Two is the magic number

Pillar of quantum mechanics stands up to new experiment

Extending an experiment at the foundation of quantum physics confirms that two is company and three is a crowd. In a new twist on the famous double-slit experiment, researchers have verified a basic tenet of quantum mechanics by showing that adding a third slit doesn’t create additional interference between packets of light.

NOT A TRIPLE THREAT The interference pattern (bottom) of a stream of photons (blue) traveling through three slits can be entirely explained by the combination of single and double interference patterns, a new study shows. Science/AAAS

The double-slit experiment embodies the mystery at the heart of quantum mechanics, the famous physicist Richard Feynman observed in his Lectures on Physics. The experiment illustrates some of the strangest predictions of quantum mechanics, including the dual particle-wave nature of tiny objects.

In the 1920s, German physicist Max Born proposed that particle pairs — and not triplets, quadruplets or more — can interfere, causing their wavelike forms to boost and diminish one another. Born’s math puts the interference contribution of the third slit (and any additional slits) at exactly zero. Although the reason why quantum interference stops at two isn’t clear, Born’s postulate has been widely accepted and used by physicists, yet until now it hadn’t been explicitly tested in experiments.  

“It’s important that you test all the postulates of quantum mechanics,” says study coauthor Urbasi Sinha of the Institute for Quantum Computing at the University of Waterloo in Canada. “What is the point of just advancing a theory in its theoretical form if you don’t have experiments backing things up?”

In the new study, Sinha and colleagues made three parallel slits in a stainless steel plate, each 30 micrometers wide and 300 micrometers tall. Light was sent through the slits, and detectors on the other side tallied up the photons that passed through each. A blocking mask allowed the researchers to open and close the three slits independently.

If existing quantum mechanics equations (and Born’s postulate) are right and three-party interference doesn’t happen, then the interference pattern when all three slits were open could be explained entirely by the combined patterns of single and double slits being open. So Sinha and her colleagues shot photons at the triple slits with all eight combinations of slits open and closed. Subtracting the interference pattern caused by all seven of the other possibilities from the pattern formed with three open slits resulted in a number very close to zero. That result, published in Science July 23, leaves very little room for Born’s postulate to be incorrect.

“Just because you’ve added a third slit doesn’t mean that you have any further interference coming in,” Sinha says. “You can explain it all in terms of single and double slit contributions.”

Detecting third-party interference would have had tremendous consequences, says theoretical physicist Fay Dowker of Imperial College London in England. “If a non-zero result were ever to be obtained, it would mean that quantum mechanics is wrong, in the same way that the double-slit experiment proves that classical physics is wrong.”

Most physicists expect that as more triple-slit experiments are conducted with other particles such as electrons and buckyballs, the case for Born’s postulate will get stronger, Dowker says. But she adds that there is a small chance that the value might get stuck at a small number hovering just above zero. “That’s the exciting thing.”

Some physicists have wanted to tweak Born’s rule to better mesh quantum mechanics with gravity. But doing so in a way that still agrees with experiments has been a challenge. The new study shows that to solve some of the outstanding mysteries, theorists will probably have to modify another piece of the puzzle. But having a value from an actual experiment in a lab provides a “good lead toward what is possible and what is not in these unification attempts,” Sinha says.

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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