What is Quantum Key Distribution?

Introduction to Quantum Computing

Quantum Key Distribution (QKD) provides for secure communication.  It is a quantum communication protocol that allows the secure generation of a shared secret key between two parties.  Thus, QKD is a communication method which uses unique features of quantum mechanics to securely exchange a secret key to encrypt and decrypt messages.  This is contrasted with classical methods of key exchange, such as the RSA cryptosystem, where security is based on the classical computing complexity of the order-finding problem. 

For QKD, the keys are generated via a quantum channel between two parties.  The principles of non-disturbance enable the detection of potential eavesdroppers on their quantum channel. Once a secure key is generated, it can, in turn, be used to encode and securely transmit information. This method of generating a secure key is known as quantum key distribution. 

QKD protocols include:

• BB84 (Prepare and Measure): encodes information in the polarization of single photons; makes use of superposition;
• Ekert91 (Entanglement-based): encodes information using polarization-entangled Bell states; makes use of entanglement;
• BBM92 (Entanglement-based): equivalent to BB84 

QKD uses fundamental quantum-mechanical properties of systems, usually implemented with light, to securely exchange keys over an unsecure public channel.  There are two general classes of QKD:

1. based on single photon sources and detectors
2. based on entangled photons and Bell state measurements