What is quantum cryptography, and how does it improve security?

Quantum cryptography is a method of securing communication by leveraging the principles of quantum mechanics, primarily to protect the exchange of encryption keys. Unlike classical cryptography, which relies on mathematical complexity, quantum cryptography uses the behavior of particles like photons to ensure keys are shared securely. A key example is Quantum Key Distribution (QKD), where two parties (e.g., Alice and Bob) generate a shared secret key by transmitting photons encoded in specific quantum states. If an eavesdropper (Eve) attempts to intercept these photons, the act of measuring them alters their quantum states, alerting Alice and Bob to the intrusion.

The security improvement comes from two quantum properties: the Heisenberg Uncertainty Principle and quantum entanglement. The Uncertainty Principle ensures that measuring a quantum system disturbs it, making eavesdropping detectable. For instance, in the BB84 QKD protocol, photons are polarized in one of four orientations (horizontal, vertical, or diagonal). Alice and Bob compare a subset of their measurements to check for discrepancies. If inconsistencies exceed a threshold, they discard the key. Quantum entanglement, used in protocols like E91, allows pairs of photons to share correlated states. Any attempt to observe entangled photons breaks their correlation, providing another layer of detection. This makes quantum cryptography inherently resistant to attacks, even from adversaries with quantum computers.

While quantum cryptography is promising, practical implementation faces challenges. Current QKD systems often rely on fiber-optic cables, limiting transmission distances (typically under 100 km without repeaters). Projects like China’s Micius satellite have demonstrated longer-distance QKD (over 1,200 km) using free-space communication, but infrastructure costs remain high. Real-world use cases include securing government communications and financial transactions—for example, Swiss elections used QKD to protect voting data in 2019. However, quantum cryptography only secures key exchange; the actual data encryption still uses classical algorithms like AES. Developers should also note that post-quantum cryptography (classical algorithms resistant to quantum attacks) is a separate but related field. Quantum cryptography’s value lies in its ability to provide future-proof security for critical systems, even as computational power grows.