A team of researchers at the University of California–Irvine has developed a novel technique to measure ultrafast relaxation processes in individual molecules. Their findings, published on July 28, 2025, in the journal Physical Review Letters, explore quantum stochastic rectification, a phenomenon where random quantum fluctuations convert an oscillating signal into a steady output. This breakthrough could significantly advance the understanding of molecular dynamics at the atomic level.
Quantum stochastic rectification has been previously observed in systems like magnetic tunnel junctions, where it is driven by both quantum mechanics and randomness. The team, led by Wilson Ho, aimed to replicate and leverage this effect on a single molecule, specifically focusing on a pyrrolidine molecule interacting with a copper surface. By applying a periodic oscillating voltage, researchers were able to investigate the intrinsic relaxation dynamics of the molecule, revealing insights that traditional microscopy techniques could not capture.
In a detailed explanation, Ho noted, “We used a home-built, low-temperature (8 K) scanning tunneling microscope in ultra-high vacuum to measure the rectification current as a transducing signal through a single pyrrolidine.” This setup allowed them to monitor random quantum transitions between two molecular states, while simultaneously applying a sinusoidal periodic voltage at varied frequencies.
The study’s primary objective was to observe quantum randomness in the molecule. The results indicated a Lorentzian-like transition in the frequency response of the rectification current. This transition correlated with an exponential decay in time, linking the frequency to the population relaxation time of the molecule.
The implications of this research extend beyond theoretical understanding. The ability to probe rapid processes in single molecules could have practical applications in quantum computing and other quantum technologies.
“Understanding how random quantum noise can enhance signals by modulating with a sinusoidal periodic drive could potentially help to combat environmentally induced errors for quantum devices,”
Ho emphasized.
Looking ahead, the researchers plan to extend their methods to study single-molecule dynamics at the picosecond scale by utilizing terahertz frequencies. This approach could facilitate the examination of ultrafast processes such as vibrational relaxation and proton motions. Ho added, “Our method of probing single molecules could reveal the relation between stochasticity and coherence, which is a fundamental yet largely unexplored aspect of quantum systems.”
The findings from this study not only pave the way for future research into molecular dynamics but also provide a powerful tool for other research teams. By advancing the methodology for investigating quantum stochasticity in single molecules, the team is contributing to the broader field of quantum technology, where minimizing errors is critical for practical applications.
As the research community continues to explore these groundbreaking findings, scientists anticipate that this innovative technique will yield significant insights into the behavior of molecules at the quantum level, opening new avenues for exploration in both fundamental and applied sciences.
