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George and John’s Excellent Adventures in Quantum Entanglement, Part 2 [Video]

The views expressed are those of the author and are not necessarily those of Scientific American.


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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

George Musser About the Author: is a contributing editor at Scientific American. He focuses on space science and fundamental physics, ranging from particles to planets to parallel universes. He is the author of The Complete Idiot's Guide to String Theory. Musser has won numerous awards in his career, including the 2011 American Institute of Physics's Science Writing Award. Follow on Twitter @gmusser.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. jtdwyer 8:12 am 03/17/2013

    How is it determined that the act of measurement actually causes the particles to be coincident, rather than simply exposing their preexisting coincident condition?

    It seems that if the polarization property is identically determined when the entangled particles are produced, then the same measurement results would apply. The property may still be random among all photons, just not between the entangled pairs. Unless, that is, it can be demonstrated that the polarization among entangled pairs is not identical prior to measurement…

    Link to this
  2. 2. George Musser in reply to George Musser 8:14 am 03/17/2013

    Excellent insight – this is what Einstein thought was the case. But the Bell inequalities rule this out. The first video gives an example.

    Link to this
  3. 3. ottokrog 6:11 pm 03/17/2013

    Entanglement is instant “communication”.

    It conflicts with Einsteins relativity theory.

    What if the speed of light varies through time and space?

    That would create some interesting theory. At least I think so.

    Quantum mechanics might not be confusing at all. If you split the entire universe into the physical universe (positive energy) and infinite separate mental parallel universes (negative energy), the wave collapse might be explainable as where all the universes meet in the quantum field. Matter and antimatter is created in this wave collapse, and that gives us the reality we observe.

    You are your own universe.

    Reality is where the minds (negative energy) meets and creates the physical universe. Our inner self is the observer and the the creator, and that is what we consider as consciousness.

    The most interesting about it all is, that if the speed of light is infinite in vacuum, then Einstein is right again. He said that the speed of light is constant locally.

    Interested? Then read my philosophical multiverse theory.

    http://www.crestroy.com

    Link to this
  4. 4. jtdwyer 7:55 pm 03/17/2013

    Thanks – I had missed Part 1, so I reviewed it. I admit that I have briefly stepped into this muck before. I’m mathematically disabled, so I’ll just do my best to explain further…

    My suggestion is that when photons are entangled by passing an originating photon beam through an optical beam-splitter, for example, it produces a separate beam of photons whose spin orientation is the complement of its entangled partner. While in each beam, individual photons’ spin is somewhat randomly oriented, not all entangled photons can be detected. As a result, detections of the actual entangled pairs cannot be collated – the two beams’ detected spin orientations can only be statistically correlated, producing the observed probabilistic distribution. I can’t attempt any analysis of the resulting distributions of detected orientations, however. Thanks for any consideration…

    Link to this
  5. 5. George Musser in reply to George Musser 11:05 pm 03/17/2013

    It’s a good intuition and, indeed, the first thing any experimenter should try. In this case, however, that’s already been done. The statistics would be subtly different and would conform to a Bell’s inequality, and therefore conflict with experiment.

    Link to this
  6. 6. jtdwyer 7:01 am 03/18/2013

    Thanks – I didn’t really expect to solve this problem after so many brilliant people have failed. Philosophically, accepting any unphysical causation is equivalent to accepting some mystical causation. I can only reasonably conclude that there is some unknown physical process responsible for these results which is simply not yet understood… In the meantime, IMO, the experimental process should be continuously scrutinized and evaluated in an attempt to discover some underlying physical process involved. Thanks very much for clearly demonstrating the results!

    Link to this
  7. 7. George Musser in reply to George Musser 8:14 am 03/18/2013

    My next book will discuss possible physical processes that underlie this phenomenon. So stay tuned!

    Link to this
  8. 8. Bryan Sanctuary 9:50 am 03/20/2013

    When George Musser got to the point in the video of trying to explain entanglement between his separated photons, it was clear he was having difficulty finding the words. Clearly he is not convinced, and definitely he does not know how it happens. I hate the words “quantum weirdness” in physics.

    I have recently shown that a quantum mechanical local realistic model of a spin resolved the EPR paradox and restores locality to Nature.

    I have a talk http://bit.ly/WHj32D which explains the model. I am close to submitting the paper and posting the simulation program.

    I think physics will sigh in relief when locality is restored and quantum weirdness debunked: that is all but those who make a living from spookyness.

    Link to this

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