Quantum entanglement is one of the most intriguing, and also misunderstood, phenomena in physics. Albert Einstein, in a letter to Max Born in 1947, referred to it saying: “I cannot seriously believe in it because the theory cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky action at a distance” [1] Quantum entanglement describes a special connection between particles. It is a quantum mechanical phenomenon at the quantum level, where the quantum states of two particles must be described with reference to each other, even if these particles are spatially separated. The properties of one particle become instantaneously affected by measurements conducted on the other, so they are co-dependent. [2] Let’s try to explain this concept with a simple example from the real world. You have a pair of socks in two separate gift boxes. If you open one and see it is the left sock, you instantly know the other box contains the right sock—even if it’s on the other side of the world! However, with entanglement, it’s as if the socks don’t decide which they are until you open the box. This sounds weird and paradoxical, but that’s the realm of quantum physics.
In quantum mechanics, if we measure physical properties, we might find correlations. For example, if two entangled particles exist such that their total spin is zero, and one particle is found to have spin up, the spin of the other particle, measured on the same axis, will be spin down. However, this leads to a paradox: measurements of the properties of the particles seem to result in an irreversible wavefunction collapse of the particle, changing the original quantum state. And for entangled particles, this would change the whole entangled system.
In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen published a paper that gave birth to what is known as the EPR paradox. According to Einstein, everything exists independently of us, and no signals can travel faster than the velocity of light, carrying information at the same time. If they do, it would contradict the special theory of relativity. So he argued that the two particles must already know which state the other was in when they were separated, conserving this knowledge. However, and surprisingly, Einstein was wrong! It is important to note that there is no conflict with special relativity that forbids communications faster than light. In fact, quantum entanglement over vast distances does not imply that this information is actually transmitted in real-time between particles.
Article Headline (The New York Times, 4 May 1935): “Einstein Attacks Quantum Theory”, via Wikimedia Commons (CC BY-SA 4.0).
The first clues to an answer came with John Bell, an Irish physicist. Bell formulated an equation—the Bell’s inequality—that is always and only correct for hidden variables theories (not always the case for quantum mechanics). Therefore, it could be used to explain quantum entanglement. Later, in 2022, Alain Aspect received the Nobel Prize for his first tests of Bell’s inequality. He used entangled photons to show the existence of hidden variables as an attribute that predetermines the states of entangled particles. Therefore, we can say that objects can somehow be correlated over long distances in ways that classical physics cannot explain. [3]
Ball on a table and ball on Jupiter, AI-generated image.
With this historical debut still ongoing, we once again see a division between two worlds: classical physics and quantum mechanics. In classical physics, everything follows predictable rules – if we place a ball on a table, it stays there unless something moves it. In the quantum world, the ball can somehow end up on Jupiter, and it can exist both on the table and on Jupiter at the same time. Yes, another paradox. Today, rather than being mystical or unexplainable, these behaviours form the foundation of modern technologies. Quantum cryptography, quantum computing, and quantum teleportation all rely on entanglement, demonstrating that while quantum mechanics may challenge our everyday understanding, it is a consistent and powerful framework for describing reality. Today, none of these technologies would exist without entanglement! [4]
Probably I have captured your attention with quantum teleportation, but again, no, it is not the idea that we have from science fiction. It is a continuous exchange of quantum information, photons, atoms, and so on, between two particles. Why teleportation? Because it allows quantum computers to work in parallel! Power consumption is now reduced by up to 100 to 1000 times. Data are exchanged over a classical channel (which is the opposite of quantum cryptography, which exchanges classical data through a quantum channel).
Recently, researchers at the University of Oxford built a scalable quantum supercomputer capable of quantum teleportation. In the next decades, scientists predict that classical bits will be replaced in machines with quantum bits, which can act as a one and a zero simultaneously through superposition. [5]
“In our study, we use quantum teleportation to create interactions between these distant systems. By carefully tailoring these interactions, we can perform logical quantum gates—the fundamental operations of quantum computing—between qubits housed in separate quantum computers.”
— Main, Dougal. Department of Physics, University of Oxford.
Cairns, John. Photograph of Dougal Main and Beth Nichol working on the distributed quantum computer. University of Oxford, 2025.
In conclusion, quantum entanglement may seem mysterious or even “spooky”, as Einstein said, but it is far from being illogical or magic. It reveals the non-classical nature of the Universe and opens doors to revolutionary technologies. Far from contradicting the laws of physics, entanglement simply challenges our classical understanding of reality, offering deeper insight into the interconnected world of quantum mechanics. So, while it may be strange, it’s definitely not as spooky as it seems!
Written by Carlotta Piras.
References:
[1] Born, Max, Albert Einstein, and Hedwig Born. 1971. The Born-Einstein Letters: Correspondence Between Albert Einstein and Max and Hedwig Born from 1916–1955, with commentaries by Max Born. London: Macmillan.
[2] Wong, Bertrand. 2019. “On Quantum Entanglement.” International Journal of Automatic Control System 5 (2): 1–7.
[3] Astronomy.com. n.d. “What Is Quantum Entanglement? A Physicist Explains Einstein’s Spooky Action at a Distance.” Accessed April 6, 2025. https://www.astronomy.com/science/what-is-quantum-entanglement-a-physicist-explains-einsteins-spooky-action-at-a-distance/
[4] NASA. n.d. “What Is the Spooky Science of Quantum Entanglement?” Accessed April 6, 2025. https://science.nasa.gov/what-is-the-spooky-science-of-quantum-entanglement/
[5] University of Oxford. “First Distributed Quantum Algorithm Brings Quantum Supercomputers Closer.” Last modified February 6, 2025. https://www.ox.ac.uk/news/2025-02-06-first-distributed-quantum-algorithm-brings-quantum-supercomputers-closer.
