‘Spooky action at a distance.’ Perhaps more unnerving than his classical laws of motion, this quote by Albert Einstein sums up what is expected as a first reaction to quantum physics. But despite his initial disregard of the discipline, Einstein is considered to be one of quantum theory’s most prominent contributors.

The term quantum is derived from the Latin word quantus; it describes the minimum amount of a substance involved in an interaction. In this context, substance can mean matter, energy, radiation, or any other physical quantity for that matter. Entanglement is an act of getting twisted or intertwined with something, like the barbed wires when we have plugged far too many of them in a single socket. Put together, quantum entanglement is an occurrence that results in two particles becoming interconnected to the point that they inherently behave as a single entity. They can either be clumped together or come into existence millions of miles apart!

To better understand the physical properties of such an entangled system, let us first turn towards some fundamental definitions, and an important thought experiment. A quantum system is a chunk of the world obeying the laws of wave-particle duality: it behaves as a wave and a particle simultaneously. Therefore, its position can never be determined definitively; rather, it is associated with a range of possible values. These various possibilities are called quantum states, and the system is said to exist in a superposition of these states. Superposition can be thought of as an overlap, or the quantum system’s ability to exist in multiple quantum states simultaneously. Making a physical measurement on these superposed states will always find the system in a single definitive state; how the system evolves from that point onwards depends on the state it was found in. This can be perceived as the measurement causing the superposition to collapse and resulting in the value registered by our detection devices.

This image shows a visualization of quantum entanglement, where two atoms are shown to have a line connecting them.
Visualising transmission across an entangled channel. Image Credits: University of Massachusetts Amherst

Making a measurement on one of two particles entangled with each other essentially improves our understanding of the second, as of yet isolated, particle. Imagine you place a red ball and a blue ball in a box. Both Alice and Bob pick out a ball each without peeking at its color. Bob is then sent off to Mars, and all communication with Alice ceases. If Bob was to then check the color of his ball, the color of Alice’s ball would also be revealed. For example, if Bob found he had a blue ball, Alice’s ball would have to be red and vice versa. This would mean some kind of instantaneous communication between Alice and Bob — faster than light — despite an arbitrary distance between them.

Even the suggestion of an interaction occurring faster than the speed of light was met with hostility from Einstein, because it opposed his determination of the speed of light as an upper limit for the exchange of information. Together with Boris Podolosky and Nathan Rosen, Einstein went on to publish the Einstein-Podolosky-Rosen (EPR) papers, explicitly stating the non-existence of a communication channel within an entangled system. Their invalidation was nonetheless a pioneering effort in the development of the hidden-variable theory. According to this theory, the supposedly complementary properties of an entangled system are associated with hidden variables that simply lie beyond the scope of human observation.

The contradiction now apparent, John Stewart Bell endeavored to unite these claims. He asserted that, in order for quantum occurrences to physically manifest, the classical world would have to be non-local. Such non-locality is characterized by the existence of particular events either too close in time or too far apart in space to exchange information at a rate confined by the speed of light. Bell’s Theorem was experimentally proven using atomic cascades, wherein collisions were induced using energetic particles immersed in solids and liquids and is now an empirical test used to verify the existence of entanglement in the discovery of new modes of quantum communication.

On 13 July 2019, physicists at the University of Glasgow managed to record the first image of a viable representation of quantum entanglement: an interaction between two photons. A photon is a massless elementary particle that travels at the speed of light, and is the force-carrier of electromagnetic radiation (e.g. light). Paul-Antoine Moreau told the BBC that the image was “an elegant demonstration of a fundamental property of nature…”. Quantum entanglement has subsequently been observed using atomic spectroscopy to track elemental compositions of photosynthetic reactions in light-harvesting proteins and energy-conversion pigments in plants.

Quantum entanglement is not just a fragment of someone’s vivid imagination; in fact, it brings us closer to an inevitable new technology. In the words of Michael Crichton, “Quantum technology turns ordinary reality upside down.” Leading this new wave of technology, quantum computing has the potential to make extraordinary leaps in statistical predictions and logistical deductions, far exceeding the ability of classical computers. Industrial giants like Hitachi, Pasqal, and Toshiba have overtaken their competitors in the race to employ quantum technology.

This image shows a visualization of quantum communication between two qubits.
Visualising quantum communication between qubits. Image Credits: MIT Technology Review

A qubit (or quantum bit) is the most fundamental unit of quantum information processing, much like the bit in classical computers. A qubit is an example of a quantum system that occupies two states simultaneously, as opposed to a classical bit existing in either one or the other state. Quantum technologies harness the power of the qubit to establish entanglement: they exist in a superposition of 0s and 1s, and any external measurement is exposed as an intrusion. The nature of entanglement can be considered monogamous — no third party has access to the information exchanged between qubits — thereby guaranteeing the channel’s privacy.

The applicability of quantum entanglement does not end here, because it is also the principle behind the enigma that is quantum key distribution (QKD). In the summer of 2019, a team of Chinese scientists employed the Micius satellite to transmit entangled photon pairs to optical telescopes located in ground stations at Delingha in the Qinghai province and Nanshan in the Xinjiang province of China at a staggering rate of nearly 6 million pairs per second. A pair of entangled photons on either end of a communication channel generates and distributes a secure key which can then be introduced into a classical encryption/decryption algorithm. This is the principle of QKD, but what sets it apart from classical communication is its inherent security. While it is possible to replicate a classical key without raising an alarm, that is no longer the case for a hacker anticipating an undetected intrusion in an entangled channel. Remarkably so, the Chinese team succeeded in establishing entanglement between detections 1,120 km apart – a first evidence of recording QKD at large-scale distances.

Though not immediately backed by intuition, quantum entanglement is indeed a gateway to a robust and unbreakable technology, lying at the very heart of quantum communication. More often than not, quantum entanglement deviates from our existing understanding of the classical laws of physics, but rightly so. There comes a point when classical physics can no longer empirically support physical occurrences on the micro scale; it is then that we resort to quantum physics for an explanation, as astonishing as that may end up being!

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