Using a molecule as a quantum sensor

A new quantum sensing technique could enable measurements of single protein structures and other important molecules, with potential applications in drug discovery and structural biology. 

The research team from the Institute for Quantum Computing (IQC) at the University of Waterloo developed the new method that uses a single molecule as a quantum sensor. Quantum sensors use unique properties of quantum mechanics to make ultra-precise measurements that traditional sensors cannot achieve.  

This technique has never been demonstrated before and uses a class of molecules for this purpose for the first time. Precise imaging of single molecules helps researchers understand how single proteins and other biomolecules behave. It can help reveal more information about how diseases develop and the ways drugs interact to potentially design more effective treatment. 

The sensors probe the spin, or local magnetic environment of atoms. Because each atom has a unique resonance frequency, if two are close together, they magnetically affect each other.  

The researchers used a class of molecules called trityl-OX063 as the quantum sensor. Its spin is isolated and protected, preserving its quantum properties (University of Waterloo).

“Our experiment establishes a new paradigm in nanoscale quantum sensing,” said Dr. Raffi Budakian, a professor in the Department of Physics and Astronomy as well as faculty at IQC. “There are other milestones to reach before we have a sensor sensitive to single molecules, but we know how to get there, and this is a huge demonstration. We’re not too far away now because of this advancement.” 

A well-established quantum-sensing technique uses synthetic diamonds designed with atomic-scale defects and involves light to read out signals. Budakian’s group used a class of molecules called trityl-OX063 as the quantum sensor. The spin of the sensor is isolated and protected, preserving its quantum properties. And, compared to the diamond sensing technique, it can be placed closer to the target it is sensing to achieve higher sensitivity. 

Their method uses the electron spin of the OX063 molecule. The magnetic fields from nearby nuclear spins change the evolution of that electron spin. The resulting signal is detected mechanically using nanowire probes that are 100 nanometers in diameter, about the size of a virus, and 20 microns long, about the diameter of a human hair.  

“One key challenge was keeping the OX063 spins coherent long enough to make them useful as sensors,” said Sahand Tabatabaei, a PhD candidate at IQC and first author on the paper. “The new control sequence we developed extended the coherence time to 400 microseconds, so it maintained its quantum state roughly 60 times longer than what is achieved with standard spin-echo techniques in the same system.” 

Sahand Tabatabaei and Pritam Priyadarsi, both PhD candidates, stand next to a low temperature scanning force microscope, the instrument that was used for taking experiment measurements (University of Waterloo).

Budakian said the sensor can already detect the magnetic state of about 10 spins. They know how to get to single spins, which is a key step toward mapping the structure of single molecules.  

Current molecular-imaging techniques, while powerful, don’t reveal unique molecular structure because their process requires a larger sample. 

“Using force detection in quantum sensing is new and opens up a whole new avenue of approaches we can take to get us closer to mapping single protein structures,” Budakian said. 

The paper Long-Lived Mechanically-Detected Molecular Spins for Quantum Sensing appears in Physical Review X. 

This project is supported in part by the Canada First Research Excellence Fund through the Transformative Quantum Technologies program.