Speaker: Wenlu Shi

Host: Chandan Setty

Title: Quantum Tunneling and Stochastic Dynamics of Single Molecules

Abstract: Quantum tunneling is a fundamental phenomenon of quantum mechanics, enabling particles to cross energy barriers that are classically forbidden. It plays a central role in phenomena as diverse as superconductivity, magnetoresistance, proton transfer in chemistry, and enzymatic catalysis in biology. Beyond its role in physical, chemical, and biological processes, tunneling between two states in a double-well potential also serves as a model for two-level dynamics that underpin qubit operation in quantum computation and sensing. While electrons and light-atom tunneling are well established, the probability of quantum tunneling decreases exponentially with mass, making direct observation of large-scale atomic tunneling exceedingly rare. Conformational changes in molecules, which are the structural rearrangements that underpin reactivity, catalysis, and recognition, involve the collective motion of many atoms. Whether such structural transitions can proceed by tunneling has remained unresolved, limiting our understanding of molecular dynamics across physics, chemistry, and biology. Here, we used variable-temperature scanning tunneling microscopy to directly observe conformational transition of a single pyrrolidine molecule on Cu(001). By combining temperature- and bias-dependent measurements with isotopic substitution, we identify a crossover from thermally activated transitions to mass-sensitive tunneling dynamics at cryogenic temperatures. Our results provide experimental evidence of intramolecular tunneling involving multiple heavy atoms, overturning the view that tunneling is confined to electrons or protons.  This work extends the concept of tunneling in double-well potentials, which has long served as a model for two-level dynamics in quantum computation and sensing, to structural motions in complex molecules. By connecting large-mass molecular tunneling with the general framework of two-level quantum systems, this work establishes a platform for probing quantum effects in complex molecular systems and suggests new routes by which quantum mechanics may be harnessed for both chemical reactivity and quantum technology.

Reference: [1] J. Yao, S. Chen, W. Shi, and W. Ho, Quantum Stochastic Rectification in a Single Molecule, Phys. Rev. Lett. 135, (2025). [2] J. Yao, Y. Park, W. Shi, S. Chen, and W. Ho, Origin of photoinduced DC current and two-level population dynamics in a single molecule, Sci. Adv. 10, eadk9211 (2024). [3] W. Shi, J. Zeng, K. Kumar, and W. Ho, Single-Molecule Conformational Change Induced by Quantum Tunneling of Large Mass (in preparation)

Bio: Wenlu Shi received his B.S. in Physics in 2018 from Nankai University, China, and his Ph.D. in Physics in 2025 from the University of California, Irvine.