computer games works.

There’s nothing that distinguishes two particles that share a quantum state. You can’t say it’s the same particle and you can’t say it’s not.

Particles (fermions, anyway) share quantum states all the time. They just can't be part of the same quantum system when they do so, for definitions of "quantum system" that some people made up at one point and are (apparently) free to redefine at will.

This is "teleportation" in the same sense that Chinese skateboards with electric motors and debatably-safe lithium batteries duct-taped to them are "hoverboards."

"Teleportation" is such a misleading term. Information (in the form of a quantum state) is transmitted, not matter. And it requires a lot of setup and preparation including classical transmission as well.

How else would we teleport matter, if it wasn't via information representing that matter?

Star Trek "does it" by literally converting matter into energy, beaming that energy to a remote point, then converting it back into matter... in place... somehow by teching the tech. That's usually what people mean when they say "teleportation" - actually somehow instantaneously moving a thing from point A to B, rather than transferring its state.

I'm not certain whether somehow mass-entangling an entire object with something else that receives its state and becomes a perfect (or near perfect because real life doesn't have "Heisenberg compensators" like Trek, to compensate for the Heisenberging) copy of the original is more or less ridiculous than Star Trek's version.

> That's usually what people mean when they say

Not everyone thinks in terms of Star Trek...

Is there a more widely held image of teleportation held by lay people?

This should be called what it was originally "spooky action at a distance". Calling it teleportation is marketing.

But it's still the closest proxy for the average person.

> actually somehow instantaneously moving a thing from point A to B, rather than transferring its state.

Is it ever explicitly called out that teleportation instantaneously moves matter?

I'm not a huge fan, but from what I've seen there is a max range they can teleport. Even taking them orbiting a planet, it usually take about 2-3 seconds or so for the teleportation to go through. Transferring state via EM waves at the speed of light (300km/s) that still gets you between half a million and a million kilometers distance in 2-3 seconds. Seems like plenty range to not need anything instantaneous.

>Is it ever explicitly called out that teleportation instantaneously moves matter?

No. Maybe "relatively instantaneously" would be more accurate. Although if we're considering the concept of teleportation as a science fictional plot device in general, it depends, and even within Star Trek, the transporters worked however the plot required.

Trek style transporters did still "move" some kind of matter stream, though. In one episode, the stream bounced off some clouds when an away team was beaming off of a planet and accidentally created a duplicate of Riker.

> Transferring state via EM waves at the speed of light (300km/s) that still gets you between half a million and a million kilometers distance in 2-3 seconds. Seems like plenty range to not need anything instantaneous.

Notwithstanding the other problems involved[0], sending only state still requires that state to be transferred into some existing matter at the other end.

[0]https://news.ycombinator.com/item?id=20782106

What scientists mean by "teleportation" is more like this: you have one particle A here, one particle B there, and by "teleportation" you make particle B have the same state as particle A.

Critical point: for each particle you want to "teleport", you have recipient particle standing by to receive it.

Well, you move the state of A onto B. By the no-cloning theorem A can no longer have the same state once B has it. Hence the term "teleportation."

What happens to the first particle once its state has been transferred? Is it a swap? Or does it go to some other unique state C?

It'll have some new unique state.

The particle ends up either getting destroyed during the measurement (e.g. that free photon or electron will no longer be), or its quantum state will have been collapsed into something that can no longer be used in a quantum algorithm.

The original discoverers [1] - a really smart bunch - used the full term "teleportation of an unknown quantum state", which is correct. Subsequent work has been lazy in their terminology. [1] is also 100% clear about the resources needed in the title.

[1] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.70...

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