German scientists link two labs with ‘universal quantum network’

Communicating with entangled distant atoms

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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 Photonics.com, 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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