5 August 2010

Important step towards quantum communication

Quantum communication

The possibility of future quantum communication is to be able to safely send data from one computer to another without data being stolen or read along the way. Quantum communication networks covering large distances are only at the drawing-board stage and in lab tests, researchers have however come an important step closer to realization. The lack of an intermediate station to store the information from light has been a major obstacle. For the first time, researchers from Harvard University and the Niels Bohr Institute at the University of Copenhagen have succeeded in connecting information from light to a solid state atom which can store the information. This is a big step towards making an intermediate station. The results have just been published in the prestigious journal, Nature.

Quantum information is sent optically - which means with light - and the signal consists of photons which are the smallest parts (a quantum) of a light pulse. Quantum communication is based on light and will be performed with optical fibers.  However, the single photons are easily lost on the way. Therefore, the solution is to have intermediate stations which can store the information from the light and thereafter send it on using another light beam.

An impurity in a diamond is illuminated with a
powerful laser beam after which it emits a single
photon. The oscillations of the emitted photon are entangled with the spin of the electrons in the
impurity and this entanglement can be used to
send quantum information over long distances.
(Illustration: Yiwen Chu, Harvard University).

“We have come a significant step closer to this intermediate station by creating so-called entanglement between the information of light and a solid state atom. It is the first time ever that this has been done and it is an important step on the way towards quantum communication over long distances”, explains Anders Søndberg Sørensen. He is a quantum physicist at Quantop, center of quantum optics at the Niels Bohr Institute. Anders Sørensen has been part of the team working on the theory of how the intermediate stations can be made and in collaboration with researchers from Harvard University, they have carried out the experiments which have shown that this can really work.

Communication via light

So far researchers have worked with gas atoms to make intermediate stations. A laser beam is sent into a vacuum chamber with a single gas atom. The atom reacts by sending out a single photon and a special phenomenon in the quantum world ensures that the photon is entangled with the atom. This entanglement or synchronization can be used to send information. The quantum information is stored in the polarization of light which can be right- or left-handed.  It can be compared with an ordinary computer’s storage of information in bits consisting of the numbers 0 and 1. But a quantum bit cannot be stolen without it being discovered as it only contains a single photon. So quantum information is burglar-proof.

Network over long distances
“But a gas atom floats freely and it is difficult to trap it so you can hit it”, explains Anders Sørensen, “therefore, we wanted to find a solution with a solid state atom which we could contain. Our solution was a diamond - not the diamond itself, which consists of carbon atoms, but tiny impurities in the diamond which consists of a single nitrogen atom”.

The research results can mark a new era in creating networks for quantum communication. Anders Sørensen estimates that such intermediate stations should be installed for every 10 to 100 kilometers and the networks will, in this way, work over long distances.

Article in Nature >>