Quantum teleportation - again

[piningforthefjords]

Hi, guys.

So you may remember that, a few months ago, I started a thread on quantum teleporting being used on humans. It got a bit heated and I apologize for that. It's just that, every time I hear about some teleportation-related thought experiment, it just seems so inevitable. Plus, as I said several times before, David Darling's comments about teleportation - that, if it turns out to be merely difficult and not impossible, it will eventually become a reality - just keep echoing in my head.

However, I still have a few questions and would love it if someone could answer them.

1.) I was always under the impression that ALL interactions between particles stop at absolute zero. Is this actually not true? Do we have any concrete evidence that interactions are still going on at absolute zero? If so, is it theoretically possible to overcome this challenge?

2.) I understand that, currently, we can only teleport the state of a single atom. Isn't the next step teleporting its correlations with other atoms? Is that theoretically possible? Wouldn't it be the next big step towards macroscopic quantum teleportation?

3.) Are the internal reactions the only thing that would cause quantum decoherence if the object was in a vacuum?

I may have more later, but could anyone answer these?

[Ken G]

1.) I was always under the impression that ALL interactions between particles stop at absolute zero. Is this actually not true? Do we have any concrete evidence that interactions are still going on at absolute zero? If so, is it theoretically possible to overcome this challenge?

It depends on what you mean by "interactions". What stops at absolute zero is thermal energy transport-- where "thermal energy" is energy that has been essentially randomized, so moves around simply based on where it is most likely to be manifested. So in an environment of absolute zero temperature, no heat can be removed from the system to make something happen. However, forces do not turn off, and other rules like the Pauli exclusion principle, which are a type of "interaction" if you will, continue to appear. Indeed, one can imagine a white dwarf star at absolute zero temperature, and it would not radiate light at all, but it would still contain an enormous amount of kinetic energy in its electrons, and it would still be hard to compress (which is what keeps it from becoming a black hole). If you wanted to "teleport" a white dwarf, you'd need to find a source not only of all that mass, but also all that kinetic energy. Same for atoms at absolute zero-- they still contain a lot of kinetic energy, so there is still some kind of "stationary motion" there, it's just that everything is in its ground state.

2.) I understand that, currently, we can only teleport the state of a single atom. Isn't the next step teleporting its correlations with other atoms? Is that theoretically possible? Wouldn't it be the next big step towards macroscopic quantum teleportation?

I think teleporting entangled correlations would be possible, although the complexity of doing so increases rapidly as the entanglements increase. That's the real problem, the sheer complexity of what you have to teleport, and the difficulty of tracking if you are succeeding.

3.) Are the internal reactions the only thing that would cause quantum decoherence if the object was in a vacuum?

Any type of time-varying coupling to complex systems where the physicist is choosing to project out the things he/she cannot track is an example of the strategy of decoherence, and if this strategy works well, we can say the system is actually "decohered", even though no such thing ever "really happens," it is a way of treating a system.

[Len Moran]

3.) Are the internal reactions the only thing that would cause quantum decoherence if the object was in a vacuum?

I may have more later, but could anyone answer these?

My understanding of decoherence theory is that it is never the case that macroscopic systems are absolutely isolated – the case for even hypothetically being able to isolate an object (other than for short periods of time) seems unrealistic. Zeh 1970, Joos and Zeh 1985 (both pioneers in decoherence theory) claim that even a speck of dust lying far away in interstellar space is not totally isolated due to its interaction with cosmic background radiation.

From the point of view of quantum physics (again as I understand things) what is crucial is that the energy level of macroscopic objects are extremely close to one another, so that even an inordinately small disturbance may induce level shifts in such systems.

In other words, any macroscopic system interacts with the environment in a non negligible way as well as with its own internal environment - there is no such thing as an isolated object.

[Jens]

1.) I was always under the impression that ALL interactions between particles stop at absolute zero. Is this actually not true? Do we have any concrete evidence that interactions are still going on at absolute zero? If so, is it theoretically possible to overcome this challenge?

One thing I'm not certain about is, what is the "challenge"? Is your goal to stop the interaction between particles? If so, why is this an important challenge to overcome?

[mahesh]

...I think teleporting entangled correlations would be possible, although the complexity of doing so increases rapidly as the entanglements increase. That's the real problem, the sheer complexity of what you have to teleport, and the difficulty of tracking if you are succeeding....

Hi Mr Fjords...
Don't mean to be a 'fly in the ointment', but I saw 'The Fly', the original gem, a couple of days ago. Brought back childhood memories. Lovely colour film. I had forgotten how vivid it is.

Any type of time-varying coupling to complex systems where the physicist is choosing to project out the things he/she cannot track is an example of the strategy of decoherence, and if this strategy works well, we can say the system is actually "decohered", even though no such thing ever "really happens," it is a way of treating a system.

The Cat. The Cat. He disappeared decohered. Shucks, I can't remember its name.

Ah well, I'll just have to see it again.

[piningforthefjords]

I've also heard that you can only QT something insofar as you don't know what it is you're teleporting. Is this right? Can someone explain this to me?

[Ken G]

I've also heard that you can only QT something insofar as you don't know what it is you're teleporting. Is this right? Can someone explain this to me?

It has to do with the fact that if you teleport a superposition state, it will arrive as a superposition state, so you can't know the answer to the measurement for which that state is a superposition without collapsing it. That's the only application of QT that I've heard of-- you can send a coded message and know if the "seal" has been broken, as it were. Wax seals worked pretty well for that too.

[Noclevername]

They really need to come up with a better name for QT, it just keeps confusing people. http://xkcd.com/465/

[piningforthefjords]

One thing I'm not certain about is, what is the "challenge"? Is your goal to stop the interaction between particles? If so, why is this an important challenge to overcome?

Doesn't the interaction between particles cause quantum decoherence? Isn't that the main thing preventing macroscopic quantum teleportation?

[piningforthefjords]

It has to do with the fact that if you teleport a superposition state, it will arrive as a superposition state, so you can't know the answer to the measurement for which that state is a superposition without collapsing it. That's the only application of QT that I've heard of-- you can send a coded message and know if the "seal" has been broken, as it were. Wax seals worked pretty well for that too.

So how does that relate to quantum teleporting a macroscopic object?

[piningforthefjords]

Hi. guys. Can someone please answer my most recent questions @ posts 9 and 10?

[DrChinese]

Doesn't the interaction between particles cause quantum decoherence? Isn't that the main thing preventing macroscopic quantum teleportation?

No, particles interactions do not automatically cause decoherence. It is possible to swap entanglement from particle to particle. If Alice and Bob are entangled, it is possible to have Bob interact with Chris and then Alice and Chris are entangled. That is not always the result, but it happens too. There are also plenty of other interactions - such as reflection of light - that does not cause decoherence.

[DrChinese]

I think teleporting entangled correlations would be possible, although the complexity of doing so increases rapidly as the entanglements increase. That's the real problem, the sheer complexity of what you have to teleport, and the difficulty of tracking if you are succeeding.

You are correct (as usual): there is such thing as entangled entanglement.

http://arxiv.org/abs/quant-ph/0508183

[Jens]

Doesn't the interaction between particles cause quantum decoherence? Isn't that the main thing preventing macroscopic quantum teleportation?

I don't really understand it well enough to answer that. But I'd have another question for specialists. Aren't there interactions between the particles in our bodies anyway, even without teleportation? So isn't decoherence also taking place on an everyday basis as time passes?

[Ken G]

Aren't there interactions between the particles in our bodies anyway, even without teleportation? So isn't decoherence also taking place on an everyday basis as time passes?

The key issue is that of a "closed system." A closed system, in quantum mechanics, is a system with no interactions of any kind with its surroundings-- a mini universe if you will (clearly an impossible idealization, but that's nothing new for physics). If a closed system starts out in a pure state of some kind, it will always be in a pure state. So even if the closed system is extremely complex, and undergoing internal decoherence all the time, taken as a whole the system is still in a single pure state that could in principle be teleported, decoherences and all. But the whole reason we invoke the decoherence concept is because tracking the actual coherences is absurdly complicated in such a large system. When tracking something in detail is an impossible task, physics is forced to take a different tack, and that's when we invoke concepts like the laws of thermodynamics, concepts like entropy, the idea of a "random" event, and so forth. Physics of complex systems just couldn't work without them, for the same reason that teleporting a human is impossible as a practical matter.

[Ken G]

You are correct (as usual): there is such thing as entangled entanglement.

http://arxiv.org/abs/quant-ph/0508183

Thanks for linking to a demonstration of how entanglements entangle. It certainly clarifies that our physics will never be able to describe the universe as it actually is-- we will always be called on to make statistical idealizations of the systems we are trying to understand.