Communication networks require nodes where data is processed or rerouted. University of Basel physicists have developed a network node for quantum communication networks that can store single photons in a vapor cell and pass them on later.

Warm atom quantum network nodes
The vapor cell stores a light particle from the single-photon source (shown below) (above). A detector (right) detects a second photon emitted simultaneously, which triggers the control laser pulse and thus starts the storage process. (Photo courtesy of the Department of Physics at the University of Basel)

Single light particles transmit information in quantum communication networks (photons). Buffer elements are required at the nodes of such a network to temporarily store and later re-emit the quantum information contained in photons.

Prof. Philipp Treutlein’s group at the University of Basel has now developed a quantum memory based on an atomic gas inside a glass cell. Because the atoms do not need to be specially cooled, the memory is simple to manufacture and versatile, even for satellite applications. Furthermore, the researchers created a single-photon source, which allowed them to test the quantum memory’s quality and storage time. Their findings were recently published in the journal PRX Quantum.

Warm atoms in vapor cells

“The suitability of warm atoms in vapor cells for quantum memories has been studied for the past twenty years,” says Gianni Buser, a Ph.D. student who worked on the experiment. “In most cases, however, attenuated laser beams – and thus classical light – were used.” The number of photons hitting the vapor cell in a given period in classical light follows a statistical distribution; on average, it is one photon, but it can be two, three, or none.

Treutlein and his colleagues created a dedicated single-photon source that emits exactly one photon at a time to test the quantum memory with “quantum light” — that is, always exactly one photon. The occurrence is signaled by a second photon, which is always sent out simultaneously with the first. This enables the quantum memory to be activated at the appropriate time.

The single photon is then directed into the quantum memory, where it causes more than a billion rubidium atoms to enter a so-called superposition state, with two possible energy levels of the atoms. The photon itself vanishes in the process, but the information it contains is transformed into the atoms’ superposition state. After a certain amount of time, a brief pulse of the control laser can read out that information and convert it back into a photon.

Reducing read-out noise

“Until now, a critical point has been noise — extra light produced during the read-out that can compromise the quality of the photon,” says Roberto Mottola, another Ph.D. student in Treutlein’s lab. Using a few tricks, the physicists were able to reduce the noise to the point where the single-photon quality remained high even after storage times of several hundred nanoseconds.

“Those storage times aren’t very long, and we didn’t optimize them for this study,” Treutlein explains, “but they’re already more than a hundred times longer than the duration of the stored single-photon pulse.” This means that the Basel researchers’ quantum memory can already be used for interesting applications. It can, for example, synchronize randomly generated single photons, which can then be used in a variety of quantum information applications.

Source: Materials provided by the University of Basel. 

Reference: DOI: 10.1103/PRXQuantum.3.020349

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