Originally theorized in 1984 by Charles H. Bennett (of IBM’s Thomas J. Watson Research Center) and Gilles Brassard, quantum key distribution (QKD) is the most common type of quantum cryptography. QKD systems are not typically used to encrypt secure data itself, but rather to make a secure key exchange between two parties by collaboratively building a shared private key, which can in turn be used for traditional symmetric key encryption methods.
QKD systems work by sending individual photon light particles across a fiber optic cable. This stream of photons travels in a single direction and each one represents a single bit, or qubit, of data—either zero or one. Polarized filters on the sender’s side change the physical orientation of each single photon to a specific position. And the receiver uses two available beam splitters to read the position of each photon as they’re received. The sender and receiver compare the sent photon positions to the decoded positions, and the set that matches becomes the key.
To better understand QKD, imagine two people, Alice and Bob, who need to establish a secure connection. They can use QKD to create a secure cryptographic key by sending polarized photons over a fiber optic cable. The cable doesn’t need to be secured because each photon will have its own randomized quantum state. Should someone, let’s call her Eve, be eavesdropping, Alice, and Bob will always be able to tell because it’s impossible to observe a quantum state without also affecting it. In this way, QKD systems are considered to be unhackable. If Bob and Alice detect a change in the quantum states of the photons, they’ll know that Eve is eavesdropping. And if Eve is eavesdropping, Bob and Alice will always be able to detect it.
Although the benefits of QKD have been proven in both laboratory and field settings, there are many practical challenges preventing widespread adoption, most notably infrastructure requirements. Photons sent across fiber optic cables degrade over distances of about 248 to 310 miles. However, recent advancements have extended the range of some QKD systems across continents by using secure nodes and photon repeaters.
