One practical application of quantum optics is the possibility to exchange cryptographic keys without fear of their being broken – which is the bottleneck of all cryptography. These keys are used to encipher a message between a transmitter and a receiver. A cipher can be made unbreakable if the key is random and equally long as the message, the so-called Vernam cipher.
Quantum cryptography makes it possible to exchange keys entirely securely by transmitting faint light – single photons. Their security is guaranteed because, if anyone should try to eavesdrop the transmission by measuring the light, it will be affected and altered in accordance with the laws of quantum optics. This can be checked afterwards, so the transmitter and the receiver can easily see whether the key they have exchanged can be relied on.