What is quantum cryptography

Secure communication through quantum cryptography

Quantum cryptography uses the polarization of light, i.e. its oscillation in the plane perpendicular to its propagation. For quantum cryptography, the photons are polarized horizontally, vertically, obliquely left or obliquely right.

Understanding the direction of polarization

The transmitter and receiver define the sequence of the bases, i.e. the polarization, in which the photons are sent and received. Only if the receiver measures the light particle in the polarization selected by the transmitter will both receive information from the measurement.

Limited range

Quantum cryptographic methods can only be used over a limited distance. The previous maximum distances were 40 and 144 kilometers. The transmission via fiber optic cables or through the air disrupts the light quanta. Amplification of the signals, in turn, would change the information encoded in the particles. Researchers are working on switching to satellites in order to send the quanta in an interference-free vacuum. However, transmission over short distances would also be of interest for security-relevant applications at authorities, ministries, institutions or companies.

Listeners change the message

Due to quantum mechanics, every measurement is always an intervention in the system that changes it. The measurement with a horizontal / vertical basis codes the particle accordingly. Uninvited "listeners" can therefore easily be noticed. If someone intercepts and measures the polarized light particles during transmission, they will change their polarization if they use a “wrong” base. The sender and receiver notice this when they send test sequences beforehand. However, quantum cryptography does not offer complete security either. Because the devices of the receiver and transmitter form a point of attack.


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