Long-distance quantum communication gets closer as physicists increase light storage efficiency by an order of magnitude

Mar 01, 2010 By Lisa Zyga feature

(PhysOrg.com) -- In a new demonstration of reversible light storage, physicists have achieved storage efficiencies of more than a magnitude greater than those offered by previous techniques. The new method could be useful for designing quantum repeaters, which are necessary for achieving long-distance quantum communication.

Physicists Thierry Chaneličre of the Laboratoire Aimé Cotton - CNRS in Orsay, France, and his coauthors have published their results of the new light storage method in a recent issue of the .

The new technique involves mapping a light field onto a thulium-doped crystal. Compared with other kinds of rare-earth ions, thulium has an interaction wavelength that makes it more easily accessible with laser diodes, allowing for a better preparation of the tool used to store the light - an atomic frequency comb.

“I would say that the most important factor [in achieving high-efficiency light storage] is the ability to properly prepare the atomic comb from a very absorbing medium,” Chaneličre told PhysOrg.com. He explained that there is a tradeoff involved: “the absorption allows the storage, but is also a source of loss during the retrieval process.”

To prepare the atomic frequency comb, the physicists filtered preparation pulses into evenly spaced absorption peaks, which resulted in an absorption comb with a specific periodicity. The scientists then sent a weak signal pulse into the comb to be stored. The signal’s spectrum was covered by many of the comb peaks, which temporarily held the signal and delayed its retrieval.

Using this technique, the physicists estimated that the total light storage efficiency was about 9%, which is a significant improvement over previous demonstrations’ efficiencies of less than 1%.

“The efficiency is the probability of retrieval,” Chaneličre explained. “In our case, for 100 storage trials, we only retrieve our photon nine times. So we need to repeat the operation to be sure that something is transmitted. This is the way a quantum repeater will work. A strong advantage of the atomic frequency comb protocol is its large intrinsic repetition rate that has already been demonstrated experimentally. The ‘quantum data rate’ of a quantum repeater will be at the end directly proportional to the efficiency and the intrinsic repetition rate. That's why it is so important.”

The scientists also found that the total light efficiency strongly depends on the shape of the frequency comb, which can be controlled by varying the relative intensity of the preparation pulses. Using this information, the physicists hope that by controlling the spectral properties of the atomic frequency comb, they will be able to improve the design of quantum repeaters.

“The main application of the protocol is quantum repeaters,” Chaneličre said. “This is the future of quantum cryptography, which is an active field of research but suffers from the limitation of current optical networks. The range of this fully-secured communication is currently limited to 100km typically because of residual absorption in the optical fibre. The goal of a quantum repeater is to extend this range toward longer distances (thousands of km). This is what we mean by ‘long-distance .’”

Explore further: Researchers find qubits based on trapped ions offer a promising scalable platform for quantum computing

More information: T. Chaneličre, J. Ruggiero, M. Bonarota, M. Afzelius, and J-L Le Gouet. “Efficient light storage in a crystal using an atomic frequency comb.” New Journal of Physics, 12 (2010) 023025. www.iop.org/EJ/abstract/1367-2630/12/2/023025/

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User comments : 10

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baudrunner
3.4 / 5 (5) Mar 01, 2010
I think the goal of long distance quantum communication should be to develop the ability to trade information in almost real time with Mars colonists and other deep space exploration probes. No joking! I would shift focus from quantum cryptography to real-time data exchange.

Efficiency can be improved by eliminating as much of the closed optical fiber system as possible and expanding optical laser interaction. The quantum repeater described above is integral to this plan.
robvoodoo
2.3 / 5 (3) Mar 01, 2010
i did immediately wonder if quantum entaglement could be used for faster-than-light real-time communication over stellar distances. Presumably people have already thought about this, but with recent developments involving entaglement (multi-qubit quantum computing) are we moving closer?
holoman
Mar 01, 2010
This comment has been removed by a moderator.
podizzle
4.5 / 5 (4) Mar 01, 2010
pay attention seti, "advanced" civilizations would not use radio waves to communicate distance. it'd be like me writing a letter, folding it into a paper airplane, and launching it into the sky hoping it gets to the space station.
dustydude
4 / 5 (3) Mar 01, 2010
I'm afraid faster than light communication is not possible... As we currently understand entanglement, if a photon has a equal probability of being either spin up or spin down. By entangling two photons and separating them by a distance, when we measure the spin of one photon and the result in spin up then we immediately know that the other is spin down. There is no way to predetermine which way the spin will be so not possible to transmit a signal or bit stream.......
Well not yet! But hopefully! v>c = startrek
Raygunner
Mar 01, 2010
This comment has been removed by a moderator.
otto1923
1.5 / 5 (2) Mar 01, 2010
I'm afraid faster than light communication is not possible... As we currently understand entanglement, if a photon has a equal probability of being either spin up or spin down. By entangling two photons and separating them by a distance, when we measure the spin of one photon and the result in spin up then we immediately know that the other is spin down. There is no way to predetermine which way the spin will be so not possible to transmit a signal or bit stream.......
Well not yet! But hopefully! v>c = startrek
Communication is possible, but information transfer is not. This is what they tell me. Any takers?
Auxon
3.3 / 5 (3) Mar 01, 2010
I'm afraid faster than light communication is not possible... As we currently understand entanglement, if a photon has a equal probability of being either spin up or spin down. By entangling two photons and separating them by a distance, when we measure the spin of one photon and the result in spin up then we immediately know that the other is spin down. There is no way to predetermine which way the spin will be so not possible to transmit a signal or bit stream.......
Well not yet! But hopefully! v>c = startrek
Communication is possible, but information transfer is not. This is what they tell me. Any takers?


It's the same thing, but yes it is possible (at least in theory, to communicate over light years); but you have to set up initial communications. It has already been done, the record is 113 Km AFAIK. For example: http://www.cosmos...ode/1361
bluehigh
2.5 / 5 (6) Mar 01, 2010
Deduction allows FTL communication of information.

For example: When I know where an object is at any given time and I know the direction of travel then I can deduce the objects position at a future time. I do have to wait for the light to arrive because I know where the object will be.

We do this every day just catching a ball in play.

With a galaxy a billion light years away, I do not need to wait for the light to arrive at the galaxys postion NOW to know where it is likely located. I can deduce the galaxys postion. Thereby having information a billion years before the light gets to me.

I expect that we will find a way to deduce information provided by entanglement that will allow communication of information much faster than light.
antialias
3.5 / 5 (2) Mar 02, 2010
Entaglement cannot be used to convey information at faster than light speeds. The entangled entities are created close together. Separating them could be done at (maximally) speed of light. In order to do FTL information transfer you would have to _modulate_ one part and then the other changes - but this doesn't happen once separated the interaction between them is broken).

So once you separate them forcing one part to the "1" or "0" state will not change the other part.

You _can_ measure one particle of an entangled system and then know what the other part will be - but since you don't know beforehand what the result of the measurement will be there is no way of encoding any information in it.
PPihkala
not rated yet Mar 02, 2010
I don't know if this idea will work or not, but this is what I thought up. Let's say we make two boxes of particles so that each particle has it's entangled pair in the other box. Then fly one of the boxes to let's say mars. When you want to send some info to mars, you examine particles in your box here in earth. That way you also convert corresponding particles in mars to their determined state. The question is is it possible in mars to determine the interval of entanglement chances happening? If it is, then the interval can be used to encode information. Let's say 1 millisecond between changes is 1 and 2ms is 0. Or what ever is more suitable way to encode the information to those intervals. This will use up the entangled particle pairs, but given enough at beginning, it should not matter.
baudrunner
5 / 5 (1) Mar 03, 2010
Entaglement cannot be used to convey information at faster than light speeds. The entangled entities are created close together. Separating them could be done at (maximally) speed of light. In order to do FTL information transfer you would have to _modulate_ one part and then the other changes - but this doesn't happen once separated the interaction between them is broken).

In that one paragraph you have contradicted the entire principle behind the idea of entanglement. The initial link must be acquired at c, but once the link is established, the entangled photons interact instantaneously. That's the whole idea.

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