150
8 Conclusion and Outlook
At the same time, the book attempts to provide a rather comprehensive overview
on the topic of QKD, from its origins to its most advanced protocols. New or improved
QKD protocols are being developed at an astonishing pace and it is quite difficult to
cover all of them in a satisfactory way.
One of the promising new protocols we focus on is TF-QKD, which has arguably
become the new benchmark for far-distance QKD. We showed that TF-QKD is a
major candidate for being implemented in near-future quantum networks. Additionally, we dedicate a Chapter to device-independent (DI) protocols, which guarantee
the highest level of security in quantum cryptography. In this context we focus on the
security aspects of the protocols and illustrate novel theoretical tools for the security
of multipartite DI protocols [13].
CKA is arguably in its infancy and there is still much to be done in order to
concretely make CKA protocols the ultimate solution for secure multi-user communication. Here we briefly outline some research directions that may be pursued.
As discussed in the book, multipartite entanglement seems to be a necessary
ingredient of CKA protocols. However, the distribution of multipartite entangled
states to the participating parties is not an easy task and it often requires the coincident
arrival of each photon to the corresponding party. This fact limits the maximum
distance between any pair of parties due to photon loss. Advances both in CKA
design and distribution of multipartite entanglement can mitigate this issue and allow
CKA to achieve longer distances (see e.g., Sect. 6.5).
From an experimental point of view, the described CKA experiments are only
the first step towards a fully fledged CKA which can serve the needs of secure
multi-user communication. We point out two aspects that should be addressed to
meet such a goal. Firstly, one should increase the generation rate of the distributed
multipartite entangled state in order to speed up the resulting secure communication.
Additionally, the future field implementation of CKA should be performed in the
existing telecommunication infrastructure. This would remove the need for dedicated
fibre networks linking the users.
A theoretical aspect where CKA is still quite underdeveloped are DI protocols
(DICKA protocols). The tools developed in [13] lay the ground for the derivation
of entropy bounds for the security of multipartite DI cryptographic protocols. We
remark that a careful estimation of these entropies is of paramount importance for the
experimental feasibility of DI protocols, as it increases noise tolerance and relaxes the
experimental requirements. Of particular interest are the existing DICKA protocols
[14, 15]. Indeed, they currently lack a tight bound on the relevant entropy, which
severely penalizes their performance. Based on the results of [13], one could aim to
develop a theoretical framework which enables the derivation of tight entropy bounds
for the existing and for future DICKA protocols. This would optimize the security
analyses of DICKA protocols and boost their potential application in the upcoming
quantum networks.
In the context of DICKA, another potential research line is the characterization
of the essential requirements for a successful protocol implementation. To be more
specific, in Chap. 7 we conjectured that genuine multipartite entanglement (GME)
shared by all the participants is necessary if the inequality being tested is the Mermin-
8 Conclusion and Outlook
At the same time, the book attempts to provide a rather comprehensive overview
on the topic of QKD, from its origins to its most advanced protocols. New or improved
QKD protocols are being developed at an astonishing pace and it is quite difficult to
cover all of them in a satisfactory way.
One of the promising new protocols we focus on is TF-QKD, which has arguably
become the new benchmark for far-distance QKD. We showed that TF-QKD is a
major candidate for being implemented in near-future quantum networks. Additionally, we dedicate a Chapter to device-independent (DI) protocols, which guarantee
the highest level of security in quantum cryptography. In this context we focus on the
security aspects of the protocols and illustrate novel theoretical tools for the security
of multipartite DI protocols [13].
CKA is arguably in its infancy and there is still much to be done in order to
concretely make CKA protocols the ultimate solution for secure multi-user communication. Here we briefly outline some research directions that may be pursued.
As discussed in the book, multipartite entanglement seems to be a necessary
ingredient of CKA protocols. However, the distribution of multipartite entangled
states to the participating parties is not an easy task and it often requires the coincident
arrival of each photon to the corresponding party. This fact limits the maximum
distance between any pair of parties due to photon loss. Advances both in CKA
design and distribution of multipartite entanglement can mitigate this issue and allow
CKA to achieve longer distances (see e.g., Sect. 6.5).
From an experimental point of view, the described CKA experiments are only
the first step towards a fully fledged CKA which can serve the needs of secure
multi-user communication. We point out two aspects that should be addressed to
meet such a goal. Firstly, one should increase the generation rate of the distributed
multipartite entangled state in order to speed up the resulting secure communication.
Additionally, the future field implementation of CKA should be performed in the
existing telecommunication infrastructure. This would remove the need for dedicated
fibre networks linking the users.
A theoretical aspect where CKA is still quite underdeveloped are DI protocols
(DICKA protocols). The tools developed in [13] lay the ground for the derivation
of entropy bounds for the security of multipartite DI cryptographic protocols. We
remark that a careful estimation of these entropies is of paramount importance for the
experimental feasibility of DI protocols, as it increases noise tolerance and relaxes the
experimental requirements. Of particular interest are the existing DICKA protocols
[14, 15]. Indeed, they currently lack a tight bound on the relevant entropy, which
severely penalizes their performance. Based on the results of [13], one could aim to
develop a theoretical framework which enables the derivation of tight entropy bounds
for the existing and for future DICKA protocols. This would optimize the security
analyses of DICKA protocols and boost their potential application in the upcoming
quantum networks.
In the context of DICKA, another potential research line is the characterization
of the essential requirements for a successful protocol implementation. To be more
specific, in Chap. 7 we conjectured that genuine multipartite entanglement (GME)
shared by all the participants is necessary if the inequality being tested is the Mermin-