Quantum communication without entanglement could perform faster than previously thought possible

Oct 29, 2012 by Lisa Zyga feature
Illustration of the new quantum communication scheme, where information is directly transmitted from one node to another using a transmitter unit and a receiver unit. The scientists calculated that the new approach could potentially enable communication rates several orders of magnitude faster than other approaches. Credit: W. J. Munro, et al. ©2012 Macmillan Publishers Limited

In order to build a quantum internet – a network that is faster and more secure than the current internet – the key is the ability to transmit quantum information between remote quantum computers (i.e., nodes). The most familiar approach involves entangling the links between nodes and then using quantum repeaters at intermediate locations to provide entanglement swapping, extending the range of entanglement across km-long networks.

In such a system, the performance is inherently limited by the time it takes to establish entanglement between nodes. This time is at best the classical signaling time between the nodes, but with many schemes it is even longer, and increases as network size increases. Since the qubits that store the are unstable and quickly decohere, quantum memories are required to store quantum information for milliseconds or longer while they wait for entanglement. The result is a theoretical limitation on speed due to the system's design and the need for additional components – quantum memories – to enable a functioning network.

In a new study published in , scientists from Japan and the UK have presented an alternative design for a network that requires neither entanglement between nodes nor quantum memories. Instead, the scheme transmits quantum information in encoded form directly across the network, using devices that act as quantum repeaters (without entanglement) to transmit and receive information between each other. Because this approach doesn't use entanglement, the communications rate is not limited by the conventional restraints that limit the rates of entanglement-based approaches.

"The significance of this work is that we have removed the need for classical messages to herald the generation of entanglement between adjacent nodes," coauthor Bill Munro of NTT Basic Research Laboratories in Kanagawa, Japan, and the National Institute of Informatics in Tokyo, Japan, told Phys.org. "This classical signal means our qubits at the remote nodes had to wait until it could be found out whether they were entangled or not. With repeater nodes separated by tens of kilometers, this waiting time was on the order of hundreds of microseconds. This means we needed long-lived quantum memories (milliseconds or longer life times) to store the information (entanglement) that was in those qubits.

"Our new scheme removes the need for long-lived quantum memories, as we do not aim to establish entanglement between the remote nodes. Instead we send an encoded signal between the nodes. Once the message has left one node to be transmitted to the next node, the resources available in the first node are freed up and the next message can be sent down the channel (even before the second node has received the first message), thus increasing the rate at which information can be transmitted. Entanglement-based approaches cannot do this, so their only route to higher communication rates is by paralleling resources within the node."

In the new approach, information is stored in matter qubits, such as electron spins, in the transmitter and receiver units. The transmitter, which contains a single-photon source, transfers the information from the matter qubits to the photonic qubits. Then it sends the photonic qubits down an optical fiber to the receiver, which contains a single-photon detector. The receiver operates in reverse, transferring the information from the photonic qubits back to matter qubits.

Unlike traditional schemes, the new scheme doesn't require entanglement between the matter qubits at the two remote nodes. As soon as the transmitter moves the information from the matter qubit to the photonic qubit, the matter qubit is no longer needed, so quantum memory is not required to store its information.

After the completion of one cycle, the final matter qubit is checked for errors using a redundant quantum parity code. Since channel losses and source/detector inefficiencies are inevitable, quantum states can easily be degraded during quantum communication. The error correction code protects against this degradation by measuring the states of several matter qubits that all carry the same information. The physicists showed that this encoding process can tolerate photon loss in excess of 50% in the quantum channel between nodes, which allows for nodes to be spaced further apart than usually thought.

After the error correction code verifies successful transmission of a qubit, the receiver then acts as a transmitter and sends the information on to the next node in the network. The scientists calculated that the scheme could transmit data at a rate of 107 quantum states per second, and operate over a distance of at least 17 km between nodes. With 48 nodes, quantum information could be transmitted over 800 km with a success probability of greater than 98%. The scientists calculated that the new approach could potentially enable communication rates several orders of magnitude faster than other approaches. In addition to speed, the new scheme also has other routing advantages.

"Our solution also makes it very easy to see how one can send information on a more complicated quantum network," Munro said. "Each node would have a telephone-like number and then we can think of telephone-like exchanges that route the information to where it needs to go. The node that sends the original message does not need to know the route to the final destination. Instead it just needs to get to a local exchange which can route it through the chain using country, region, and city parts of the telephone-like number."

In the future, the researchers plan to tackle some of the technical challenges facing the new quantum communication scheme.

"Our future plans are an experimental implementation of this approach," Munro said. "Currently we are working on ways to improve our local gates within our physical system – a necessary step to showing our new communication scheme.

"The key requirement in this new approach is the need for efficient quantum gates within the repeater node," he added. "In the normal -based approach, we can work with gates that fail a lot of the time in a heralded way. Currently, efficient quantum gates are experimentally challenging to achieve but they need to be achieved if distributed quantum computers are going to be achieved."

Explore further: MRI for a quantum simulation

More information: W. J. Munro, et al. "Quantum communication without the necessity of quantum memories." Nature Photonics DOI: 10.1038/NPHOTON.2012.243

Journal reference: Nature Photonics search and more info website

4.8 /5 (13 votes)

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antialias_physorg
4.8 / 5 (4) Oct 29, 2012
Doesn't this mean that the inherent security due to entanglement isn't present in these kinds of transmissions?
kevinrtrs
2.3 / 5 (6) Oct 29, 2012
My question exactly. Usually when people are so hyper about a newfound advantage, they tend to minimise the accompanying disadvantage.
Tektrix
5 / 5 (1) Oct 29, 2012
The error correction scheme mentioned in the article may be able to detect eavesdropping. The quantum states are still being sent and are still as sensitive to external eavesdropping as entangled states. The problem is detecting the eavesdropping. In classic error correction schemes, patterns show up that are peculiar to a specific type of error. Perhaps the same is true for this quantum system- where eavesdropping would have a particular error signature.
MikeGroovy
1 / 5 (1) Oct 29, 2012
I don't need my streaming video from CrunchyRoll to be encrypted, I just need it to be fast! At the same time I would prefer for my sexy communications with my wife and my purchases from woot to be secure! There will always be a need to make things faster, that is until they are instant, (then we just need better prediction algorithms.) Hopefully a quantum network of the future will be dynamic enough to handle fast data and secure data.
My favorite form of FTL quantum communication is featured in the world of Eve Online.
community.eveonline.com/background/communication/
Hopefully only a matter of time since a method has been devised to measure a quantum state without changing it http://phys.org/n...ion.html Something like this could make remote control on Mars real time.. or better yet instant communication with an interstellar probe.
TheKnowItAll
5 / 5 (1) Oct 29, 2012
10^7 a typo?
Nydoc
1 / 5 (1) Oct 29, 2012
"The scientists calculated that the new approach could potentially enable communication rates several orders of magnitude faster than other approaches."
Does anyone know if this refers to bandwidth or latency?
visualhawk
1 / 5 (1) Oct 30, 2012
And we currently have record transfer rates of close to 2* 10^11 bit/s - thus 20000 times faster than this approach. Interesting academic achievement but not of real practical importance at this stage.
El_Nose
4 / 5 (1) Oct 30, 2012
If you want fast -- tell movie production companies that HD is the most resolution you will ever need. Unfortunately movie companies have realised that 3D cannot be recorded by camera and the format is so data intense that they are willing to bet people will accept huge data streams... same thing with the new 4000*2000 super higher definition --- we do not need these newer standards - they are there simply to prevent pirating -- while your cable company is now putting limits on how much you can download in a month.
johanfprins
1 / 5 (2) Oct 30, 2012
It would have been better if they have first demonstrated their scheme by experiment, and only then showed by mathematical means why it works: If it will ever work!
pjvc
not rated yet Oct 30, 2012
Unfortunately movie companies have realised that 3D cannot be recorded by camera...while your cable company is now putting limits on how much you can download in a month.


One in the same lately!
hal_fisher1
not rated yet Oct 31, 2012
El Nose, yes, we do need the higher def. With screens getting ever bigger 1080 dots are way too big. You want that wall sized tv or holo-deck or not?

They have a possible solution to the Heisenberg Uncertainty Principle and that is to illuminate the desire particle from two sides at once. This is hoped to not cause it's rotation to change.

LagomorphZero
not rated yet Oct 31, 2012
With out entanglement, isn't this just spintronic communication? By that I mean, the information is stored in electron spins, and sent down (optical) wire by photon spins?

Except the use of the word qubit, this sounds exactly like modern electronic communication replaced by spin states, which I understand to be called spintronics..
Pkunk_
1 / 5 (1) Nov 02, 2012
I don't need my streaming video from CrunchyRoll to be encrypted, I just need it to be fast!

My favorite form of FTL quantum communication is featured in the world of Eve Online.
Something like this could make remote control on Mars real time.. or better yet instant communication with an interstellar probe.

I agree completely , the most important facet of quantum communication is not the security aspect. It is the almost magical ability to have almost zero-latency communication across cosmic distances. Theoretically you could achieve 0 latency across any distance in the known universe.
The ability to send probes to far away Solar systems and get back instant data when they reach there is priceless. Without instant communications , society would once again fragment when we go out into space.
antialias_physorg
5 / 5 (2) Nov 02, 2012
Without instant communications , society would once again fragment when we go out into space.

Fragmentation depends upon your lifespan. If you have very much prolonged lifespans then even very long travel times will not fragment a society. (And if humans want to go anywhere in the universe we'll definitely have to expand of lifespans one way or another. But that seems a problem that is more easily tackled than building spaceships, anyhow)

With out entanglement, isn't this just spintronic communication?

No, because a spintronic current only has two states (spin up, spin down). Quantum communication on the other hand can still make use of superposition - even without entanglement.
ValeriaT
1 / 5 (2) Nov 02, 2012
In particle model the vacuum enables the spreading of both longitudinal (gravitational), both transverse (electromagnetic) waves. In the water surface analogy the light waves correspond the surface waves and the longitudinal gravitational waves correspond the sound waves spreading through underwater in much higher speed than the surface waves. But the underwater waves do manifest itself like the indeterministic noise at the water surface. They don't result from exchange of energy with observer and observer objects, they result from shielding of this interaction with all surrounding objects at the same moment. Therefore the entanglement cannot be used for deterministic information transfer, i.e. the transfer, where you can be always perfectly sure with the source and detector at the single moment. It's similar like with communication through sound waves: you can be never sure, who is talking with you, until you see him talking.But as we know this way is still quite feasible and reliable
ValeriaT
2.3 / 5 (3) Nov 02, 2012
This model of vacuum is symmetric: it enables both particles, both waves. The particles can be both blobs, both bubbles. And the waves can be both transverse both longitudinal. Here are many reasons, why mainstream physics managed to hide whole half of explanation before layman publics or even before its peers. The effects mediated with longitudinal waves are weaker and indeterministic by their very nature. But the mainstream physics is strictly deterministic and formal math based. It requires lotta time to learn it, so it's very conservative too. The teaching of formal models is a great business for many high school teachers, so they form a lobby, which is hostile toward all alternative and more general attempts for reality explanation as a single man. Their reluctant stance can be as fanatical, as the stance of religion fanatics. These guys really BELIEVE, they're doing public service, when they prohibit deeper explanation of reality in similar way, like the medieval preachers.
Pkunk_
3 / 5 (2) Nov 02, 2012
@antialias_physorg wrote -
Fragmentation depends upon your lifespan. If you have very much prolonged lifespans then even very long travel times will not fragment a society.

Take present society for example. The pace of change with newer technology, change in culture, attitudes means someone from my Grandfathers generation wouldn't even understand the language of today , leave alone the concept of "computers".
Without communications , the "disconnected" would revert to the middle and ancient ages where the pace of change was glacial due to the lack of exchange of ideas.