Photons on roundabout route could get caught in action

Light sometimes prefers to take the scenic route. Now, a new twist on a classic experiment could trace photons’ wandering ways. In the Sept. 19 Physical Review Letters, researchers propose a version of the double-slit experiment that encourages light to take weaving paths before striking a screen.

It’s no surprise that the double-slit experiment is so popular in physics classrooms. The experiment encapsulates the wonder of quantum mechanics. It also is simple to execute, requiring only a light source, a screen and a plate with two thin parallel slits. Illuminate the slits and a pattern of bright and dark bands shows up on the screen. The striped pattern suggests that light travels in waves that interfere with each other behind the slits.

The strange part comes when the experimenter turns down the intensity of light so low that light strikes the screen as individual particles, or photons. Particles seemingly would not influence each other as they passed through the slits one by one. Nonetheless, the same light-dark pattern gradually builds up on the screen. “It’s been called the most beautiful experiment in physics,” says Urbasi Sinha, a physicist at the Raman Research Institute in Bangalore, India.

Yet the double-slit experiment has some misconceptions attached to it, Sinha says. Physicists generally assume that understanding the striped pattern depends only on the contribution of light traveling via one of two paths: through slit A or through slit B. But quantum mechanics doesn’t limit particles to those routes, she says. Photons could take a road less traveled, particularly in an apparatus with three slits — a common variation of the classic experiment. The light could perhaps pass through one slit, wind out the second slit and then loop back in through the third before finally hitting the screen (see illustration).

Sinha and her colleagues set out to quantify the share of particles that take a meandering route. They found that in a three-slit experiment using visible light, photons taking roundabout paths account for about one-millionth the intensity of the interference pattern on the screen — a contribution that’s too small to measure directly. But the researchers could raise that fraction significantly by tweaking the experimental design. An experiment using microwave photons, which have a longer wavelength than visible light, along with relatively wide, spread-out slits would make the looping photons detectable, at about one-thousandth of the total intensity, their calculations show.

“It’s a cool calculation,” says Howard Barnum, a physicist at the University of New Mexico in Albuquerque. “And it would be cool just to observe the contribution [from these paths] experimentally.” Sinha is on the case, having already performed some early runs of the microwave three-slit experiment. Other physicists use the three-slit experiment to search for new quantum phenomena, Barnum says, and this study will help them cancel out the subtle influence of weaving light.