Scientists demonstrate the use of a hydrogen molecule as a quantum sensor

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What if we could use a hydrogen molecule as a quantum sensor in a scanning tunneling microscope equipped with a terahertz laser? This would allow us to measure the chemical properties of materials at unprecedented temporal and spatial resolutions.

This new technique has now been developed by physicists at the University of California, Irvine, according to a statement released by the institution on Friday.

In the ultra-high vacuum of a scanning tunneling microscope, a hydrogen molecule is held between the silver tip and the sample. Femtosecond bursts of a terahertz laser excite the molecule, turning it into a quantum sensor. Source: Wilson Ho Lab, UCI

A much more sensitive quantum microscope

“This project represents a breakthrough both in the measurement technique and in the scientific question that the approach allowed us to explore,” said co-author of the new study Wilson Ho, Professor Donald Bren of physics, astronomy and chemistry.

“A quantum microscope that relies on exploring the coherent superposition of states in a two-level system is much more sensitive than existing instruments that are not based on this principle of quantum physics.”

Scientists were able to achieve a superposition of two states through a laser pulse that caused the newly designed system to transition from a ground state to an excited state in a cyclic fashion. Even though the duration of the cyclic oscillations lasted only a few tens of picoseconds, the scientists were able to see how the hydrogen molecule interacted with its environment.

A fused hydrogen molecule with the quantum microscope

“The hydrogen molecule became part of the quantum microscope in the sense that wherever the microscope scanned, the hydrogen was there between the tip and the sample,” Ho said. “It’s an extremely sensitive probe, which allows us to see variations down to 0.1 Angstroms. At this resolution, we could see how the charge distributions change on the sample.

Ho further added that this experiment represents the first demonstration of chemically sensitive spectroscopy based on terahertz-induced rectification current through a single molecule. The new technique can now be applied to the analysis of two-dimensional materials that could be used in advanced energy systems, electronics and even quantum computers.

This study was published in the journal Science.

Abstract:
A scanning tunneling microscope (STM) combined with a femtosecond terahertz (THz) pump-probe laser can enable coherence measurements of single molecules. We report THz pump-probe measurements that demonstrate quantum sensing based on a hydrogen (H2) molecule in the cavity created with an STM tip near a surface. Atomic-scale femtosecond spatial and temporal resolutions have been obtained from this quantum coherence. H2 acts as a two-level system, with its coherent superposition exhibiting extreme sensitivity to the applied electric field and the underlying atomic composition of monolayer islands of copper nitride (Cu2N) grown on a Cu(100) surface. We acquired time-resolved images of the THz rectification of H2 on Cu2N islands for varying pump-probe delay times to visualize the heterogeneity of the chemical environment at the sub-angstrom scale.


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