Licentiate Thesis: Quantum technologies for secure communications

Quantum technologies promise to revolutionize a variety of fields such as metrology, cryptography or computing, among others. The resources that quantum mechanics offers allow us to design faster optimization algorithms, to provide unconditional security in communications and to measure quantities beyond the grasp of current technology. However, these exciting applications are in most cases proof of principles, and they have been tested only under the optimal conditions of a laboratory.


The increasing interest in making quantum technologies part of the daily life pushes the state of the art of platforms and ideas. In the case of light-based technologies, the search for new and more efficient sources of single photons is a rapidly developing area and a keystone for quantum technologies. Nonetheless, having sources and powerful physical hardware are not the only elements needed to make quantum technologies feasible. Analytical tools, protocols and algorithms need to be further developed in order to achieve this goal.


In this work we have addressed and investigated two issues that are relevant for the development of quantum technologies. The first one is the search for more efficient quantum-dot-based devices, capable of delivering single photons on-demand and efficiently generating entangled photon pairs. The second problem is the limited detection efficiency for device independent quantum key distribution (DI-QKD) protocols.


Regarding the development of single photon sources, we investigated a quantum-dot-planar microcavity system. We showed that the quantum dot emission is enhanced due to the cavity, as was confirmed by performing lifetime measurements. These measurements also show a moderate Purcell enhancement of up to 1.7. Also, we demonstrated that a self-assembled microcavity, naturally formed during the growth process, is able to improve the collection efficiency of the source, achieving a collection efficiency of 0.17. When we generated pairs of entangled photons, we do so using the time-bin degree of freedom. The density matrix obtained after performing quantum state tomography measurements yields a concurrence of 0.70 and fidelity with respect to the maximally entangled state of 0.84.


The second part of this work focuses on device-independent quantum key distribution. Particularly, we addressed the problem of the limited detection efficiency of single photon detectors and their implications on the key rate of DI-QKD protocols. We used numerical techniques to study new DI-QKD protocols based on higher dimensional states, which display certain features that can benefit DI-QKD protocols. Our findings showed that a DI-QKD protocol with three inputs and four outputs has a lower detection efficiency threshold than the most studied protocol with two inputs and two outputs using maximally entangled states. We also considered, again for the protocol with three inputs and four outputs, the case in which noise disturbs the correlations of the system. We found in this case, that under the assumption of perfect detection efficiency a previous experiment was already able to distill a secret cryptographic key.