Chinese team builds first quantum router

August 7, 2012 by Bob Yirka, report

(A) Illustration of a genuine quantum router. The control photon can be in arbitrary superposition states with coecients c0; c1 that determine the path of the signal photon. (B) The entanglement-based approach to implementation of a genuine quantum router. With a bit of pre-shared entanglement, the quantum router can be realized with linear optical devices. The control coecients c0; c1 are imprinted through operation on the control photon alone with a polarization rotator and a lter. The routing is realized with a polarization beam splitter (PBS) and a wave plate on the signal photon. Image from arXiv:1207.7265v1 [quant-ph]
( -- With all the talk of quantum computers, little notice has been made of work on what is known as a quantum Internet, which is where data is sent across a web of computers via devices that work at the quantum, rather than atomic level, thereby increasing the speed of the whole system. The holdup at this point is in creating devices capable of routing such information. Now it appears that a team of physicists working from Tsinghau University in China have proven that it’s possible to do so. They have, as they describe in the paper they’ve uploaded to the preprint server arXiv, built a working quantum router capable of routing one cubit.

The trick in routing quantum data is that reading the information from a signal that tells a router where to send data, causes that data or signal to be destroyed; that’s just how quantum mechanics works, so the ordinary way of routing data on a network won’t work. To get around that problem, the researchers used two of the special properties of quantum particles, namely, entanglement, whereby whatever happens to one, automatically happens to another and the fact that a particle is capable of representing two states at once (i.e. both 1 and 0).

To build their router the team first generated a photon with superposition (one that has both horizontal and vertical polarization states); they then converted the photon to two entangled photons that also had superposition states. Then they treated one of the entangled pair as the control signal and the other as the data signal. When the control signal is read, and destroyed, the router gains the information it needs to know regarding which of two optical fiber cables to send the data signal, and thus, routes the data signal down the desired path.

The researchers aren’t claiming they’ve come up with a solution for building a quantum Internet, as clearly their router is only capable of routing a single cubit, but it does demonstrate that quantum data is possible, and that’s something that until now, no one else has been able to do. And it also gives hope to researchers that someday a new and different type of quantum router will be created that really will allow for a true , and if that happens, data transmission will likely become so fast, that it will cease to be a topic of conversation.

Explore further: Physicists build first single-photon router

More information: Experimental demonstration of an entanglement-based quantum router, arXiv:1207.7265v1 [quant-ph]

We report an experiment that demonstrates full function of a quantum router using entangled photons, where the paths of a single-photon pulse are controlled in a coherent fashion by polarization of another single photon. Through a projective measurement, we prepare the polarization of the control photon in arbitrary superposition states, leading to coherent routing of the target photon in quantum superposition of different paths. We demonstrate quantum nature of this router through optical measurements based on quantum state tomography and show an average fidelity of $(93.24pm 0.23)%$ for the quantum routing operation.

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Aug 07, 2012
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5 / 5 (1) Aug 07, 2012
I'm a bit confused by this: Are they entangling a different property of the photon or the same one that is carrying the qbit information of the original one?

If its the same one then they just have broken quantum key distribution (because they can make a copy of the key being sent with a man-in-the-middle attack) - so I'm assuming they use a different property (?)
...I'm not really seeing where it says either inthe paper.

Also: It seems that the information for the router is destroyed by this (i.e. the routing information diminishes each time you cross a router). This may not be a showstopper, since I have no idea how a quantum internet would ultimately look like, but it does seem to limit the number of 'hops' you can do along the route.
Phil DePayne
not rated yet Aug 07, 2012
What theyre saying is that they are slightly 'imprinting' the polarization of the control photon on the signal photon to a such a degree that a polarization beam splitter can 'route' the signal photon, not enough to destroy information. Problem is, it only works with their setup that is limited to one photon at a time.
not rated yet Aug 07, 2012
What theyre saying is that they are slightly 'imprinting' the polarization of the control photon on the signal photon to a such a degree that a polarization beam splitter can 'route' the signal photon,

That is what they are doing. But in a 'real world' application the photon would already have to carry that information when sent (its target IP-Address) and not just get it at the router.

As for 'just one bit': I imagine that could either be solved via superposition, or simply classically via sending packets of qbits where the first few have the routing information and the others are just shuffled along the same channel.

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