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.4 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.
13 thoughts on “How to create entangled photon pairs”
Chad Orzel’s article “How Do You Create Quantum Entanglement?” also discusses “how you get them entangled in the first place. … you can’t just arbitrarily entangle two particles that have no common history.”
Chapter 1 “The Light that Shines Straight” of Charles H. Townes‘ book How the Laser Happened: Adventures of a Scientist — discusses the myriad uses of the laser (as well as how lasers work).
And here’s an interesting note regarding the history of quantum mechanics:
This YouTube Veritasium1 channel video “Quantum Entanglement & Spooky Action at a Distance” is an interesting visualization of experiments which tested Bell’s Theorem using spin of electrons. There is no introduction comparing the context to other common statistical models such as coins or to a game of cards with a demonic quantum dealer. 2 Is presentation of what each side of the proposal predicts/expects (with analogies), the no-go criteria, and the (tabulated) results clear enough? I like the approach, however, and have thought of doing something similar with tennis balls.
 Veritasium is a channel of science and engineering videos featuring experiments, expert interviews, cool demos, and discussions with the public about everything science.
 The best TV documentary on this topic which I’ve viewed is Episode 1 of The Secrets of Quantum Physics, a two-part TV series for BBC in 2014 [the discussion of quantum entanglement and Bell’s Theorem starts ~31′ into the video]. The documentary uses the analogies of (entangled) spinning coins and a (hidden in boxes) pair of gloves; discusses the “shut up and calculate” mantra (avoiding the debate about quantum reality) after WWII; then John Bell’s work in the 1960’s (“how do you look behind the curtain without pulling it open”), an analogy using a game of cards (2-card reveal) with a demonic quantum dealer and testing whether the dealer is rigging (à la Einstein) the deck, conclusion that “at the fundamental quantum level reality is truly unknowable,” Bell’s equation 1964, hippie physicists, 1972 experimental testing (John Clauser), photon polarization laser apparatus (4 settings, 4 runs).
This June 15, 2017, Space.com article “New Quantum-Entanglement Record Could Spur Hack-Proof Communications” discusses a fascinating experiment in long-distance entangled photon transmission.
July 14, 2017 – Here’s another article on long-distance entangled photon transmission: Chinese scientists say they have “teleported” a photon particle from the ground to a satellite orbiting 1,400km (870 miles) away.
July 15, 2017 – Chinese Scientists Just Set the Record for the Farthest Quantum Teleportation.
In his January 26, 2016, talk “Quantum is Different: Part 2 – One Entangled Evening,” physicist John Preskill speculated that entanglement is really the fundamental notion that underlies space.
Followup: Going beyond entangled pairs for quantum computing: Space.com, April 30, 2018, “These ‘Spooky’ Entangled Atoms Just Brought Quantum Computing One Step Closer.”
Astrophysicist Paul Sutter explains why entangled photos do not permit communication faster than the speed of light in this Space.com article “Quantum Weirdness May Seem to Outrun Light — Here’s Why It Can’t” (September 29, 2018).
Here’s an article by Chad Orzel celebrating the laser. He mentions that using laser beams to push atoms around formed the basis for his career in physics – as in optical tweezers and laser cooling. And he cites the 2018 Nobel Prize in physics as illustrative of advances in laser applications.
Forbes > “‘Light Under Flawless Tutelage Knows No Limits’: Sixty Years Of Lasers Finding New Problems To Solve” by Chad Orzel (May 15, 2020).
A tabletop optical bench demonstration of nonclassical correlation between two photon streams is memorable. That shows violation of the Bell inequality – in a probabilistic manner. Quantum information applications, however, require indistinguishable and entangled photon pairs on demand – in a deterministic manner.
So, how do you create a single photon on demand? Without any bunching. How about a single entangled pair on demand? A way to transfer entangled quantum states on demand?
These challenges remain a focus of research in quantum computing and communications.
Using a probabilistic generation process, spontaneous parametric down conversion (SPDC) has a low conversion efficiency (how many pump photons undergo the SPDC process) and limited entanglement fidelity (how strongly photons are entangled).
So, this 2018 Optical Society article caught my attention awhile ago.
The Optical Society > Research News > “Entanglement On Demand” by Stewart Wills (June 19, 2018).
 Nature > “Highly-efficient extraction of entangled photons from quantum dots using a broadband optical antenna” (July 31, 2018).
 arXiv.org > “Single photon sources: ubiquitous tools in quantum information processing” (2019).
There’s a useful table in this article comparing properties and yield of various types of single photon sources.
 This 2010 arXiv.org article has more technical information on parametric down-conversion, double slit experiments – including double-slit quantum eraser, and photon entanglement (in the PDF paper): “Spatial correlations in parametric down-conversion” (October 6, 2010).
This is a premise of quantum physics which I’ve pondered for years. Namely, that fundamental “particles” are identical: “the fundamental indistinguishability of all particles of the same kind.” Consequences. This article raises the question of entanglement without local interaction (direct or mediated).
• SciTechDaily > “Quantum Entanglement of Independent Particles Without Any Contact – Ever” by Polish Academy Of Sciences (April 8, 2020)
 Cf. John Wheeler’s quip that there really is only one electron in the universe.
More research on single photon emitters …
• Phys.org > “Devil in the defect detail of quantum emissions unravelled” by University of Technology, Sydney (Nov 2, 2020)
Here’s a simplified non-technical piece about quantum entanglement: “Despite their vast separation, a change [e.g., measurement] induced in one will affect the other.”
• Space.com > “Quantum entanglement: A simple explanation” by Jesse Emspak, contributions from Kimberly Hickok (March 16, 2022)
Only the cartoon contains the word correlated. So, the context may be unclear.
Comments are closed.