# Polarization in quantum cryptography?

An unpolarized photon when passed through (+) polarizer, what will be the output ?

1. +

2. |

3. --

Plus, if anyone have something that could make understanding quantum cryptography http://en.wikipedia.org/wiki/Quantum_cryptography easier, please do share.

Relevance

unpolarized light source emits photons in random polarity.

so "unpolarized" photon a photon which is polarized but you dont know where.

a "+" polarizer is to my knowladge will be transperant to either "up" or "left" (let say left) and reflective for the other one (let say up).

if a photon with a combination of the 2 will come, then it depends on how close it is to up or left. the closer it is to left, the higher the chance it will go through the polorizer. the closer it is for the up, the higher the chance it will be reflected.

the quantum measurement makes the photon polorization up in case it passes through and left in case it is reflected, no matter what direction it had before.

if for instance u'd rotate your polarizer by 45 degrees, it will be like u're measureing (x) - measuring wether the photon is 45 degrees above or 45 degrees below. etc.

so polarizer makes a quantum measurement in a base that depends on its allignment. (x,+ or whatever angle you chose)

now.. about quantum cryptography. it is about sending a string to someone, and knowing if someone is listening.

and in that part i'm less sure... i hope someone else will explain. if not i'll try myself.

+

Cryptography, art and science of preparing coded or protected communications intended to be intelligible only to the person possessing a key. Cryptography (Greek kryptos, “secret”; graphos, “writing”) refers both to the process or skill of communicating in or deciphering secret writings (codes, or ciphers) and to the use of codes to convert computerized data so that only a specific recipient will be able to read it using a key . Cryptographers call an original communication the cleartext or plaintext. Once the original communication has been scrambled or enciphered, the result is known as the ciphertext or cryptogram. The enciphering process usually involves an algorithm and a key. An encryption algorithm is a particular method of scrambling—a computer program or a written set of instructions. The key specifies the actual scrambling process. The original communication may be a written or broadcast message or a set of digital data.

Selected Ciphers and Codes Secret messages may be hidden or disguised in many ways. Encrypting, or coding, a message means changing it from words everyone can see and understand into a special set or particular order of symbols known only to a few. Concealment is a simple kind of cryptography, because the message is written normally and merely hidden. Although they are hard to break, codes are also easy to use because words and symbols are predetermined. (The reason codes are so difficult to break is that there is no way to figure them out logically. There is no clear link between F5 and the message.) In substitution ciphers, messages are completely rewritten. A set of new letters or numbers is assigned to the alphabet (upper right) or the numerical value of letters may be used with a repeating key word (lower right

In its broadest sense, cryptography includes the use of concealed messages, ciphers, and codes. Concealed messages, such as those hidden in otherwise innocent text and those written in invisible ink, depend for their success on being unsuspected. Once they are discovered, they frequently are easy to decipher. Codes, in which predetermined words, numbers, or symbols represent words and phrases, are usually impossible to read without the key codebook. Cryptography also includes the use of computerized encryption to protect transmissions of data and messages.

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