1.1 Background and Motivation
3
CKA plays an important role in this book. We introduce the reader to CKA by
providing an intuitive explanation of its development from existing QKD protocols
[20]. We generalize the security framework of QKD to include CKA and focus
on a multipartite version of the popular BB84 protocol [21]. We also discuss the
recent experimental realizations of CKA protocols, with particular emphasis on the
implementation of the multipartite BB84 protocol [22].
Founded on the working principle of TF-QKD, we discuss a novel CKA protocol where multiple users distil a conference key through single-photon interference
events [23]. Thanks to this feature, we show that the protocol significantly outperforms previous CKA schemes over long distances, as it employs a W-class state as
its entanglement resource in place of the conventional GHZ state.
The information-theoretic security of QKD and CKA protocols holds as long as the
assumptions made when proving their security are actually met by their experimental
implementation. This requires the users to verify the trustworthiness of their quantum
devices, which might be a quite daunting task.
A possible solution is provided by the device-independent (DI) paradigm. Indeed,
the security of DI protocols, such as DIQKD and DI randomness generation (DIRG)
protocols, holds independently of the actual functioning of the devices used to implement them [24, 25]. This remarkable fact relies on the observation of non-local
correlations certified by a Bell inequality violation.
In this book we carefully review all the steps leading to the security of DI protocols
starting from the observation of a Bell violation. In doing so, we revisit some wellestablished results in DIQKD [26] from the perspective of their recent generalization
to multipartite DI protocols [27]. This allows us to introduce the latest developments
in the security analyses of multiparty DI protocols and apply them to a specific
tripartite DI scenario, as well as discuss the challenges in proving the security of
DICKA protocols.
1.2 Book Structure
The contents of the book are organized as follows.
• In Chap. 2 we set the theoretical framework by introducing all the concepts of quantum information theory that are necessary for the understanding of the remainder
of the book. We place particular emphasis on the various entropy definitions that
capture different measures of information.
• We introduce quantum key distribution (QKD) in Chap. 3. After discussing the
purely quantum features on which the security of QKD is based, we describe the
paradigmatic BB84 protocol. We then consider a generic QKD protocol and prove
its security under the most general circumstances. We conclude the Chapter by
listing some important state-of-the-art QKD experiments.
• By generalizing the BB84 protocol to a multipartite scenario, in Chap. 4 we introduce multipartite QKD, also known as quantum conference key agreement (CKA).
3
CKA plays an important role in this book. We introduce the reader to CKA by
providing an intuitive explanation of its development from existing QKD protocols
[20]. We generalize the security framework of QKD to include CKA and focus
on a multipartite version of the popular BB84 protocol [21]. We also discuss the
recent experimental realizations of CKA protocols, with particular emphasis on the
implementation of the multipartite BB84 protocol [22].
Founded on the working principle of TF-QKD, we discuss a novel CKA protocol where multiple users distil a conference key through single-photon interference
events [23]. Thanks to this feature, we show that the protocol significantly outperforms previous CKA schemes over long distances, as it employs a W-class state as
its entanglement resource in place of the conventional GHZ state.
The information-theoretic security of QKD and CKA protocols holds as long as the
assumptions made when proving their security are actually met by their experimental
implementation. This requires the users to verify the trustworthiness of their quantum
devices, which might be a quite daunting task.
A possible solution is provided by the device-independent (DI) paradigm. Indeed,
the security of DI protocols, such as DIQKD and DI randomness generation (DIRG)
protocols, holds independently of the actual functioning of the devices used to implement them [24, 25]. This remarkable fact relies on the observation of non-local
correlations certified by a Bell inequality violation.
In this book we carefully review all the steps leading to the security of DI protocols
starting from the observation of a Bell violation. In doing so, we revisit some wellestablished results in DIQKD [26] from the perspective of their recent generalization
to multipartite DI protocols [27]. This allows us to introduce the latest developments
in the security analyses of multiparty DI protocols and apply them to a specific
tripartite DI scenario, as well as discuss the challenges in proving the security of
DICKA protocols.
1.2 Book Structure
The contents of the book are organized as follows.
• In Chap. 2 we set the theoretical framework by introducing all the concepts of quantum information theory that are necessary for the understanding of the remainder
of the book. We place particular emphasis on the various entropy definitions that
capture different measures of information.
• We introduce quantum key distribution (QKD) in Chap. 3. After discussing the
purely quantum features on which the security of QKD is based, we describe the
paradigmatic BB84 protocol. We then consider a generic QKD protocol and prove
its security under the most general circumstances. We conclude the Chapter by
listing some important state-of-the-art QKD experiments.
• By generalizing the BB84 protocol to a multipartite scenario, in Chap. 4 we introduce multipartite QKD, also known as quantum conference key agreement (CKA).