New method could link quantum computers over long distances ^{CEFR B2}

Researchers at the University of Chicago propose a theoretical extension of quantum links that could let quantum computers share information across unprecedented distances. The new paper by Assistant Professor Tian Zhong in Nature Communications argues that with improved quantum coherence times, fiber connections could reach about 2,000 km (1,243 miles), far beyond the previous practical range of a few kilometres.

The team achieved the improvement by increasing coherence of individual erbium atoms from roughly 0.1 milliseconds to more than 10 milliseconds, and in one experiment to 24 milliseconds. In principle, that longer coherence would permit links up to about 4,000 km, a distance the paper cites from UChicago PME to Ocaña, Colombia. Rather than introducing new substances, the researchers changed the crystal-growth technique: they adapted molecular-beam epitaxy (MBE) for rare-earth doped crystals instead of using the high-temperature Czochralski melt-and-cool process. MBE builds thin layers and produced material with very high purity and strong coherence properties, with help from UChicago PME Asst. Prof. Shuolong Yang.

Outside specialists welcomed the result. Institute of Photonic Sciences Professor Hugues de Riedmatten called the approach highly innovative and said it offers a scalable route to produce many networkable qubits in a fiber-compatible device. Next steps are experimental: Zhong plans to test connectivity by linking two qubits in separate dilution refrigerators through 1,000 kilometres of spooled cable and to build a third fridge to form a local network and simulate a future long-distance link.