Quantum Cryptography

Motivation

A future quantum computer will be able to solve certain complex problems such as the factorization of large numbers much faster than a regular computer. This leaves many current cryptographic schemes insecure. Specifically, the RSA algorithm which is used in today's browsers will be rendered unsafe.

Fortunately, quantum mechanics provides a solution to this problem as well: Quantum Cryptography. An encrypted communication even sent over an classical channel can be proven to be absolutely safe if a random cryptographic key is used by both parties. This is true once the key is as long as the message and it is only used once (one-time-pad). The task at hand is to devise a means of distributing random keys in a secure manner over a quantum channel. Thus, quantum cryptography really means quantum key distribution (QKD). Many protocols exist that enable such a QKD. An example is the BB84 protocol.

The problem is that one needs powerful sources of entangled photons or single photon sources which produce single photons on demand. We are interested in:

1. generation of entangled photons in parametric downconversion and related schemes
2. single photon generation in doped photonic crystals