Nobelpreiskolloquium
Prof. Dr. Philip Moll
Max Planck lnstitut für Struktur und Dynamik der Materie, Hamburg

"Quantum mechanics goes big"

The 2025 Nobel prize recognized outstanding work on quantum mechanical behavior in macroscopic objects. Generically, quantum mechanics describes the world of the small. Its invention solved outstanding puzzles of physics such as atomic spectra, yet it evidently does not capture the classical world we live in. Generally, when macroscopic numbers of quantum objects interact, they lose their coherence and with it the ability to interfere - the key distinction between wave and particle physics. John Clarke, Michel Devoret and John Martinis showed that macroscopic superconducting currents, flowing in circuitry well visible by eye, can still be fully coherent quantum objects. In particular, the system can tunnel between two states in classically forbidden ways. This well-known phenomena was well known for single particles such as individual electrons tunneling through a barrier in semiconductors. Here, macroscopic numbers of electrons forming the superconducting condensate tunnel between two states at once - which was believed to be impossible at the time. This marked the onset of macroscopically phase coherent phenomena in superconducting structures, formed the basis of many quantum computing architectures today and posed conceptual questions about long-range coherence in manybody systems. Our research is active in this area, as we search for new phase coherent states in metals besides superconductivity - hoping to elevate the operation temperature of phase coherent behavior in applications. Here a recent discovery of long-ranged coherence oscillations in the Kagome family CsV3Sb5 is an interesting new step along the path started by Clarke, Devoret and Martinis.