Quantum Entanglement

An illustration of photon entanglement (Mliu92 via Wikimedia Commons).

In entry six of this encyclopedia,* we considered Schrodinger’s cat, a thought experiment justifying the Copenhagen Interpretation and quantum superposition; as I had little knowledge of physics at the time, the studying was much more strenuous, the rabbit holes much deeper. As such, I must admit that I was mistaken through portions of the entry. After several entries on astrophysics and quantum mechanics, I am beginning to understand, to an extent, the basics. 

Nevertheless, I thought I’d write this entry to both humble myself and demonstrate that I still lack a solid understanding of quantum physics–and, indeed, physics as a whole. Accordingly, in our relentless pursuit of knowledge, we stumble upon the greatness of quantum entanglement, a tremendous topic of innovation and paradox.

An overview and history of quantum entanglement

Quantum entanglement occurs when two or more quantum particles exist and interact in such a way that the quantum states of the particles are inherently interdependent. The implications of this concept–which could include superluminal☨ information exchanges–places quantum entanglement at the epicenter of the conflicts between classical and quantum mechanics. 

Entanglement was first considered by physicists Albert Einstein, Boris Podolsky, and Nathan Rosen, in their famous thought experiment known as the Einstein-Podolsky-Rosen (EPR) paradox, which argued that descriptions of quantum systems given by Schrodinger’s wave function are not complete, although correct. The three scientists later speculated that separate particles could become “entangled,” leading them to both be measurable✢ merely by observation of one particle. The paper written on the EPR paradox eventually reached Erwin Schrodinger, who ultimately wrote back to the original authors; in his response, he first coined the term “entanglement.” Interested in building on the idea, Schrodinger wrote a paper, later discovering and writing himself that quantum entanglement was “the characteristic trait of quantum mechanics.” Ultimately, as entanglement became more mainstream, Einstein and Schrodinger were still dissatisfied with the fact that entanglement seemed to violate the theory of relativity, as the entangled particles seemed to transmit information instantaneously–a paradox that, to this day, cannot be explained through special relativity. 

Decades after the EPR paradox and Schrodinger’s groundbreaking paper on entanglement, another physicist by the name of John Stewart Bell, demonstrated that the principle of locality, which states that an object is directly influenced by only its immediate surroundings, was mathematically incongruent with quantum theory; this both opened the door to other external influences such as entanglement, but it also eventually complicated many detection experiments. Not much later, the first observational evidence for entanglement came when physicist Carl Kocher observed two photons emitted from a calcium atom that were entangled. 

Many subsequent experiments concluded that entanglement existed, but all ultimately cast questions about its nature, often as a result  of locality as well as detection; as such, significant doubt was cast even on the simplest entanglement hypotheses until the 21st century. It wasn’t until 2015 that those questions were finally closed, when an experiment proved beyond all reasonable doubt that two photons were entangled with one another, as the correlations in their properties could never be explained by locality. At this point, entanglement became a cohesive concept in quantum mechanics. But there remains one single loophole, superdeterminism, which is also a loophole to Bell’s Theorem. According to superdeterminism, one can evade Bell’s Theorem and entanglement itself by arguing that the quantum system does not have “free will” when it is measured; essentially, such quantum measurements are biased in such a way that they force particles to act accordingly. Nevertheless, superdeterminism will always be a loophole to entanglement; as the experimenter Alain Aspect said himself, "no experiment, as ideal as it is, can be said to be totally loophole-free." 

With the groundbreaking 2015 experiment came the meteoric rise in quantum entanglement as a quantum mechanical concept, and also the equally meteoric rise of entanglement in popular culture (e.g. “Scientists Discover Phenomenon That Allows Matter to Travel at Speeds Faster Than Light”).

The concept of entanglement

To understand entanglement in the broadest sense, let us first consider a photon (the massless particle that mediates the electromagnetic force): Picture an experiment in which a concentrated laser is directed at two potential crystals; the two such potential crystals are BBO, otherwise known as beta-barium borate, consisting of one barium atom, two boron atoms, and four oxygen atoms, and lithium niobate, consisting of one lithium atom, one niobium atom, and three oxygen atoms. When a concentrated laser beam makes contact with these crystals, the crystals exhibit unusual optical effects that in rare cases result in a single photon from a beam being “split” into two photons, each with a fraction (generally half) of the energy of the original photon.** 

The two new photons, maintaining the same properties of the original photon, are now entangled. These two are essentially mirror images of one another: the characteristics of the two photons complement one another. For example, one photon may have an up-spin, while the other has a down-spin. Such is the case with many quantum systems, including quarks and leptons. 

Entanglement seems to arise in nature as often as it does in the laboratory. One notable study, “Billions of Quantum Entangled Particles Found in Strange Mineral,” found exactly what was said in the title. 

As such, the billions and billions of particles discovered to be entangled in this strange metal arose one more hypothetical–and existential–question: Could all the particles in the universe be entangled? For the moment, little evidence appears to back this idea up, and as more particles are “measured,” we will be able to determine whether an “entangled universe” exists or not.

Without evidence for or against this idea, one further question arises: Could the beginning of the universe have initiated the particle interactions that would entangle the entire universe? According to the peer-reviewed paper Everything is Entangled, by physicists Roman Buniy and Stephen Hsu, Big Bang cosmology is the primary source of evidence for a universe dominated by quantum entanglement. The two argued that, considering every particle in the universe derives from the same place, it is plausible that they all could be entangled with one another; furthermore, as demonstrated by Stephan Hoyer, the strength of entanglement does decay over time, but it is a horizontal asymptotic decay; as a result, although entanglement between two particles may weaken over time, it can never be lost. If such is the case, all particles that existed during the Big Bang could be entangled, although such entanglement is weaker than it was shortly after the Big Bang.

An additional concept of entanglement is waveform collapse. Waveform collapse occurs when a particle is observed; a photon in wave-particle duality, for example, would be observed as a particle or as a wave after waveform collapse. Waveform collapse affects quantum entanglement in the same way it affects superposition: Entangled particles become disentangled. When a particle interacts with its environment beyond its interactions with other entangled particles, it is separated from the particles it was once entangled with. 

Wrapping it up

Entanglement is a phenomenon that links particles together, even at vast distances, such that two particles can be mirror images of one another. The concept of entanglement attracted many of the greatest physicists in human history, including Albert Einstein and Erwin Schrodinger, yet many questions remain unanswered regarding the idea. As with most models in quantum physics, there is yet so much to be understood. As always, take care and stay curious.

* Entry six is available via What is Schrodinger's Cat? The Thought Experiment Behind Quantum Mechanics.

✢ Measurement, in this case, is purely hypothetical, for any such measurement will lead to waveform collapse, decoherence, and loss of entanglement. 

** Conceptually, photon splitting, although rare, is possible, as such splits can occur while conserving momentum and energy.

If you have any questions, comments, or corrections, please comment on this post or email learningbywilliam@gmail.com with your concerns. Thank you.

References

Boyd, J. (2020, January 16). Billions of quantum entangled electrons found in 'strange metal'. ScienceDaily. Retrieved January 15, 2023, from https://www.sciencedaily.com/releases/2020/01/200116144105.htm

Buniy, R. V., & Hsu, S. D.H. (2012, December 5). Everything is Entangled. Physics Letters B, 718(2), 233-236. https://doi.org/10.48550/arXiv.1205.1584

Cartwright, J. (2010, July 28). Photons meet with three-way split. Scientific American. Retrieved January 15, 2023, from https://www.scientificamerican.com/article/photons-meet-with-three-split/

Myrvold, W., Genovese, M., & Shimony, A. (2004, July 21). Bell's Theorem (Stanford Encyclopedia of Philosophy). Stanford Encyclopedia of Philosophy. Retrieved January 15, 2023, from https://plato.stanford.edu/entries/bell-theorem/

Piccard, J. (n.d.). Einstein–Podolsky–Rosen paradox. Wikipedia. Retrieved January 15, 2023, from https://en.wikipedia.org/wiki/EPR_paradox

Quantum entanglement. (n.d.). Wikipedia. Retrieved January 15, 2023, from https://en.wikipedia.org/wiki/Quantum_entanglement

Quantum superposition. (n.d.). Wikipedia. Retrieved January 15, 2023, from https://en.wikipedia.org/wiki/Quantum_superposition

Rethinking Superdeterminism. (n.d.). Frontiers. Retrieved January 15, 2023, from https://www.frontiersin.org/articles/10.3389/fphy.2020.00139/full

Schirm, C. (2020, January 7). What is locality in physics? In plain Englishlocality. Quantum Physics Lady. Retrieved January 15, 2023, from http://www.quantumphysicslady.org/glossary/locality/

Superposition, Interference and Decoherence - Quantum Theory and the Uncertainty Principle. (n.d.). The Physics of the Universe. Retrieved January 15, 2023, from https://www.physicsoftheuniverse.com/topics_quantum_superposition.html