Quantum Key Distribution Protocols

As described in the Principle of Quantum Key Distribution, quantum key distribution consists of three steps:

  1. Key Exchange: qubits are exchanged between the two parties. This process is known as Raw Key Exchange (RKE) and is the only quantum part of quantum key distribution. It leads to the generation of the raw key.
  2. Key Sifting: The RKE is by followed by the Key Sifting in which only certain cases are selected. After the sifting step, both parties share a sequence of bits, called the sifted key.
  3. Key Distillation: After sifting, the emitter and the receiver jointly process the sifted key to distill a secure sequence of bits called the secret key. The process — detailed in the Key Distillation article — consists itself of three steps:
    1. error correction,
    2. privacy amplification and
    3. authentication

QKD protocols define only the first two steps mentioned above, namely the raw key exchange and the key sifting. Although several protocols exist, only two quantum cryptography protocols will be mentioned: BB84 and SARG.

The BB84 protocol is extremely useful to illustrate the principle of quantum cryptography. It was the first protocol to be invented. It was created in 1984 by Charles Bennett of IBM Research and Gilles Brassard of the University of Montreal. In spite of this, it is still widely used and has become a de facto standard.

The SARG protocol was devised in 2004 by Valerio Scarani, Antonio Acin, Nicolas Gisin of University of Geneva, and Grégoire Ribordy of id Quantique.

In the next articles, we will see that these two protocols are very similar. They only differ by the bit value associated to each quantum state for the Raw Key Exchange and the Key Sifting. This difference makes SARG more efficient in term of distance and secret key rate than BB84. BB84 will be fully described in the following articles, whereas just a few clues will be given to explain the principle of SARG protocol. Both protocols are implemented in id Quantique’s Quantum Key Distribution systems. However, as it yields better performance, only SARG is used in the SwissQuantum network.