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

The development allows them to measure the chemical properties of materials at unprecedented time and spatial resolutions.
Loukia Papadopoulos
3D rendering of a hydrogen molecule.Ackun/iStock

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

Scientists demonstrate the use of a hydrogen molecule as a quantum sensor
In the ultrahigh vacuum of a scanning tunneling microscope, a hydrogen molecule is held between the silver tip and 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 an advance in both the measurement technique and the scientific question the approach allowed us to explore,” said in the press release co-author of the new study Wilson Ho, Donald Bren Professor of physics & astronomy and chemistry.

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

The scientists were able to achieve a superposition of two states through a laser pulse that coaxed the newly-engineered system to go from a ground state to an excited state in a cyclical fashion. Even though the duration of the cyclical oscillations lasted only mere tens of picoseconds, the scientists were still able to see how the hydrogen molecule was interacting with its environment.

A hydrogen molecule merged 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 in between the tip and the sample,” said Ho. “It makes for an extremely sensitive probe, allowing 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 which 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 pump-probe femtosecond terahertz (THz) 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 spatial and femtosecond temporal resolutions were obtained from this quantum coherence. The 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 the copper nitride (Cu2N) monolayer islands grown on a Cu(100) surface. We acquired time-resolved images of THz rectification of H2 over Cu2N islands for variable pump-probe delay times to visualize the heterogeneity of the chemical environment at sub-angstrom scale.

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