A major field of research at the current time is that of implementing Quantum Key Distribution over large distances using satellites. If this protocol works with this technology, it will have huge implications on future information security. In order for a satellite to implement this idea, there are many aspects that must be taken into account. One of the big issues that comes up for this type of system is that of propagating light through the turbulent atmosphere and its effects on the acquisition, pointing and tracking system. The projects studied in this thesis study some of the effects of the atmosphere on certain detectors, try to develop pointing schemes for better accuracy as well as develop knowledge in free space propagation of other single photon experiments. \n \nIn the first experiment, I study the spatial correlations of the daughter photons created in spontaneous parametric down conversion. I look at the effect of altering the pump beam on the positions of the down converted photons and see if the pump can be manipulated in a way to control the directions of the daughter photons. I begin to utilize a deformable mirror and Shack-Hartmann wavefront sensor which are generally used in adaptive optics, but we plan to use them to alter the pump beam in the spontaneous parametric down conversion process to analyze the correlations between the pump and down converted photons. \n \nThe second experiment investigates the effects of laser scintillation on the performance of a possible tracking device that could be implemented on a satellite. This quad sensor tracks the position of a beam and a system will be developed to move the sensor to keep the beam in the center where there is a hole for the quantum single photons to stream through. In order to create the effects of scintillation, a turbulence simulator box was built and characterized. This box combines wind turbulence with a heat gradient to mimic atmospheric turbulence on a small scale. \n \nFinally, my contributions to a large scale, long distance free space quantum optics experiment are explained and the overall goal of the experiment is discussed. This experiment exposed me to actual free space transmission issues as well as many fundamental techniques for performing long distance optics experiments. In this experiment there was no correction for atmospheric turbulence, but in the future, techniques could be implemented which might increase the efficiencies of the free space links.
A major field of research at the current time is that of implementing Quantum Key Distribution over large distances using satellites. If this protocol works with this technology, it will have huge implications on future information security. In order for a satellite to implement this idea, there are many aspects that must be taken into account. One of the big issues that comes up for this type of system is that of propagating light through the turbulent atmosphere and its effects on the acquisition, pointing and tracking system. The projects studied in this thesis study some of the effects of the atmosphere on certain detectors, try to develop pointing schemes for better accuracy as well as develop knowledge in free space propagation of other single photon experiments. \n \nIn the first experiment, I study the spatial correlations of the daughter photons created in spontaneous parametric down conversion. I look at the effect of altering the pump beam on the positions of the down converted photons and see if the pump can be manipulated in a way to control the directions of the daughter photons. I begin to utilize a deformable mirror and Shack-Hartmann wavefront sensor which are generally used in adaptive optics, but we plan to use them to alter the pump beam in the spontaneous parametric down conversion process to analyze the correlations between the pump and down converted photons. \n \nThe second experiment investigates the effects of laser scintillation on the performance of a possible tracking device that could be implemented on a satellite. This quad sensor tracks the position of a beam and a system will be developed to move the sensor to keep the beam in the center where there is a hole for the quantum single photons to stream through. In order to create the effects of scintillation, a turbulence simulator box was built and characterized. This box combines wind turbulence with a heat gradient to mimic atmospheric turbulence on a small scale. \n \nFinally, my contributions to a large scale, long distance free space quantum optics experiment are explained and the overall goal of the experiment is discussed. This experiment exposed me to actual free space transmission issues as well as many fundamental techniques for performing long distance optics experiments. In this experiment there was no correction for atmospheric turbulence, but in the future, techniques could be implemented which might increase the efficiencies of the free space links.
Long-range quantum communication requires sending qubits, usually photonic cubits, over long distances through free space. Photons propagating through free space encounter optical turbulence, causing scattering and entanglement degradation. In recent decades, adaptive optics has been used to correct imaging errors introduced by turbulence. An atmospheric turbulence simulator has been constructed to measure the effect of optical turbulence on single photon propagation, and an adaptive optics system has been incorporated. The turbulence simulator uses two rotating phase plates to emulate the effects of random dispersive media and two afocal systems to scale long propagation distances down to lengths realizable in a laboratory. The adaptive optics system consists of a Shack-Hartman wavefront sensor used to measure wavefront error, and a deformable mirror used to correct this error. The use of adaptive optics allows us to correct wavefront errors introduced by turbulence. Adding an adaptive optics system into this simulator facilitates more robust characterization of the strength of the simulated turbulence, as well as correction of some of the negative effects the turbulence introduces. This presentation will discuss the effects of turbulence on single photon propagation when an adaptive optics system is used and how these results compare to previously measurements taken without the use of adaptive optics, allowing us to quantify the improvement adaptive optics introduces.
Many fundamental and applied experiments in quantum optics require transferring nonclassical states of light through large distances. In this context the free-space channels are a very promising alternative to optical fibers as they are mobile and enable to establish communications with moving objects, using satellites for global quantum links. For such channels the atmospheric turbulence is the main disturbing factor. The statistical properties of the fluctuating transmittance through the turbulent atmosphere are given by the probability distribution of transmittance (PDT). We derive the consistent PDTs for the atmospheric quantum channels by step-by-step inclusion of various atmospheric effects such as beam wandering, beam broadening and deformation of the beam into elliptic form, beam deformations into arbitrary forms. We discuss the applicability of PDT models for different propagation distances and optical turbulence strengths in the case when the receiver module has an annular aperture. We analyze the optimal detection and correction strategies which can improve the channel transmission characteristics. The obtained results are important for the design of optical experiments including postselection and adaptive strategies and for the security analysis of quantum communication protocols in freespace.
This article offers a solution to the fragmentation problem through a unified model and simulation showing how Adaptive Optics (AO) improves both classical and quantum communications. Free-space optical links can carry Quantum Key Distribution (QKD) over distances that cannot be reached with fiber; however, atmospheric turbulence corrupts the wavefront. For a single-mode quantum receiver, this drives the quantum bit error rate above the security threshold before detector noise can even be considered. AO addresses this by restoring the wavefront and recovering the coupling loss. The theory, mitigation techniques, and quantum security analysis needed to understand this are spread across three research communities that rarely cite one another: atmospheric turbulence and AO theory, classical modulation over fading channels, and BB84 quantum key distribution. We consolidate and develops all three topics in order to trace the chain from wavefront error to bit error rate and secret key rate. We survey six families of turbulence mitigation and provide an assessment of their suitability for single-mode quantum links. In addition, we survey the free-space QKD experimental record, from terrestrial hops to satellite downlinks. A wave optics case study of a representative 500-km Low Earth Orbit (LEO) downlink, corrected by a closed-loop Shack–Hartmann and deformable-mirror receiver at 1.55 μm ties the review together. Across a turbulence sweep, the correction stabilizes single-mode coupling near its physical ceiling, while the uncorrected channel varies by more than an order of magnitude. When carried into the downlink budget, this lifts the quantum link across the 11% security bound at 12 of 15 turbulence conditions where the uncorrected link yields no key, thereby restoring a secure quantum bit error rate in a narrow 4.2–4.9% band throughout and a secret key rate of 7.6–11.2 kbit/s while simultaneously widening the margin of the co-propagating classical channel above its forward error correction threshold.
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
Scattering by atmospheric turbulence is one of the main challenges in creating long free-space optical links, and specifically links of entangled photons. Classical compensation methods are hard to apply to entangled photons, due to inherently low signal to noise ratios and the fragility of entanglement. We have recently shown that we can use a bright laser beam that pumps spontaneous parametric down conversion to control the spatial correlations between entangled photons for compensating their scattering. In this work, we apply the pump-shaping technique to compensate for the scrambling of correlations between entangled photons that scatter by emulated atmospheric turbulence. We use a spatial light modulator and Kolmogorov’s turbulence model to emulate atmospheric turbulence in the lab, and enhance the entangled photons’ signal by a factor of fifteen using pump optimization. We show this for both a static and dynamic emulated atmosphere, and also demonstrate the compensation of the scattering of a higher-order mode. Our results can open the door towards realizing free-space quantum links with entangled photons, used in applications such as quantum key distribution.
The quantum space age has officially begun and many important milestones and achievements have recently been demonstrated, such as the exciting launch and results of the first quantum demonstration satellite, Micius. Previously with terrestrial applications, quantum key distribution was limited in distance to a few hundred kilometers through either free space or optical fiber. This had dampened progress towards a global quantum cryptographic network, but with the recent progress towards space implemented quantum systems, the door has been opened once again. \n \nIn this thesis, we begin by studying the effect of using an adaptive optics system to improve the efficiency of a free space link to a satellite for quantum key distribution. Adaptive optics has been used extensively in astronomy and has the potential to increase the average optical intensity received by the satellite. We study the effect of the atmosphere on the beam as it propagates from the ground station to the satellite. In the up-link configuration, the atmosphere is of special concern as it affects the beam at the beginning of the propagation, making the end effect worse. \n \nOne of the important components of a free space quantum key distribution satellite system is a fine pointing unit. We have, along with industry partners, designed and implemented such a unit for free space optical links. The device was designed to have little to no effect on the polarization of the photons used to transmit the key bits. The device was tested, both in the laboratory and outside and quantum key distribution was successfully performed while the fine pointing was active. \n \nThe main experiment of the thesis demonstrates quantum key distribution to a moving airplane from a ground station. The components of a quantum key distribution receiver prototype were tested locally around the University of Waterloo campus as well as some tests using private airplanes. The collaboration with the National Research Council of Canada really allowed the project to take flight by granting us access to a research aircraft to deploy our receiver prototype. This project spanned over three years and culminated in a two week flight campaign out of Ottawa and Smiths Falls Ontario. \n \nUsing only five flight hours we were able to successfully transmit finite size quantum secure keys from our optical ground station, located at Smiths Falls--Montague Airport to a Twin Otter Research Aircraft housing our quantum key distribution receiver prototype. Many of the components implemented in the receiver were designed and built with spaceflight in mind and have a clear path to flight for space application. \n \nFinally, we study the feasibility of implementing a quantum key distribution receiver onto a nano satellite. In partnership with the University of Toronto Institute for Aerospace Studies Space Flight Laboratory, we studied the various aspects such as optics, detection, cooling, power, mass, etc., to determine if it would be possible to perform quantum key distribution to a nano satellite. The main difference of this project from the previous Quantum Encryption and Science Satellite is the simpler pointing system, which doesn't utilize fine pointing. \n \nThrough various studies, experiments, and component design, we have shown the feasibility of implementing quantum key distribution to a moving aircraft in an up-link configuration. This work contributes to the long line of achievements leading towards satellite implementations of quantum key distribution for eventual global quantum cryptography.