Transferring Information Between Matter and Light

By Roland Piquepaille

Physicists from the Georgia Institute of Technology have successfully transferred "quantum information from two different groups of atoms onto a single photon." Apparently, it's the first time that anyone has exchanged information between matter and light. This breakthrough opens the way to the development of large-scale quantum networks. But according to the researchers, there shouldn't be any practical applications of their research before at least 2010. Read more...

Here is what these physicists have achieved.

The researchers, Assistant Professor Alex Kuzmich and graduate student Dzmitry Matsukevich, report transferring atomic state information from two different clouds of rubidium atoms to a single photon. In the photon, information about the spatial states of the atom clouds was represented as vertical or horizontal optical polarization.

"A really big issue in quantum information systems today is distributed quantum networks, and for that, you have to be able to convert quantum bits of information based on matter into photons," Kuzmich said. "This is the first step, one building block. What we have done is create a quantum network node, and now the next step is to create a second quantum network node and connect them."

Here is a photograph of two Georgia Tech physicists using their optical equipment to transfer information from two different groups of atoms onto a single photon (Credit: Georgia Tech; Photo: Gary Meek). And here is a link to a larger version.

Please read the university news release for the technical details about the experiments. And let's focus here on how this discovery will help to build future quantum communications systems?

Conversion of quantum states from atomic-based systems to photonic systems is necessary for long-distance communication. While the matter-based systems can provide long-term storage of information, efficient transfer of information requires that it be converted into a photonic state for transmission across optical fiber networks.

Once converted into a photonic qubit, the information can be processed and may not need to be converted back to a matter-based qubit.

"If you want to realize a quantum repeater, you must have two such quantum nodes," Kuzmich explained. "But in this quantum communications approach, you don't ever need to convert the photon back to atomic format."

The research work has been published by Science in its October 22, 2004 issue under the title "Quantum State Transfer Between Matter and Light." And thanks to arXiv, for those of you who are not subscribers to Science, here are two links to the abstract and to the full paper (PDF format, 14 pages, 198 KB).

Here is the short abstract.

We report on the coherent quantum state transfer from a two-level atomic system to a single photon. Entanglement between a single photon (signal) and a two-component ensemble of cold rubidium atoms is used to project the quantum memory element (the atomic ensemble) onto any desired state by measuring the signal in a suitable basis. The atomic qubit is read out by stimulating directional emission of a single photon (idler) from the (entangled) collective state of the ensemble. Faithful atomic memory preparation and read-out are verified by the observed correlations between the signal and the idler photons. These results enable implementation of distributed quantum networking.

Sources: Georgia Institute of Technology news release, via PhysOrg.com, October 21, 2004; arXiv website