The quantum world... is weird. Physicists have a pretty good handle on it mathematically, but that math can sometimes point to things that just seem wrong. Like a particle zooming towards a seemingly-impenetrable barrier and then just — poof — appearing on the other side.
Yet, not only does this happen, it’s essential for things as fundamental as photosynthesis and solar fusion - aka how the Sun makes light and heat. Physicists call this phenomenon quantum tunneling. And for a long time, they’ve argued about the details behind how it works. Like, do particles spend time in the barrier, or are they somehow teleporting straight across it?
Now, though, there’s finally some hard evidence. And it seems like quantum tunneling can actually be kind of slow… at least, by quantum standards.
To know how tunneling works, it helps to know a little about quantum physics in general. In the world of the really tiny, physicists use a mathematical function called the wavefunction to describe the properties of particles.
For instance, if you’re looking at an electron, its wavefunction would describe where it is, how fast it’s moving, and so on. And one way we can use wavefunctions is in the Schrödinger equation. This takes a wavefunction and uses it to predict what will happen to a particle next. If you’re familiar with Newton’s laws of motion— it’s like those, but for the ultra-microscopic.
What exactly a solution to the Schrödinger equation means has been debated for decades. But, here, just know that most physicists agree that the resulting wavefunction is a description of the probability of finding an object in a given place, at a given time. The wavefunction is, well, a wave, so it can be larger at some points and smaller at others. And the bigger the wave is at a given point, the more likely your object is to be in that place when you look for it. Even the location of large things like you and me are theoretically described by these waves, but the effect is way too small to be measurable.
But with really small objects, on the scale of a few atoms, this becomes super important. So, coming back to quantum tunneling — one way waves get weird is when they approach obstacles. Waves — even familiar ones — have a kind of curious property: They rarely stop dead in their tracks when hitting something.
Like, think about sound. If sound waves were perfectly blocked by solid objects, then the inside of your car would be a zone of complete silence, even if someone was jackhammering the sidewalk next to you. And all that sunlight hitting your windshield would just… stop, leaving you in total darkness.
The same basic idea is true for the wavefunctions that describe quantum particles. An object’s wavefunction can extend into — or even past — an obstacle. And since that function describes the probability of finding a particle in a given location, sometimes the particle ends up there, too. That’s quantum tunneling in action. And it’s as strange as it sounds.
Physicists discovered that electrons could do this in 1927, in the very early days of quantum mechanics. And ever since, they’ve wondered what objects do while they’re tunneling through the barrier. And like, how long does tunneling take? Some researchers have argued that it’s instantaneous. Others have wildly different ideas.
Well, a team at the University of Toronto went on a 20-year quest to find out and published the long-awaited results in the journal Nature in July 2020.
In their experiment, they sent atoms of super-cold rubidium to tunnel through a barrier that normally should have reflected them. When you’re dealing with individual atoms, you need incredible precision, so the researchers used laser beams instead of a physical obstacle like a wall.
They used one beam to control the motion of the atoms, and a second to act as a barrier for the atoms to tunnel through. The choice of rubidium also wasn’t random— because rubidium has an interesting property: Its spin can be altered by lasers.
So in the study, when the atoms passed through this laser barrier, their spin would change. And the longer this took, the more their spin was affected. To be clear, “spin” doesn’t actually mean “rotation,” but the details are messy enough that even the researchers have referred to it this way. So here, it’s a fine approximation.
The larger point is, by measuring the atoms’ spin axis before and after they entered the barrier, scientists could tell how long the atoms took to tunnel. And their final result was an average of 0.61 milliseconds. Which, sure, is silly fast, considering a blink of an eye is a few hundred milliseconds. But it’s also pretty long for something that some people have suggested is instantaneous!
None of our current technologies actually use rubidium atoms or lasers like this, so the actual number is less important than the fact that it was possible to measure it. But measuring it at all is a big deal. Because knowing how long quantum tunneling takes could be really useful not just for understanding the world, but for building newer, better technologies.
Like, quantum computers aren’t quite ready yet, but in theory, they have the potential to process more information than we could ever imagine. Except, these devices work by timing and tracking individual particles. So if you don’t know how long tunneling takes… it can be hard to plan what your computer will actually do.
So, this measurement is a key step in resolving an argument that’s been raging for almost a century. But it also has the potential to unlock a new world of awesome tools.