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. (In fact, some types of magnetic hard drives rely on tunneling for reading out data.) And the Nobel Prize-winning scanning tunneling microscope relies on electrons tunneling through a forbidden no man's land between a tiny fingerlike probe and a conducting surface. Still, no one has ever seen a macroscopic object tunnel through an obstacle.
But Mika Sillanpää and colleagues at Aalto University in Finland say it might be possible to do just that using a tiny widget that resembles a trampoline as they reported 8 November in Physical Review B. Researchers would fashion the micrometer-wide trampoline out of graphene, a superstrong, superflexible sheet of carbon only one atom thick. They would suspend the membrane—small but much larger than the atoms and molecules that are the usual domain of quantum physics—over a metal plate. When experimenters applied an electrical voltage, the membrane would have two stable positions: one in which it bows slightly in the middle and one in which it bends enough to contact the plate below. In the Finnish team's design, the electrical and mechanical forces on the membrane create an energy barrier between these two positions. If researchers could lower the membrane's energy by cooling it to a temperature of less than a thousandth of a degree above absolute zero, then the only way it could get between the two positions is quantum tunneling. The experimenters could then observe the membrane's change of configuration by looking for a change in the system's capacitance, a measure of how well it can store electrical charge. Sillanpää says achieving the low temperatures required may take several years, but the team is moving forward with an experiment.
Quantum tunneling in a mechanical system is "the kind of holy grail that people are looking for now," says physicist Walter Lawrence of Dartmouth College, but the experiment is likely to be difficult. Gil-Ho Lee, a physicist at Pohang University of Science and Technology in South Korea, says the proposed experiment would be an important first step toward demonstrating quantum tunneling. But he cautions that it might not be conclusive because the membrane might perform similar flip-flops when it absorbs a little extra energy in the form of heat. "A more sophisticated test must be done," Lee says. He says that searches for quantum tunneling in electrical systems known as Josephson junctions faced similar issues in the 1980s before experiments eventually confirmed tunneling.
So why can't you use quantum tunneling to walk through a wall? Quantum mechanical calculations show that for something as big as a person, the probability is so small that you could wait until the end of the universe and most likely still not find yourself on the other side.