• antibunching
• Crittografia Quantistica
• giunzioni p-n planari
• p-n planar junction
• Quantum Cryptography
• single-photon emission
• singolo-fotone

Sharing information between individuals through secure channels has always been an extremely important issue that led to the implementation of increasingly efficient cryptographic systems. Nowadays, the most widespread secure communication protocols are based on public-key approaches whose security is guaranteed by the complexity of the algorithms needed to decript the message without knowing the private key. These offer the so-called computational security: computer available nowadays are not powerful enough to break the cyphertext in any reasonable amount of time.
Anyway, computationally-secure approaches cannot be considered inherently safe for at least two reasons: if an efficient algorithm is found, the security of these protocols immediately fails; given the ever-growing computing power, ciphers that are impossible to break now, will -- most likely -- became insecure in a few years.

The research field of Quantum Cryptography offers a different approach to secure information sharing: bits are encoded in the quantum state of a single particle, typically the phase or the polarisation state of single photons, and the security is guaranteed by the fact that an eavesdropping attempt -- a measurement on the state in the quantum-mechanics language -- would alter the state and leave a detectable trace.

Since its first proposal, in 1984 by Bennet and Brassard, many quantum-cryptography protocols have been proposed and some of them also implemented in commercial systems. The widespread diffusion of these is anyhow hindered by the lack of real, practical single-photon sources.

My thesis work took place in this context. It was focused on the realisation of planar light-emitting junctions and their study in terms of single-photon emission and was carried out at the NEST laboratory of Scuola Normale Superiore, where researchers have been involved in several EU-level projects focused on the realisation of single-photon sources for quantum-cryptography applications. The activity in this field led to the recent demonstration of a novel kind of p-n planar junction based on semiconductor heterostructures and characterised by a very high operation bandwidth and electron-to-photon conversion efficiency. My project was motivated by the observation that these sources, when driven close to their conduction threshold, emits from selected spots along their perimeter. The sub-micron size of these spots suggested that single-photon emission could be achieved due to Coulomb-blockade effects from each spot. During my work I have verified this hypotesis by fabricating devices with a reduced junction area, in order to limit the number of emission spots.
The analysis of the emitted-photon statistic performed by means of a dedicated Hanbury-Brown-Twiss setup that I have realised during my thesis allowed me to observe for the first time emission from these devices in the antibunching regime, demonstrating that they can be operated as single-photon sources and are suitable for the implementation of QKD systems.


This work is organised in four chapters.

Chapter 1 is a brief introduction to Quantum Cryptography and reports on the state-of-art of the main components of a QC setup. At the end of this chapter I shall describe the experimental setup needed to characterise a source in terms of single-photon emission.
Chapter 2 introduces the semiconductor heterostructures and the lithographic processes used during my work to realise my planar p-n junctions.
Chapter 3 I shall describe the experimental setup and report the preliminary characterisation of the sources in terms of electrical and optical properties.
Chapter 4 reports the observation of light-emission with sub-Poissonian statistics from my devices, which constitutes the main result of my work.