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Researchers at the Institute for Quantum Computing (IQC) at the University of Waterloo have developed a quantum sensing technique that utilises a single molecule to map the magnetic environments of nearby atoms
Published in Physical Review X, the experiment establishes a new paradigm in nanoscale sensing that could eventually enable scientists to map single protein structures for drug discovery and structural biology.
Led by Dr Raffi Budakian, a professor in the Department of Physics and Astronomy, the team demonstrated a mechanically detected molecular spin sensor capable of measuring ultra-weak local magnetic interactions.
Overcoming the Limits of Diamond Quantum Sensors
Existing nanoscale quantum sensors often rely on synthetic diamonds engineered with atomic defects (such as nitrogen-vacancy centres) and read out using optical methods. While effective, these diamond-based sensors have physical limitations:
Proximity constraints:
Diamond sensors cannot always be placed close enough to a target biomolecule, limiting spatial resolution.
The molecular advantage:
The Waterloo team used trityl-OX063, a class of synthetic organic molecules, as the quantum sensor. Because it is a discrete molecule, it can be positioned significantly closer to target samples than solid-state diamond defects, maximising sensitivity.
How the molecular sensor works
The technique relies on tracking the electron spin of the trityl-OX063 molecule, which changes state in response to magnetic fields generated by nearby atomic nuclear spins:
Nanowire force detection:
To read out the signal, researchers used custom mechanical nanowires measuring 100 nanometers in diameter and 20 microns in length. These ultra-thin probes mechanically register the minute forces exerted by the changing electron spin states.
Extending coherence time:
A major hurdle in molecular quantum sensing is maintaining quantum coherence before environmental noise disrupts the measurement. Lead author Sahand Tabatabaei developed a control sequence that extended the sensor’s coherence time to 400 microseconds, roughly 60 times longer than standard spin-echo techniques in the same system.
Path toward single-protein mapping
Conventional structural imaging techniques require bulk biological samples consisting of millions of molecules, which masks individual structural variations.
The Waterloo team’s molecular quantum sensor can currently detect the magnetic state of about 10 nuclear spins. With the path to single-spin sensitivity now mapped out, the approach offers a viable route toward imaging individual protein structures and observing how drugs interact with biomolecules at the single-molecule scale.
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