4. QKD procedure

In this section, we describe what happens in a P&M scheme, specifically in the BB84 protocol [33]. In this protocol, Alice and Bob are connected by two communication channels, namely an insecure quantum channel and an authenticated classical channel [2]. The quantum channel is used for the transmission of qubits and is controlled by the eavesdropper. The classical channel is authenticated so that the eavesdropper can only listen to the communication but cannot alter the messages being transmitted. This ensures that Alice and Bob can prove that they are communicating between each other. Otherwise, an eavesdropper could simply block all quantum and classical communication between Alice and Bob and perform QKD with Alice while taking on Bob's role and vice versa. Therefore, Alice and Bob have to identify each message they send as originating from themselves before any post-processing can begin.

#### 4.1. Quantum phase

In the quantum phase, Alice and Bob make use of the quantum channel. They employ the quantum mechanical signals (i.e. qubits) and they also perform measurements. Three subprotocols take place which are as follows:


Afterwards, Alice keeps the record of signal choices; Bob keeps the record of his basic choices and the corresponding measurement results.

#### 4.2. Classical phase

3.1. Prepare and measure (P&M) scheme

6 Advanced Technologies of Quantum Key Distribution

[27], six-state [34] and SARG04 [35] protocols.

3.2. Entanglement-based (EB) scheme

4. QKD procedure

4.1. Quantum phase

protocols take place which are as follows:

In a P&M scheme, Alice encodes some classical information into a set of quantum states and sends them via an insecure quantum channel to Bob. Bob then performs measurements on the quantum states he receives. This results in classical data generated by quantum means being shared between Alice and Bob. Examples of protocols that use this scheme are BB84 [33], B92

In an EB scheme, a source prepares and distributes a maximally entangled quantum state where one system is sent to Alice and another to Bob. Alice and Bob then perform measurements in two mutually unbiased bases on their system, respectively. Upon measurement, they obtain perfectly correlated outcomes which are completely random. Since the source prepares a pure state, it means that this state cannot be correlated with an eavesdropper. This implies secrecy of the key. An example of a protocol which uses this scheme is the E91 protocol [36].

In this section, we describe what happens in a P&M scheme, specifically in the BB84 protocol [33]. In this protocol, Alice and Bob are connected by two communication channels, namely an insecure quantum channel and an authenticated classical channel [2]. The quantum channel is used for the transmission of qubits and is controlled by the eavesdropper. The classical channel is authenticated so that the eavesdropper can only listen to the communication but cannot alter the messages being transmitted. This ensures that Alice and Bob can prove that they are communicating between each other. Otherwise, an eavesdropper could simply block all quantum and classical communication between Alice and Bob and perform QKD with Alice while taking on Bob's role and vice versa. Therefore, Alice and Bob have to identify each message

In the quantum phase, Alice and Bob make use of the quantum channel. They employ the quantum mechanical signals (i.e. qubits) and they also perform measurements. Three sub-

a. Signal preparation: Alice prepares a random sequence of strings which are drawn from a set of four signal states and encodes each bit value in the state of a quantum system. The

c. Measurement: Bob applies a quantum measurement on the quantum system to decode a bit value. The signals are measured in a random sequence of polarisation bases, either in

b. Transmission: The encoded quantum system is sent to Bob via the quantum channel.

they send as originating from themselves before any post-processing can begin.

basis states are horizontal, vertical, diagonal and anti-diagonal.

the horizontal/vertical or diagonal/anti-diagonal bases.

In this phase, Alice and Bob use some classical communication protocol in order to distil a secret key from their correlated data. They achieve this by means of a discussion over the authenticated classical channel. The key extraction procedure is described as follows:

