Quantum Technology · Structural Biology
Every drug that works in your body first has to fit a protein's exact three-dimensional shape — but actually seeing that shape, one molecule at a time, has always been close to impossible.
Physicists at the University of Waterloo just built a sensor small enough to try.
Researchers at the Institute for Quantum Computing turned a single organic molecule into its own quantum sensor, then read it out mechanically instead of optically — extending its useful quantum lifetime 60-fold and opening a real path toward imaging individual protein structures instead of averaging over millions of molecules at once.
Dr. Raffi Budakian beside the low-temperature scanning force microscope used to detect the molecular spin sensor. Credit: University of Waterloo.
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A team at the University of Waterloo's Institute for Quantum Computing has shown, for the first time, that a single organic molecule can act as its own quantum sensor for imaging biomolecules. This article walks through why single-molecule imaging has resisted every conventional technique, how the team's molecular sensor works and how it differs from the diamond-based sensors that came before it, the coherence breakthrough that made the measurement possible, and what it could mean for drug discovery once the method reaches its ultimate goal: sensing one molecule at a time.
Start Here
1. A New Way to See a Single MoleculeUnderstand
2. Why Seeing One Molecule at a Time Is So Hard 3. What a Quantum Sensor Actually Measures 4. Diamond's Long Reign in Quantum Sensing 5. Meet Trityl-OX063: A Molecule Built to Be a SensorEvaluate
6. Listening for Spins Through Vibration, Not Light 7. Buying Time: The Coherence Breakthrough 8. From Ten Spins to OneFinal Take
11. Manohar InsightA New Way to See a Single Molecule
Proteins are the machines that carry out almost everything a living cell does, and how well a drug works usually comes down to how precisely it fits the shape of the protein it's meant to act on. Understanding that shape, in fine detail, is one of the foundational problems of modern biology and medicine.
Physicists at the Institute for Quantum Computing (IQC) at the University of Waterloo have now demonstrated a new way to approach that problem: using a single molecule, tuned to behave as a quantum sensor, to measure the magnetic environment around individual atoms with extraordinary precision.
This is the first time this particular class of molecule has been used this way, and the results, published in Physical Review X, describe a technique the research team believes puts single-molecule structural imaging within genuine reach for the first time.
Why Seeing One Molecule at a Time Is So Hard
Most techniques used to determine the structure of proteins today, including X-ray crystallography and cryo-electron microscopy, are remarkably powerful, but they share one important limitation: they need a bulk sample containing millions of copies of the same molecule, all averaged together to produce a single structural picture.
That averaging works well for molecules that behave uniformly, but it quietly erases something important. Individual protein molecules don't all sit frozen in one identical shape; many flex, bend, and shift between subtly different configurations, and those variations can matter enormously for how a protein actually functions or how a drug interacts with it.
A technique sensitive enough to measure a single molecule, rather than an averaged crowd of them, could reveal that hidden variation directly — but getting a sensor small enough, and precise enough, to do that has remained a persistent challenge in the field.
What a Quantum Sensor Actually Measures
Quantum sensors don't work like ordinary measuring instruments. Instead of directly detecting light, heat, or force in the everyday sense, they exploit delicate quantum-mechanical properties, such as spin, that are extraordinarily sensitive to their immediate surroundings.
In this context, spin refers to a quantum property of atoms and electrons that behaves like a tiny internal magnet. Every atom's spin has its own characteristic resonance frequency, and when two atoms sit close enough together, their spins subtly influence one another through magnetic coupling.
A quantum sensor is essentially built to detect that subtle magnetic influence. By placing a spin with well-understood, controllable properties near a target of interest and carefully tracking how that spin's behavior shifts, researchers can infer detailed information about the magnetic environment around it — and, by extension, about the atomic structure producing that environment.
Diamond's Long Reign in Quantum Sensing
Until now, the most well-established platform for this kind of quantum sensing has been the nitrogen-vacancy, or NV, center: a specific type of atomic-scale defect engineered into synthetic diamond. NV-center sensors have driven major advances in nanoscale magnetic imaging over the past two decades, and they remain a workhorse technology in the field.
Their readout method is optical — researchers shine laser light onto the diamond defect and interpret the light it emits in response to infer the sensor's quantum state. That approach is well understood and highly refined, but it comes with a practical constraint: the diamond housing the defect has a physical size, which limits how close the sensor itself can actually get to whatever it's trying to measure.
Since sensitivity in this kind of measurement falls off sharply with distance, that limitation matters a great deal. It's part of why researchers have long been interested in finding an alternative sensor that could be brought closer to its target without sacrificing the delicate quantum properties that make the whole technique work.
Meet Trityl-OX063: A Molecule Built to Be a Sensor
The team, led by physics professor Raffi Budakian, took a different route entirely. Rather than engineering a defect into a crystal, they used an existing class of stable organic molecules called trityl-OX063 — already familiar to chemists, but never before pressed into service as a quantum sensor in this way.
Each trityl-OX063 molecule carries a single unpaired electron sitting at its center, surrounded by three sulfur-substituted rings that shield it from magnetic noise in its surroundings. That shielding is what makes the molecule useful: it protects the electron's fragile quantum spin state from the electron's own environment, while still leaving it sensitive enough to pick up the faint magnetic fields produced by nearby atomic nuclei.
Because it's a molecule rather than a solid-state defect, it can also be chemically attached to a specific site on a target protein, giving researchers a level of targeting precision that a diamond sensor, bound to its crystal, simply can't offer. Compared with the diamond-based approach, the team's molecular sensor can also be positioned significantly closer to whatever it's measuring, which directly improves its sensitivity.
Listening for Spins Through Vibration, Not Light
Perhaps the most distinctive part of the Waterloo team's approach is how they chose to read the sensor out. Instead of using light, as diamond-based sensors do, their platform detects the molecule's spin state mechanically — through vibration rather than photons.
As nearby atomic nuclei generate shifting magnetic fields, those fields alter the evolution of the trityl-OX063 molecule's electron spin. That change in the electron spin, in turn, produces a minuscule mechanical force, and it's that force the researchers actually measure, using custom-built silicon nanowire probes roughly 100 nanometers across, about the width of a virus, and 20 microns long, close to the diameter of a human hair.
Mechanical, force-based detection like this isn't entirely new to physics — it has roots in a technique called magnetic resonance force microscopy — but applying it to a single molecular spin sensor of this kind, with this level of precision, had not been demonstrated before this work.
PhD candidates Sahand Tabatabaei and Pritam Priyadarsi beside the low-temperature scanning force microscope used to take the experiment's measurements. Credit: University of Waterloo.
Buying Time: The Coherence Breakthrough
Getting the physics to work in principle was only part of the challenge. Quantum states are famously fragile, and a spin sensor is only useful for as long as it stays coherent — meaning it maintains its delicate quantum properties before environmental noise scrambles them beyond use.
Sahand Tabatabaei, a PhD candidate at IQC and the paper's first author, developed a new control sequence specifically to extend that window. The result was a coherence time of 400 microseconds — a number that sounds tiny, but represents roughly 60 times longer than what standard spin-echo techniques typically achieve in the same experimental system.
That improvement matters because a longer coherence time gives the sensor more time to accumulate a usable signal before its quantum state degrades, directly translating into a more sensitive, more reliable measurement. Without it, the sensor simply wouldn't hold its quantum information long enough to be practically useful.
From Ten Spins to One
In its current form, the sensor can already detect the collective magnetic state of roughly ten nearby nuclear spins — a genuinely difficult measurement in its own right, and a substantial improvement over prior molecular approaches.
Budakian described the milestone in direct terms: "Our experiment establishes a new paradigm in nanoscale quantum sensing." He was equally clear, though, that reaching single-spin sensitivity, the level of precision genuinely needed to map an individual molecule's structure, still requires further work.
What makes the team optimistic is that the remaining gap is now a matter of engineering refinement rather than an open scientific question. They understand, in principle, what needs to improve — sensitivity, coherence time, and signal isolation — and see a credible path toward closing that gap in the coming years.
What This Could Mean for Drug Discovery and Disease Research
If the technique reaches single-molecule sensitivity, the payoff could be substantial. Many diseases trace back to proteins that misfold, malfunction, or interact abnormally with other molecules in the cell, and those failures often play out differently across individual protein copies rather than uniformly across an entire population.
A sensor capable of resolving single-molecule structural detail could let researchers directly observe that kind of variation, rather than inferring an average behavior from a bulk sample. That distinction matters enormously for understanding how a disease-linked protein actually misbehaves, and for designing a drug precise enough to correct it.
Because the molecular sensor can also be chemically tethered to a specific site on a target protein, the approach could eventually let researchers probe particular regions of interest directly — the exact binding pocket a drug is meant to engage, for instance — rather than only capturing a protein's overall shape.
The Road Still Ahead
It's worth being clear-eyed about what this result is, and isn't. The team has demonstrated a genuinely new sensing paradigm and a large, verified jump in coherence time — but they have not yet imaged a single protein structure, and they say so themselves.
Getting from roughly ten detectable spins down to one is not simply a matter of scaling up an existing dial. It will likely require further advances in coherence protection, mechanical sensitivity, and probably fabrication precision on the nanowire probes themselves, each of which represents real, unresolved engineering work.
There's also the broader question of translating a delicate, low-temperature laboratory demonstration into something closer to a routine structural biology tool. This transition has tripped up promising sensing techniques before. Cautious confidence, rather than certainty, is the appropriate read on the timeline from here.
Manohar Insight
What stands out to me about this work isn't just that a new sensor material was found — it's that the team changed how they were willing to listen to it. For twenty years, quantum sensing largely meant reading spin states with light, because that's what diamond-based platforms were built around. Choosing to read a spin out mechanically instead, through the faint physical force it produces, is a genuinely different way of approaching the same underlying problem.
That shift is what let a fragile organic molecule, of all things, become a viable competitor to one of the most robust materials in physics. Diamond earned its dominance because it's an exceptionally stable host for a quantum state. A soft, chemically delicate molecule succeeding in that same role, largely because it can be read out differently and placed closer to its target, says something important: the sensor material may matter less than how cleverly you extract information from it.
If that principle holds as the field pushes from ten spins toward one, it suggests the eventual breakthrough in single-molecule imaging may come less from finding an even more exotic sensor, and more from continuing to rethink how existing ones are read.
Join the Signal
If quantum sensors eventually let researchers watch individual protein molecules flex and shift in real time instead of averaging millions together, what disease do you think would benefit most from that kind of detail first? Share your thoughts in the comments.
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About the Author
K M Manohar is an independent Sci-Tech writer and research curator who explains breakthrough discoveries that are shaping the future of technology.
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