To test the basics of quantum theory, physicists recently pulled out an antique. In a paper published today in *Science*, they confirmed a staple of quantum mechanics, using a test derived from a classic nineteenth century light experiment. In particular, the researchers questioned how particles move through three slits, something previously too difficult to measure. They found that the particles behaved just like quantum theory--or more specifically the Born Rule--would have predicted. As physicist Chad Orzel describes in his blog, that's bad news for theorists hoping to tweak this rule to solve Nobel Prize-worthy problems related to quantum gravity or Grand Unifying Theories.

[The study is good news if] you're the ghost of Max Born, or the author of an introductory quantum book.... This was disappointing news for some theorists, though, as there are a number of ways to approach problems ... that would require some modification of the Born rule. [Uncertain Principles]

But how did they do it? **Step 1: Watching Light Waves ** Throw a pebble in a pond and it creates waves. Throw two pebbles in a pond and they will create waves that interact. Where the peaks of two waves meet, they will create an even bigger wave. Where the peak of a wave meets the trough of another, they will cancel each other out--as if there is no wave at all. Thomas Young's 1800s double-slit experiment involves shining one color of light through two open slits to hit a screen. If light is a particle, Young imagined, then you get two streaks, like spray-paint through a stencil. That's not what he saw. Invisible ripples created visible effects. On the screen, bright lines appeared where the waves built on one another. Other places the light waves canceled each other out leaving only darkness. **Step 2: Watching Particles Wave, Too ** In the 20th century, quantum physicists did a similar experiment with particles, including electrons, firing them through two open slits. Classical physics would predict that the particles would land in two streaks on the other side. Instead, they saw a sight just like Young's interference pattern. The particles were somehow interfering with each other, and more amazingly, even a particle fired alone created the pattern. It was interfering with itself.

This surprising effect provided one of the first clues to the weird world of quantum mechanics. Now precise measurements have been made on a version with three slits--and they again confirm the predictions of quantum mechanics. [New Scientist]

Why would you even bother trying three slits? That gets into the specifics of quantum mechanics and the Born Rule. **Step 3: Watching Probability Waves ** So what type of waves are crashing into one another when a particle passes through a slit? Probability waves. The value of a probability wave in various experiments is in part calculated by the Born Rule. In a double slit experiment--the probability waves values show that the electron is *more likely* to appear in one of the "bright" spots of the interference pattern and *less likely* to appear in one of the dark spots. The Born Rule says that that we need to look at the interactions of probability waves only from two slits at a time--as opposed to looking at how ripples from all three slits interact at once. If the probability could include an extra value from interactions including all three slits at once, then interference pattern would change.

There was no experimental verification of this proposition until now.... "The existence of third-order interference terms would have tremendous theoretical repercussions--it would shake quantum mechanics to the core," says [coauthor Gregor] Weihs. [ScienceDaily]

**Step 4: Adding and Subtracting Slits** Urbasi Sinha of the University of Waterloo in Ontario, Canada and his team made a comparison. First they looked at the probability values formed by all three slits. Then, by covering up each of the slits in turn, they looked at the pattern formed from two slits at at time. Adding up the values from each of the two slits, they got the overall pattern formed by three--meaning the Born Rule was right for as close as they could measure.

[T]he three-path interference term came to more or less zero. Co-author Ray Laflamme of the University of Waterloo in Ontario, Canada, "always hoped for three-path interference", says Weihs. "But then he's more of a theoretician. If there was three-path interference, there would be a Nobel prize waiting." [Nature News]

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