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A Short Introduction to Quantum Theory

To understand, why terms such as ‘Quantum Theory’, ‘Quantum Mechanics’ or ‘Quantum Physics’ were introduced and why these fields of scientific research had an astonishing big impact on Cryptography, we need to go back to the year 1799.

 


Thomas Young

In 1799 the English polymath Thomas Young carried out an experiment, in which he sent light through 2 narrow slits and expected to see two bright stripes on a screen, that he had positioned in front of the slits at some distance. Instead he observed that the light fanned out from the slits and formed a pattern of light and dark stripes on the screen.


Ducks demonstrating the interference of waves

The explanation of this phenomenon came into his mind some time later, when he observed several ducks at a pond. The ducks were swimming behind each other, each producing a trail of small ripples and it was obvious that those ripples were somehow interfering with each other. If two wave peaks arrived at a spot simultaneously, then the result was an even higher peak whereas two troughs seemed to produce an even deeper trough. Therefore Young assumed, that light was some kind of wave and that the different stripes on his screen were the result of some interaction of the light rays. The ducks gave Young a deeper insight into the nature of light and inspired him to publish his masterpiece “The Undulatory Theory of Light”. 
More than a hundred years later physicians were able to repeat Young’s experiment with single photons and were puzzled by the fact, that they again observed a pattern of light and dark stripes on the screen. It was impossible to explain, how one single photon could interact with itself  using the laws of classic physics and so the ‘Quantum Theory’ was born.


Double slit experiment as carried out by Thomas Young in 1982.



Erwin Schrödinger

The idea of so called Superposition was introduced.
Erwin Schrödinger explained this term with the following analogy:
Remember a cat in a box. There are two possible states of the cat: dead or alive. At the beginning we know that it is alive, so there is no superposition of state, but if you put a vial of cyanide into the box and close the lid, you don’t know, whether the cat is still alive or not. So the animal is in a superposition. When we open the box, we force the cat to leave superposition and it must be either dead or alive.
This concept is also valid for small particles like photons.

 

Links:

Please consult Wikipedia.org to find out more about Quatum Mechanics:

en.wikipedia.org/wiki/Quantum_theory