Transporting Stored Light

A team of physicists led by Patrick Wind­passinger at Johannes Gutenberg University Mainz has success­fully transported light stored in a quantum memory over a distance of 1.2 millimeters. They have demons­trated that the controlled transport process and its dynamics has only little impact on the properties of the stored light. The researchers used ultra-cold rubidium-87 atoms as a storage medium for the light as to achieve a high level of storage effi­ciency and a long lifetime.

For the experiment, atoms of rubidium-87 are first pre-cooled and then transported to the main test area, which is a custom-made vacuum chamber. There they are cooled to temperatures of just a few microkelvins. (Source: AG Windpassinger, U. Mainz)

“We stored the light by putting it in a suitcase so to speak, only that in our case the suitcase was made of a cloud of cold atoms. We moved this suitcase over a short distance and then took the light out again. This is very interesting not only for physics in general, but also for quantum communi­cation, because light is not very easy to capture, and if you want to transport it elsewhere in a controlled manner, it usually ends up being lost,” said Wind­passinger, explaining the compli­cated process.

The controlled mani­pulation and storage of quantum information as well as the ability to retrieve it are essential prere­quisites for achieving advances in quantum communi­cation and for performing corres­ponding computer operations in the quantum world. Optical quantum memories, which allow for the storage and on-demand retrieval of quantum infor­mation carried by light, are essential for scalable quantum communication networks. For instance, they can represent important building blocks of quantum repeaters or tools in linear quantum computing. In recent years, ensembles of atoms have proven to be media well suited for storing and retrieving optical quantum information. Using a technique known as electro­magnetically induced trans­parency (EIT), incident light pulses can be trapped and coherently mapped to create a collective exci­tation of the storage atoms. Since the process is largely reversible, the light can then be retrieved again with high effi­ciency.

Now, Wind­passinger and his colleagues described the actively controlled transport of such stored light over distances larger than the size of the storage medium. Some time ago, they developed a technique that allows ensembles of cold atoms to be trans­ported on an optical conveyor belt which is produced by two laser beams. The advantage of this method is that a rela­tively large number of atoms can be trans­ported and positioned with a high degree of accuracy without signi­ficant loss of atoms and without the atoms being uninten­tionally heated. The physicists have now succeeded in using this method to transport atomic clouds that serve as a light memory. The stored information can then be retrieved elsewhere. Refining this concept, the develop­ment of novel quantum devices, such as a racetrack memory for light with separate reading and writing sections, could be possible in the future. (Source: U. Mainz)

Reference: W. Li et al.: Controlled Transport of Stored Light, Phys. Rev. Lett. 125, 150501 (2020); DOI: 10.1103/PhysRevLett.125.150501

Link: Experimental Quantum Optics and Quantum Information, Institute of Physics, Johannes Gutenberg-University Mainz, Mainz, Germany

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