# Quantum Cryptography

## Quantum cryptography is a way of encrypting data with naturally occurring properties of quantum mechanics that cannot be hacked.

Quantum cryptography is how quantum mechanical properties are used to perform cryptographic tasks. In general, these tasks would be impossible or infeasible with classical (non-quantum) systems. Quantum mechanics is the science of the very small. It explains how matter and energy behave at the scale of atoms and subatomic particles.

Quantum cryptography relies on two principles:

- If two photons are created at exactly the same moment in time and space, they will remain perfectly identical for all time, even if separated by great distances.
- Quantum physics prohibits eavesdropping on a message without being detected.

# Quantum Key Distribution

The best known example of quantum cryptography is quantum key distribution, which exchanges encryption keys for secure communications between shared parties. Properties found in quantum physics help to exchange cryptographic keys so that security is guaranteed.

# Vulnerabilities

While in principle quantum key distribution offers perfect security, all real world implementations are subject to practical limitations that introduce vulnerabilities. The most important vulnerability is the one that allows an eavesdropper to measure the photons without disturbing them. Although this can’t be used to learn anything about the secret key, it’s still a big problem because it means that there’s a way for an attacker to know whether they’re being observed or not.

## For example, weak laser pulses can be measured without disturbing them and thus reveal their content.

This approach is called quantum non-demolition measurements. Because it uses weak laser pulses, it’s also…