The lenient laws of quantum mechanics permit a lone electron to be in two places at once--as long as it strictly avoids any contact with its surroundings. Now, for the first time, physicists have built an environment with a knob so they can dial up the quirky quantum world or tune it out. The report, appearing in today's issue of Nature, begs the question of where the quantum world ends and the quotidian begins.
The table-top device, built by physicist Mordehai Heiblum and colleagues at the Weizmann Institute for Science in Rehovot, Israel, pulls electrons through two adjacent corridors atop a tiny microchip. If not watched--that is, if it can skirt through the hallways without interacting with them--a single electron will go through both at the same time and "recombine" when the pathways merge. But add something that can detect which path the electron takes, and suddenly it cleans up its act, taking one corridor or the other as a rat would in a maze.
The Weizmann researchers rigged a kind of adjustable electric dam in one of the channels to impede the flow of electrons. The higher the dam, the more accurately they could tell whether an electron had passed by. Raise it, and more electrons should be forced to take a single path. "You can tell 'oho!'--one went by," says Heiblum. Lower it, he says, and all the electrons should duplicitously slide through both channels at once.
By watching the flow of electrons where the two corridors merged, the researchers could count how many electrons had taken the single route and how many had taken the double route. When they set the dam to detect 5% of the electrons, about that percentage took a solitary corridor. As they lowered the dam, the strange hand of quantum mechanics took over again until all electrons were taking both paths.
"It's a beautiful experiment," says Ned Wingreen, a physicist at the NEC Research Institute in Princeton, New Jersey. In the everyday world, he says, the environment that kills quantum behavior is unfathomably complex, but "here, it's something you can understand perfectly" with the laws of quantum mechanics. That suggests that any environment is just a big quantum system, which brings up the strange question of whether the universe itself is forever splitting off, taking multiple paths at once. That's the logical conclusion, he says, "but it makes me ill to think about it."