The main argument of this thesis is the application of advanced techniques for the optimization of single photon communication and in general of single photon applications. The work is inserted in the contest of various projects that involve the departments of Information Engineering and Astronomy of the University of Padua. In particular my contribution has been the development of a quantum cryptography setup that we called QuAKE. The system has been designed and implemented in our labs and include in the hardware some advanced temporal and spatial filtering techniques. These features has been realized respectively with an ad hoc electronics and with an adaptive optics system, the latter developed entirely in our department. The high level software for quantum cryptography has been also implemented and many optimizations have been realized both in the logical design and in the single algorithms. The last part of this thesis describes an astronomical instrument, called AquEYE, developed by our group and capable of time tagging single photons coming from celestial sources. In particular a description of the time and frequency distribution unit is given since this has been my contribution to the AquEYE instrument so far. The thesis is organized as follows: after an introduction to quantum cryp- tography (chapter 1), the QuAKE system is presented (chapter 2), the electronics and the optical setup are described (chapter 3) as well as the adaptive optics system (chapter 4), it follows a description of the results obtained testing the adaptive optics system outdoor and on the QuAKE system (chapter 5) and the description of the high level software and the related results (chapter 6), last a description on the timing and frequency unit of AquEYE is presented as well as some of the early results of the instrument (chapter 7).
The main argument of this thesis is the application of advanced techniques for the optimization of single photon communication and in general of single photon applications. The work is inserted in the contest of various projects that involve the departments of Information Engineering and Astronomy of the University of Padua. In particular my contribution has been the development of a quantum cryptography setup that we called QuAKE. The system has been designed and implemented in our labs and include in the hardware some advanced temporal and spatial filtering techniques. These features has been realized respectively with an ad hoc electronics and with an adaptive optics system, the latter developed entirely in our department. The high level software for quantum cryptography has been also implemented and many optimizations have been realized both in the logical design and in the single algorithms. The last part of this thesis describes an astronomical instrument, called AquEYE, developed by our group and capable of time tagging single photons coming from celestial sources. In particular a description of the time and frequency distribution unit is given since this has been my contribution to the AquEYE instrument so far. The thesis is organized as follows: after an introduction to quantum cryp- tography (chapter 1), the QuAKE system is presented (chapter 2), the electronics and the optical setup are described (chapter 3) as well as the adaptive optics system (chapter 4), it follows a description of the results obtained testing the adaptive optics system outdoor and on the QuAKE system (chapter 5) and the description of the high level software and the related results (chapter 6), last a description on the timing and frequency unit of AquEYE is presented as well as some of the early results of the instrument (chapter 7).
Quantum Key Distribution (QKD) is a cryptography technique that allows two parties to share a secret common key that can be used for a secure exchange of information. To meet the requirements and interests of different final users, the technological development of this technique has rapidly led toward commercial systems for a global quantum infrastructure, namely a quantum network. Communication among parties requires an optical channel, which for short distances is typically provided by optical fiber. However, the limitation of optical losses on longer fiber links, or the unavailability of an exploitable deployed fiber, requires the use of a different QKD medium, i.e. free-space. At the University of Padova we are investigating, both theoretically and experimentally, how Adaptive Optics (AO) can be used to improve the performance of free-space quantum communication, with a particular focus on QKD. Following our theoretical studies on the optimal design for an intermodal free-space/ fiber link, we were able to realize different QKD field trials along a 600m-long free-space link exploiting polarization encoding. Besides the metropolitan area of Padova, we are now working on the development of an extra-urban free-space link, which is more prone to atmospheric turbulence effects. For this link, we equipped a radar tower located on a hill in Teolo with a remotised optical transmitter, and we are now developing the AO system for the receiving telescope, located at the Department of Information Engineering in Padova, with which we plan to realize QKD during day and night time.
The aim of my thesis is to demonstrate the feasibility of Quantum Communication in free space and space, pointing out how ESA Galileo constellation could be strengthens into an Optical Quantum Communication Network (OQCN), by the employment of a compact and low cost prototype (SaNe-QKD OPT). Considering table 2 according the guidelines of [70] (2012) for the European Quantum Information Processing and Quantum Communication, three crucial long term goals are fulfilled, (Satellite Quantum Communication, besides 1000 kilometers of spatial Quantum Cryptography, multi-node Quantum Network), with reference to Galileo constellation: the new devised OQCN will perform Quantum Communication between approaching satellites of the constellation in multi-node mode, and Quantum Key Distribution scheme will be employed over large distances, definitely above 1000 kilometers. My thesis summarizes three years of researches at the Luxor Laboratories in Padova. It is dedicated to two core topics, which are twofold aspects of the same issue: beam propagation in Quantum Communication over ground and over space, with the final realization of a Quantum Communication Network in which concretized the acquired knowledges in Quantum field. And Inter-island Canary links have been chosen as representing a worse case scenario for Quantum Communication experiments, an ideal test bed to investigate beam propagation in view of the space applications, where crucial aspects are the huge distances that Communication beams have to cover. The thesis is divided in two complementary parts: Ground beam propagation Beam propagation in free space along horizontal links in Canary Islands, performing long distances (143 kilometers) and aiming to exploit beam propagation through a turbulent medium. The results pointed out optical configurations and specifics in order to obtain an effective and stable communication link. These researches are within of the strategic project Quantum Future of the University of Padova, "the shift in the Quantum paradigm" Space beam propagation. The second part is dedicated to space beam propagation, regarding design and arrangement of the Quantum prototype SaNe-QKD to fuse with the Optical Communication prototype OPT (by Thales Alenia Space). The resulting system, SaNe-QKD OPT, will be positioned on board of Galileo satellites for performing Quantum Optical Inter-satellite links. Simulations about inter-satellite links, network topology features and key length evaluation, have allowed to obtain results and specifications about operative wavelengths and telescope apertures to use for inter-satellite communications. Then, the prototype SaNe-QKD have been arranged and is here shown in each part. This researches are part of the project "Application of Optical Quantum Communication Links for GNSS" by the European space Agency (ESA). My thesis is divided in four chapters, as follows Ground beam propagation Chapter 1 introduces an overview of main concepts of photonics, atmospheric models and turbulence parameters, optics and fiber-optics and telescopes, used throughout the treatment. The Newtonian telescope is investigated in order to analyze the optical path arriving at the prototype SaNe-QKD for space purpose and the Canary Telescope in order to realize an optical system implementing the centroid spot following system. Zemax simulations are presented for both to check aberrations and for collimation issues. For Newtonian off-axis field, coma and field curvature are revealed. For Canary Telescope chromatic aberration is revealed and the arrangement scheme of the experiment of beam propagation in free space in Canary Islands with co-propagating beams control is described. Kolmogorov atmospheric turbulence models with Hughnagel Valley model is then briefly reported too. It will be used in Canary experiments over 143 kilometers and in chapter 3 for up/downlink quantum communication simulations between a ground base transceiver and a satellite. Chapter 2. It is known that an unconfined optical mode propagating in a turbulent medium suffers of distortions. In Quantum Communication the information is encoded and transmitted as a train of single photons with mean average of about one photon for pulse; it follows that the link losses of a beam propagating in atmosphere increase with the distance; in contrast they cannot be reduced by increasing the signal power as in optical communication. Consequently, it is crucial to exploit beam propagation over long ranges, in order to analyze photon statistics, the transformations on the beam due to the ground in order to prevent the unleashing of the beam between transmitter and receiver. The turbulence introduces two contributes according the eddies size [51] [52]respect to the optical beam: • Beam wandering, which occurs when a laser beam is refracted by eddies with size larger than the beam diameter, causing a displacement of the beam center (centroid). • Beam spreading, which is due to the laser beam being refracted by eddies smaller than the beam diameter. The short-term beam spread is an additional spread with respect to the standard spread due to the free space laser beam propagation (without turbulence effects). These effects appears in relation to the exposition time: on short time scales the beam wandering is the dominant effect, while on long time scales the beam spread-ing is the dominant effect. Using turbulence as a resource, the research of beam propagation in free-space communication links opens to test a new equipment, the custom Canary telescope (Chapter1). The team performed two free-space propagation links with the telescope: the former, a ‘short’ local range (about 20 km) test link between Asiago and Monte Grappa (Italy) in order to examine the Canary Tele-scope and check the communication equipment at the transmitting part; the latter, a ‘long’ range (143 km long) link between La Palma and Tenerife (Canary Islands) for free-space propagation experiments with severe turbulence conditions. Developments and data analysis are presented, pointing out methodologies for turbulence characterization in Ground Quantum Optical Links. The results of the propagation of single, double beams along 143 kilometers have demonstrated that it is possible to optimize an optical system in order to reconstruct the beam long-term diameter and by techniques of beam co-propagating technique, the loss link could be reduced. This is promising as link losses are crucial aspect in Quantum Communication, when in a noisy channel the quantum signal (the in-formation encoded in single photons) cannot be improved by increasing the signal power. Besides we observed that the statistic of arrival of single photons over a free-space 143 km optical link confirms that there is a transformation from Poissonian to log-normal distribution: There is still the evidence of consecutive subintervals of low losses allowing us to envisage the exploitation of turbulence as an SNR improvement technique. Space beam propagation Chapter 3 according the requests of the ESA project, a feasibility study for Quantum Communication applications to Galileo constellation is shown. The chapter starts with an overview of the merits of Optical Communication (data rate exchanging, lightness, compactness, low power consumption..) Pointing out that the intrinsically secure is the added value that only the Quantum counterpart could supply. A brief part recalling orbital motion in space in order to check satellite orbital motion. Then we open to up/downlink simulations overview: recalling the simulations in literature of ground to space or space to ground beam propagation through the atmosphere, the feasibility study shows that unfortunately Galileo constellation can realize only inter-satellite communication links, as the atmosphere and the altitude of the satellite constellation prevents any effective and real single photon transmission with the current technologies. However, simulation results of inter-satellite beam propagation show us that the huge distances can be overwhelmed (derived requirements are telescope diameters >20 cm and operating wavelength <532nm), and time interval within perform Quantum Communication can be calculated: respect to a reference satellite of different orbital plane, the time interval in which the intra-satellite distances are within 15000 kilometers are • For satellite called three of about 176 minutes • For satellite called two of about 168 minutes Then exploiting the relative motion of satellites lying in different orbital planes, we show that it is possible to target communication between satellites that are closer in turns, and to achieve appreciable transmission rates: the distance intervals in which we calculated the raw key rate values are 6000, 10000 and 19000 kilometers, and the best values in raw key rates (18M bits/s at 6600 kilometers, related to lower attenuation values) are obtained for a telescope diameter of 50 cm and wavelength of 50nm, while the worse values in raw key rates (2.2 Kbits/s at 6600 kilometers, related to higher attenuation values) are for a telescope diameter of 20 cm and wavelength of 800nm. Finally, today information-based society security is of paramount importance: Galileo Optical Quantum Network will guarantee intrinsically secure key exchanging, free of PSN attack, within a satellite distance signed by decoy scheme approach. After a brief review of a model for a Quantum Communication System, simulations about key rate are shown in the final section, showing that for same wavelengths and within same beam propagation distances, larger apertures present higher values of key rate[bit/s]. At the same time we have observed that shorter operating wave- lengths presents higher values of key rate, assuring that in merit to Galileo OQCN decoy schemes could be applied in order to beat PNS attack over well defined communication links in relation to the link distance covered, the operative
This dissertation reports on an experimental quantum optics project performed in the group of Prof. Jurgen Eschner at ICFO - The Institute of Photonic Sciences in Spain, to advance in the research of long-distance quantum communications. It describes in detail several crucial engineering contributions to the project and puts them in the framework of the physical background and of the first successful proof-of-principle experiments. The document starts with an introduction about long-distance quantum communication, discussing some of its applications and its state of the art, and introducing the different scenarios to create distant entanglement with an apparatus like the one presented here. Then, it follows the technical description of the setup, which is the main part of the document. This is consistent with the percentage of time of the PhD dedicated to its design and construction, because the experiment was started from scratch. This technical description has a small part which explains all the work the group has done as a team, e.g. the vacuum setup and the laser system, and then it focuses in my individual contribution to this technical work, with the Cavity Locker and the pulse sequencer called Hydra. The latter is a highly complex project which took two years of development, and consequently it occupies the biggest portion of this document. After the technical description, the first operations with single trapped ions are explained, finishing with an implementation of a bandwidth-tunable single-photon source for quantum networking applications. The experimental setup consists of two ions traps -so far the only ones in Spain, and one out of a handful similar setups worldwide- located in two different vacuum chambers which are separated by approximately one meter of distance. In each apparatus, single trapped ions are manipulated with laser light, allowing us to control the energy level of their valence electron to perform quantum information processing. One main goal of the experiment is to create entanglement between two distant ions -one in each trap-, a mandatory resource to perform quantum communications between them. Therefore, each vacuum chamber can be understood as a transceiver, or node, in a quantum network scenario. Such a network could be used, for example, to create more secure communication links using quantum cryptography; or to extend the power of two quantum processors located at a distance by means of the interchange of quantum information between them, e.g. using quantum teleportation. The quantum transceiver technology studied in this thesis will also help establishing the basis for space-based quantum communication links using satellites, enabling the creation of global quantum networks in a not too far future.
This thesis work is about the realization of hardware and software systems for Quantum Random Number Generation (QRNG) and Quantum Key Distribution (QKD). Such systems were developed to guarantee a full functionality for a broader investigation of these two cutting edge applications of Quantum Information field. The thesis describes in details both the hardware and the software that were developed for FPGA-CPU board, Time-to-Digital converter (TDC) devices and computers, along with QRNG and QKD specific applications and their results. Randy was the first FPGA-based QRNG device to be developed; it uses a light source attenuated to single-photon level and one single-photon avalanche diode (SPAD). From the sampling of the SPAD electrical signal, the device produces random numbers through dedicated generation protocols and through the Peres unbiasing algorithm in order to maximize the output generation bit rate. Furthermore, the device allows to generate real time random numbers. This feature is used for the time setting of electro-optical components for extending Wheeler’s delayed-choice experiment to space. The same techniques were applied to a second device, LinoSPAD; it combines an FPGA-chip and a CMOS-SPADs array. Moreover, in this device, a TDC improves the photon detection time accuracy. Along with a dedicated post-processing based on Zhou-Bruk algorithm, the TDC allowed to reach a final bit rate equivalent to 300 Mbit/s. As far as QKD systems are concerned, within the collaboration among the University of Padova, the Italian Space Agency (ASI) with the Matera Laser Ranging Observatory (MLRO) and the Chinese Academy of Sciences (CAS) a TDC device management software was developed. The project aim is to realize a quantum cryptographic key exchange between the Chinese satellite Micius and MLRO. The software was designed to manage the entire data acquisition synchronized with UTC time. Furthermore, another software was designed to deal with electro-optomechanical and electro-optical components. The software is aim at the time-variant compensation of the beam angular changes through the optical path. Once again, within a collaboration between ASI and University of Padova, a full free space QKD system over tenths of kilometers was developed. It required the design of various components. This work describes the QKD source along with the dedicated FPGA board design. Such board generates the electrical impulses to control the qubit laser along with the electro-optic phase and intensity modulators.
Very accurate and stable time tagging capabilities are fundamental for Quantum Astronomy and Quantum Key Distribution. The main task of Quantum Astronomy is to find particular signatures of different astrophysical emission mechanisms or scattering processes by measuring the statistics of the arrival time of each incoming photon. This line of research will be particularly important with future extremely large telescopes. On the other hand, Quantum Key Distribution (QKD) assures a secure cryptographic key sharing between optical transmitters and receivers through the synchronous exchange of quantum states (e.g., single polarized photons). \nBoth technologies need to detect the arrival times each photon with very high temporal resolution in order to discriminate the signal photons from the background ones. In this article, we present the activities of our research group on Quantum Astronomy and Quantum Key Distribution, taking into account their very strict requirements. \nFor Quantum Astronomy, we have developed an instrument called AquEYE capable of time-tagging each incoming single photon using as detectors four Single Photon Avalanche Diodes (SPAD). In this experiment, we need to maintain an absolute time scale reference with a maximum error phase less than 1 ns for measurements lasting more than 30 minutes. For the development phase of our instrument, we used an available rubidium oscillator disciplined by a GPS receiver. This system will supply the reference clock and trigger signal to our acquisition electronics, which are based on a time-to-digital converter. In the paper, we will present the more advanced solution we have under evaluation. \nAs we are also working in the realization of a QKD prototype, we will present our study on time and frequency stability of the local oscillators of the electronics. For the moment, we are using two simple quartzes, but in the article we will show the results of several mathematical simulations. With them, we will analyze the performance tradeoffs in terms of the final cryptographic key rate, through different synchronization techniques and frequency reference sources.
Quantum information and quantum optics offer powerful innovative tools for secure communication and cryptography, with Quantum Key Distribution (QKD) becoming a standard technology in the cryptographic world. By exploiting fundamental principles such as quantum superposition, entanglement, and the no-cloning theorem, QKD enables two users to establish a shared secret key with information-theoretic security. Quantum communication is a fundamental element for the realization of the quantum internet, a network of distant nodes connected by quantum channel. The realization of such networks requires methods to distribute states and correlations with high fidelity and low losses. This thesis explores the use of innovative photonic technologies for the realization of experiment aiming at improving quantum communication and quantum channels. Single photons based on QD are generated on-demand and show high purity, near-optimal indistinguishability and low multi-photons emission. By exploiting the features of these photons, a method to realize entangled states is implemented. Using the polarization degree of freedom, these entangled states are ideal candidates for free-space QKD protocols based on the Bell’s inequality. To address the problem of the reference frame alignment using the polarization, hybrid states are studied and implemented. The specific combination of polarization and orbital angular momentum (OAM) can produce photons that are not modified by rotation of the reference frame. We implemented an experiment to realize a QKD that is independent on the relative rotation of the two user. The final goal of my thesis was devoted to the in-field demonstration of quantum teleportation as a fundamental building block of distributed quantum networks, enabling the reliable transmission of quantum information between remote nodes. Exploiting the free-space infrastructure in the "Sapienza" University of Rome, a link of around ∼ 300 m, we implemented the quantum teleportation between two different QDs showing the possibility of using this technology into real-case scenario. The results obtained show not only that QDs are ideal sources for quantum communication, but also the possibility of using new states to improve current technologies, advancing the development of efficient and reliable quantum networks.