Quantum cryptography is the science of data encryption based on the laws of quantum mechanics. Unlike traditional cryptography, quantum encryption methods use the physical properties of elementary particles to protect and transmit data.
Quantum cryptography applications
In quantum cryptography, information is encoded in the states of quantum particles. The most commonly used are photons, which can be transmitted over an optical cable. A quantum particle cannot be completely copied, and any attempt to measure its state will cause it to change. In theory, this makes it possible to establish with absolute certainty whether the data was intercepted en route.
Most often, quantum particles are used to generate and transmit cryptographic keys. If a third party attempts to intercept the key, the state of the particles changes, alerting the sender and the recipient that the key has been compromised.
Quantum encryption protocols
The first protocol to use photon-based key distribution was developed in 1984 by scientists Gilles Brassard and Charles Bennett. There are currently several such protocols for quantum key exchange. The most common of them are:
- The Bennett-Brassard 1984 (BB84) protocol. This measures photon polarization (a characteristic that describes the geometry of a photon’s oscillations) and the ability of quantum particles to be in a superposition of states (that is, in all possible states simultaneously) until the moment of measurement.
- The Ekert (E91) protocol. Developed by Artur Ekert in 1991, this protocol likewise measures photon polarization, but instead of superposition, it relies on quantum entanglement — the interdependence of the states of two (or more) particles.
Quantum cryptography challenges
Much like with quantum computers, there are several challenges hindering the widespread adoption of quantum cryptography. The top two challenges are as follows:
First, building and maintaining networks for quantum communication is expensive.
Second, there are issues regarding range of data transmission when using encryption based on the physical properties of quantum particles: the greater the distance over which a private key needs to be transmitted, the higher the risk that not all photons will reach the recipient in their original state.
Despite these challenges, quantum cryptography methods are already being used in practice by research centers and commercial companies. For example, the technique of quantum random number generation is implemented in certain smartphones.