Berlin research moves closer to the diamond-based quantum internet

Brain City Berlin

What if the internet of the future were completely secure against eavesdropping? That is the promise of the quantum internet: a global network based on the laws of quantum physics that transmits information in a way that makes interception fundamentally impossible. Researchers at Humboldt-Universität zu Berlin and the Ferdinand-Braun-Institut in Brain City Berlin have now taken an important step in that direction.

In today's internet, data is transmitted as classical bits: ones and zeros. In the quantum internet, individual particles of light, known as photons, take on this role. They can carry quantum information and make use of a property called quantum entanglement: two entangled particles influence each other regardless of the distance between them. This makes transmission not only faster but also fundamentally secure against eavesdropping. Such infrastructure could become critically important in the future for fields such as medicine, finance and government communications.

The challenge: Generating individual photons in a controlled way

Making the quantum internet a reality requires reliable sources of individual photons. That sounds simpler than it is. Until now, it has been difficult to cleanly separate the control signals, the laser pulses that excite the system, from the actual information carriers, the emitted photons. Conventional methods require complex filtering technology that reduces the efficiency of the system.

A research team led by Prof. Dr Tim Schröder at HU Berlin has now demonstrated, together with colleagues at Technische Universität Dortmund, a more elegant solution: the SUPER method (Swing-UP of the quantum EmitteR population). Two precisely coordinated laser pulses in the femtosecond range, a quadrillionth of a second, excite a diamond-based quantum system. The result: photons can be generated more cleanly and efficiently, without complex filtering. The study has been published in the renowned journal Nature Communications.

Why diamond?

The researchers use diamond crystals with deliberately introduced defects in their atomic structure, known as tin-vacancy centres. These tiny structures are remarkably stable and well suited as quantum bits, the smallest units of quantum information.

Doctoral researcher Cem Güney Torun, one of the study's first authors, explains: "With ultrafast pulses, we can control the quantum state on entirely new timescales. This opens the path to faster and more complex quantum operations based on diamond." His co-first author Mustafa Gökçe adds: "Our method allows us to excite the system efficiently while keeping the emitted single photons clean and usable. This is a central prerequisite for building networks for quantum communication."

Brain City Berlin as a location for quantum research

The study is the result of close collaboration between the Integrated Quantum Photonics research group at the Institute of Physics at HU Berlin and the Joint Lab Diamond Nanophotonics at the Ferdinand-Braun-Institut, Leibniz Institute for High Frequency Technology, also in Berlin. The fact that both institutions are located close to each other and are closely connected within the Berlin research network is no coincidence: Brain City Berlin, with its strong universities, non-university research institutions and targeted support for collaboration, provides precisely the conditions in which breakthroughs like this can emerge. The quantum internet is still some way off, but in Berlin, intensive work is already under way to make it a reality.

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