Hacker News new | past | comments | ask | show | jobs | submit login
Complex quantum teleportation achieved for the first time (phys.org)
98 points by lelf 56 days ago | hide | past | web | favorite | 60 comments

On why entanglement in general cannot be used for FTL communication, BUT you can get some FTL "correlations" out of it which are impossible with purely classical methods: https://medium.com/@ahelwer/walking-the-faster-than-light-ti...

Nice of them to decide to call this "multidimensional quantum teleportation" too. We'll be chasing the "OMG whatif scientists teleport in monsters from other dimensions didn't anyone play doom" fire down for the next 20 years.

What sort of technologies could (theoretically) be built relying on "quantum teleportation"?

Uninterceptible communication would be one application

> With this, the international research team has also made an important step towards practical applications such as a future quantum internet, since high-dimensional quantum systems can transport larger amounts of information than qubits. "This result could help to connect quantum computers with information capacities beyond qubits", says Anton Zeilinger, quantum physicist at the Austrian Academy of Sciences and the University of Vienna, about the innovative potential of the new method.

I wonder how a quantum internet would be different than the binary one.

Incorrect. You still need to transfer classical bits of information from source to receiver for teleportation to work - without which the success probability is provably 0.

If these bits are blocked, no teleportation happens.

1. Size of the Particle teleported?

2. Displacement distance & velocity?

3. Energy consumed?


4. Is it really a teleportation?

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.

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....

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."

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


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.


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...

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

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.

"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.

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...

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

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?

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.

It'll have some new unique state.

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.

> 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.


> That's usually what people mean when they say

Not everyone thinks in terms of Star Trek...

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

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

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

Interesting, is this teleportation thing restricted only by space or particles can also be tangled being in different time periods?

It's only called teleportation, it has nothing to do with teleporting matter in the sci-fi sense.

Quantum teleportation is about preparing two particles so that that by destroying the original particle's state by measuring it, and then transmitting a specific bit pattern as regular information, constrained by the speed of light, the other particle can be made to have the identical quantum state as was measured on the now destroyed particle.

The quantum state is (destructively) "teleported" from one particle onto another (preallocated) particle, obeying the rather mundane rules of classical physics.

In theory, could that be used on a block of random matter and a, say, banana, so that you can get lightspeed transmission of banana-ness?

Realistic theory? No, not even remotely.

The problem comes from having to measure the quantum state of every quantum particle in that banana and transmitting the bits necessary to perform the state transfer. We're talking destructively reading out well over 10^25 particles, turning that into tens of yottabytes of data, and then transmitting that data with error correction over distances that you can't just... you know... ship a banana over.

And then you'd have to receive that data in a way that either lets you perform the transfer in real time, or in a way that lets you store that data so you can transfer the quantum states one by one or in small batches.

And that's if you're okay with "Scanning" the original banana in a way that destroys the banana as yous scan it. Not the quantum states, the actual banana: the destructive read of particles will cause nuclear reactions and so very quickly you're no longer scanning "a banana" but "messed up subatomic partilces".

I don’t like the term “observer” because it implies it’s an entirely passive thing.

Why is it that we cannot passively observe quantum states? Why does observation necessarily have to be destructive?

There is no such thing as "passive" observation: something has to change in order for information to become known. Without a transfer of energy, no observation can be made, so either we absorb the particle to register that energy (and then it is literally gone after measurement) or we stimulate the particle into emission (drastically changing its quantum state), or we "poke" the particle by throwing things at it (changing its quantum state in the process).

To “observe” something, you need to interact with it in some way, such as bouncing a photon off it. That interaction changes the quantum state.

How is that different or better than just transmitting information at light speed over fiber? Excuse the ignorant question.

It is (in theory) capable of transmitting the entire quantum state.

It's the star trek transporter version of teleportation for quantum mechanical systems.

  It's the star trek 
  transporter version of 
  teleportation for quantum 
  mechanical systems.
No. No it isn't. It's not teleportation, because transit still occurs at non-instantaneous rates, and it is effectively slower than light speed travel when you include the overhead of the protocol.

A near-light-speed rocket ship could literally accomplish the same thing.

It is not a gateway technology that opens a portal to a destination. It clones state, and can replicate an infinite series of clones at a station, via transmission of state.

At best it could be called a matter replicator, but it is not teleportation.

Oh, I see - the transmitted information is far "less" than the quantum state itself.

indeed. To convey the single quantum state |φ> we only need to send 2 bits, provided we pre-allocated our "receiving" particle correctly (which as far as we currently know has to be one of an entangled pair).

how did they get from this to quantum internet in once sentence?

In contrast to conventional internet where classical 1s and 0s are transmitted via some medium like copper or fiber optics or radio waves, qbit states are far too fragile to survive such transportation and so would be transmitted via the quantum teleportation protocol.

Physical networking infrastructure will still be necessary; teleporting a qbit requires a pre-existing entangled qbit pair in addition to two bits of classical information, so you'll need some method of sending one half of an entangled qbit pair to the recipient. It's okay if this process is lossy, because you can easily generate & re-send an entangled qbit if it gets lost. Once you successfully manage to get the entangled qbit through, you can use it to teleport the qbit you actually want to send (whose state is presumably the product of some long, expensive computation and therefore far too valuable to risk sending via such a lossy channel).

In a nutshell, why are we trying to do this?

maybe... ultra-secure communication of very large data?

First known teleportation device was called the Fax which brings us to the reality that teleportation can be achieved by transferring the information through space rather than translation through space as most people imagine it.

This, once reality (at human scale), will bring an important ethical dilemma since for a short time there will be 2 of the same person, which for all intents and purposes are the same, even more so than cloning. Obviously, after transfer is complete, the source would have to be killed.

For non organic matter this could be great and perhaps the basis of some sort of matter to energy converter (it takes energy to create the duplicate), in time, storing the information of stuff you have transferred in a database can do without the initial step, and simply recreate the stuff.

As with any tech, there is a double edge sword to be mindful of.

Be careful what you wish for then push that teleport button as needed.

Guidelines | FAQ | Support | API | Security | Lists | Bookmarklet | Legal | Apply to YC | Contact