Hardware for Quantum Networks Ultra-Fast Laser Pulses Elminate the Need for Complex Optical Filters

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Researchers demonstrate a new method for generating individual photons in a diamond-based quantum system. A new laser process based on diamond nanostructures helps to achieve this.

Diamond crystals with specifically introduced defects, so-called tin-vacancy centers (SnV centers), are considered promising building blocks for stable quantum bits (qubits). They can store and process quantum information and transfer it to photons. However, there is a massive hurdle for optoelectronics developers: in order to control these qubits, they have to be excited with light. The single photon emitted by the qubit must then be clearly identified as an information carrier.

In conventional setups, however, there is the problem that the bright light of the exciting control laser can only be separated from the weak individual photons using very complex optical filter techniques. These filters attenuate the signal, drastically reduce the efficiency of the system and stand in the way of scaling the hardware for commercial applications.

Two Laser Pulses Instead of Complex Filters

A research team from Humboldt-Universität zu Berlin and the Ferdinand-Braun-Institut (FBH), in collaboration with TU Dortmund University (Germany), has now demonstrated that this hardware problem can be solved using a new control method. The method is called SUPER and stands for Swing-UP of the Quantum EmitteR Population.

Instead of filtering the light afterwards, SUPER starts at the source: Two specially coordinated, extremely short laser pulses in the femtosecond range excite the quantum system. This ultra-fast optical control—one of the fastest ever demonstrated for diamond-based systems—makes it much easier to separate the control laser from the emitted photons.

Better Signal Quality, More Complex Operations

The researchers involved from HU Berlin also emphasize the great advantage this has for future system design. "With ultrafast pulses, we can control the quantum state on completely new time scales. This opens the way to faster and more complex quantum operations based on diamond," explains Cem Güney Torun, PhD student at the Institute of Physics and one of the two lead authors of the study.

However, the real optoelectronic bottleneck is the yield and signal quality, as his colleague Mustafa Gökçe adds: "Our method enables us to excite the system efficiently while keeping the emitted single photons clean and usable. This is a key prerequisite for building networks for quantum communication."

High Efficiency for Future Quantum Repeaters

Another decisive advantage for future network architectures: the SUPER method preserves the internal quantum spin state of the system. This is absolutely essential in order to create quantum entanglement between remote network nodes.

The combination of modern diamond nanofabrication and ultra-fast laser optics provides solid-state quantum technology with a new tool. The scaling of components such as quantum repeaters, which are required for a lossless quantum internet over long distances, is thus moving a significant step closer from the pure basic laboratory towards practical system integration. (heh)

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