The first time I ever saw quantum entanglement for myself was in August 2011 on a road trip to Colgate University. Goodness knows how many blog posts and magazine articles have been written about the quantum realm, invariably describing it as weird. But I'd never actually seen this supposed mind-blowingness with my own eyes, which was mildly embarrassing, since I'd written a number of those posts and articles myself. In graduate school, I'd taken a quantum-mechanics class and filled two avocado-colored spiral notebooks with equations, but not once did the professor actually show us the phenomena the equations described. So when we pulled out of my driveway, I felt like a pilgrim on a voyage for which I'd spent much of my life preparing.
This video shows the result. It's part two of a video project I've been working on with John Matson, Sci Am's associate editor for physics, and Eric Olson, the magazine's video guru. In part one, we and our colleague Mary Karmelek dramatized what quantum entanglement means, metaphorically. Now you get to see the non-metaphorical version.
I'd gotten to know Colgate professor Enrique Galvez a decade ago for his studies of the orbital angular momentum of light. I went back to him because of his reputation as a pioneer of quantum experiments that college students could do in a lab course, and he kindly set aside a day to demonstrate them for us. The video focuses on the famous EPR experiment that Einstein devised and published in a famous paper with Boris Podolsky and Nathan Rosen in 1935. At the end, it mentions the elaboration developed by physicist John Bell in the mid-1960s, which proved that entanglement represents a type of nonlocality--or, as Einstein put it, "spooky action at a distance."
The experiment entails creating pairs of photons that must then run a gauntlet of polarizing filters (shown in photo above). The polarizers are oriented so that an individual photon has a 50% chance of getting through. When both photons get through their respective polarizers, the equipment registers a "coincidence." For a pair of unentangled photons, that has a 25% chance of happening--it's equivalent to flipping two coins and seeing two heads. For entangled photons, however, the probability ranges from 0% to 50% depending on the relative polarizer orientation. The photons are correlated in a way the ordinary laws of chance do not allow. It is as if you flipped two coins and both always landed on the same side.
Like many physics experiments, when you first see the setup, you focus on the taking in all the complexity. Much of the equipment on the lab bench is technically essential but conceptually irrelevant; it ensures the alignment of light, for example. The data readouts require some interpretation, too: to translate coincidence rates to a probability, you need to account for the efficiency of the particle detectors. "The actual doing of an EPR measurement is not very glamorous," Galvez admits. But then it dawns on you what you're seeing. The photons are acting in unison even though no known force or influence links them. And they do so despite being separated by the width of a hand, which, for an infrared photon, might as well be a million miles.
In fact, I was so inspired by Galvez's and others' efforts to streamline these experiments that I recently developed my own el-cheapo version, which you can do at home for a few hundred dollars.
Photo credit: Eric R. Olson/Scientific American
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