Discrete Variable QKD (DV-QKD)

Discrete Variable QKD encodes information in distinct, separate quantum states - like the difference between whole numbers (discrete) versus decimal numbers (continuous). Typically this means single photons with polarization (vertical or horizontal, diagonal or anti-diagonal) or phase states. Bob measures each photon with single-photon detectors that either click (detected) or don't (not detected). The "discrete" isn't just technical jargon - it means the information is digital-like, either-or, which maps naturally to bits.

DV-QKD includes the famous BB84 protocol and entanglement-based protocols like E91. The detectors need cooling to reduce noise (false clicks when no photon arrives), and key rates are limited by detector dead time. But DV-QKD holds the distance records - researchers have demonstrated it over 300+ kilometers and even satellite-to-ground links.

DV-QKD has the longest track record and the strongest security proofs. When governments and defense organizations deploy QKD, they typically choose DV-QKD because the security analysis is mature and well-understood. The physics is also cleaner conceptually - one photon equals one bit of information.

For maximum-distance links or highest-security requirements, DV-QKD is the standard. It's what protects classified communications, connects quantum network nodes separated by cities, and will eventually enable satellite quantum key distribution for global coverage.