Quantum cryptography is the use of quantum computers to perform cryptographic tasks. It was first proposed in the early 1970s by physicist Stephen Wiesner at Columbia University and has since been developed by researchers around the world.
The advantages of using a quantum computer for cryptography, rather than a classical computer, arise from the inherent properties of quantum mechanical communications. For instance, in a quantum informational system, it is impossible to observe data without changing it, so that eavesdropping would be impossible. This type of guaranteed cryptographic secrecy is referred to as unconditional security.
In recent years, scientists have constructed portable quantum computers that transmit encoded data using polarized beams of light. The one shown in the image is from the QAP (Quantum Applications Project) in Europe.