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Image of entangled photons?

John Healy

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Today’s news cycle contains articles about research by some physicists at the University of Glasgow who claim to have imaged entangled photons. Looks like they used a precision laser-based, table-top optical bench system. This Cnet article is a basic summary of the research: “Einstein called it ‘spooky action.’ Here’s an image of it for the first time — Quantum entanglement is one of the weirdest phenomena in all of science, so physicists tried to take a picture of it” (July 12, 2019).

Physicists at the University of Glasgow set up a complicated experiment to capture in a single image what Einstein called “spooky action at a distance.” A pair of photons were shot from a laser, split and sent on very different journeys before being captured by a special camera. The resulting image consistently showed what looks like a pair of photons mirroring each other to form a ring shape.

The article cites the paper on the process published in Science Advances: “Imaging Bell-type nonlocal behavior.” The paper contains images of 4 phase filters with different orientations. The paper has links to supplementary materials such as the detailed experimental setup.

The violation of a Bell inequality not only attests to the nonclassical nature of a system but also holds a very unique status within the quantum world. The amount by which the inequality is violated often provides a good benchmark on how a quantum protocol will perform. Acquiring images of such a fundamental quantum effect is a demonstration that images can capture and exploit the essence of the quantum world. Here, we report an experiment demonstrating the violation of a Bell inequality within observed images. It is based on acquiring full-field coincidence images of a phase object probed by photons from an entangled pair source.

This experiment … illustrates that Bell-type nonlocal behavior can be demonstrated within a full-field quantum imaging protocol. Because we do not close all the various loopholes, our demonstration cannot be interpreted as another absolute demonstration that the world is behaving in a nonlocal way. However, these loopholes are not fundamentally associated with the experimental paradigm presented here and could be, in principle, closed with technically more advanced detectors and phase-image displays.

We see no easy way of qualitatively reproducing our imaging results using only classical correlations, let alone the quantitative violation of a Bell inequality that we report here, which requires entanglement.

… we proposed and demonstrated the use of an imaging scheme to perform a demonstration of a Bell-type inequality.

The Cnet article contains a YouTube video of The Bell Test. See the comments as to whether that visualization was helpful or not.

This video tells a story of ‘quantum love’ to illustrate of how the Bell Test works, and how loopholes affect it. The video illustrates the Bell Test as described in the paper: ‘Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres’ by B. Hensen et al. at the Delft University of Technology (published on Oct 21, 2015).

The history of quantum entanglement is fascinating. Many science communicators have tried to explain the evidence in a non-technical way. Central to that story, however, is understanding the role of statistical correlation, which can be technically challenging.

A system can be tested for entanglement. The University of Glasgow physicists demonstrated that quantum imaging can be used to detect the presence of Bell-type entanglement.

My understanding is that in a standard SPDC table-top optical bench system, we cannot say whether two particular photons are entangled (e.g., as to polarization). We can only say statistically over many many detections of photons from a laser stream that the Bell Inequality is violated, as well as estimate the efficiency of the lab system in generating those entangled photons (as a percent of all photon events).

And in general not all entanglements are the same — the amount of entanglement can vary between quantum states. And there are various ways to quantify that.

Other posts

• How to create entangled photon pairs

Wiki references

• Quantum entanglement

Quantum entanglement is a physical phenomenon that occurs when pairs or groups of particles are generated, interact, or share spatial proximity in ways such that the quantum state of each particle cannot be described independently of the state of the others, even when the particles are separated by a large distance.

… all interpretations agree that entanglement produces correlation between the measurements and that the mutual information between the entangled particles can be exploited, but that any transmission of information at faster-than-light speeds is impossible.

Note that the state of a composite system is always expressible as a sum, or superposition, of products of states of local constituents; it is entangled if this sum necessarily has more than one term.

Entanglement is broken when the entangled particles decohere through interaction with the environment; for example, when a measurement is made.

The electron shell of multi-electron atoms always consists of entangled electrons. The correct ionization energy can be calculated only by consideration of electron entanglement.

• Principle of locality

In physics, the principle of locality states that an object is directly influenced only by its immediate surroundings. A theory which includes the principle of locality is said to be a “local theory”. This is an alternative to the older concept of instantaneous “action at a distance“. Locality evolved out of the field theories of classical physics. The concept is that for an action at one point to have an influence at another point, something in the space between those points such as a field must mediate the action.

• Action at a distance vs. physical interaction by contact (collision)

In physics, action at a distance is the concept that an object can be moved, changed, or otherwise affected without being physically touched (as in mechanical contact) by another object. That is, it is the nonlocal interaction of objects that are separated in space.

This term was used most often in the context of early theories of gravity and electromagnetism to describe how an object responds to the influence of distant objects. For example, Coulomb’s law and Newton’s law of universal gravitation are such early theories. 

More generally “action at a distance” describes the failure of early atomistic and mechanistic theories which sought to reduce all physical interaction to collision. The exploration and resolution of this problematic phenomenon led to significant developments in physics, from the concept of a field, to descriptions of quantum entanglement and the mediator particles of the Standard Model.

• Spontaneous parametric down-conversion (SPDC)

4 thoughts on “Image of entangled photons?”

  1. More articles in the news cycle on this post:

    • Scientists unveil the first-ever image of quantum entanglement by University of Glasgow (July 13, 2019)

    They set up a super-sensitive camera capable of detecting single photons which would only take an image when it caught sight of both one photon and its entangled ‘twin’, creating a visible record of the entanglement of the photons.

    • Scientists unveil image of quantum entanglement for the first time ever — The Bell entanglement depicts two photons sharing a physical state (July 12, 2019)

    •

  2. Regarding entangled photons: Phys.org > “Researchers develop practical method for measuring quantum entanglement” by Rochester Institute of Technology (August 26, 2019).

    When two quantum particles—such as photons, electrons or atoms—become entangled, they have special correlations that show up in their measurements even when the particles are separated by an enormous distance. This unique property, which can only be explained through quantum mechanics, is at the heart of many of the technologies as part of the National Quantum Initiative.

    As quantum technologies become more complex, users will need a way to calculate how much quantum entanglement exists within a given system. For the system in this study — involving spatially entangled photon pairs — the new technique needed a million-times fewer measurements than previous methods. And because the technique is based on information theory, the measurement technique has the added benefit of never overestimating how much entanglement is in a system.

  3. Caltech Magazine > “Untangling Quantum Entanglement” by Whitney Clavin (Fall 2019)

    So, entangling 2 photons is one thing, but how do you entangle 100 atoms?

    The electrons, photons, and other particles that make up our universe can become inextricably linked, such that the state observed in one particle will be identical for the other. That connection, known as entanglement, remains strong even across vast distances.

    “When particles are entangled, it’s as if they are born that way, like twins,” says Xie Chen, associate professor of theoretical physics at Caltech. “Even though they might be separated right after birth, [they’ll] still look the same. And they grow up having a lot of personality traits that are similar to each other.”

    “It may be tempting to think that the particles are somehow communicating with each other across these great distances, but that is not the case,” says Thomas Vidick, a professor of computing and mathematical sciences at Caltech. “There can be correlation without communication.” Instead, he explains, entangled particles are so closely connected that there is no need for communication; they “can be thought of as one object.”

    Or, to simplify, consider two “entangled” quarters, each hidden under a cup. If two people, Bob and Alice, were each to take one of those quarters to a different room, the quarters would remain both heads and tails until one person lifted the cup and observed his or her quarter; at that point, it would randomly become either heads or tails. If Alice were to lift her cup first and her quarter was tails, then when Bob observed his quarter, it would also be tails. If you repeated the experiment and the coins are covered once more, they would go back to being in a state of superposition. Alice would lift her cup again and might find her quarter as heads this time. Bob would then also find his quarter as heads. Whether the first quarter is found to be heads or tails is entirely random.

    According to Manuel Endres, an assistant professor of physics at Caltech, one of the first steps toward understanding many-body entanglement is to create and control it in the lab. To do this, Endres and his team use a brute force approach: they design and build laboratory experiments with the goal of creating a system of 100 entangled atoms.

    Another factor in creating and controlling quantum systems has to do with their delicate nature. Like Mimosa pudica ,a member of the pea family also known as the “sensitive plant,” which droops when its leaves are touched, entangled states can easily disappear, or collapse, when the environment changes even slightly.

    The problem is that entangled particles become entangled with the environment around them quickly, in a matter of microseconds or faster. This then destroys the original entangled state a researcher might attempt to study or use. Even one stray photon flying through an experiment can render the whole thing useless.

    “You need to be able to create a system that is entangled only with itself, not with your apparatus,” says Endres. “We want the particles to talk to one another in a controlled fashion. But we don’t want them to talk to anything in the outside world.”

    “Up until about 20 years ago, the best way to explore entanglement was to look at what nature gave us and try to study the exotic states that emerged,” notes Painter. “Now our goal is to try to synthesize these systems and go beyond what nature has given us.”

    While entanglement is the key to advances in quantum-information sciences, it is also a concept of interest to theoretical physicists, some of whom believe that space and time itself are the result of an underlying network of quantum connections.

  4. As a follow-up to my comment on Caltech Magazine’s Fall 2019 article “Untangling Entanglement,” the Letters to the Editor in the Spring 2020 issue are interesting.

    Still entangled?

    That was a fascinating article about quantum entanglement (“Untangling Entanglement,” Fall 2019). It mentioned how a single photon could upset a delicate experiment. But if quantum gravity and gravitons are real, why don’t gravitons get entangled with experimental setups? Wouldn’t gravitons be part of the environment? A curious mind wants to know. – BILL HOLLAND (BS ’77)

    If a particle like an electron is in a superposition of two different positions, a single emitted photon could reveal the position of the particle, causing that delicate superposition to decohere. In principle, emission of a single graviton could have the same effect. In practice, however, gravity is such a weak force that we never need to worry about this source of decoherence in today’s laboratory experiments. — John Preskill, Richard P. Feynman Professor of Theoretical Physics

    As part of the article on quantum entanglement, I wish you had mentioned some of the history and the role played by Caltech alumni. The first experiment to demonstrate entanglement was done by my Caltech classmate John Clauser (BS ’64) [later at Lawrence Berkeley Labs]. Following this original experiment, numerous more refined versions have been carried out. – FRANK WINKLER (BS ’64)

    The recent article on quantum entanglement misunderstood the nature of the perfect correlation between spin measurements of entangled particles with net zero spin (e.g., superposition of horizontal and vertical spin). If one particle is measured to have horizontal polarization when the polarizer is at angle alpha, its twin will instantly adopt vertical polarization if measured at the same angle. If the twin also adopted horizontal polarization, angular momentum would not be conserved. – JERRE LEVY (PHD ’70)

    Re Preskill’s answer: So, when gravity becomes important, when gravity is strong as in a black hole (vs. a terrestrial lab experiment), might “gravitons” decohere an electron in a state of superposition?

    What might happen if two entangled electrons approach or cross the event horizon? Or, what happens when entangled photons cross (or approach) the event horizon? – as if emitted from a spaceship doing that lab experiment nearby.

    Generally, is there anything different regarding entanglement near or inside the event horizon? Theoretically.

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