2
1 Introduction
In the long term scientists envision the creation of large-scale quantum networks
where, thanks to quantum entanglement, QKD-enabled secure communication will
be possible among any subset of users in the network. With a broader perspective,
such networks could be linked together in a quantum internet [11, 12] that would
serve much more scopes than just secure communication, e.g.. secure access to
remote quantum computers [13, 14].
On the one hand, this book aspires to be a beginner’s guide to QKD and other
related topics. Nonetheless, it aims at carrying the reader from the fundamentals of
the subject up to the latest research results. The book is characterized by a pedagogical approach, enriched by intuitive explanations and detailed calculations, sometimes
hard to find in published papers. The ultimate goal is to prepare the reader to eventually take part to the research efforts on this flourishing field.
1.1 Background and Motivation
Quantum cryptography beautifully combines ideas and contributions coming from
various fields of study, ranging from quantum information and quantum communication, to computer science and classical cryptography. The interplay between these
diverse disciplines leads to theoretical advancements that can be of broad interest
and applicable to other research fields.
Nevertheless, because of the significant commercial appeal of quantum cryptography and in particular of QKD, the on-going research is also guided by more practical
purposes. For instance, combined theoretical and experimental efforts are constantly
devoted to: stretc.hing the maximum distance at which QKD can be performed,
increasing the key-generation rates, simplifying the experimental setups, and so on.
To this aim, the book addresses a novel QKD protocol that has recently received
a lot of attention from the scientific community and is considered to be the new
benchmark for long-distance QKD in fibre. The protocol, named twin-field (TF)
QKD [15, 16], allows two parties to establish a secret key over long distances with
single-photon interferometric measurements occurring in an intermediate relay. In
this context, we analyse in detail realistic implementations of TF-QKD by employing
recent theoretical results and simulations [17, 18].
With bipartite QKD links becoming the norm in several research centres and
on-field installations all over the world, the next big step would be to interconnect
such isolated links into quantum networks in order to perform more sophisticated
multi-user tasks [12].
A natural application of future quantum networks is certainly the generalization
of QKD to multiple users with multipartite QKD, also known as quantum conference key agreement (CKA) [19]. A CKA protocol is employed when a confidential
message needs to be securely broadcast within a group of users. The users, upon
performing a CKA protocol, share a common secret key—the conference key—with
which they can encrypt and decrypt the secret message.
1 Introduction
In the long term scientists envision the creation of large-scale quantum networks
where, thanks to quantum entanglement, QKD-enabled secure communication will
be possible among any subset of users in the network. With a broader perspective,
such networks could be linked together in a quantum internet [11, 12] that would
serve much more scopes than just secure communication, e.g.. secure access to
remote quantum computers [13, 14].
On the one hand, this book aspires to be a beginner’s guide to QKD and other
related topics. Nonetheless, it aims at carrying the reader from the fundamentals of
the subject up to the latest research results. The book is characterized by a pedagogical approach, enriched by intuitive explanations and detailed calculations, sometimes
hard to find in published papers. The ultimate goal is to prepare the reader to eventually take part to the research efforts on this flourishing field.
1.1 Background and Motivation
Quantum cryptography beautifully combines ideas and contributions coming from
various fields of study, ranging from quantum information and quantum communication, to computer science and classical cryptography. The interplay between these
diverse disciplines leads to theoretical advancements that can be of broad interest
and applicable to other research fields.
Nevertheless, because of the significant commercial appeal of quantum cryptography and in particular of QKD, the on-going research is also guided by more practical
purposes. For instance, combined theoretical and experimental efforts are constantly
devoted to: stretc.hing the maximum distance at which QKD can be performed,
increasing the key-generation rates, simplifying the experimental setups, and so on.
To this aim, the book addresses a novel QKD protocol that has recently received
a lot of attention from the scientific community and is considered to be the new
benchmark for long-distance QKD in fibre. The protocol, named twin-field (TF)
QKD [15, 16], allows two parties to establish a secret key over long distances with
single-photon interferometric measurements occurring in an intermediate relay. In
this context, we analyse in detail realistic implementations of TF-QKD by employing
recent theoretical results and simulations [17, 18].
With bipartite QKD links becoming the norm in several research centres and
on-field installations all over the world, the next big step would be to interconnect
such isolated links into quantum networks in order to perform more sophisticated
multi-user tasks [12].
A natural application of future quantum networks is certainly the generalization
of QKD to multiple users with multipartite QKD, also known as quantum conference key agreement (CKA) [19]. A CKA protocol is employed when a confidential
message needs to be securely broadcast within a group of users. The users, upon
performing a CKA protocol, share a common secret key—the conference key—with
which they can encrypt and decrypt the secret message.