Quantum frequency conversion, crucial for a global quantum Internet, is being advanced by the HiFi project, which tackles wavelength mismatches and stabilizes quantum communications. Credit: SciTechDaily.com
Fraunhofer IAF achieves record output power with VECSEL for quantum frequency converters.
The expansion of fiber optics is progressing worldwide, which not only increases the bandwidth of conventional Internet connections, but also brings closer the realization of a global quantum Internet. The quantum internet can help to fully exploit the potential of certain technologies. These include much more powerful 
VECSEL setup for the development of a low-noise pump source for quantum frequency conversion. Credit: © Fraunhofer IAF
Project HiFi: Enabling technologies for quantum frequency conversion
In the joint project “HiFi — Highly integrated quantum frequency converter of highest fidelity based on innovative laser, fiber and production technology” funded by the German Federal Ministry of Education and Research (BMBF), researchers are working on the realization of all necessary technologies to provide quantum frequency converters (QFK) with high efficiency and low noise for initial test tracks. The Fraunhofer Institute for Applied Solid State Physics IAF has contributed to the project with the successful development of disk lasers (also known as vertical-external-cavity surface-emitting lasers, VECSELs) based on gallium antimonide (GaSb). These are optically pumped, surface-emitting semiconductor lasers with an external resonator and intracavity filter for wavelength selection.
Single-mode VECSEL module with up to 2.4 W output power for the frequency range between 1.9 and 2.5 µm, developed as a pump source for quantum frequency converters. Credit: © Fraunhofer IAF
2.4 W output power with absolute frequency stability below 100 kHz
“The VECSELs we developed as part of HiFi are spectrally narrow-band pump sources which, depending on the output wavelength of the qubits used, specifically cover a wavelength between 1.9 and 2.5 µm and achieve an output power of up to 2.4 W with an absolute wavelength stability of less than 2 fm. This corresponds to a frequency stability of less than 100 kHz and clearly falls below the frequency stability class 1E-9. The result represents an international record for this type of laser,” explains Dr. Marcel Rattunde, HiFi sub-project coordinator and head of the optoelectronics department at Fraunhofer IAF.
“The result was made possible by the close cooperation with project partner MENLO Systems GmbH. Together, we locked the disk laser to a frequency comb, which in turn was coupled to a 10 MHz reference,” emphasizes Rattunde.
In their experiments, the researchers set the emission wavelength exactly to the target wavelength for demonstration experiments at the fiber link of Saarland University (2062.40 nm), to which Fraunhofer IAF has handed over the laser module. In addition to power scaling, the most important research tasks of Fraunhofer IAF in the HiFi project are the precise understanding of the mode behavior of the lasers and the identification and elimination of noise sources.
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