Hello again, lovely readers. If you missed Part One, here’s the short version: quantum entanglement lets two particles share a connection so deep that measuring one instantly tells you something about the other, no matter the distance between them. Einstein hated it, called it “spooky action at a distance,” and spent decades convinced something deeper had to be going on underneath. Turns out he was wrong about the “something’s missing” bit but, as we found out, also sort of right that something deeper was going on, just not in the way he expected. If you want a full recap of the article click here.

This one was a toughie folks, and I’m glad I took the extra week to get it right, and believe me, it gets weirder!

OK. Here we go. Now, where were we? Ah yes…

We left things with entanglement’s strangest party trick: the correlation between two particles is instant, but you still can’t use it to send a message faster than light. Relativity stays safe. Causality stays intact. Crisis averted.

So, if entanglement can’t be used to break physics, what is it actually good for? As it turns out, it’s good for quite a lot as from here on, things get genuinely practical, and then genuinely weird, in roughly that order.

We’re going to look at the real technology entanglement already powers, then follow the rabbit hole all the way down to black holes, wormholes, and the unsettling possibility that the universe itself might be built out of nothing but connections.

So, if you’re ready, buckle up, it’s time for an amazingly quantumly entangled ride…!

For decades, entanglement was just a philosophical curiosity, whereas these days it’s an actual working tool. Thanks to entanglement, quantum cryptography is possible. Entangled particles can be used to generate secure encryption keys, and the process appears almost magical! Any attempt to eavesdrop disturbs the system, so if you attempt to spy, you are immediately detected, clapped in irons, and locked in a dark room for ever (or spend hours and hours and days and days writing a blog piece trying to explain quantum entanglement as punishment. Obviously, I jest. I haven’t been spying. Honest. I wouldn’t know how to).

Something else that entanglement gives us is quantum computing, which again, is something else we are always hearing about, without actually hearing about it. Classical computers use bits, that is zeros and ones. Quantum computers use qubits (no, not Q*Bert, he’s the cute orange dude from the Nintendo game, which was originally an arcade game by someone else. I might be wrong on that one though, answer in the comments if you’re so inclined). Using qubits means each one can be zero, one, or both, at the same time, behaving as a single system with many possible simultaneous states, allowing problems to be solved dramatically faster.

And there’s more! Entanglement also gives us quantum teleportation; we can beam people from orbit to M class planets just like Star Trek!!! Sorry, got a bit carried away there, we sadly can’t do the Star Trek stuff, and it’s very likely we never will. Although it’s not anywhere near as exciting as Star Trek, quantum teleportation is still remarkably awesome as we can transfer the exact quantum state of a particle from one place to another without physically sending it.

Let me try and explain that with an analogy! Bob and Betty both share an entangled pair, Betty measures her particle in a way that reveals its information but destroys its original state in the process and sends the result to Bob as an ordinary classical message. Bob then uses that message to transform his own particle, the other half of the entangled pair, into the exact state Betty’s particle used to hold.

Hopefully that all makes sense, as we’re moving on to an even bigger idea now. Entanglement as the fabric of reality. How mind bendingly cool does that sound?

Let me try and explain that in my rambling way without rambling too much.

Some physicists suspect entanglement isn’t just a feature of the universe, but that it might be the thing that everything else is built on. Let’s take a look at black holes and entropy. You know what entropy is even if you don’t think you do. Rudolf Clausius, the German physicist, said (in 1850…?), when describing the second law of thermodynamics, that the universe’s natural state moves from order to disorder, which is known as entropy. You can see this in action just by watching an ice cube you have taken out of the freezer and left on the kitchen work top, as it naturally melts in a warm room, the puddle of water it leaves behind, however, never spontaneously freezes back into an ice cube. I have a feeling that I’m going to have to add thermodynamics and entropy to the list of future blog articles. At least I’ll be keeping myself busy!

Sorry, went off on a tangent there, back to black holes and entropy (you already know about black holes as you’ve read my Black Holes article, haven’t you?).

Black holes have entropy, but the strange thing is their entropy is proportional to their surface area, not their volume, which suggests something profound. It suggests that the information content of a region of space might live on its boundary and that space may be more like a hologram than a solid volume.

Let me try and explain that better.

When scientists analyse this, they find that the entropy of a black hole behaves like entanglement entropy, a bit like entanglement acting as a sticky information glue. The inside and the outside of a black hole are deeply entangled, which means the structure of spacetime within may emerge from these connections.

A modern theory suggests that entangled particles may be connected to tiny wormholes, not like the Star Trek wormhole near Bajor that you can travel through and start an intergalactic war, think of them more like mathematical bridges linking them together.

Right, here’s the bit I said in Part One I would attempt to explain later, this stuff is all relatively new, but I’m going to have a good stab at it.

To attempt to explain this, we’re going back to Einstein. ER = EPR (yes, another equation, this is new stuff though, real, current research and I love it!).

EPR is the Einstein-Podolsky-Rosen effect and refers to the bit in quantum mechanics where two particles become linked so that measuring one will instantly affect the other, regardless of distance.

ER is the Einstein-Rosen bridge (this is what actually got me interested in all this stuff way back in 1996 just after I bought my first house). The Einstein-Rosen bridge is a type of wormhole, a hypothetical tunnel connecting two different points in spacetime.

In simple terms, this means that when two particles are entangled, it’s as if they are connected by an invisible geometric link, in this case, think of it as a teeny tiny wormhole, not in the sense of travelling as I stated above with my Star Trek analogy, but just as a theoretical way to describe the connection. This is important because it tries to unify quantum mechanics with general relativity and may help to explain what happens in black holes.

The way it was first explained to me (by my father back in said nineties) is one of those things that you often see in sci-fi movies when the science boffin is trying to explain space travel (or something similar) to us regular folk. Imagine two particles as two dots left by a marker pen, on a piece of A4, one at the top of the page, the other at the bottom. Looking at the page, they are far apart, but you can fold the paper in half, so the two dots are touching. This “fold” is essentially the wormhole (the ER bit of the above equation. The Einstein-Rosen bridge).

This was Einstein’s description of quantum entanglement. His “Spooky action at a distance.” As I mentioned in my last article, he thought entanglement was weird and suspicious. These days the teeny tiny physics dudes (quantum physicists, not small people made out of physics, although that is technically not far off what we actually are, albeit all different shapes and sizes) have a different, more boring way of describing it.

They just say, “what if it’s not spooky, what if the particles are literally connected through spacetime?”

Don’t just take my word for it, this isn’t just theory, it has been engineered in labs and I have more equations to demonstrate coming up.

To create entanglement there is this simple recipe:

First off, you start with two qubits:

|0> |0>

Then you apply a Hadamard gate (a fundamental logic gate) which puts one qubit into a superposition. Next you apply a CNOT (Controlled not gate, another fundamental logic gate). This links the second qubit to the first. The result is:

(|00> + |11>) / √2

And that, folks, is an entangled state.

And it gives us real world hardware with different technologies that can achieve this physically. Superconducting qubits are controlled with microwave pulses, trapped ions use shared motion to couple particles and photons can be entangled through nonlinear optics. These processes, however, are precise, delicate and extremely sensitive to noise. Which is partly why we don’t see this in everyday use. With entanglement everywhere, why does the whole world look so normal? The answer to that is decoherence.

When quantum systems interact with their environment, such as air molecules, light, and heat, these interactions spread entanglement everywhere and destroy any clean quantum correlations, resulting in quantum weirdness fading as classical reality re-emerges. Because of entanglement, we are forced to rethink some deeply held assumptions. Here’s a little table to help visualise it:
 

What we expect	What quantum physics says
Objects are separate	Systems can be fundamentally inseparable
Properties exist already	They may only form during measurement
Influences are local	Correlations can be nonlocal

This in turn leaves us with three uncomfortable truths.

• Reality isn’t local.
• It isn’t fully predetermined.
• Information plays a central role in physics.
  
  This forces us to think of the universe in a new way, which isn’t very easy if you are entrenched in the old ways. Instead of thinking of the universe in terms of things, think of it in terms of relationships. What entanglement is telling us is that what matters most isn’t what something is but how it is connected to everything else.

Poor old Einstein worried that entanglement meant that physics had gone off the rails, and it bothered him for years, when instead it revealed something so much deeper: the universe isn’t just a collection of isolated objects moving through space, it is a web of connections where space and separation may emerge from something more fundamental underneath.

Quantum entanglement shows us that reality is built not from independent pieces, but from shared states that link distant parts of the universe into a single, unified whole.

I suppose this could lead us into a discussion on dark matter and dark energy, but along with lots of other stuff I’ve mentioned while writing these articles, that is for another time. First, I need to crack on with some of the stuff I’ve teased already. Bell’s Inequalities next methinks, which will be another biggie!