The Quantum Nose
On Quantum Tunneling in Olfaction
My dad has always been a deeply curious person. Curious about archeology, art, history, space, and physics. So when he watched "Let There Be Life," the second episode of Jim Al-Khalili's BBC series The Secrets of Quantum Physics, he of course shared it with me .
He knows I'm obsessed with smell. He's obsessed with physics. The Venn diagram of our interests doesn't always overlap, but when it does, really interesting conversations happen. Like this. The idea that quantum mechanics, the framework physicists use to describe the behavior of subatomic particles, might help explain how the nose works. That the act of smelling something could involve electrons tunneling through energy barriers at the quantum scale. That your nose might, in some sense, be listening to molecules rather than recognizing their shapes.
I know my side of this better than I know his. I’m familiar with how contested the theory he was describing actually is in olfactory research. I’ve worked with scientists who are deeply skeptical of it. But I realized I didn’t understand the quantum side well enough to know what I actually thought, or to be able to explain any of it clearly to you. So I decided to do what I usually do when I don’t understand something: read about it and work through it here, in public, to see where it leads. So let’s start at the beginning.
What We Think We Know About Smell
For most of the history of olfactory science, the dominant explanation for how the nose works has been some version of shape. The idea is that an odor molecule drifts into your nose, encounters a receptor, and if its shape of the molecule matches the receptor’s binding site, something clicks. Signal sent. Brain interprets. You smell coffee.
This is known as the lock-and-key model, and it’s been the foundation of olfactory science since Linda Buck and Richard Axel won the Nobel Prize for their work on odorant receptors. I wrote about a significant 2025 update to this picture in an earlier piece, where a team at the University of Geneva managed to map far more receptor-ligand pairs than had ever been achieved before. That research reinforced the shape-based framework while adding important nuance about how primary receptors drive core scent perception.
But shape theory has a problem it has never quite solved. Some molecules with nearly identical shapes smell completely different. Ethanol smells like vodka. Ethanethiol, which has almost the same molecular structure, smells like rotten eggs. The lock-and-key model struggles to explain why two keys that look the same open such different doors. That gap is what makes room for alternative theories. And the most ambitious of those alternatives, the vibrational theory, is the one that involves quantum physics.
A Detour into the Quantum World
I want to be honest before I go further. Quantum physics is not my native language. I understand enough to follow the broad shape of what’s being said, not enough to argue the theories. What I’m going to offer here is my best attempt at a working understanding.
Quantum physics, which describes the behavior of particles at the subatomic scale, operates by different rules than classical physics. One of the strangest of these is quantum tunneling. At the quantum scale, a particle encountering a barrier it doesn’t have enough energy to cross doesn’t simply stop. There is a real, calculable probability that it will appear on the other side anyway. Through the barrier. Not over it.
It’s measurable, predictable, and already built into technologies we use. The scanning tunneling microscope, which gave us the first images of individual atoms, works because of this effect. So does the nuclear fusion happening inside the sun. Without quantum tunneling, protons in the sun’s core wouldn’t have enough energy to fuse, and the sun wouldn’t shine.
What makes quantum biology interesting, and what Al-Khalili’s documentary explores, is the possibility that living systems have evolved to exploit these quantum effects. Photosynthesis may rely on quantum coherence to move energy through cells with unusual efficiency. Migratory birds may navigate using quantum entanglement in their eyes. And, according to a theory that has been generating argument for nearly thirty years, the human nose may detect smell through quantum tunneling.
Malcolm Dyson proposed a version of this theory way back in 1928, suggesting that molecular vibrations, rather than shape, might be what the nose detects. Robert Wright expanded it in the 1950s. Then it was largely set aside, overshadowed by the lock-and-key framework that came to dominate olfactory research through the rest of the twentieth century.
In 1996, a biophysicist and perfumer named Luca Turin revived it, with a specific proposed mechanism. Turin’s version holds that the receptors in your nose detect the vibrations of a molecule’s chemical bonds, using a quantum process to do it. The molecule enters the receptor’s binding site, shape first, fitting like a key. Then an electron inside the receptor attempts to cross from a donor site to an acceptor site. For that crossing to succeed, the electron needs to shed a bit of energy. It can only do that if the molecule sitting in the gap has a vibrational frequency at exactly the right level to absorb it. If the frequencies match, the electron tunnels through, the molecule vibrates, and that vibration is what the receptor registers as a signal. In this model, the nose acts more like an ear reading frequencies. Every molecule has a vibrational fingerprint, a characteristic pattern determined by its atomic composition, and ,Turin argues, that it is this fingerprint you smell.
Turin and colleagues later developed this into the “swipe card” model, a refinement that tries to honor both theories rather than discard one. Shape gets the molecule through the door. Vibration is what actually triggers the signal. The two are not mutually exclusive. They operate at different stages of the same process. This is important, because the vibrational theory is often framed as a direct opponent of shape theory, when the more current version of the argument is really about what happens after the shape fits.
This theory can explain some things that shape theory doesn’t. Ethanol and ethanethiol have similar shapes but smell nothing alike. Their vibrational spectra are quite different. That asymmetry is easier to account for under the vibrational model. Jennifer Brookes, whose 2017 paper in Proceedings of the Royal Society is one of the key academic anchors for this piece, situates olfaction within a broader argument that life has been exploiting quantum effects all along. Enzymes, photosynthesis, magnetic navigation in birds. Olfaction might be one piece of a larger and increasingly credible picture of quantum biology.
The Arguments
Here is where I should point out that I have worked with Leslie Vosshall, a neurobiologist at Rockefeller University who is one of the leading figures in olfactory research and one of the most rigorous and consistent critics of Turin’s theory. Her skepticism is specific, methodological, and worth understanding in detail.
The primary experimental test of the vibrational theory involves isotopes. If Turin is right, then a molecule and its deuterated version, identical in shape but different in vibrational frequency because deuterium is heavier than hydrogen and vibrates more slowly, should smell different. In 2004, Vosshall and Andreas Keller ran this test using acetophenone and its fully deuterated analog. Subjects could not tell them apart. The similarity scores were indistinguishable from those for identical pairs. Her conclusion: the result shows that molecular vibrations alone cannot explain the perceived smell of a chemical. The study wasn’t designed to prove shape theory, just to test a specific prediction of the vibrational theory, and that prediction failed in humans with that particular molecule.
Turin’s team later ran their own tests and found mixed results. Humans couldn’t distinguish deuterated acetophenone from its normal counterpart, confirming Vosshall’s findings. But they reported that subjects could distinguish a different molecule, cyclopentadecanone, from its deuterated version. Turin argued the difference was about the number of hydrogen atoms involved. More hydrogens mean more vibrational modes, and more chances for the receptor to detect the difference.
Vosshall raised a further concern. The olfactory membranes are full of enzymes that metabolize odorants, changing their chemical identity before they even reach the receptors. Deuterated molecules interact differently with these enzymes than their non-deuterated counterparts. So even if subjects in a study could tell two isotopomers apart, that difference might be explained by enzymatic activity rather than vibrational detection. The experiment is harder to interpret cleanly than it looks.
Further, by 2015, a team led by Eric Block published what they described as a comprehensive attack on the theory’s core mechanism, testing the electron transfer model at the receptor level and finding no evidence to support it.
Turin’s response to this line of criticism has been that testing receptors in isolated cell systems doesn’t adequately capture what happens inside a living organism, where olfaction is embedded in a vastly more complex biological environment. It’s a reasonable objection, though it also makes the theory increasingly difficult to test.
Where This Leaves Us
I started this piece to understand what was actually being proposed in the quantum perspective on olfaction. What I've come to understand is that the vibrational theory, and the quantum tunneling mechanism at its center, involves coherent physics. It has been taken seriously by physicists who have no stake in olfactory debates. Quantum tunneling is real and consequential in biological systems. Whether it is happening specifically at olfactory receptors, in the way Turin describes, remains unclear
What I also understand now is that Vosshall's skepticism is a form of rigor, not closure. Her position has always been that the evidence, as it stands, doesn't support the theory in humans, not that the theory is impossible to imagine. Those are different claims, and the difference matters.
The debate between shape and vibration isn’t settled. It may not be settable with the experimental tools currently available, given how difficult it is to isolate receptor-level quantum effects inside the warm, messy, enzymatically rich environment of a living nose. What we have, for now, is an open question sitting at the edge of two incredibly compelling fields. My dad would call that exciting, and I would have to agree.
References
Al-Khalili, Jim. “Let There Be Life.” The Secrets of Quantum Physics, BBC, 2014.
Block, Eric, et al. “Implausibility of the Vibrational Theory of Olfaction.” Proceedings of the National Academy of Sciences, vol. 112, no. 21, 2015, pp. E2766–E2774. https://www.pnas.org/doi/10.1073/pnas.1503054112
Brookes, Jennifer C. “Quantum Effects in Biology: Golden Rule in Enzymes, Olfaction, Photosynthesis and Magnetodetection.” Proceedings: Mathematical, Physical and Engineering Sciences, vol. 473, no. 2201, 2017, pp. 1–28. https://www.jstor.org/stable/44683206
Keller, Andreas, and Leslie B. Vosshall. “A Psychophysical Test of the Vibration Theory of Olfaction.” Nature Neuroscience, vol. 7, no. 4, 2004, pp. 337–338. https://www.nature.com/articles/nn1215
Turin, Luca. “A Spectroscopic Mechanism for Primary Olfactory Reception.” Chemical Senses, vol. 21, no. 6, 1996, pp. 773–791.
Vosshall, Leslie B. “Laying a Controversial Smell Theory to Rest.” Proceedings of the National Academy of Sciences, vol. 112, no. 21, 2015, pp. 6525–6526. https://www.pnas.org/content/112/21/6525
I’ve been telling myself for weeks now that I should finally take the time to understand quantum physics better. Your post gave me a really on point first glimpse into it thank you for that.