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Quantum stochastic rectification is a process observed in some physical systems, which entails the conversion of random quantum fluctuations (i.e., quantum noise) and a small oscillating signal, such as a weak alternating current or AC voltage, into a steady output (e.g., a direct current, or DC). This quantum effect has been previously reported in magnetic tunnel junctions that are driven by both quantum mechanics and randomness (i.e., stochastic processes).

Researchers at the University of California–Irvine recently showed that the quantum stochastic rectification observed in individual molecules can be leveraged to study their intrinsic relaxation dynamics. Their approach, outlined in a paper in Â鶹ÒùÔºical Review Letters, could inform the future study of molecular dynamics and advance the measurement of rapid processes that take place in single molecules at the atomic scale.

"A few years ago, I served on a Ph.D. Advancement committee and the graduate student discussed his thesis research involving stochastic processes in nm-scale magnetic tunnel junctions," Wilson Ho, senior author of the paper, told Â鶹ÒùÔº. "The signal in his experiment was affected by the thermal noise and showed a transition when the driving frequency was varied.

"It occurred to me that we should observe a similar effect, but fully quantum mechanical in our scanning tunneling microscopy (STM) probing of a single molecule. I mentioned these ideas to Jiang Yao, graduate student at the time in my group, and our discussion led to the publication of this paper."

The main objective of the recent study by Ho and his students was to successfully observe an inherent quantum randomness (i.e., quantum stochasticity) in a single molecule. To do this, the researchers applied a periodic oscillating voltage to an individual pyrrolidine molecule adsorbed on a copper surface, which interacted with random state switching in the molecule due to quantum effects.

They then observed and measured the molecule's responses to the voltage oscillating frequency, particularly focusing on structural changes (i.e., conformations). This ultimately allowed them to measure how fast the molecule relaxes (i.e., returns to its original state after being disturbed), picking up fast processes that were not picked up using microscopy tools alone.

"We used a home-built, low-temperature (8 K) STM in ultra-high vacuum to measure the rectification current as a transducing signal through a single pyrrolidine, which allowed us to monitor the stochastic, history independent, random quantum transitions between two molecular states and subjected simultaneously to a sinusoidal periodic voltage drive of varied frequencies," explained Ho.

"A Lorentzian-like transition in the frequency response of the rectification current, corresponding to an exponential decay in time, was shown to match the quantum stochastic dynamics, relating the transition frequency to the population relaxation time."

The results gathered by Ho and his colleagues demonstrate that quantum stochastic rectification processes can be leveraged to probe the quantum stochasticity of individual molecules. Using their methods, the researchers were able to probe rapid processes that occurred in a single pyrrolidine molecule at the atomic scale for times too short to be followed by STM electronics.

"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," said Ho. "From a methodological perspective, our frequency-dependent rectification spectroscopy offers a powerful method to probe fast relaxation processes in two-level systems by using a sinusoidal periodic drive that significantly simplifies instrumentation requirements."

In the future, the experimental methods employed by Ho and his colleagues could be used by other research teams to study the dynamics of individual molecules, while also potentially helping to advance quantum technologies by reducing errors resulting from the interaction of quantum states with the surrounding environment. As part of their next studies, the researchers plan to probe single-molecule dynamics on the picosecond scale, by extending their approach to THz frequencies.

"Besides measuring ultrafast processes such as vibrational relaxation and proton motions, 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," added Ho. "These two phenomena often coexist, but current techniques have struggled to probe them simultaneously."

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