Learn about the 5 most common types of Quantum Key Distribution protocols. Explore how each works to provide a highly secure method of generating and distributing keys.

5 Most Common Types of Quantum Key Distribution Protocols

Quantum Key Distribution (QKD) is a cryptographic technique that uses the principles of quantum mechanics to establish a secure key between two parties. The fundamental idea behind QKD is that measuring a quantum system in an unknown state will always change that state. This means that if an eavesdropper tries to intercept the communication, they will inevitably leave a trace that can be detected. QKD protocols come in various forms, and here we will discuss the five most common types.

1. BB84 Protocol

The BB84 protocol is one of the earliest and most well-known QKD protocols. It was developed by Charles Bennett and Gilles Brassard in 1984. The BB84 protocol uses two non-orthogonal states to transmit the key, which can be either horizontal/vertical or diagonal/circular polarization of photons. The sender randomly chooses one of the two bases and sends the corresponding photon to the receiver. The receiver measures the photon in one of the two bases randomly. After the transmission, the sender and the receiver publicly announce their bases but not their outcomes. The key is then generated by keeping only the bits for which both parties used the same basis.

2. E91 Protocol

The E91 protocol, also known as the Ekert protocol, was proposed by Artur Ekert in 1991. It uses entangled particles to distribute the key. The sender sends one half of an entangled pair to the receiver, while keeping the other half. The sender and the receiver then measure their particles in one of two bases. The key is generated by keeping only the bits for which both parties used the same basis and their measurement outcomes are the same.

3. B92 Protocol

The B92 protocol was developed by Charles Bennett in 1992. It uses only two non-orthogonal states, unlike the BB84 protocol. The sender randomly chooses one of the two states and sends the photon to the receiver. The receiver measures the photon in one of the two bases randomly. Unlike the BB84 protocol, the sender and the receiver announce their measurement bases and outcomes publicly. The key is then generated by keeping only the bits for which both parties used the same basis and the sender’s bit is equal to the receiver’s bit.

4. DPS Protocol

The Differential Phase Shift (DPS) protocol was proposed by Wang and Qi in 2005. It is a modified version of the BB84 protocol, where the sender sends phase-shifted coherent states instead of single photons. The receiver then measures the phase of the received states using a homodyne detector. The key is generated by keeping only the bits for which both parties used the same phase and their measurement outcomes are the same.

5. MDI Protocol

The Measurement-Device-Independent (MDI) protocol was proposed by Lo et al. in 2012. It is designed to overcome potential flaws in the devices used for QKD. In the MDI protocol, the sender sends one half of an entangled pair to each of two untrusted nodes, which perform the measurements independently. The key is then generated by comparing the measurement results of the two untrusted nodes.

In conclusion, QKD protocols use the principles of quantum mechanics to establish a secure key between two parties. There are various QKD protocols available, and we have discussed the five most common ones. Each protocol has its advantages and disadvantages, and the choice of the protocol depends on the specific application.