Quantum network nodes with hot atoms

Communication networks need nodes at which information is processed or redirected. Physicists at the University of Basel have now developed a network node for quantum communication networks that can store single photons in a steam cell and transmit them later.

In quantum communication networks, information is transmitted by single particles of light (photons). At the nodes of such a network, buffer elements are needed to temporarily store, then retransmit later, the quantum information contained in the photons.

Researchers at the University of Basel from the group of Professor Philipp Treutlein have now developed a quantum memory based on atomic gas inside a glass cell. The atoms do not need to be specially cooled, which makes the memory easy to produce and versatile, even for satellite applications. In addition, the researchers made a single photon source that allowed them to test the quality and storage time of quantum memory. Their results were recently published in the scientific journal Quantum PRX.

Hot atoms in steam cells

“The relevance of hot atoms in steam cells for quantum memories has been studied over the past twenty years,” says Gianni Buser, who worked on the experiment as a PhD student. “Generally, however, attenuated laser beams – and therefore conventional light – were used.” In classical light, the number of photons striking the vapor cell during a certain period follows a statistical distribution; on average it is one photon, but sometimes there may be two, three or none.

To test quantum memory with “quantum light” – meaning always precisely one photon – Treutlein and his colleagues developed a dedicated single-photon source that emits exactly one photon at a time. The moment this happens is announced by a second photon, which is always emitted simultaneously with the first. This allows quantum memory to be activated at the right time.

The single photon is then directed into quantum memory where, using a control laser beam, the photon causes more than a billion rubidium atoms to enter a state known as a superposition of two energy levels. possible atoms. The photon itself disappears in the process, but the information it contains is transformed into the superimposed state of the atoms. A short pulse from the control laser can then read this information after a certain storage time and transform it back into a photon.

Reading noise reduction

“So far, a critical point has been noise – the extra light that is produced during reading that can compromise the quality of the photon,” says Roberto Mottola, another PhD student in Treutlein’s lab. Using a few tricks, the physicists managed to reduce this noise enough so that after storage times of several hundred nanoseconds, the quality of the single photon is still high.

“These storage times are not very long, and we haven’t really optimized them for this study,” says Treutlein, “but they are already more than a hundred times longer than the duration of the stored single-photon pulse. ” This means that the quantum memory developed by the Basel researchers can already be used for interesting applications. For example, it can synchronize randomly produced single photons, which can then be used in various quantum information applications.

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Material provided by University of Basel. Note: Content may be edited for style and length.

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