In the basements of Munich's research institutes, a new kind of signal is traveling through superconducting cables at temperatures colder than outer space. Physicists at the Walther-Meißner-Institute and Technical University of Munich have achieved something previously thought nearly impossible: they've successfully teleported quantum microwave states between separate cooling chambers, carrying the delicate quantum properties over distances that shatter previous constraints.
The breakthrough matters because quantum networks—interconnected machines that share quantum information—could one day become the backbone of unhackable communication systems or even a quantum version of the internet. Building these networks requires solving a fundamental problem: quantum signals are fragile creatures, easily destroyed by heat and thermal noise. Until now, quantum microwave systems were trapped inside individual cooling machines called dilution refrigerators, with no practical way to connect them.
The Munich team developed what they call a cryogenic link, essentially a bridge of superconducting cables connecting multiple dilution refrigerators. The key innovation was demonstrating that quantum entanglement—the spooky quantum property that Einstein famously doubted—could survive propagation through superconducting coaxial cables at temperatures up to 4 Kelvin, a temperature still cold enough to liquefy helium but warmer than previous systems required. The research, published in Physical Review Letters as part of the Quantum Microwave Communication and Sensing project, proves that what seemed theoretically impossible could work in practice.
"One would naively expect quantum entanglement at frequencies of around 5 GHz to vanish during the propagation through the thermal channel at 4 Kelvin with corresponding dozens of thermal photons," explained Wun K. Yam, the Ph.D. student who led the experimental work. Yet it didn't vanish. The reason lies in elegant physics: while the superconducting cables do carry random thermal fluctuations, these fluctuations only corrupt the quantum signal if they're absorbed into it through losses. By using extremely high-quality niobium-titanium superconducting cables with losses on the order of decibels per meter—nearly negligible—the team preserved the quantum information as it traveled.
Kirill G. Fedorov, the group leader who conceived the approach, framed the achievement in historical terms: "Our paper was inspired by an original idea to demonstrate that microwave communication can operate in the quantum regime and over macroscopic distances, beyond the constraints of an mm-size chip." The team built a unique prototype system of three cryostats—two dilution refrigerators reaching temperatures around 20 to 50 milliKelvin and an intermediate cold node at approximately 3 Kelvin—all connected by multilayered superconducting cables.
The physics underlying their success is something Fedorov called the fluctuation-dissipation theorem, a principle that has guided quantum mechanics for decades but had never been demonstrated this way in a practical network. In essence: quantum information survives in channels where noise exists as long as that noise cannot leak into the signal. The Munich researchers proved this isn't merely theoretical—it's engineerable.
This work opens a path toward what researchers call Q-LAN: quantum local area networks where superconducting quantum computers separated by significant distances could finally talk to each other, sharing the power of quantum information without degradation. The vision of large-scale quantum networks, once confined to laboratory daydreams, just moved a significant step closer to reality.