German scientists link two labs with ‘universal quantum network’ • The Register

Andre Kesteloot andre.kesteloot at
Thu Apr 12 11:10:43 CDT 2012

-------- Original Message --------
Subject: 	German scientists link two labs with ‘universal quantum 
network’ • The Register
Date: 	Wed, 11 Apr 2012 23:02:55 -0400
From: 	André Kesteloot <andre.kesteloot at>
To: 	Kesteloot André <andre.kesteloot at>

>     German scientists link two labs with ‘universal quantum network’
> Communicating with entangled distant atoms
> By Richard Chirgwin 
> <> 
> • Get more from this author 
> <>
> Posted in Science <>, 11th April 
> 2012 23:48 GMT <> German 
> researchers have demonstrated a technique that allows them to create 
> entanglement between atoms in different places, using photons to put 
> the atoms into an entangled state.
> Quantum effects have already crept into the cryptography world, in 
> which entangled pairs of photons are used for key exchange. However, 
> in the new experiment, the researchers have gone a step further: 
> they’ve combined two kinds of quantum systems to crate a more general 
> purpose network.The setup works like this: a single rubidium atom is 
> trapped in a reflective optical cavity, at each node of the network, 
> with nodes connected via an optical fibre. Each of those rubidium 
> atoms can act as a qubit (ie, able to store a quantum state).
> When the atom emits a photon, the qubit – that is the state of the 
> atom emitting the photon – is encoded into the photon’s polarization, 
> and the destination node then takes on the state of the qubit that 
> emitted the photon.
> This arrangement means that atoms can be used to /store/ qubits, while 
> the photons are use to transmit state. It solves a challenge in 
> quantum communications, since while photons work very well to transmit 
> quantum states, they’re very difficult to store.
> Researcher Stephan Ritter of the Max Planck Institute of Quantum 
> Optics explained 
> <> 
> to /Scientific American/ that the combination of atomic and photonic 
> qubits was proposed 15 years ago, but it’s difficult to achieve in 
> practice because "if you want to use single atoms and single photons, 
> as we do, they hardly interact".
> That’s where the reflective cavity comes in: when the photon arrives 
> at its destination, it can be reflected past the rubidium atom tens of 
> thousands of times, improving the chance that the desired interaction 
> will actually happen.
> “The cavity enhances the coupling between the light field and the 
> atom,” Ritter says.
> Hence the experiment achieves the genuinely spooky: a read-write 
> operation across two laboratories connected by around 60 meters of 
> fibre, in which the receiving atom becomes entangled with the 
> transmitter, even though there’s been no direct interaction between them.
> That, Ritter says, could extend the application of the network even 
> further: once two atoms are entangled, the quantum state of one 
> depends on the quantum state of the other.
> As noted <> at 
> /,/ it only takes a microsecond to achieve the 
> entanglement, but the state lasts 100 microseconds. That means it 
> would be possible to build a network of “quantum repeaters” that use 
> quantum teleportation, rather than photons, to transmit information 
> between different places. “Entanglement of two systems separated by a 
> large distance is a fascinating phenomenon in itself. However, it 
> could also serve as a resource for the teleportation of quantum 
> information. One day, this might not only make it possible to 
> communicate quantum information over very large distances, but might 
> enable an entire quantum Internet”, Ritter said.
> The work is published <> in 
> /Nature./ ®

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