Quantum Entanglement: Immerse Yourself into Mechanics

In 1935, Einstein called it “spooky action at a distance.” He found it problematic as it went against his other theory, the theory of relativity. In fact, it was such a revolutionary concept that Einstein claimed it wasn’t a true description of reality.

Entanglement. The quantum phenomenon where quantum states become linked. If you’re into quantum computing, chances are you’ve heard of the term “entanglement,” and it typically is just seen as states being connected and that’s it. And that opinion is exactly where most people are wrong.

Entanglement is the fact that two particles can share a single quantum state, regardless of the distance between them. The best analogy I can think of for entanglement is this: imagine flipping a switch a million miles away and instantly knowing whether the light turned on or not. You don’t know if the light is on or off because the signal traveled that fast, you know the answer to that fact because the switch and the light were never independent in the first place.

But here’s where it gets confusing: we don’t know if the light is on or off until we actually look.

Before measurement, neither particle has a definite state. As you may already know, measuring a quantum state collapses it into a state of 0 or 1 (just like a normal bit). According to our switch analogy, we don’t know if the light is actually on or off until someone looks at it (somebody looking at it forces it to collapse).

Now let me confuse you a little more. From what I just said it might seem that only the light switch causes the light to turn on or off, but instead, it goes both ways. Instead, this time if we were to take a look at the light (thus collapsing it), and the light were on, the switch would turn on. So it’s a two-way street.

If you’ve ever heard of a CNOT gate, a gate which entangles two qubits (a control qubit and a target qubit), you might assume that when two qubits are entangled that the control qubit is always in “control,” influencing the target but not the other way around. And, when a CNOT gate is created, that theory is right. However, CNOT gate is not entanglement, it’s the process which creates entanglement.

Yes, I know we are really getting to the confusing stuff now.

Imagine this process:

1. Start with one qubit (plain old |0> state)
2. Apply a Hadamard gate to it (now its in superposition like a blur of the two states)
3. Create a second qubit (|0> state)
4. CNOT gate: applying it to the first qubit as control and second as target

At step #4, the first qubit is in superposition (neither 0 or 1 right now). Then, the CNOT is saying if the control qubit is |1>, flip the target. But, we don’t know what the current state of the control qubit is. So, the target qubit is now spinning as well since we don’t know what state it’s in. Now since both qubits are spinning, they are entangled.

The moment you measure either of those qubits, both of them stop spinning. Measure the first qubit and it collapses to 0 or 1, and immediately the second qubit will collapse to its correlated state, regardless of the distance between them. There was no communication between them, not even a signal. This is why Einstein was confused, since in his theory of relativity he couldn’t explain how two particles coordinated their states instantly regardless of distance.

Now using quantum gates, we can influence the probability of collapsing to a specific state. What if you could weight the situation so a qubit had a 99% chance of collapsing to the |1> state? Quantum gates let you do that, so that you can tilt the odds. However, it’s important to note that you cannot guarantee an outcome, but you can make the randomness negligible using gates.

And this is why entanglement is such a valuable asset to quantum computing. It’s beyond just interlinking two qubits’ states. Entangled qubits don’t have to be opposite states either, they can be manipulated so that they both collapse to the same state. Entanglement isn’t just a strange aspect of quantum mechanics, it’s science, and that’s why understanding it is so important to quantum and its usefulness.

Thanks for reading. I post articles every Monday talking about quantum computing and its applications. If you enjoyed reading this article, it’d mean a lot to me if you liked and followed me.