When you hear the word "quantum," what do you think of?

Maybe quantum computers or quantum physics, or maybe your box set of Quantum Leap, or Daniel Craig in Aviators, or, maybe, the word quantum just sounds complicated, so you've ignored everything I've said and you're sitting there thinking about pizza.

Well, it turns out there's quantum physics at work right in the middle of your face.

and it helps you enjoy things like... the smell of pizza.

So stay with me...

In the past, we believed our nose could tell the difference between smells in a pretty straight forward way.

For example, when a molecule enters your nose, it fits into a specific combination of receptors, activating or deactivating them, based on its shape, just like a key in a lock.

That, combined with environmental cues from your other sensors, tells your brain what you're smelling.

So, if you smell this molecule, called Butyric acid, and you see pasta, you think "Parmesan."

But if you smell it outside the exit to a roller coaster, you think... vomit.

Because Butyric acid is present in both, it fits into the same receptors, no matter where it's coming from.

But, there's a problem with this lock and key theory.

Take molecules that contain both sulfur and hydrogen, for example.

These compounds can take a number of different sizes and shapes.

And according to the lock and key theory, because the range of shapes is so wide, the compound should also fit into a wide range of different olfactory receptors and produce different smells.

But that's not what we see in real life.

The truth is, sulfur and hydrogen molecules just smell like rotten eggs.

So, what if your nose tells you the difference between smells in a separate, more quantum way?

That's right!

Your nose could be using quantum mechanics to give you your sense of smell.

It's called the Vibration Theory of Olfaction.

This theory is less like a lock and key and more like a swipe card.

It says your nose may take advantage of a quantum physics effect called Tunneling.

Quantum Tunneling is when a particle tunnels through a barrier that it wouldn't be able to based on purely classical physics.

Imagine you're pushing a ball so it rolls up a small hill.

If you give it enough energy-that is, you push it hard enough- the ball can roll up and over the hill.

If you don't push it hard enough, it just rolls back down.

This is true in classical physics, but at the quantum level, it's a little different.

Even if the ball doesn't have enough energy to get up and over the hill, there's a small probability it may borrow energy from its surroundings and use that to tunnel through to the other side.

Our noses may use this to differentiate older molecules based on their molecular vibrations.

They have to be compatible with their receptor in a way that allow for Quantum Tunneling to happen.

If the vibration theory is correct, it means our noses should be able to tell the difference between different odor molecules and different types of the same molecule.

To test the vibration theory, researchers used a number of different odoring compounds.

Some of those compounds were typical and contained hydrogen.

But for each of those, they also had the same odorant containing deuterium, or, heavy hydrogen.

The nucleus of a typical hydrogen atom contains just one proton, while the nucleus of a deuterium contains a proton and a neutron.

The result is two odorants that are identical in size and shape, but different in mass and molecular vibration.

If our noses are like a lock and key, participants shouldn't be able to tell those odorants apart, but... they did.

Basically, the human nose can smell the presence of extra neutrons in a molecule, which is totally amazing!

And our noses aren't the only example of quantum mechanics in biology, which is then actually just called quantum biology, which sounds really cool.

When birds navigate through the sky and when plants carry out photosynthesis, they use quantum biology, too.

Some of the most amazing natural phenomenon that we experience every day happens at this teeny tiny scale, including how we experience the delightful smell of pizza.

Quantum interactions happening in your nose right now.