Have you ever counted the number of things that you own which use lasers?1 One of the best know devices emerging from our understanding of quantum physics is the laser. Remember the meaning of the acronym? Laser pointers are cool, eh. We rely on lasers for communications, entertainment, health & safety, defense, retail services, manufacturing, and research.2
One spooky use of lasers which is unlikely to be found in your home is to produce entangled photon pairs. (Chad Orzel says, however, that “these days the technology required is well within the reach of an undergraduate laboratory.” 3) The laser apparatus employs spontaneous parametric down-conversion (SPDC). Say what? Well, SPDC can be used to demonstrate why some sci-fi stories make use of faster-than-light communications. And why Einstein never accepted quantum mechanics. And why quantum physics remains mind boggling.
A nonlinear crystal is used to split photon beams into pairs of photons that, in accordance with the law of conservation of energy and law of conservation of momentum, have combined energies and momenta equal to the energy and momentum of the original photon and crystal lattice, are phase-matched in the frequency domain, and have correlated polarizations.
SPDC is stimulated by random vacuum fluctuations, and hence the photon pairs are created at random times. The conversion efficiency is very low, on the order of 1 pair per 10^12 incoming photons. However, if one half of the pair (the “signal”) is detected at any time then its partner (the “idler”) is known to be present.
SPDC allows for the creation of optical fields containing (to a good approximation) a single photon. As of 2005, this is the predominant mechanism for experimentalists to create single photons (also known as Fock states). The single photons as well as the photon pairs are often used in quantum information experiments and applications like quantum cryptography and Bell test experiments.
As noted in the Wiki quote above, we’ll need SPDC to discuss “spooky action at a distance” and Bell’s Theorem.
In the mean time, here’s an animated video which explains the topic: “Einstein’s brilliant mistake: Entangled states” by Chad Orzel.
Published on Oct 16, 2014 – When you think about Einstein and physics, E=mc^2 is probably the first thing that comes to mind. But one of his greatest contributions to the field actually came in the form of an odd philosophical footnote in a 1935 paper he co-wrote — which ended up being wrong. Chad Orzel details Einstein’s “EPR” paper and its insights on the strange phenomena of entangled states. Lesson by Chad Orzel, animation by Gunborg/Banyai.
 Here’s a (partial) list of my laser things: DVD player/drive, laser printer (obviously, eh), FIOS Internet service, …
“What Has Quantum Mechanics Ever Done For Us?” by Chad Orzel.
Last week in an episode (4-21-2017 rerun of #10 “Pliers”) of the rebooted MacGyver TV series (which I rarely watch), MacGyver grabbed a solar path light from a yard, hopped in a car, yanked the entertainment console from the dashboard, extracted the laser diode (output in the 3 to 5 mW range) from the CD player, connected the solar sensor as an input to the car radio, pointed the laser (through the windshield) at a window of a house, and listened to a man talking inside. Really? Unlikely, assuming MacGyver figured a way to keep the laser on without anything to track, due to a laser diode’s short (mm) coherence length. Just do a Google search for “How to build a laser microphone.”
 When I was doing research at Hughes Aircraft, I visited Hughes Research Laboratories in Malibu CA a few times. Afterwards named to HRL laboratories, I’d not remembered that HRL was where the first working model of the laser was created in 1960. Really liked that research vibe.
 Here’s a photo of the apparatus required to study SPDC (below). But at over $10,000 something that’s unlikely to be in your home lab, eh.
The general topic involves experiments with correlated photons. In the Immersion we will cover the following lab exercises, which include full hands-on setup and alignment: Spontaneous parametric down-conversion, single-photon interference, quantum eraser, Hanbury-Brown-Twiss test, entanglement, Bell inequality violation. … Thirty years ago, such experiments represented a tour de force of technology and equipment; today they can be done in a few afternoons in a junior-level optics lab, thanks to current photon-counting technology and the use of nonlinear crystals to produce entangled photon pairs. Yet these experiments are still closely related to active research in quantum information and the fundamentals of quantum mechanics.
 “How Quantum Randomness Saves Relativity” – “In physics, Albert Einstein is famous for two things: developing the theory of relativity, and hating quantum mechanics.” — Chad Orzel, Associate Professor in the Department of Physics and Astronomy at Union College; author of How to Teach Physics to Your Dog and How to Teach Relativity to Your Dog and Eureka: Discovering Your Inner Scientist.
In this article Orzel explains why photon state correlation does not permit faster-than-light communication: “This might seem like it opens the possibility of faster-than light communication between Alice and Bob. They simply share entangled photons with each other, and then measure their polarizations, calling one outcome ‘0’ and the other ‘1.’ This lets them transmit messages in binary code, and violate the restriction from relativity that nothing can exceed the speed of light. But this is where quantum randomness, the divine dice-throwing that Einstein derided, steps in to save the day.” And linked science blog posts discuss the consequences for the idea of causality.