I would like to start with a joke: Cat walks into a bar… and doesn’t.
That is Schrodinger’s cat which shows us how bizarre the quantum world is. Some people understand Schrodinger’s cat experiment in a way that you can not know if the cat is dead or alive until you open the box. But in reality, this experiment shows that the cat is actually dead and alive at the same time, and after opening the box it defines the state – being alive or dead. So you can actually kill the cat by opening that box. But how can something be dead and alive at the same time? And how can the state change only by looking at it? That is the point of Schrodinger’s idea to point out that the quantum world is completely different from our view of the world. If a quantum particle is in manystates at the same time it is called in superposition between these states.
Now let’s introduce quantum probabilities. To do this, we can use our everyday quantum measurement device – polarized sunglasses. If the photon reaches the polarized filter of sunglasses and it is polarized on the same axis as the filter, it has a 100% chance to go through the filter. Contrariwise, if the polarization of the photon is 90 degrees to the filter, it has 0% chance to go through. And finally, if the angle is 45 degrees it has a 50% chance to go through. So, from 1000 photons, around 500 photons stop and 500 photons go through. But the quantum weirdness is that we can’t actually know before this measurement which photon will come through and which one stops. We only know the probabilities. And of course, the measurement changes the polarization of photons. This quantum non-determinism worried physicists for decades, Einstein commented it with a famous quote “God doesn’t play a dice”. He and many more physicists thought that there must be some “hidden variable” which we have to find, or we can‘t, but still has to be there. But the Bells inequality test experiment showed us that there is no hidden variable.
Quantum entanglement. In the 1930s, Albert Einstein was upset about quantum mechanics. He proposed the thought experiment where according to the theory, an event at one point in a universe could instantaneously affect another event arbitrarily far away. He called it “Spooky action at a distance” because he thought it was absurd. It seems to imply faster than light communication – something his theory of relativity ruled out. But nowadays, we can do this experiment and what we find is indeed spooky. Let’s imagine two entangled photons and we are going to measure them at a 45 degree angle from their polarization. We find out that if we measure them at the same time, in the same direction we get the same result. Both photons stop or both go through. But it is strange that entanglement works on any distance instantaneously. Measuring one photon instantaneously affect the result of measuring the second photon at any distance in our universe.
The quantum internet and quantum computers are based on these strange principles. That is the reason why they are so different from anything we are used to and why we can do things we cant before.