Entanglement...
Distance no object!
Two qubits are entangled when their states are correlated in lock step. When you measure one qubit (say, its spin), the joint wavefunction “collapses” and you find the other qubit’s result perfectly correlated. The point being that once we measure the state of one of the cubits, we can predict with 100% certainty the state of the other cubit.
There was much controversy over this with Einstein famously being totally opposed to the idea. You’ve likely read somewhere that he really wasn’t having any truck with entanglement. ‘Spooky action at a distance’ is what he is supposed to have called it. However, he was wrong, though to untangle all of this properly we’ll need stronger Math so we’ll have to deal with this in a further post, especially when we look much deeper at Bell’s Theorem and the ‘EPR’ paper.
To generalise we can say that Einstein was of the opinion that the recorded result was due to some shared or ‘hidden’ information and indeed a theory of hidden information held sway for a while. Until Bell came along and demonstrated a theorem that showed that such correlated cubits were actually outside what physicists call ‘local’ reality - classical reality to you and me.
You’ll see the phrase ‘Bell State’ and ‘Bell Inequality’ in the literature a lot and it’s all to do with proving ‘beyond all reasonable doubt’ that one or more cubits are entangled. In a way Bell’s Theorem is the ‘flux capacitor’ of the quantum world. It makes the magic work and if we want to get back to the future, we need to understand it properly. And we will.
Entanglement of many correlated cubits is one of the core contributors to the astounding power of quantum computation, along with the superposition initially discussed in the last post.
How can we tell two qubits are entangled?
The classic example used to discuss this is the coin toss example and you’ll see it throughout discussions on entanglement. Presumably because we can correlate heads and tails with spin up and spin down in our imaginations.
Imagine two coins when flipped always land in the same way. Now imagine taking one of them to another room and flip both. The result will be the same. Hop on a plane to Alaska with one coin and repeat the flip. The same result occurs. You get the idea. Using satellites and ground stations as part of the kit, entanglement of qubits to date has been measured to around 1200 kilometers. This promising start opens up the likelihood of global quantum networks communicating at speeds and parallelism that would leave our current networks in the dust!
Just while we’re mentioning speed; it has to be noted that none of the entangled qubit information is transmitted from one qubit to the other. It would be alluring to think some faster than speed of light communication was going on but that is not the case. Experimental evidence has shown that whilst some quantum entanglement information may be encoded at the moment the entangled pair are created, no actual communication takes place between the entangled pair.
A better way of looking at this is to say that once two qubits are entangled, their joint wavefunction is one inseparable object—even if you carry one qubit miles away. There’s no hidden communication traveling between them. You simply treat them as “Qubit A” and “Qubit B” wherever they happen to be. Their physical separation only matters when you need to apply gates or compare measurement results in the lab; it doesn’t break the entanglement itself.
Personally speaking, if there are any spooky vibes still to be had its that you might say the entire universe and its correlates are just one humongous information system and we’re just beginning to grasp how to interact with it. Imagine being able to know with 100% certainty, the state of a quantum system on the other side of the andromeda galaxy or the magellanic clouds!
Why it all matters
Quantum communication relies on these entangled, ‘non-local’ links but when we delve deeper into it in future posts we’ll see that even in exponentially faster quantum networks unfortunately the speed of light constraint still obtains.
Finally, quantum security protocols depend on the fact that no eavesdropper can mimic quantum correlations. This is a silent arms race in itself and will be the subject of a future post in our monthly intel stream. I need to investigate with more concrete examples of who is doing what before then. Cryptographic security and current RSA protocols are starting to look vulnerable, for sure. The rumour mill has it that some state actors are already collecting secrets on the basis of ‘Harvest Now - Decrypt later. i.e. When the technology has matured. And on that chilling note…
That’s all for now! If you like my efforts to make quantum computing more accessible to everyone; please consider recommending it on your substack. Or share it with a friend or colleague. Every recommend makes the project grow. Thanks for Reading!