A Single Molecule Quantum Sensor That Sees Inside Individual Proteins

A modified XYXY dipolar decoupling sequence. This new control sequence suppresses electron–electron dipolar interactions across a broad range of control fields, extending coherence times to ~400 microseconds — roughly 60 times longer than standard spin-echo techniques in the same system . "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 .

Nanowire-based mechanical readout. Instead of the optical readout used in diamond NV-center sensors, the SQUINT platform detects quantum signals mechanically. The nanowire probes are 100 nm in diameter (about the size of a virus) and 20 microns long (about the diameter of a human hair) . This mechanical detection approach is entirely new for quantum sensing and allows the sensor to be placed much closer to target molecules than diamond-based sensors can achieve .

Implications for Single-Protein Imaging and Drug Discovery

The ability to locate a sensor molecule directly adjacent to a target — without being locked into a crystal lattice like diamond NV centers — gives trityl-OX063 a fundamental proximity advantage . The researchers envision several powerful applications:

• Single-protein structure mapping. Current molecular imaging techniques require large, averaged samples and cannot reveal the unique structure of a single protein. This technique could measure the magnetic environment of individual atoms within a single protein, enabling direct structural mapping at the molecular level .
• Drug discovery. Precise imaging of single proteins could reveal how diseases develop at the molecular scale and how drug candidates interact with their targets, potentially enabling the design of more effective treatments .
• Proximity advantage. Because trityl-OX063 molecules are not locked into a crystal lattice, they can be chemically attached or positioned within a few nanometers of the target molecule, offering molecular-level control over sensor placement that existing platforms cannot match .

Current Status and Roadmap

The SQUINT sensor can already detect the magnetic state of about 10 nuclear spins . Dr. Raffi Budakian, who led the research, said the team has a clear path to reaching single-spin sensitivity — the critical step needed to map the structure of single molecules . The work represents a practical step toward using quantum sensors to answer questions in biology and medicine that have been out of reach with classical techniques.