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Lesson 1, Topic 8
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# Quantum Cryptography

##### digitalscotland July 13, 2021
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Quantum Cryptography

In this video, Josefina Vivanco describes Quantum Cryptography. So at 0:26 sec She stated that a regular Computer uses speech a unit of information with two states like zero or one, up and down, heads or tails.
0:35 Sec; She states that a Quantum Computer uses qubits which have the capacity of being zero, one and a Superposition of these values at the same time.
At 0:40 sec She Asked that if we spin a coin, what do I get?
Its Heads or Tails and off course we don’t know. It can be a 50% heads or 50% tails the moment she asked. So, we Could say that when the coin is spinning it can be heads and tails at the same time. And when it stops it adopts a state.
(1:00 sec) She describe the Definition of Quantum Superposition.
The ability of a Photon to be in different states at the same time. And when we observed collapses in one is called Quantum Superposition.
Imagine we want a Combination of three digits that are one and zero like:
000
001
011
111
110
100
101
010
but with the superposition of qubits all these options become one Because each digit can be one and zero at the same time.
Code: 010001010111100100101101010001110101100
We can use Code lots of times longer so imagine how difficult it would be to try to figure out one key.
(1:30 min); So How do we send the messages?
First we will use Quantum mechanics t randomly create the key. We will send the sequence of polarized Photons that can mean four different directions. Then we have to measure in which direction they are polarized using two detectors randomly. And they will transfer photons into bits like a horizontal could be registered as one and a vertical as zero on the first detector. And the second diagonal right as one and diagonal left are zero but, If we use a vertical and horizontal Detector on a Diagonal Photon, we will have the same chance of measuring a one or a zero. That’s why the both detectors need to match.
Consequently, you will have a sequence of ones and zeros and then we compare and you tell me which detector you used and I will tell you when you are right and when you are wrong based on which filters I use to send the Photons after that we eliminate the wrong results so we get a sequence of polarized and measured Photons. And we have our key now I can send the actual encrypted message and we can use the Quantum Key to Decrypt it.
2:30 min; Why is this so Secure?
If a spy measure one or more of the photons with the wrong detector. The state will change. When we take the detectors, we will see that even if you use the correct one then the number was wrong. If we also used our wrong detector, we would not tell that someone was trying to salvage the chest, but for a long time the probabilities of a spy of not being caught decrease a lot. If we detect a spike we can sell the key and our information is safe because I would not send it until I’m sure there is no one spying. 