Another Win for Quantum Mechanics: Passing the Triple-Slit Test

By Joseph Calamia | July 23, 2010 3:26 pm

rippleTo 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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Image: Wikimedia / Copyright © Armedblowfish, all rights reserved.

  • nick

    “more or less zero” sounds like the results could be experimental error.

    Lets not forget the standard model was recently proved incorrect in its prediction that neutrinos didn’t have any mass (and anyone could have looked at E=MC² and made an assumption that yes, if they have energy they do indeed have mass). So… until the answer is “zero” and not “more or less zero” there is still wiggle room 😀 I suspect we’ll hear more on this when our measuring instruments improve further.

  • Wilson

    It’s not as simple as throwing a tennis ball at two holes in a wall.

    At the atomic level there are really no “slits” as we think of them., The atoms in the “solid” material and the atoms in the air in the “slits” form a continum (all atoms are mostly empty space anyway) so there are no clearly defined “edges” to the “slits”

    You’r firing waves at waves through waves. There could be a lot of other interactions going on there.

  • NevTheTech

    Jeeze… All I said was “Could you switch the litghts on?”

  • G-Max

    If this sort of thing is to be believed, it would imply that a person at the receiving end of those light waves could see light coming through said slits even when there is no line of sight from the eye to the light source… which is why I won’t believe it until I see it for myself. However, if the quantum psychos aren’t just making this all up, then it promises a whole new approach to holography and stealth technology.

  • Tech STAC82

    My understanding of the DOUBLE slit experiment is that… Through a a series of tubes and mirrors a single quanta of energy (photon) is given the “choice” of two paths to follow. At the end of its journey(s) is a photo sensitive plate. Through thousands if not millions of such emissions a pattern of interference emerges. Being as the emission is a single quanta (photon), the pattern expected would be two bright “patches” not a wave like interference pattern.

    The key in this is that it is a single photon being emitted. So its wave function should have little or no affect on its “choice” or its final location on the plate. What the evidence suggests is that the photon “chooses” to take BOTH PATHS AT THE SAME TIME causing the interference pattern, but is also negated by the fact that single points are measured on the plate that form this pattern.

    I do not fully understand the whole of the implications, I only have a little high school physics. Nor do i really understand the reason to give a third “option” to the photon, but man, I love this stuff!

  • tricia

    What about the observer in the experiment. One of the experiments previously done like this also used a vertical blind with 2 telescopes behind it which was closed and opened to see the photons action of predicting what it did. This kind of led to the theory of the photon going back in time because of the observer
    What are your comments about this thanks. I am still learning so much it’s great

  • Brian

    @nick: There is always wiggle room in science. So yes, there is a degree of uncertainty regarding their findings. As for your claim that the Standard Model predicted that neutrinos have mass, this is incorrect. It predicts no such thing. The neutrinos were given 0 mass by hand — it is no great stretch to imbue them with mass in the SM. Also, your assertion that physicists should have realized that neutrinos have mass via the celebrated high-school level equation E=mc^2 is, again, incorrect. This expression does not say that all particle with energy also have mass; that’s a gross misinterpretation. The same theory that gives us E=mc^2 is also perfectly consistent with massless photons (which have energy).


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