Quantum Entanglement: Physics at its “Spookiest?”
Science & Technology - 15 Minute Read (+ 10 Minutes of Further Details)
An Overview
As the modern world braces for a technological revolution spearheaded by the offshoots of quantum mechanics, the role that quantum entanglement plays in our society grows increasingly significant. It is the weirdness, counter-intuition and, as Einstein believed, apparent impossibilities of entanglement that power these revelatory developments. However, at its core, quantum theory and its elusively enigmatic principles offer insights into realities that far surpass modern society.
Succinctly, quantum entanglement describes an intimate link between two particles that, at first glance, seem to have an influence over each another no matter the distance separating them. This link results in both of the entangled particles perpetually displaying corresponding properties if they are measured the same way, even if they are situated millions of light-years apart.
This results in a glaring inconsistency. It seems as though, given this inherent influence that two entangled particles have over each other, that an external observer would be able to obtain information on both of the particles by measuring the properties of just one of them. For a simplified example, say a particle can either have the property X or the property Y. This particle is then entangled with another particle, meaning that now the two properties must correspond, say for instance, they must always be opposite the other. As a result, given that quantum entanglement holds across all distances, an external observer would only have to measure one particle in the entangled pair: if they detect the property X, then they know that the other particle, wherever in space-time it may be, must have the property Y. Incredibly, if this other particle is then retrieved at some point and is sent through the same detector as the first particle (with property X), then this retrieved particle will always display the property Y, agreeing with quantum entanglement.
The true violatory inconsistency is that the observer would obtain such information instantaneously upon measuring just one of the two entangled particles, no matter how far across space the other particle is situated. In other words, it seems as though upon being measured, an entangled particle sends a “signal” of sorts across immense distances impossibly quickly to inform its distant partner particle which property it should display in order to sustain the entanglement correspondence.
In short, it is through this seemingly impossible relationship that entanglement and entangled particles have catalysed a revolution, both cognitively and practically, in the realm of science and technology. From encrypting national data to computing at unforeseen capabilities, quantum theory has induced a paradigm shift.
It should also be noted that the terms ‘quantum theory’ and ‘quantum entanglement’ are occasionally used interchangeably and although entanglement is certainly a subset of quantum theory, the immense significance of the phenomenon is what resulted in Erwin Schrödinger decreeing entanglement as ‘not one of but the feature of quantum theory.’ This has only been reinforced by the works of 2022 Physics Nobelists Alain Aspect, John Clauser, and Anton Zeilinger, with Zeilinger publishing ‘Dance of the Photons: Einstein, Entanglement and Quantum Teleportation’ to further publicise the true potential of these discoveries. (You can find my full book review to “Dance of the Photons” here)
Alan Aspect, John Clauser, and Anton Zeilinger were awarded the 2022 Nobel Physics Prize for their extensive works into quantum entanglement, particularly with the implementation of the Bell Tests.
Image Source: https://www.quantumlah.org/about/highlight/2022-10-physics-nobel-quantum
It is through this astonishing first impression of quantum entanglement that the concept was able to achieve its then-dismissive yet presently-iconic title of ‘spooky action at a distance.’ However, just how ‘spooky,’ if at all, is quantum entanglement?
In order to answer this one must establish the grounds upon which entanglement stands, the grounds upon which the entirety of quantum theory stands: superposition.
Superposition
Superposition is a property of a quantum system that enables it to be in multiple states, or more accurately, in all possible states, at any given time, represented by a wave function of all possibilities. Once the system is observed, the ‘real’ state is confirmed and the wave function collapses to leave behind a distinct reality.
A familiar example for such a quantum system would be a coin toss, but not a regular coin toss found in our macrocosmic world since such coin tosses are entirely predictable with classical mechanics. Instead, a ‘quantum coin toss’ would involve the outcome of the toss to be dictated by quantum processes, or random processes that with our current model of physics, we are unable to predict.
As a result, these ‘quantum coin tosses’ are entirely unpredictable, with the chances of heads and tails being precisely 50/50. When the coin is tossed and the result is not yet seen, the coin is said to be in a superposition of states, meaning that until observed, the coin has landed on both heads and tails. This is the heart of quantum theory and with this troubling and anti-deterministic fundamental that it posits, it is no surprise it caused a great division amongst the greatest of scientific minds.
The act of observation itself is what determines the state of a quantum system by breaking down the superposition wave function.
© Kabeer Hans, 2024
Furthermore, the act of ‘observing’ and the ‘observer’ themselves are other contested topics amidst quantum theory, with our current understanding having no conclusive definition for either. However, to grasp the concept of quantum entanglement, it is, if not the understanding, the acceptance of superposition that is required.
Entanglement
As mentioned, entanglement cannot exist without the idea of superposition and this is due to the fact that entanglement is a more complex and even further “rebellious” form of the concept. Rather than the superposition of all possible states in one place that was seen in the ‘quantum coin toss’ example, entanglement refers to the superposition of two quantum systems in all possible states in two different points in space.
However, as was briefly described in the introduction, entanglement is not merely two quantum coin tosses occurring in tandem independent of one another but instead, two quantum systems with a deep-rooted connection between them that results in them always displaying corresponding results when measured the same way.
Unfortunately, it is here where the ‘quantum coin toss’ analogy begins to break down as we employ quantum properties that function differently to the classical properties of ‘heads’ or ‘tails.’ Instead, the example utilised henceforth will be one with photons, the smallest units of light.
When two photons of light are emitted by a quantum process, such as a high-energy collision or excitation of ‘quantum dots’ (extended explanations can be found in the Further Details section below), they are entangled in a way that connects their properties on a fundamental level. To elaborate, when one entangled photon is measured for a certain property, such as its ‘spin’ with the two outcomes being ‘spin up’ or ‘spin down,’ the corresponding property of the other photon can be deduced with certainty.
When these photons are measured, they are passed through detectors measuring the photon’s ‘spin’ in certain orientations relative to the photon’s polarisation (the propagation of the wave). If the orientation of the detector matches the polarisation, the photon will be recorded as having ‘spin up’ while if the detector’s orientation is opposite to that of the photon’s spin, it will be recorded as having ‘spin down.’ With entanglement, we are able to predict the spin of a photon, ‘up’ or ‘down’, on the basis of the recorded spin of its entangled partner.
This is due to the laws of conservation of angular momentum and energy, which dictate that no energy can be created nor destroyed and systems must exist in energetic equilibrium. As a result, if the first photon is measured with ‘spin up,’ the second photon must have ‘spin down’ to neutralise angular momentum to zero and comply with the universal laws.
Photons, and all other fundamental particles, have an intrinsic “spin.” Although this is not actually the particles rotating, the analogy is appropriate in this case and the “up spin” or “down spin” is determined by detectors in certain orientations.
© Kabeer Hans, 2024
Hence, the hidden connections between particles in entanglement allow for a plethora of advancements and innovations, making use of the strangeness of these infinitesimally small systems in our growing societies. However, the consequences of quantum entanglement also pose serious questions from those within the scientific community as it threatens to uproot our present understanding of physics. Despite this, entanglement and quantum theory will continue to resist conjectures for their abolishment and are unlikely to face a greater adversary than the one they were able to overcome in the 20th century: Albert Einstein.
Implications & Applications
At first glance, the imperative question that entanglement raises is does it allow for information to be communicated faster than the speed of light? This concern, vocalised by Einstein, was first seen through the following school of thought: entanglement seems to enable the instantaneous influence over the measured properties of a particle due to the measured property of its entangled counterpart, no matter the distance between the two systems.
To elaborate, if two photons are emitted by a quantum process and thereby are entangled, with one confined on Earth while the other reaches Mars, it should, by way of Einstein's theory of relativity, take any signal at least three-and-a-half minutes to reach Mars from Earth (given that Mars is 3.5 light-minutes from Earth and lightspeed is established as the “universal speed limit” by Einstein’s relativity). This means that upon measuring the Earth-photon’s spin, the entangled Martian-photon should not be able to ‘know’ what its spin ‘should be’ for at least 3.5 minutes.
Puzzlingly, or, as per Einstein, spookily, this is not the case as the two measurements will always coincide and comply with conservation laws no matter when measured or how far apart, suggesting that some quantum information has been transmitted to the Martian photon at speeds exceeding the astronomical limit.
However, fortunately for quantum theory, or perhaps fortunately for relativity, this speed limit is not technically violated; information is not technically travelling at greater-than-lightspeed. This is because the information known by the observer on either end of this quantum experiment, Earth or Mars, is completely random. Recall that it is impossible to predict whether the photon will be ‘spin up’ or ‘spin down,’ meaning that the result itself is meaningless until compared with the result of the other entangled photon.
Furthermore, the observer that first identifies the spin of a photon is not able to act upon it faster than the speed of light, meaning that even if the observer on Earth knows that the Martian photon will be ‘spin down’ after recording their photon as ‘spin up,’ they cannot send any signal in response that will reach Mars faster than the speed of light, or before the Martian photon arrives.
And so, with a galactic sigh, Einstein’s theory of relativity and quantum entanglement remain intact but does this then not eradicate all the ‘spookiness’ of entanglement? Not entirely. Although this does manage to resolve the pressing inconsistency that Einstein dubbed “spooky,” entanglement still very much retains its abnormal features and thereby the potential for its innovative applications in the modern world. However, the “spookiness” with which the phenomenon is so often described, may need to be reevaluated. In fact, there have been calls from the scientific community to phase out the usage of ‘spooky’ to describe the theory, with the adjective gaining notoriety for only portraying quantum mechanics as increasingly intimidating and immortalising Einstein’s flawed perception of the theory.
Nevertheless, the slight loss of mysticism about quantum theory should not detract from the far-reaching implications of quantum entanglement in our modern world. For instance, the work of the likes of Anton Zeilinger, part of the three 2022 Physics Nobel Prize Winners for developments into quantum entanglement, has proliferated numerous innovations as offshoots of quantum theory.
Within these include the utilisation of entanglement in encryption, with the measurement of one of a pair of entangled particles being used as a safety feature, triggering immediate alerts for security breaches with the particles’ entanglements. Furthermore, entanglement has been implemented into quantum computers, which process data at immense powers, given their ability to implement qubits, which are computing bits with superpositioned states of ones and zeroes as opposed to the regular binary system of bits used in most computers.
IBM’s recently-developed quantum computer which employs characteristics of quantum entanglement to allow for immense processing capabilities, ushering in a new age for technology and computing.
Image Source: https://www.fierceelectronics.com/electronics/ibm-releases-quantum-computing-software-stack
Moreover, recent developments in the field of quantum entanglement include a new view on the fabric of time itself, led by the work of Alessandro Coppo and his colleagues at the National Research Council of Italy. Birthed from the ideas of a 1980s theory, the recent experiments investigate the possibility that an object is seen changing over time only because it is entangled with time itself. Due to this, the theory posits that each and every observation of the object is a quantum measurement and that the act of observing these objects continually is, in fact, the progression of time. In short: time itself may be a byproduct of entanglement and not necessarily an indivisible fundamental of our universe.
Conclusion
Despite the immense prevalence of ‘quantum’ tehcnologies in modern society, the implementation of this enigmatic concept is set to indefinitely grow increasingly pertinent in today’s world, especially with the implications of quantum entanglement. Although it may be time for entanglement to shed its descriptor of being ‘a spooky action at a distance’ that was once thrust upon it by the greatest mind in science, the possibilities and humanity’s capabilities with it are forecasted to only develop further, meaning that entanglement may be the pathway to fully incorporate quantum mechanics into our lives.
Further Details
How Entangled Particles are Formed
The entanglement of particles can only be achieved by certain events or processes that result in the emission of particles that, by way of conservation laws, are connected at a quantum level. As mentioned earlier, two methods of achieving this entanglement include high-energy emissions, discussed previously as the conservation of angular momentum, and quantum dots.
While the former relies solely on the conservation of energy principle, resulting in the emitted photons having to cancel each other out to maintain equilibrium, quantum dots work in a different fashion. Quantum dots are semiconductor particles that, when decayed from an excited state, release entangled photon pairs as electrons jump to lower energy bands within the atom.
Quantum dots are another example of more contemporary implications of quantum theory, with them being implemented into various technologies and their development winning the 2023 Chemistry Nobel Prize (you can read more about Quantum Dots and 2023 Nobel Prizes here).
A third form of emitting a pair of entangled particles includes the splitting of a photon itself. This involves passing a photon through a nonlinear crystal, which, in comparison to normal crystals, responds differently to electromagnetic fields and it is not affected by the strength of the waves. Once the photon passes through, it is split into two lower-energy photons, known as signal and idler photons, which are entangled. This is achieved as photons are massless particles, allowing them to be divided into identical particles with the only differences being their energies, a phenomenon stemming from Einstein’s famous equation relating mass and energy: e = mc2.
Entangled pairs of photons can be emitted through a variety of methods, including the two depicted above of the decay of quantum dots from excited levels and the splitting of photons themselves through the use of nonlinear crystals.
© Kabeer Hans, 2024
Bell Tests vs Einstein
Before discussing the procedures and implications of Bell tests in response to Einstein’s defiance towards quantum entanglement, further elaboration is required on the aforementioned measurement process for a pair of photons. As was previously stated, detectors oriented in line with the polarisation of a photon would measure ‘spin up’ while detectors oriented oppositely with the polarisation would measure ‘spin down.’
However, the detector can also be aligned perpendicular to the propagation of the photon, meaning that it is midway between being perfectly aligned, which would result in ‘spin up’, and being oppositely aligned, which would result in ‘spin down.’ Due to this, there is a 50% chance that the photon will be recorded with ‘up spin’ and a 50% chance it will be recorded with ‘down spin.’
It is here where the strangeness of quantum entanglement is seen once again. When entangled photons are measured through these halfway detectors that create 50/50 chances, the results show that the photons can not have had a predetermined spin. This means that their spin was determined only as they were measured and prior to being measured, they were in a superposition of both ‘spin up’ and ‘spin down.’
This is seen through the following proof by contradiction. If the photons did have their spin predetermined, with photon A being ‘spin up’ and photon B being ‘spin down’ before being measured, then a critical problem arises: each photon has a 50% chance of being recorded as ‘spin up’ in the halfway-oriented detectors and a 50% chance of being recorded as ‘spin down.’ Therefore, 50% of the time, both photons will be recorded as ‘spin up’ or both will be recorded as ‘spin down’ when measured in the same orientation, violating conservation laws.
The diagrams above illustrate how the idea of the spin of particles being predetermined is false, given how predetermination gives rise to the possibility of two entangled particles having matching spins and violating the laws of conservation of energy and angular momentum.
© Kabeer Hans, 2024
As a result, these photons do not have a predetermined spin and rather are in entangled superpositions of opposite spins before being measured.
Returning to the mid-1900s, given that this negation of predetermination sustained entanglement’s validity, Einstein launched his own first attack towards the theory. He suggested that opposed to having their spins predetermined, the photons must have been formed with some hidden information within them, information inaccessible to us, that ‘programmed’ them to display certain spins when tested under certain orientations.
For decades, this was accepted by many scientists as an indicator of information that we were unable to know, hidden secrets within the photons that resulted in the perceived entanglement. It was only in the 1960s that this hidden-information theory posited by Einstein was tested, challenged, and ultimately, disproved.
Physicist John Bell developed a way to test whether or not a pair of entangled particles did contain hidden information that programmed them to have specific spins under specific orientations. He achieved this by having detectors that randomised their orientations.
To elaborate, Bell tests consisted of two identical spin detectors, able to detect spin in three different orientations: upright, at a 120º angle and at a 240º angle. In the 120º and 240º-angled orientations relative to the polarisation of a photon, the photon has a ¼ chance of being aligned with the detector and recorded as ‘spin up’ and a ¾ chance of being opposite to the detector and recorded as ‘spin down.’ (This can be visualised as the orientation is 3 times closer, in terms of angular measure, to being exactly opposite to the photon’s polarisation, at a relative 180º angle, than it is to being upright and aligned with the photon’s polarisation)
The four diagrams above depict the probabilities of obtaining SPIN UP and SPIN DOWN measurements for different orientations of the measurement detectors (identified by the direction of the arrows).
© Kabeer Hans, 2024
With this, once again, proof by contradiction can showcase why Einstein’s theory of hidden information is false. This is because if the photons do, in fact, have predetermined ‘plans’ for which spin to display under specific orientations, then the following results would be seen:
Each photon would go through the detector with their ‘plan.’ These ‘plans’ to ensure that the photons are always displaying opposite spins would be between two types. The first type would simply be a plan where no matter the orientation, photon A would display ‘spin up’ while photon B would display ‘spin down.’ The second type of plan would be for photon A to always display ‘spin up’ for the upright orientation but always display ‘spin down’ for the 120º and 240º orientations. On the other hand, photon B in this case would plan to always be ‘spin down’ for the upright orientation of measurement and always ‘spin up’ for the 120º and 240º orientations.
(Note here that all other combinations of plans are mathematically equivalent to plan type 1 or plan type 2 since ‘spin up’ and ‘spin down’ can be used interchangeably, as can ‘photon A’ and ‘photon B’)
Given this, by way of Einstein’s hidden-information theory, there is a chance that plan type 2 outcomes do not display opposite spins and are instead matching. Recall that the orientations of the detectors are chosen randomly, and so, in the example type 2 plan, this matching would occur, for instance, if photon A was tested in an upright orientation, resulting in ‘spin up’ but photon B was tested in the 120º orientation, also resulting in ‘spin up.’ It is important to recall here, that is is not a violation of conservation laws and neither is it a violation of quantum entanglement theory, since the two photons can only have identical ‘spins’ if they are measured in different orientations (in this case, upright for photon A and at a 120º angle for photon B).
After identifying all outcomes, it is seen that 4 out of 9 times the spins of the photons will be matching instead of being opposites in type 2 plans and will inherently never be matching in type 1 plans. As a result, if Einstein’s theory is correct and the photons do contain this hidden information through these ‘plans,’ then the results of these experiments should show that the photon spins are opposite at least 5/9 of the time (~55%). However, when these Bell Tests are carried out, the spins are only opposite 50% of the time. Einstein’s approach to entanglement does not work.
Einstein’s defiance to entanglement led him to propose the theory that entangled photons have some sort of intrinsic planning that enables them to always display opposite spins. The diagram above however, proves how Einstein’s theory failed to match experimental results.
© Kabeer Hans, 2024
In contrast, quantum theory explains this 50%-50% result perfectly. This is seen as the following is considered:
Take that photon A is measured as ‘spin up’ in the upright detector orientation, meaning that photon B will now always be ‘spin down’ in the upright orientation, but not always ‘spin down’ in the 120º and 240º orientations. This is because particle B, which is now always ‘spin down’ if measured upright, flips the relative orientation of the 120º and 240º detectors. Instead of being 120º away from the photon’s polarisation either side, given that photon B has now effectively ‘flipped’ the orientations, the detectors measure at 60º away from the photon’s polarisation on either side.
As a result, instead of having a 1/4 chance of being ‘spin up’ as was seen in the 120º and 240º orientations, photon B now has a 3/4 chance of being ‘spin up’ in the 60º and 300º orientations. Given that these two orientations are used 2/3 of the time, the total probability that photon B will be ‘spin up’ and will not display opposite spin to photon A is 3/4 x 2/3 which is 1/2, precisely matching the experimental results of Bell tests.
As opposed to Einstein’s mistake, quantum theory elegantly illustrates how the 50/50 experimental results are obtained, with one of the outcomes (all of which are mathematically equivalent) being shown above.
© Kabeer Hans, 2024
Following the development of Bell tests in the 1960s, it was John Clauser in the 1970s and Alain Aspect in the 1980s that carried out the experiments and proved that hidden-information theory was, in fact, false and quantum entanglement was to be embraced. For this, along with Anton Zeilinger, they were awarded the 2022 Physics Nobel Prize.
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