The science of cryptography has existed in one form or another for centuries.
Cryptography is the art of encoding and decoding messages for transmission between two parties, while keeping the message secret from unwanted viewers. There are many common implementations of cryptography in use today such as the HTTPS protocol on the Internet. All forms of data encryption and cryptography require a key in some form or another for use in encoding and decoding data. There are even some modern methods of encryption that have been mathematically proven to be unbreakable if they are
implemented properly.The inherent problem with any form of cryptography is in the method of key distribution.To date, all forms of message encryption have possessed this major vulnerability. As such, it makes no difference whether a secret message is sent via homing pigeon, a radio transmission, a penciled message on a notepad, or whispering to another, there is no physical way to get a cryptographic key to another without running the risk of  the key being intercepted.To solve this dilemma, a radical solution has been proposed called quantum cryptography. This method essentially uses photon light particles to send a key to an intended recipient. The difference with this method of key distribution is that it implements quantum mechanic's uncertainty principle as a means to guarantee the privacy of a key during distribution. Practically, quantum cryptography has the ability to afford complete privacy during the entire broadcast of an encoded message.
In quantum computing, we use the concept of qubits, superposition and coherence. A normal bit can be in only two states - 0 and 1. But a qubit it can be in 0, 1, or in the superposition of 0 and 1.
Quantum computers, will be much much faster than traditional silicon based compuers