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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.
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.
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.
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.