If you've ever tried the experiment, you know you can't walk through a wall. But subatomic particles can pull off similar feats through a weird process called quantum tunneling. Now, a team of physicists says that it might just be possible to observe such tunneling with a larger, humanmade object, though others say the proposal faces major challenges.
If successful, the experiment would be a striking advance toward fashioning mechanical systems that behave quantum mechanically. In 2010, physicists took a key first step in that direction by coaxing a tiny object into states of motion that can be described only by quantum mechanics. Tunneling would be an even bigger achievement.
So how does quantum tunneling work? Imagine that an electron, for example, is a marble sitting in one of two depressions separated by a small hill, which represent the effects of a sculpted electric field. To cross the hill from one depression to the other, the marble needs to roll with enough energy. If it has too little energy, then classical physics predicts it can never reach the top of the hill and cross over it.
Tiny particles such as electrons, however, can still make it across even if they don't have enough energy to climb the hill. Quantum physics describes such particles as extended waves of probability—and it turns out that there is a probability that one of them will “tunnel” through the hill and suddenly materialize in the other depression, even though the electron can't occupy the high ground between the two low spots.
It sounds unlikely, but scientists and engineers have amply demonstrated quantum tunneling in semiconductors in which electrons tunnel through nonconducting layers of material.