# Quantum Cryptography - Santa Rosa Junior College

By: Lacey De Bois, Luis Molina, Vanessa Nielsen What is Quantum Cryptograph? Quantum cryptography uses the polarization of photons to send the secret messages. The

system prevents secrecy because anyone who intercepts the photon will disrupt the polarization. The system ensures secrecy because anyone intercepting a transmitted photon will disrupt its polarization, and affect the rate at which the receiver can correctly measure it. So the sender and receiver can detect the eavesdropper by noticing a spike in the transmission error rate. They can then stop communicating or try again on a different network The process is usually explained with two people, Alice

and Bob, who want to send each other secure messages. Alice sends Bob a key, a random sequence of bits that represent either 1 or 0. The key is a stream of bits that are traveling linearly and oscillating. The angle of the oscillations is the polarization of the particles. Alice will send Bob bits using one of two polarizers in a random order. Bob must guess which of the two polarizers to use

when receiving Alice's bits. If he uses the correct one, he will receive the correct bit number. If Bob chooses the wrong polarizer, then a random number will appear (1 or 0). Bob will record his data and send it back to Alice so she can compare results. Alice will then send Bob a message letting him know which bits were guessed correctly. They will both remove the incorrect bits, giving

them a network key that is unique to them. Ideally, Bob will have a 75% success rate. 50% of the time he will choose the correct polarizer, but when he chooses the wrong polarizer he has a 50% chance of getting the correct number out of the wrong polarizer. This puts Bob at 50% and 25%

success, or 75%. If their nemesis, Eve, is intercepting their information then Bob will have a lower success rate. Eve will check 50% with the correct polarizer and 50% with the incorrect polarizer. Bob will receive 50% of the bits in their original format, and 50% will be random bits. Bob will still have a 75% success rate with the Eve's

correct bits and a 50% success rate with her random bits he received. This gives Bob 37.5% and 25%, or a total of 62.5% success because Bob has a success rate is less than 100% after they got rid of the bits that didnt work initially they know Eve was spying.

http://youtu.be/3DGlEvC78iE http://www.csa.com/discoveryguides/crypt/overview.php http://en.wikipedia.org/wiki/Cryptography http://en.wikipedia.org/wiki/Quantum_cryptography

http://www.thenewnewinternet.com/2010/08/04/researchers-claim-unbreakable- cryptography/ http://www.hitachi.com/rd/yrl/people/info_security/04.html http://www.nature.com/news/2010/100520/full/news.2010.256.html

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