Quantum Physics, Predictions, and Tunneling
How Long Does Quantum Tunneling Take?
+ The phenomenon known as “tunneling” is one of the best-known predictions of quantum physics, because it so dramatically confounds our classical intuition for how objects ought to behave. If you create a narrow region of space that a particle would have to have a relatively high energy to enter, classical reasoning tells us that low-energy particles heading toward that region should reflect off the boundary with 100% probability. Instead, there is a tiny chance of finding those particles on the far side of the region, with no loss of energy. It’s as if they simply evaded the “barrier” region by making a “tunnel” through it.
So, there is absolutely no debate among physicists about whether quantum tunneling is a thing that happens. Physicists get a bit twitchy without something to argue over, though, and you don’t have to dig into tunneling (heh) very far to find a disputed question, namely “How long does quantum tunneling take?”
+ The big advantage this offers is that unlike electrons, which are point particles, atoms have complicated internal structure and can be put in a bunch of different states. This lets them make an energy barrier out of a thin sheet of laser light that increases the energy of the atom in the light. They can control the energy shift by adjusting the laser parameters to get any height they want— they can even “turn off” the barrier without turning off the laser, by making a small shift in the laser frequency, which is crucial for establishing the timing.
+ The laser also changes the internal state of the atoms in a way that varies in time, letting them use the atoms as a kind of clock. They prepare a sample that’s exclusively in one particular state, and set the laser up in such a way that it drives a slow evolution into a different internal state. They separate the two different states on the far side of the barrier, and measure the probability of changing states. Once they have that, it’s relatively easy to convert that into a measurement of how much time the atoms spent interacting with the laser.
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