Quantum Research Advances in Molecular Coherence and Network Limits

Researchers have achieved significant milestones in quantum physics, addressing both the stability of quantum emitters and the limitations of long-distance communication networks.

At the Max Planck Institute for the Science of Light, a team led by Vahid Sandoghdar reached the ultimate quantum limit of optical coherence using individual molecules on a surface. By utilizing a sublimation cleaning technique within a cryogenic vacuum chamber, the researchers removed surface contaminants that typically disrupt delicate quantum states. This allows molecules on a prepared crystal surface to maintain quantum states for the maximum physically permitted duration, facilitating better manipulation via scanning tunneling microscopy.

Simultaneously, researchers at the University of Waterloo identified a fundamental limitation in long-distance quantum communication. Their study demonstrates that quantum repeater chains relying solely on Gaussian operations, homodyne measurements, and classical communication cannot outperform direct transmission through lossy optical channels, such as fiber optic cables. To analyze these constraints, the team introduced a new mathematical framework termed fractional extendibility for Gaussian states, which provides a method for evaluating the capabilities and inherent limits of Gaussian quantum networks.