Decoy state protocol

Definition:
In quantum cryptography, the Decoy State Protocol is an advanced method used in Quantum Key Distribution (QKD) to counteract vulnerabilities introduced by imperfect light sources. It is one of the most widely implemented QKD schemes, especially in systems using weak coherent pulses instead of ideal single-photon sources.

Scientific context:
Unlike the original BB84 protocol, which assumes perfect single-photon sources, practical QKD implementations often emit multi-photon pulses. These can be exploited by photon number splitting (PNS) attacks, where an eavesdropper selectively captures and stores extra photons to gain partial key information without introducing detectable errors.

The decoy state protocol overcomes this vulnerability by having the transmitter (Alice) randomly vary the intensity of pulses—sending a mix of:

• Signal states (standard intensity),

• Decoy states (lower or higher intensity), and

• Vacuum states (no photons).

Because these intensity levels result in different photon statistics, any attempt to intercept or manipulate the channel will affect the detection patterns. By comparing these across different states, the receiver (Bob) can detect anomalies and estimate the secure key rate with greater confidence.

Did you know?
Decoy state QKD systems have enabled secure key exchange over more than 100 km of fiber and are foundational to real-world quantum communication networks, including satellite-based experiments.