Project HiFi Addresses 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 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. Courtesy of Fraunhofer IAF.

“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,” said Marcel Rattunde, HiFi sub-project coordinator and head of the optoelectronics department at Fraunhofer IAF.

The result, he said, an international record for this type of laser, was made possible through collaboration with MENLO Systems. “Together, we locked the disk laser to a frequency comb, which in turn was coupled to a 10 MHz reference,” Rattunde said.

In their experiments, the researchers set the emission wavelength exactly to the target wavelength for demonstration experiments at the fiber link of Saarland University (2062.4 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.

VECSEL setup for the development of a low-noise pump source for quantum frequency conversion. Courtesy of Fraunhofer IAF.

In quantum frequency conversion, the energy of the pump photon is subtracted from the signal photon by a difference frequency process in a non-linear optical crystal. To ensure a low-noise process, the energy of the pump photons must be below the target wavelength (usually 1550 nm), otherwise the pump laser can generate photons in the output signal due to parasitic effects.

In combination with the MENLO frequency comb, the VECSELs developed at Fraunhofer IAF meet the high requirements of quantum frequency conversion, as their narrow bandwidth and wavelength stability prevent fluctuations in the pump wavelength and consequently changes in the target wavelength of the qubits. If there is a deviation above the natural linewidth, the qubits would no longer be indistinguishable, which would eliminate a basic requirement for subsequent quantum mechanical processing.

The research is being presented at SPIE Photonics Europe in Strasbourg, April 7-11.