The expansion of fiber optics is making progress around the world, not only increasing the bandwidth of traditional internet connections but also bringing the realization of a global quantum internet closer. The quantum internet could help unlock the full potential of certain technologies. These include more powerful quantum computing through the linking of quantum processors and registers, more secure communications through quantum key distribution, or more precise time measurement through atomic clock synchronization.
However, the difference between the glass fiber standard of 1550 nm and the system wavelengths of the various quantum bits (qubits) implemented so far poses an obstacle, as these qubits are mostly in the visible or near-infrared spectral range. Researchers hope to overcome this obstacle with the help of quantum frequency conversion, which specifically changes the frequency of photons while preserving all other quantum properties. This enables conversion to the 1550 nm telecommunications range for low-loss, long-distance transmission of quantum states.
HiFi Project: Implementation Technology of Quantum Frequency Conversion
In the joint project “HiFi – Highly integrated quantum frequency converters of the highest fidelity based on innovative laser, optical fiber and production technologies” funded by the German Federal Ministry of Education and Research (BMBF), the researchers are working to achieve all the necessary ’s technology delivers high-efficiency, low-noise quantum frequency converters (QFKs) for initial test tracks.
The Fraunhofer Institute for Applied Solid State Physics IAF contributed to the project by successfully developing a gallium antimonide (GaSb)-based disk laser (also known as a vertical external cavity surface emitting laser, VECSEL). These are optically pumped, surface-emitting semiconductor lasers with external resonators and intracavity filters for wavelength selection.
2.4 W output power, absolute frequency stability below 100 kHz
“The VECSELs we developed as part of HiFi are spectrally narrowband pump sources that specifically cover wavelengths between 1.9 and 2.5 µm, depending on the output wavelength of the qubit used, and achieve output powers up to 2.4 W with absolute wavelength stability The stability is less than 2 fm. This corresponds to a frequency stability below 100 kHz, which is significantly lower than the frequency stability class 1E-9. This result represents an international record for this type of laser.” HiFi Subproject Coordinator and Fraunhofer IAF Optoelectronics Department explains the person in charge, Dr. Marcel Rattunde.
“This result was achieved in close collaboration 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 the experiment, the researchers set the emission wavelength precisely to the target wavelength for demonstration experiments at the Saarland University fiber link (2062.40 nm), to which the Fraunhofer IAF has handed over the laser module. In addition to power scaling, the most important research tasks of Fraunhofer IAF in HiFi projects are the precise understanding of the modal behavior of lasers and the identification and elimination of noise sources.
Quantum frequency conversion using pump lasers
In quantum frequency conversion, the energy of the pump photon is subtracted from the signal photon through a difference frequency process in a nonlinear 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 will produce photons in the output signal due to parasitic effects.
In combination with the MENLO frequency comb, the VECSELs developed by the Fraunhofer IAF meet the high requirements of quantum frequency conversion because their narrow bandwidth and wavelength stability prevent fluctuations in the pump wavelength and thus the target wavelength of the qubit. Variety. If there is a deviation above the natural linewidth, the qubits will no longer be indistinguishable, which would eliminate an essential requirement for subsequent quantum mechanical treatment.