Chinese group breaks distance record for teleporting qubits

May 14, 2012 by Bob Yirka report
Bird's-eye view and schematic diagram for free-space quantum teleportation. Image from: http://arxiv.org/abs/1205.2024

(Phys.org) -- A team of Chinese physicists has broken the distance record for teleporting qubits, extending it from 16 to 97 kilometers. They did so, as they explain in their paper uploaded to the preprint server arXiv, using the phenomenon known as entanglement.

In this context, teleportation is used to denote the exchange of information describing the states of two separate entities without having to move any actual information through the space between them. It’s important to note that teleportation in this context does not imply that an object is actually moved from one place to another, or disassociated and re-associated as seen in Star Trek, etc.

is where two participles are entangled, i.e. connected in a way that still cannot explain, though it can be shown that whatever happens to one, happens automatically and instantaneously, to the other. Thus, if one of a pair of entangled particles were made to represent one element of a stream of data that comprised a single letter of the alphabet, for example, the other would take on that value as well, allowing for instant communication; one that would also offer a means of communicating free from the worry of eavesdropping.

Unfortunately, there is still the problem of creating the pair with distance between them; that’s the part that has been difficult, and it’s the part that this new Chinese team has found a new way to increase. They used a 1.3-watt laser and some optic tricks to cause a pair of entangled photons to appear at two separate locations (across a lake from one another) at the same time, and then used a classical channel to measure the results.

In so doing the team showed that it might be possible to create a system using teleportation for communication purposes, based on communications through the air, to special satellites. Doing so would have two very significant advantages over conventional systems. The first, and most obvious would be to allow for nearly perfect cryptography applications. The second would be communications free from latency lags.

There is still a long way to go however, as the Chinese team has also demonstrated - their efforts still required the use of a classical channel to measure the results. Having to do so nullifies the latency lag effect and doesn’t allow for teleporting whole messages, though it should allow for sending such messages through a classical channel while sending encryption keys via .

Explore further: Quantum physics just got less complicated

More information: Teleporting independent qubits through a 97 km free-space channel, arXiv:1205.2024v1 [quant-ph] arxiv.org/abs/1205.2024

Abstract
With the help of quantum entanglement, quantum communication can be achieved between arbitrarily distant places without passing through intermediate locations by quantum teleportation. In the laboratory, quantum teleportation has been demonstrated over short distance by photonic and atomic qubits. Using fiber links, quantum teleportation has been achieved over kilometer distances. Long distance quantum teleportation is of particular interest and has been one of the holy grails of practical quantum communication. Most recently, quantum teleportation over 16 km free-space link was demonstrated. However, a major restriction in this experiment is that the unknown quantum state cannot directly come from outside. Here, based on an ultra-bright multi-photon entanglement source, we demonstrate quantum teleportation, closely following the original scheme, for any unknown state created outside, between two optical free-space links separated by 97 km. Over a 35-53 dB high-loss quantum channel, an average fidelity of 80.4(9) % is achieved for six distinct initial states. Besides being of fundamental interest, our result represents an important step towards a global quantum network. Moreover, the high-frequency and high-accuracy acquiring, pointing and tracking (APT) technique developed in our experiment can be directly utilized for future satellite-based quantum communication.

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antialias_physorg
3.6 / 5 (10) May 14, 2012
In this context, teleportation is used to denote the exchange of information describing the states of two separate entities without having to move any actual information through the space between them.

Calling this exchange of information is incorrect. Information requires a priori knowledge of the encoded information. This is not true of entangled entities (measuring/forcing their state before spearation breaks entanglement). The abstract is completely misrepresented in this piece.

It is 'spoky action at a distance' or 'non locality' but it isn't information transmission. The distinction is a very subtle one.

What you CAN do with this is have perfect encryption.
The qbits added for encryption are only required to be the same, but not to be in any definite state (i.e. you can encrypt, but the encyption proces itself does not need to carry infomation)

The passing of the actual infomation itself, though, is still limited by the speed of light.
Vendicar_Decarian
2.4 / 5 (9) May 14, 2012
Antialias provides the consensus view on the matter of information exchange through widely separated, entangled particle pairs.

There are a growing number of physicists that are questioning the premise that ftl communication is not possible since the disentanglement of entangled particles appears to be communication itself.

antialias_physorg
3.4 / 5 (5) May 14, 2012
Note that the abstract does not make reference to FTL information transmission but to quantum communcation (which does not include the concept of FTL information transmission, but 'merely' the concepts of secure communication and quantum teleportation)

The abstract very specifically states
"However, a major restriction in this experiment is that the unknown quantum state cannot directly come from outside."

and

"Here, based on an ultra-bright multi-photon entanglement source, we demonstrate quantum teleportation, closely following the original scheme, for any unknown state created outside"

Notice the operative word 'unknown'. That is: you do not known WHAT the entanglement will be (which would be information) - just THAT it will be entangled (which is not).

There are a growing number of physicists that are questioning the premise that ftl communication is not possible

And I hope they find a way around it. But this (article) isn't it.
powersquared
3.7 / 5 (3) May 14, 2012
This whole idea of spooky action at a distance is nothing more than modern day alphysics. The two "entangled" particles are what they are from the moment they are created.
Noumenon
1.7 / 5 (6) May 14, 2012
Antialias provides the consensus view on the matter of information exchange through widely separated, entangled particle pairs.

There are a growing number of physicists that are questioning the premise that ftl communication is not possible since the disentanglement of entangled particles appears to be communication itself.



How so? To each observer he is seeing only random results. Only when those results are brought together at < C does one notice a entangled correlation.
Deathclock
2 / 5 (4) May 14, 2012
This whole idea of spooky action at a distance is nothing more than modern day alphysics. The two "entangled" particles are what they are from the moment they are created.


I tend to agree, there is nothing spooky about it
Noumenon
1.8 / 5 (5) May 14, 2012
This whole idea of spooky action at a distance is nothing more than modern day alphysics. The two "entangled" particles are what they are from the moment they are created.


That doesn't explain anything. If they were "what they are from the moment they are created" they would act as independent entities, but they do not,... they act as one thing being effected.

I refer you to the double slit quantum eraser experiment.
antialias_physorg
not rated yet May 14, 2012
Spooky action in quantum mechanics is a bit more complicated.

Consider you create two entangled photons with the same polarization (vertical or horizontal - but you don't know which, because that would be measurement and would break the entanglement)
You get send those photons to two separate locations. You measure them using a polarization filter that is either vertical or horizontal (same at both locations). Fine. You get the same result at both places. So far, so good, so far everything according to classical mechanics.

Quantum mechanics however is about probabilities.
Example: take a single photon with horizontal or vertical polarization and send it through a polarization filter that is angled at 45 degrees - what happens? 50% of the time it will pass through 50% of the time it will not. Still sort of understandable. Passing throug is probabilistic, right?

(cont...)
antialias_physorg
5 / 5 (4) May 14, 2012
Now comes the spooky part:

Take the two entangled photons of the first example and don't push them through a vertically or horizontally aligned filter at the target location but through one that is turned 45 degrees. "Common sense" from the second eyxperiment would say that the outcome is probabilistic: 25% cases you get both to pass, 25% you get none to pass, 25% you get one to pass at location 1 and the other at location 2 not to pass and 25% no pass at location 1 and a pass at location 2.

However this is not what happens. If they are entangled either both pass or both don't. And that's what's spooky. The probabilities aren't 'computed locally' at the filter as with single photons, but are computed over the entire (nonlocal) system.
powersquared
not rated yet May 14, 2012
Spooky action in quantum mechanics is a bit more complicated.

Consider you create two entangled photons with the same polarization (vertical or horizontal - but you don't know which, because that would be measurement and would break the entanglement)
(cont...)

How could you not help from know which is which when you created them?
powersquared
not rated yet May 14, 2012
Now comes the spooky part:

Take the two entangled photons of the first example and don't push them through a vertically or horizontally aligned filter at the target location but through one that is turned 45 degrees. "Common sense" from the second eyxperiment
However this is not what happens. If they are entangled either both pass or both don't. And that's what's spooky. The probabilities aren't 'computed locally' at the filter as with single photons, but are computed over the entire (nonlocal) system.

I do not see anything spooky about this. What might be spooky would be if one passed threw and the other did not.
Remember you are filtering BOTH by 45 degrees since both photons are 90 degrees out of phase with each other BOTH have an equal chance of passing or being filter out.
antialias_physorg
not rated yet May 14, 2012
I was trying to simplify. Usually you take it so that one photon is vertically and the other is horizontally polarized and you have the filters at 45 and 135 degrees and you have to consider the superposition of polarizations.

Check here (forget about the formulae and just read the text, it explains it clearly enough, but I just tried to recap but for the life of me I can't fit it into 1000 characters)
http://en.wikiped...nglement

Remember you are filtering BOTH by 45 degrees since both photons are 90 degrees out of phase with each other BOTH have an equal chance of passing or being filter out.

But since passing through a 45 degree filter from a horizontal/vertical polarization should be 50% the passing through (or not) should be independent of what happens at the other location. But it turns out they are correlated.
vacuum-mechanics
1 / 5 (1) May 14, 2012
This whole idea of spooky action at a distance is nothing more than modern day alphysics. The two "entangled" particles are what they are from the moment they are created.


I tend to agree, there is nothing spooky about it


Now, maybe it is the time for Einsteins hidden variable to come back for solving the problem!

http://www.vacuum...id=19=en
CHollman82
1 / 5 (1) May 14, 2012
However this is not what happens. If they are entangled either both pass or both don't. And that's what's spooky.


This still makes perfect sense to me, and doesn't seem spooky at all.

The entangled photons share the same properties, of course the same filter (or two filters with the same properties) will affect each in the same way...
antialias_physorg
5 / 5 (5) May 14, 2012
Unfortunately the Bell theorem seems to show that no matter how many hidden variables you add to a theory (or to quantum mechanics) non-locality can't go away.

That's pretty vexing, because we mostly think that information transmission is limited by the speed of light.
As of now entanglement and non locality does not allow for transmitting information. There is correlation - but you can't use it to transmit messages. You can use it for encryption/verification that the transmission wasn't intercepted - but quirkily enough these actions don't constitute information transmission.
CHollman82
1 / 5 (1) May 14, 2012
Oh I get it, if you assume a probabilistic nature of reality then I get what you're saying, but if you assume a deterministic nature of reality then it makes complete sense that two photons with the same exact properties will respond in the exact same way to the same filter.
powersquared
not rated yet May 14, 2012

Remember you are filtering BOTH by 45 degrees since both photons are 90 degrees out of phase with each other BOTH have an equal chance of passing or being filter out.

But since passing through a 45 degree filter from a horizontal/vertical polarization should be 50% the passing through (or not) should be independent of what happens at the other location. But it turns out they are correlated.

This is to be expected, both filters are 45 degrees out of phase with the two entangled photons. It is like an on off switch both pass or both are blocked.
CHollman82
1 / 5 (1) May 14, 2012
Both photons are the same (entangled) both filters are the same (45 degree phase)... why wouldn't you expect both photons to react the same? Only if you assume non-determinism and a probabilistic nature of reality. It actually seems counter-intuitive to consider this "spooky" action.
Charon
not rated yet May 14, 2012
Why not just use it as a telegraph? You lose speed at the ends, but, say for really long distances, you would gain a lot of time.
simplicio
5 / 5 (2) May 14, 2012
It's very misleading text. They imply FTL because they mention the benefit of no latency lags (ie, instant) communication. They even say exactly "...allowing for instant communication;".

But at end they say, no not really, because you still need back channel at less than speed of light (so not instant). The last part is true, but why tease with talk of FTL? Average person would be very confused.
Pressure2
3.4 / 5 (5) May 14, 2012
This spooky action at a distance is no different than splitting a coin in half, face from tails. Mix the halves up throw them in opposite directions and check one and you always know what the other is. The photons are what they are from the moment they are created, so naturally when you check one you know the other.
antialias_physorg
5 / 5 (1) May 14, 2012
why wouldn't you expect both photons to react the same?

Because when you shoot single photons (unentangled ones) with the SAME polarization (one horizontal and one vertical) at such filters they react differently: They pass or don't pass according to probability (50 / 50 chance) - INDEPENDENTLY of each other (occasionally you get the mixed case of one passing at A and another not passing at B).

But when you do it with entangled ones that doesn't happen. They either BOTH pass or BOTH don't pass. You never get the mixed case.

The last part is true, but why tease with talk of FTL?

It's instant in the sense that you get instant encryption/decryption.

This spooky action at a distance is no different than splitting a coin in half, face from tails.

No, because in quantum physics you have superpositions (not something you have in coins) But the 45 degree filter relies on the superposition state of the polarized photons.
simplicio
5 / 5 (2) May 14, 2012
This spooky action at a distance is no different than splitting a coin in half, face from tails. Mix the halves up throw them in opposite directions and check one and you always know what the other is. The photons are what they are from the moment they are created, so naturally when you check one you know the other.

This is very wrong. It is classical explanation which does not works on quantum level. Is not the case we don't know state, but the state is not determined (or is all possible states at once).
Noumenon
1 / 5 (4) May 14, 2012
This spooky action at a distance is no different than splitting a coin in half, face from tails. Mix the halves up throw them in opposite directions and check one and you always know what the other is.


You're not including enough variables in your analogy , which is why it is not spooky, and is not representative of entanglement and the statistical inconsistencies of considering each measurement probability independently.

There is no perfect analogy, but say each coin can take on one of three values, A, B, or C,... and upon comparing observed results from many such entangled pairs, you find that, in the time order of measurement (non-communitive observables), they are always in alphabetical order, a correlation. Statistically this would not be possible considering each coin as independent, so they must be "entangled",.. one result dependent on the measurement of the other.
Noumenon
1 / 5 (4) May 14, 2012
,...well, more like clockwise order with, A-->B, B-->C, C-->A.
Pressure2
1 / 5 (1) May 15, 2012
This is very wrong. It is classical explanation which does not works on quantum level. Is not the case we don't know state, but the state is not determined (or is all possible states at once).

The only reason you do not know the states is because you do not want to know them. You create them and you could know the state of both from the start. Try that and you would find the coin anology is a perfect fit.

In other words not knowing the states is NOT proof they are in both states until detected.

Ryan1981
5 / 5 (1) May 15, 2012
I found this nice website that tries to explain quantum entanglement. http://www.davidj...ex.shtml For me the part with the polarizer on detector A and the double slid on detector B shows irrefutably that somehow there must be something that connects these entangled photons. Wether it is predetermined or instantanious is still something I have to think about (and probably won't get the answer too :P)
CHollman82
1 / 5 (1) May 15, 2012
Because when you shoot single photons (unentangled ones) with the SAME polarization at such filters they react differently

But when you do it with entangled ones that doesn't happen. They either BOTH pass or BOTH don't pass.


Right, and that makes sense, because when they are entangled they have the same properties so they react the same to the filter and when they are not entangled they do not necessarily have the same properties (but they might incidentally) so of course they may or may not react similarly to the filter...

All this means to me is that there is some property of photons that is always shared when they are what we consider to be "entangled" that are not necessarily shared when they are not entangled. This property determines how they react to the filter... we may not understand this property, but it only violates locality if you rule out determinism... but it seems the reaction to the filter is simply pre-determined by the entangled state.
CHollman82
1 / 5 (1) May 15, 2012
Consider some property "x" that we don't understand about photons. Say that when we create a pair of entangled photons this property "x" is always equal between the two entangled photons. Say this property "x" determines how a photon will react to a given filter... Now isn't it obvious that when we make entangled photons they will always react the same to the same filter, but when we don't make them entangled they may or may not react the same depending on the state of property "x" of each photon, which is not controlled when we aren't making entangled photons?

When we have two non-entangled photons each of their property "x" is in some unknown state, it may be the same between them or it may not be, so of course they may react the same to the same filter or they may not... but when we create entangled photons what we are actually doing is setting property "x" equal in each, so of course they react the same to the same filter.

This seems obvious...
antialias_physorg
5 / 5 (2) May 15, 2012
they have the same properties so they react the same to the filter and when they are not entangled they do not necessarily have the same properties


It's completely independent of the property (or your measuring device for that matter).
Here#s a good sumary of bell's theorem.
http://www4.ncsu....ell.html

CHollman82
1 / 5 (1) May 15, 2012
It's completely independent of the property


What property? I'm talking about a property that may exist that we have no knowledge of.

Again, some property "x" of photons that we have no knowledge of is equal between two entangled photons, and not necessarily equal between two non-entangled photons... this perfectly predicts the results we see if this property "x" determines the photons reaction to a filter.

Say that property "x" is binary, two entangled photons may have property "x" in state 1 or state 0, we don't know, but if they are entangled then they are equal, both are in state 1 or both are in state 0... In two non-entangled photons their property "x" MAY or may not be equal, because we aren't controlling it when we don't make them "entangled". This perfectly predicts the observation of a probability distribution with non-entangled photons and a correlation with entangled photons and it doesn't require any "spooky" action at a distance.
antialias_physorg
5 / 5 (1) May 15, 2012
[q9Again, some property "x" of photons that we have no knowledge of is equal between two entangled photons, and not necessarily equal between two non-entangled photons...
Read the link. In it the experiment describes that you actually get less correlation with a 3-way detector. No amount of hidden variable (common and unknown properties) of the entangled entities will help. I really can't put it in 1000 characters. Please read the link. It uses no mathematics (except addition and multiplications)
CHollman82
1 / 5 (1) May 15, 2012
I've actually read that entire thing before, don't remember all the details but I remember it was somewhat convincing, though without knowing the details of the testing apparatus (and, frankly, without relevant education in particle physics) it's difficult to be completely convinced. I do trust the physicists though, and if they say that this is the best explanation currently then I'll go with that, but I also understand that the bleeding edge of quantum physics is a volatile place, and what we think we know now is more than likely to change in time
Pressure2
not rated yet May 15, 2012

Bell's Theorem has not been proven. I have also read the same thing several years ago at a NASA site, I have not been able to find it lately.

http://arxiv.org/.../0208187
Pressure2
not rated yet May 15, 2012
Why doesn't someone run the polarized photon entanglement experiment knowing which polarized photon went in which direction. Then see if the same polarization arrived at the measuring point. I say it will.
Now if it does, why in the world would anyone think the polarization is unknown and in both states during transit?
slayerwulfe
not rated yet May 15, 2012
Unfortunately the Bell theorem seems to show that no matter how many hidden variables you add to a theory (or to quantum mechanics) non-locality can't go away.

That's pretty vexing, because we mostly think that information
transmission is limited by the speed of light.
As of now entanglement and non locality does not allow for transmitting information. There is correlation - but you can't use it to transmit messages. You can use it for encryption/verification that the transmission wasn't intercepted - but quirkily enough these actions don't constitute information transmission.

i'm a little lost here because no one is defining or questioning exactly what these Chinese researches have done or not done, are you able to clarify this for us.
antialias_physorg
5 / 5 (1) May 16, 2012
i'm a little lost here because no one is defining or questioning exactly what these Chinese researches have done or not done, are you able to clarify this for us.

I only have access to the abstract lniked inthe article. But from that I surmise that they have taken the method of an earlier experiment and upped the range by using a brighter source.

Read: They have significantly extended the range over which you can have entangled pairs.
DJHill
not rated yet May 31, 2012
Simple explanation: If you have 2 bowls of alphabet soup, containing identical amounts of letters, you can pick out the same letters in each, as long as you know which ones you are looking for. The trick is actually knowing in advance which ones to eliminate prior to the experiment. The ones that are left have to be the same. By charging (polarizing or depolarizing) the photons does not prove the teleportation of known information, it does prove that the shell game is alive and well.

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