They say it takes two to tango and the same is true of *quantum entanglement*, a fundamental resource in our universe – like energy, whose properties we’ve only recently started to understand. Unlike quantum superposition, which is a property that can be exhibited by a single particle, entanglement requires the existence of two or more quantum subsystems; that is, it requires a decomposition of a quantum system into parts. As such, quantum entanglement is a property shared by parts of a larger whole, like the individual electrons (fundamental particles of electricity and magnetism) in a pair with zero total spin.

Let’s take a deep dive. In quantum mechanics, the state of such a pair is written as:

What you see above is a 50-50 quantum superposition. Let me break it down for you: The two numbers, and , are called *the amplitudes. *The two expressions and , represent *the quantum states* of the two electrons. Together, the amplitudes and the quantum states can give a complete description of everything in the universe. This is powerful stuff.

Now let’s dig a little further (in case you haven’t noticed, you are learning quantum mechanics right now). The amplitudes are usually fractions and they are associated with how often you would observe the particular quantum state in front of which they chill. The association of amplitudes with probabilities is simple: Take the (norm of the) amplitude and square it (e.g. 1/2 becomes 1/4, so you would observe that particular state about one-in-four times). And this is exactly why the quantum state I wrote above is a 50-50 superposition! The amplitudes square to 1/2, so each quantum state will be observed with 50% chance. We cannot know which state we will see (it is not for lack of trying – it is intrinsically a random choice of a multiverse branch), but half the time the pair of electrons will be in the state and the other half they will be in the state .

Now, here comes the weird part: The two electrons will always be observed to have opposite spin (when one has spin up, the other one will have spin down and vice versa). In other words, one electron wouldn’t know its own spin here on Earth, until the other electron passing near the Sun’s electromagnetic field decided to be spin up. Spooky action at a distance, indeed.