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How to create entangled photon pairs

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.

[1] 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.”

[2] 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.

[3] 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.

Photo of apparatus
Experiments with correlated photons

[4] “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.


14 thoughts on “How to create entangled photon pairs

  1. 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.”

    • Entanglement from birth
    • Second-generation entanglement
    • Entanglement by accident
    • Entanglement by interaction
  2. 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:

    … Townes was in the early stages of developing what would become the laser. Independently, and with crushing definitiveness, both von Neumann and Bohr told him that his project could not work, because of the Heisenberg uncertainty principle, which prohibited the necessary perfect alignment of photons. In his autobiography, How the Laser Happened, Townes wrote, “I’m not sure I ever did convince Bohr.” — Gilder, Louisa (2008-11-11). The Age of Entanglement (Kindle Locations 4681-4684). Knopf Doubleday Publishing Group. Kindle Edition.

  3. 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.

    Video frame

    Published on Jan 12, 2015 — Quantum entanglement and spooky action at a distance are still debated by professors of quantum physics (I know because I discussed this topic with two of them).

    Does hidden information (called hidden variables by physicists) exist? If it does, the experiment violating Bell inequalities indicates that hidden variables must update faster than light – they would be considered ‘non-local’. On the other hand if you don’t consider the spins before you make the measurement then you could simply say hidden variables don’t exist and whenever you measure spins in the same direction you always get opposite results, which makes sense since angular momentum must be conserved in the universe.

    Everyone agrees that quantum entanglement does not allow information to be transmitted faster that light. There is no action either detector operator could take to signal the other one – regardless of the choice of measurement direction, the measured spins are random with 50/50 probability of up/down.

    [1] Veritasium is a channel of science and engineering videos featuring experiments, expert interviews, cool demos, and discussions with the public about everything science.

    [2] 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).

  4. This June 15, 2017, article “New Quantum-Entanglement Record Could Spur Hack-Proof Communications” discusses a fascinating experiment in long-distance entangled photon transmission.

    In a new study published online today (June 15) in the journal Science, researchers report the successful distribution of entangled photon pairs to two locations on Earth separated by 747.5 miles (1,203 km). [The 18 Biggest Unsolved Mysteries in Physics]

    In the new study, researchers used China’s Micius satellite, which was launched last year, to transmit the entangled photon pairs. The satellite features an ultrabright entangled photon source and a high-precision acquiring, pointing and tracking (APT) system that uses beacon lasers on the satellite and at three ground stations to line up the transmitter and receivers.

  5. 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.

    What has the Chinese team achieved?

    They created 4,000 pairs of quantum-entangled photons per second at their laboratory in Tibet and fired one of the photons from each pair in a beam of light towards a satellite called Micius, named after an ancient Chinese philosopher.

    Micius has a sensitive photon receiver that can detect the quantum states of single photons fired from the ground. Their report – published online – says it is the first such link for “faithful and ultra-long-distance quantum teleportation”.

    July 15, 2017 – Chinese Scientists Just Set the Record for the Farthest Quantum Teleportation.

    Chinese scientists have just shattered a record in teleportation. No, they haven’t beamed anyone up to a spaceship. Rather, they sent a packet of information from Tibet to a satellite in orbit, up to 870 miles (1,400 kilometers) above the Earth’s surface.

    More specifically, the scientists beamed the quantum state of a photon (information about how it is polarized) into orbit.

    Not only did the team set a record for quantum teleportation distance, they also showed that one can build a practical system for long-distance quantum communications. Such a communication system would be impossible to eavesdrop on without alerting the users, which would make online communications much more secure.

  6. 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.

    Now the idea of quantum error correction is even giving us new ways of thinking about space and time. I love black holes. I mean doesn’t everybody? How can you not love an object which is made out of nothing but pure warped space-time geometry. …

    Well, it turns out that 40 years ago Stephen Hawking was spending a sabbatical year at Caltech and he spent a lot of that time thinking about quantum entanglement between the inside and the outside of a black hole and realized that that gives rise to some deep questions which we still don’t know the answer to after 40 years. But the struggle with the puzzles having to do with black hole entanglement have led to a very audacious idea that we call the holographic principle.

    … it’s a speculative idea, but if our current notions of quantum gravity are on the right track, it really seems to be true. And what we’re beginning to appreciate is that this very scrambled encoding of information on the boundary is really an instance of quantum error correction — that the geometry in the room, say which seats in the auditorium are close to other seats — is encoded in the quantum entanglement on the boundary.

    So if that’s correct it means that entanglement is really the fundamental notion that underlies space. That it is quantum entanglement that holds space together. Now if that’s true, it means that the quantum structure of space is something that we ought to be able to study in the laboratory. [See the YouTube video — published on Feb 14, 2016, the IQIM Caltech channel.]

  7. Followup: Going beyond entangled pairs for quantum computing:, April 30, 2018, “These ‘Spooky’ Entangled Atoms Just Brought Quantum Computing One Step Closer.”

    “Twenty years ago, entanglement of two particles was a big deal,” study co-author Rainer Blatt, a physics professor at the University of Innsbruck in Austria, told Live Science. “But when you really go and want to build a quantum computer, you have to work with not just say five, eight, 10 or 15 qubits. In the end, we will have to work with many, many more qubits.”

    The team managed to entangle 20 particles together into a controlled network — still short of a true quantum computer but the largest such network to date. And while they still need to confirm that all 20 are fully entangled with each other, it’s a solid step toward the supercomputers of the future. To date, qubits have not outperformed classical computer bits, but Blatt said that moment — often called the quantum advantage — is coming.

  8. Astrophysicist Paul Sutter explains why entangled photos do not permit communication faster than the speed of light in this article “Quantum Weirdness May Seem to Outrun Light — Here’s Why It Can’t” (September 29, 2018).

    Instead of picturing a hard, solid, precise point in space and time, scientists now see a particle as a cloud of fuzzy probabilities … quantum states. … the real fun begins when we get two particles to share a quantum state. In certain circumstances, we can connect two particles in a quantum way, so that a single mathematical equation describes both sets of probabilities simultaneously.

    According to quantum theory, as soon as one “choice” is made, the partner particle instantly “knows” what spin to be. It appears that communication can be achieved at faster-than-light speeds, …

    The resolution to Einstein’s question comes via an excruciatingly careful examination of who knows what and when. … while the process of disentanglement happens instantaneously, the revelation of it does not. We have to use good old-fashioned no-faster-than-light communication methods to piece together the correlations that quantum entanglement demands.

  9. 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).

    This Saturday, May 16, is the 60th anniversary of the first working laser, achieved by Theodore Maiman at Hughes Labs in 1960. At the time, one of Maiman’s colleagues, Irnee d’Haenens, famously dubbed the laser “a solution in search of a problem”— it wasn’t immediately obvious what a coherent source of light was really for.

    There are lots of ways to categorize laser applications, but I tend to favor breaking them into two rough groups: there are those applications that just need a bright source of light, and those that need the special quantum properties of laser light.

    … experiments [relying on the coherence properties of laser light] include the ultra-fast, high-intensity laser pulses … where exercising exquisite control over the frequency and intensity distribution of the laser light lets you create pulses short enough to probe the behavior of atoms and molecules on a time scale comparable to that of the electrons moving within the atom itself. These can also be used for “high-harmonic generation” experiments, making tabletop x-ray lasers by blasting short pulses into samples of particular gases and cleverly manipulating the properties of the beam as it travels through.

  10. 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).

    Sources based on parametric down conversion operate at very low efficiency per pulse due to the probabilistic generation process. Semiconductor quantum dots can emit single pairs of entangled photons deterministically but they fall short due to the extremely low-extraction efficiency.[1]

    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).

    In the past decade, researchers have made giant strides in demonstrating entanglement – the nonclassical correlation between sometimes distantly separated systems of photons or particles that constitutes a key aspect of “quantum weirdness.

    Much of the entanglement news in recent years has related to demonstrations of large numbers of entangled particles in a single system, or the transfer of an entangled state across increasing distances (stretching, in a 2017 spaceborne example, to some 1200 km). From the point of view of building a true quantum network with multiple entangled nodes, however, these examples have a problem: They tend to be built on “probabilistic” platforms. That means, in essence, that the generation and transfer of a entangled quantum state has some (possibly small) finite likelihood, to be realized through a (possibly very large) number of repeated efforts.

    A network of multiple nodes, by contrast, requires a more reliable, deterministic scheme—one that can create and transfer quantum states essentially on demand.

    The ETH Zürich method (Nature, doi: 10.1038/s41586-018-0195-y) taps into an idea first floated theoretically more than 20 years ago (Phys. Rev. Lett., doi: 10.1103/PhysRevLett.78.3221). That concept involved using laser pulses to excite an atom within a single-mode optical cavity, mapping the atom’s quantum state into a time-symmetric photon that is then sent through a transmission line to a receiving node and absorbed by a receiving atom. The approach, according to the original paper, should be able to transfer entanglement from the first atom to the second “with unit probability.”

    [1] Nature > “Highly-efficient extraction of entangled photons from quantum dots using a broadband optical antenna” (July 31, 2018).

    … photon-pair source efficiency is defined as the probability of collecting a photon pair per excitation pulse … This device achieves a combination of brightness, single-photon purity (99.8%) and entanglement fidelity (90%).

    [2] > “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.

    So, what do we gain and what do we lose when we use SPDC? There is of course a lot to gain. In spite of the efficiency of the process being low (1 in 10^9 or 1 in 10^10 pump photons undergo the SPDC process) but still SPDC process generates some of the brightest photon sources (one of the brightest being ~2 MHz). One of downsides of the SPDC process is the fact that it is probabilistic. Thus, if an application requires a photon on-demand, SPDC is not our solution. Moreover, there is a finite probability of multi-photon events which can cause security loopholes in quantum key distribution protocols; although the situation is much better than weak coherent pulses where such probabilities are much higher.

    [3] This 2010 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).

  11. 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.”[1] 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)

    What is interaction and when does it occur? Intuition suggests that the necessary condition for the interaction of independently created particles is their direct touch or contact through physical force carriers. In quantum mechanics, the result of the interaction is entanglement – the appearance of non-classical correlations in the system. It seems that quantum theory allows entanglement of independent particles without any contact. The fundamental identity of particles of the same kind is responsible for this phenomenon.

    Effects of particle identity are usually associated with their statistics having consequences for a description of interacting multi-particle systems (such as Bose-Einstein condensate or solid-state band theory). In the case of simpler systems, the direct result of particle identity is the Pauli exclusion principle for fermions or bunching in quantum optics for bosons. The common feature of all these effects is the contact of particles at one point in space, which follows the simple intuition of interaction (for example, in particle theory, this comes down to interaction vertices). Hence the belief that the consequences of symmetrization can only be observed in this way. However, interaction by its very nature causes entanglement. Therefore, it is unclear what causes the observed effects and non-classical correlations: is it an interaction in itself, or is it the inherent indistinguishability of particles?

    [1] Cf. John Wheeler’s quip that there really is only one electron in the universe.

  12. More research on single photon emitters …

    • > “Devil in the defect detail of quantum emissions unravelled” by University of Technology, Sydney (Nov 2, 2020)

    In new research, published in Nature Materials, an international multidisciplinary collaboration led by the University of Technology Sydney (UTS), has uncovered the chemical structure behind defects in white graphene (hexagonal boron nitride, hBN), a two dimensional nanomaterial that shows great promise as a platform for generating quantum light.

    The defects, or crystal imperfections, can act as single photon sources and an understanding of their chemical structure is critical to being able to fabricate them in a controlled way.

  13. 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.”

    • > “Quantum entanglement: A simple explanation” by Jesse Emspak, contributions from Kimberly Hickok (March 16, 2022)

    (image caption) Quantum entanglement is when two particles link together in a certain way no matter how far apart they are in space. Their [quantum] state remains the same [i.e., correlated]. (Image credit: MARK GARLICK/SCIENCE PHOTO LIBRARY via Getty Images.)

    (graphic caption) This cartoon helps explain the idea of entangled particles. Alice and Bob represent photon detectors, which NASA’s Jet Propulsion Laboratory and the National Institute of Standards and Technology developed. (Image credit: NASA/JPL-Caltech)

    Only the cartoon contains the word correlated. So, the context may be unclear.

  14. Here’s a twist on spontaneous parametric down-conversion, and how the non-classical correlation was verified.

    • Physics World > “Microwave photons are entangled with optical photons” by Tim Wogan (28 May 2023) – A protocol for entangling microwave and optical photons has been demonstrated by researchers in Austria.

    Entangled photon pairs are generated by a process called spontaneous parametric down-conversion, whereby one photon splits into a two. However, nobody had previously managed to generate an entangled pair of photons whose energy differed by a factor of more than 10,000. This difference encompasses a photon at an optical telecoms wavelength of about 1550 nm; and another at a microwave wavelength of about 3 cm.

    Fink’s group pumped a lithium niobate optical resonator that was part of a microwave resonator with a high-power laser at telecom wavelengths. The vast majority of the laser light simply came back out of the resonator unchanged and was filtered out. However, approximately one photon per pulse split into two entangled photons – one microwave and the other at a wavelength just slightly longer than the pump photons.

    “We verified this entanglement by measuring the covariances of the two electromagnetic field fluctuations. We found microwave-optical correlations that are stronger than classically allowed, which signifies that the two fields are in an entangled state.” says Liu Qiu, a postdoctoral researcher and joint first author on the paper describing the work.


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