Imagine you and your friend are tossing coins for fun, when suddenly your friend remembers they have always dreamed of being an astronaut and suddenly disappear into a rocket ship bound for the other side of the universe. You (sadly) resume tossing your coin when you realize something odd—the coins you and your friend have seem mysteriously connected.

The moment your friend flips their coin, even from the other side of the universe, your coin magically lands on the opposite side of theirs. Huh? This instant connection is called quantum entanglement and lies at the heart of quantum mechanics.

Let’s get into it.

The EPR Paradox

In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen published the paper “Can Quantum Mechanical Description of Physical Reality be considered complete?” which presented the paradox between quantum theory and observable reality. The paradox was this: Einstein believed that “every element of physical reality must have a counterpart in physical theory,” and the quantum mechanics theory he had helped develop simply did not (Einstein et al.,1935).

The disconnect was something Einstein called “spooky action at a distance,” or entanglement (Einstein et al.,1935). Theory supported locality, or the idea that action in a location must be contained to that location, yet experimental results suggested the opposite: if two particles were shot in opposite directions at two detectors, the measured state of one particle appeared to affect the measure state of the other. That is the essence of entanglement; quantum particles not (perceivably) obeying locality.

Einstein, Podolsky, and Rosen therefore concluded that the quantum mechanical description of physical reality could not be complete because the “correctness of theory is judged by the degree of agreement between the conclusions of the theory and human experience,” and entanglement did not agree with their human experience (🙁)(Einstein et al.,1935). Instead, they hypothesized there must be some hidden variables that allow particles to become entangled.

The only problem—what were they? EPR didn’t know. They believed that there was some unconsidered force at work between entangled particles that hadn’t been discovered yet, but they couldn’t say what those were. Hence, “hidden variables.”

Bell’s Inequality

The next development came from John Bell, a physicist who published a sassy response to the EPR paradox titled, “On the Einstein-Podolsky-Rosen Paradox.” Using a mathematical proof, he demonstrated the hidden variables EPR hypothesized actually didn’t exist (Bell, 1964).

He did this by modeling a similar scenario—two particles shot in opposite directions at two detectors measuring their state on arrival —and assuming the conditions Einstein set were true. The first condition stated that the hidden variables would be responsible for the outcome state, which had to be determined before the particles hit the detectors; the second stated the particles had to obey locality, meaning information between them could not travel faster than light. Based on these conditions, he set a limit on how correlated the two particles could be and called it “Bell’s Inequality.”

However, when he did the calculations, the correlation between the two particles shot in opposite directions exceeded the limit. The conditions Einstein set couldn’t possibly be true.

The Outcomes

Particles, and nature itself, are inherently nonlocal. It was the general assumption at the time that reality is local, but Bell proved that it is not. Action in one location does affect action at another. Spooky indeed! Bell’s proof also showed the nature of the universe itself is uncertain—it only exists in probabilities. Like an electron in orbit, it’s not really where we think it is, it just has a high probability of being where we think it is.

Entanglement is a cool way to describe how two particles interact, but it is also a mind-boggling way to think about the universe. When it comes to applications in quantum computing, entanglement allows qubits to communicate with each other instantaneously. Through entanglement, information can teleport; it is a key resource for algorithms like Shor’s or Gregor’s to exploit the relationship between correlated qubits. Along with superposition, it increases a computer’s parallelism and speed, making quantum computers that much more powerful.

Unlike the run-in you had with your last entanglement, this entanglement is fun and holds a lot of promise for the future. Tune in next time as we learn about quantum tunneling, a phenomenon that a lot of biological systems—in addition to technological systems—rely on.

​​(1) Einstein, A., Podolsky, B., & Rosen, N. (1935). Can the Quantum Mechanical Description of Physical Reality be considered Complete? Physical Review, 47, 777–780.

(2) Bell, J. S. (1964). On the Einstein Podolsky Rosen paradox. Physics Physique Fizika, 1(3), 195-200.

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