Impact of turbulence in long range quantum and classical communications

The study of the free-space distribution of quantum correlations is necessary for any future application of quantum as classical communication aiming to connect two remote locations. Here we study the propagation of a coherent laser beam over 143 Km (between Tenerife and La Palma Islands of the Canary archipelagos). By attenuating the beam we also studied the propagation at the single photon level. We investigated the statistic of arrival of the incoming photons and the scintillation of the beam. From the analysis of the data, we propose the exploitation of turbulence to improve the SNR of the signal.Comment: 5 Pages, 5 figures, 1 Table, revtex

Quantum communication channels between earth and space and space to earth

The work presented in this thesis, on the topics of global quantum network, is the result of a set of research activities done at the CNR-IFN Luxor laboratory over these three years. The main goal that we would reach is the feasibility demonstration of quantum communication in free-space, without excluding from the definition "free space" any possible scenario. This is the way that in the future may lead to a global quantum network all over the world, that involves satellites and earth station. We may identify three major link families, that should help the reader to classify the research activities presented: • Satellite to Earth and Earth to satellite link - called in the thesis Vertical link • Point to point horizontal link - called in the thesis Horizontal link • Satellite to satellite link - called in the thesis Intersatellite link In a certain way we tried to deal with all of them trough analysis and simulation or experimental campaigns. The core topics of this works however are those related to the experimental campaigns, respectively: • Laser beam propagation in a free-space horizontal link: we established an optical link of 144km between Canary islands in order to improve the knowledge on horizontal laser propagation over atmosphere and to define the setup requirements and the most suitable condition to get an effective quantum link. This work is presented in Chapter 3. • Polarization maintenance over a space quantum link: we designed, realized and installed a polarimeter to reconstruct the polarization state of photons in a space optical link in the MLRO. Our quantum information, in fact, is codified in the polarization state of photons. To pave the way for future developments and implementations (i.e. quantum transmitter and receiver hosted in satellites) we must as first prove the maintenance or the retreadability of the polarization state. The yet existing laser ranging facilities are examples of optical links between Earth and space that can be exploited for polarization analysis and reconstruction providing them with a polarimeter. This work is presented in Chapter 6. The thesis is arranged as follows: a first chapter introduce the atmospheric model and the most common quantities used in this field to describe the effects of turbulence. Follows an analysis and simulation chapter (Ch.2) on space to Earth and Earth to space quantum links. The treatment proceed with the experimental results of the Canary campaign (Ch.3) on horizontal quantum link. Chapters 4 and 5 are related on the polarimetry topic: starting from formalisms and definitions, are presented the simulations that have lead to the requirements for polarimeter design. In the second-last (Ch.6) is described the polarimeter in all its subsystems, calibration and first test results. Chapter 7, the last one, regards on intersatellite quantum link: the treatment is purely focused on model definition and feasibility analysis. A detailed abstract of the main chapters is reported below: • Chapter 2 - Vertical Link - (Analysis and numerical simulation chapter): As mentioned in the introduction: on a global space quantum network we have to deal with satellite to earth and earth to satellite links. To investigate the feasibility of quantum communication over these long distance in a space scale we have to understand the behaviour of a laser beam propagating into a turbulent media. As introduced in chapter [1], the atmospheric turbulence is a source of beam wandering and beam spreading respectively due to small or large eddies if compared with the beam size. These different effects are appreciable considering the exposure time: for short time, the beam appears spread but more over, wandering from the ideal centroid positions; instead for long time scale the wandering effects integrated over time, gives rise to a dominant beam broadening. The beam preserves its Gaussian profile, but more larger in size. A beam affected at the start of its path by the degradation induced by atmosphere will arrive at the receiver plane far away from the unperturbed expected position. On the same way, a beam affected at the end of its path will be barely modified. The results obtained show that the effect of beam wandering is strongly different for the uplink and downlink, up to about two orders of magnitude. A few ten thousand meters of beam radius are reached in a very long distance (like GEO satellite) in the uplink, while a beam radius on the order of few hundred meters is calculated in the case of downlink. From simulation of the beam displacement , appears that it is clearly negligible for downlink in which the main input affecting the beam is the broadening due to turbulence eddies, therefore related to short term beam effects. Instead for the uplink the beam displacement appears monotonically increasing with the distance, due to the atmosphere at the beginning of the path. For a downlink a greater portion of sky compared to uplink falls in the "good SNR cone", allowing a longer time-slot for quantum communication. The SNR is closed to 0.7dB at zenith for Galileo satellites; the situation is most promising for GPS satellites in which the SNR is around 2.2dB. In both cases the values, de facto prove the unfeasibility of quantum communication with the simulated setup, but gives good wishes related to further technological improvements. The actual feasibility of QKD is related to link distances in the range of LEO orbits, a good SNR can be achieved without stretching the telescope field-of-view to the technological limit. Small improvements on IFOV can shift the link distance feasibility limit toward higher orbits, allowing quantum communication even with GPS and Galileo satellites. • Chapter 3 - Free-space long distance horizontal optical link - (Canary experimental results): In the perspective of the extension of Quantum Communication (QC) in free-space to long distances, the analysis of the phenomena that occur to a visible or near-infrared beam in the propagation in atmosphere and their understanding is of crucial importance for devising the most convenient optical terminal. The investigation on the ground-ground case is used also to envisage the space QC, along with a vast area of research in satellite Classical Communications [58]. More in details, optical propagation in atmosphere in the case of links length of over 100km is affected by several transformations of the beam parameters, resulting in an increase of the link losses. Moreover, the long range optical communication at the single photon limit exploiting the quantum protocols, on the other side, differs from the classical protocols in that the signal to be transmitted cannot be intensified, being a train of very weak pulses with an average about one photon per pulse. The understanding of the effects induced by the propagation in both the irradiance at the receiver as well as in the temporal statistics is crucial to assess the quality of the communication and eventually the feasibility of the link. Moreover, for the very long links, fading and losses are induced by the decoupling of the beam with the receiver due to large wandering of the beam spot, as it was also investigated for the space channel [86]. In this perspective of free-space long range single-photon quantum link, we planned a set of experimental campaigns to study the propagation of a single or twin optical beams in scale length of several tens to a few hundreds kilometres. We address in this work the study of the phenomena induced in very long propagation by using of laser beams to investigate links whose length is of several tens to a few hundreds kilometers. In the experiments, the observation of the whole beam combined to the measure of the local irradiance at the receiver side are performed and then projected to the case of single photons links. In addition, with the aim to stabilize the centroid position at the receiver, the propagation of two beams forming a small mutual angle is studied in the framework of the isoplanatic angle spread for low and high orders of the beam spatial modes. The experimental models were realized in Alpine links as well as between the Tenerife and La Palma link in the Canary archipelagos. The whole beam at the receiver was acquired and the scintillation analysis carried out and compared to models including the meteorological data from the Canary Observatories. The design of the optical setup for long range quantum communications is crucial, and we addressed this issue by developing an optimized telescope for the experiments. The literature on horizontal propagation is quite poor mainly because for decades of years the interest was focused on vertical propagation -for astronomical purposes-. As first we needed to understand the effects of atmospheric turbulence in beam propagation: like wandering, spreading and scintillation. This led us, as last step, to identify the best design parameters for transmitters and receiver and for communication protocol. With the gained data we want to define a model or a set of criteria to be considered in designing an optical quantum link and to predict the link losses for a realistic receiver and, possibly to envisage the exploitation of the time-varying losses for the selection of the portion with the highest signal-to-noise ratio [77]. The observation of the propagation of a single or a pair of beams along very long paths of over 100km have shown that the beam is subjected to splitting into multiple spots but that its long-term diameter may be confined to a spot that is only a factor 3 to 5 the diffraction limit. This results have been obtained by using a suitable large aperture aplanatic refractive transmitter. In this way a significant reduction of the link losses for quantum communication channels in this extreme conditions are possible by the implementation of such scheme. This setup was used in both the OGS (Tenerife)-to-JKT (La Palma) propagation and the reverse. The losses reduction is a fundamental aspect to improve the communication effectiveness and reliability. In single photon quantum communications link losses require to stretch the communication time: in fact, unlike the classical channel in which high losses can be compensated increasing the transmitter power, in quantum communication we can only iterate the single photons transmission until a sufficient amount of data is reached. The correlations of the two spots in the twin beam propagation and in opposite propagation (beacon signal from receiver) have demonstrated the possibility of the centroid control of the quantum channel by the use of an auxiliary co-propagating beam or by the use of an intense beam from the receiver. Alternative or combined use of both systems is the winning solution for an effective and stable quantum link. The system presented and tested aims to be a reliable solution to establish a free space quantum channel. The techniques described allow the transmission of an intense laser (the so called "auxiliary") for turbulence probing and compensation while simultaneously being exchange single photons. • Chapter 5 - Space Quantum Communications - (Analysis and numerical simulation chapter): About a decade ago, several groups endeavoured the porting of quantum communication in general and Quantum Key Distribution (QKD) as first example outside the cradle of the lab, in the adverse outdoor conditions: [34], [62]. The natural extent of free space Quantum Communication (QC) is space, due to the restrictions imposed on the Earth surface by the Earth curvature as well as atmospheric turbulence. The experimental demonstration of the feasibility of single photon exchange between Space and a ground receiver has been also demonstrated in our group recently [94]. The demonstration of the control of the polarization state along the Space channel is important to pave the way to further steps in space QC. We estimated the link budget at the Matera site to design a polarimeter: performed the simulation for Goce we obtained the values for SNR at the minimum and maximum satellite visibility. The simulation results can be properly extended for the other satellites. • Chapter 6 - SPOLAR-M experiment - (Design and development chapter): The simulation showed the link budget required for earth-space and space-earth single photons channel, and justify the use of Laser Ranging sites to experimentally test the link. In this chapter after a brief introduction on Stokes Polarimetry will be presented the polarimeter used at the MLRO, designed and developed at the Luxor Laboratory. The system used in the SPOLAR-M experiment is a Four-Channel Polarimeter using Non Polarizing Beam Splitter. This polarimeter use two non polarizing beam splitter, a quarter-wave retarder, a polarizing beam splitter and two linear polarizers. Readings are made at four detectors. The input Stokes vector is determined from the four detector measurements and from use of a transfer Mueller matrix found during the calibration procedure. The chapter discuss the design of the instrument to be integrated with the Matera laser ranging facility. The design will be oriented in order to reduce the changes to the existing setup and to ensure the normal operation of the station. • Chapter 7 - Intersatellite link - (Analysis and numerical simulation chapter): This chapter complete the scenario of a global quantum communication network analysing the feasibility of intersatellite quantum link. In section 7.1 will me modelled the propagation of laser beam in a satellite to satellite link in order to get information on the link budget (attenuation, noise, SNR) and hence on its feasibility. As in chapter [2] for vertical link, the simulation results show that a real implementation of a QKD protocol in space, based on coherent states, requires a careful design of the transmitter: field-of-view, telescope pointing and tracking, efficient single photon detectors etc. The spot size at the target is of the order of a few tens of meter for short-wavelength transmission systems. The corresponding attenuation is rapidly increasing from the relatively good conditions of closest approach of the satellites. The level of attenuation ranges between the cases of downlink and uplink in the case with a terminal on the Earth[30]. Anyway, using the part of the orbit in which the satellites are sufficiently close, the QKD appears feasible. In general, the level of losses that are associated with terminals equipped with optics up to 500mm in diameter makes hardly feasible the double simultaneous link for the distribution of an entangled pair. The background noise is here limiting the SNR to very low levels. The feasibility is also dependent on the capacity of orbital prediction and pointing accuracy over extended intervals. In conclusion, we have seen that in some conditions the extension of quantum communications and technologies appears feasible in the extreme scenario of the satellite optical quantum networks.Il lavoro presentato in questa tesi, riguardante la comunicazione quantistica su scala globale, e’ il risultato di una serie di attivita’ di ricerca svolte presso il laboratorio CNR-IFN Luxor nel corso di questi tre anni. L’obiettivo principale che si vuole raggiungere e’ la dimostrazione della fattibilita’ della comunicazione quantistica in spazio libero, senza escludere dalla definizione "spazio libero" ogni possibile scenario che si puo’ incontrare su una rete globale. Proseguendo in questa direzione in un futuro si potrebbe arrivare ad una rete quantistica mondiale, che coinvolge satelliti e stazioni terrestri. Allo scopo di aiutare il lettore a classificare le attivita’ di ricerca presentate, possiamo identificare tre principali "famiglie" di link: • Link satellite - Terra e Terra - satellite - chiamato nella tesi Vertical link • Link orizzontale punto punto - chiamato nella tesi Horizontal link • Link satellite - satellite - chiamato nella tesi Intersatellite link Attraverso la modellizzazione e la simulazione numerica e/o campagne di test sperimentali abbiamo cercato di affrontare problematiche inerenti a tutte e tre le tipologie. I temi di maggior rilievo, tuttavia, sono quelli relativi alle campagne sperimentali, rispettivamente: • Propagazione di fasci laser in spazio libero su link orizzontale: e’ stato realizzato un link ottico di 144km tra le isole Canarie, al fine di approfondire la conoscenza sulla propagazione della radiazione laser su link orizzontali in atmosfera. I risultati ottenuti hanno consentito di definire le specifiche e la configurazione piu’ adatta per rendere stabile ed affidabile il link quantistico. Questo lavoro viene presentato nel capitolo 3. • Mantenimento della polarizzazione su un canale quantistico spaziale: abbiamo progettato, realizzato e installato un polarimetro presso il centro MLRO allo scopo di ricostruire lo stato di polarizzazione dei fotoni in un canale quantistico spaziale. La nostra informazione quantistica, infatti, e’ codificata nello stato di polarizzazione dei fotoni. Per aprire la strada a sviluppi futuri e all’effettiva implementazione di questa tecnologia (es. trasmettitore e ricevitore quantistici installati a bordo di satelliti) dobbiamo prima dimostrare che il canale preserva lo stato di polarizzazione e che e’ possibile ricostruirne il valore iniziale. Le gia’ esistenti strutture impiegate per il laser ranging sono un esempio di link ottico tra la Terra e lo spazio che puo’ essere sfruttato per l’analisi e la ricostruzione della polarizzazione, opportunamente integrando nel sistema un polarimetro. Questo lavoro viene presentato nel capitolo 6. La tesi e’ strutturata come segue: un primo capitolo introduce il modello atmosferico e le grandezze comunemente utilizzate in questo campo per descrivere gli effetti della turbolenza atmosferica. Segue un capitolo di analisi e simulazione (cap.2 ) sui link quantistici spazio - Terra e Terra - spazio. Nel capitolo 3 sono riportati i risultati sperimentali sul link ottico orizzontale, ottenuti nelle campagne di test svoltesi presso le isole Canarie. I capitoli 4 e 5 sono riguardanti il tema della polarimetria: dopo un’introduzione sui formalismi e le definizioni, sono presentate le simulazioni che hanno fornito le specifiche per la progettazione del polarimetro. Nel penultimo capitolo (cap.6 ) e’ descritto il polarimetro in tutti i suoi sottosistemi: passando dalla progettazione, alla taratura per finire con i risultati dei primi test.L’ultimo capitolo 7 riguarda i collegamenti intersatellitari quantistici: la trattazione e’ esclusivamente focalizzata sulla definizione del modello e sull’analisi di fattibilita’. Un riassunto del contenuto dei principali capitoli e’ riportato di seguito: • Capitolo 2 - Link verticale - (Capitolo di analisi e simulazione numerica): Come accennato precedentemente, su una rete spaziale quantistica globale abbiamo a che fare con link satellite - Terra e Terra - satellite. Per studiare la fattibilita’ della comunicazione quantistica su queste lunghe distanze dobbiamo analizzare la propagazione di un raggio laser in un mezzo turbolento. Come introdotto nel capitolo [1], la turbolenza atmosferica da origine al wandering - "ballamento" - e allo spreading - "allargamento" - del fascio, rispettivamente in base alle dimensioni dei vortici, piccoli o grandi rispetto alle dimensioni del fascio. Questi diversi effetti sono apprezzabili considerando il tempo di esposizione: su scale brevi il fascio appare allargato, ma ancor di piu’, vaga rispetto alla posizione ideale del centroide. Su scale di tempo lunghe gli effetti del wandering sono integrati nel tempo, dando luogo ad un consistente allargamento del fascio. Il fascio conserva il suo profilo gaussiano, ma ha dimensioni nettamente maggiori. Un fascio che attraversa l’atmosfera all’inizio del suo percorso subira’ una degradazione per cui arrivera’ al ricevitore lontano dalla posizione ideale. Allo stesso modo, un fascio che incontra l’atmosfera alla fine del suo percorso sara’ degradato in modo molto meno marcato. I risultati ottenuti dimostrano che l’effetto del wandering sul fascio e’ fortemente diverso per l’uplink e il downlink, fino a circa due ordini di grandezza: da poche decine di migliaia di metri di raggio su distanze molto lunghe (come i satelliti GEO) in uplink, a un raggio dell’ordine di poche centinaia di metri in downlink. Dalle simulazioni dello spostamento del fascio dalla sua posizione ideale, e’ chiaramente evidente che tale effetto e’ trascurabile per il downlink, in cui la degradazione principale e’ l’allargamento causato dai vortici della turbolenza, quindi legato a effetti su scala temporale breve. Invece per l’uplink lo spostamento del fascio appare crescere monotonamente con la distanza, a causa dell’atmosfera all’inizio del percorso. Per il downlink una porzione maggiore di cielo, rispetto al uplink, cade nel "cono a buon SNR", questo permette uno slot temporale piu’ lungo per la comunicazione quantistica. L’ SNR e’ circa 0.7dB allo zenith per i satelliti Galileo, la situazione e’ piu’ promettente per i satelliti GPS in cui l’ SNR e’ di circa 2.2dB. In entrambi i casi i valori ottenuti di fatto dimostrano l’infattibilita’ della comunicazione quantistica con la configurazione simulata, ma danno buoni auspici per ulteriori miglioramenti tecnologici. Nelle condizioni attuali, la fattibilita’ della QKD e’ relegata a distanze nel range delle orbite LEO, in cui un buon SNR puo’ essere ottenuto senza imporre vincoli troppo stringenti per il campo di vista del telescopio e senza spingere la progettazione al limite tecnologico. Piccoli miglioramenti sul IFOV possono spostare il limite di fattibilita’ verso orbite piu’ alte, consentendo la comunic

Standardizzazione e validazione dei dati della pianificazione urbanistica locale nella regione del Veneto

La Legge Regionale del Veneto n. 11 del 23 aprile 2004 ha introdotto varie innovazioni, tra cui l’obbligo per i Comuni di compilare in formato digitale i propri strumenti urbanistici, ed in particolare i Piani Regolatori Comunali, secondo una specifica di compilazione predisposta dalla stessa Regione, che prevede in particolare anche l’obbligo di usare un formato GIS standardizzato. According to the Land Planning Act of Region Veneto (Italy), Municipalities have to compile their Local Land Plans (PRC – Piani Regolatori Comunali) using a standardized GIS data model. The Region Veneto has issued such specifications about the GIS data model immediately after the entry into force of Law n. 11 (2004). The Veneto Region has immediately activated a system for the validation of the GIS data of Plans coming from Municipalities. The system computes two indices to measure the compliancy of the incoming data with the data model according to art. 11 of Law n. 11 (2004). For sure the operative model issued by the Law n. 11 (2004) requires skilled technicians both from the side of the Municipalities for the population of the geodatabase of the local land plan and from the side of the Veneto Region for the validation. The Veneto Region has set up a specific office staffed with GIS skilled personnel using a state-of-the-art software procedure for data validation. The workflow of data validation has been published by the Region in January 2007. The latest and improved release of GIS data model was issued in January 2010. The software procedure has been built on top of Intergraph’s GeoMedia GIS software. Currently the whole procedure for Local Land Planning is coherent with the SDI of Region Veneto - operative since 2011. According to INSPIRE Directive and some italian laws, Region Veneto has started the design of the new workflow for the GIS component of local land planning. The leading concepts is the Municipalities will be capable of validating autonomously their GIS datasets using a web service so the data uploaded by Municipalities into the Central GeoDatabase of Region Veneto’s SDI will be ready for automated mosaicking , remodelling (using local data models and INSPIRE Land Use data model) and SDI-sharing

Intersatellite quantum communication feasibility study

The shift in the Communication paradigm from the bit to the qubit is increasingly exploited in terrestrial long range links and networks, with strong potentials in secure communications, quantum computing and metrology. The space-to-ground quantum key distribution was also considered as feasible. A new different scenario for the quantum communications is that of the intersatellite link. In this study we focus on the extension of intersatellite communications into the quantum domain. The long distances involved and the fast relative motion are severe constraints, partially compensated by the absence of beam degradation due to the propagation in the atmosphere as well as the relatively low background noise level. We address the conception of the optical terminal and the predicted performances in the case of constellations of LEO and MEO satellite including the quantum communications and quantum teleportation

Feasibility Analysis for Quantum Key Distribution between a LEO Satellite and Earth

Terrestrial QKD channels can connect two links with a maximum distance of few hundred kilometres. In the case of fibre links, this is due to the signal attenuation in the fibre; in the case of free-space link the losses are due to atmospheric turbulence and absorption. Free-space optical terminals exploiting satellite-based relays are the only resource that can enable global scale quantum key distribution, since single photon propagation is for the main part in vacuum with no turbulence or absorption, and just a small part of the path is through the atmosphere. Several proof-of-principle experiments have been carried out recently: among these the feasibility of singlephoton exchange between a satellite and an optical ground station was demonstrated in 2008 [1]

Engineering a Long Distance Free-Space Quantum Channel

We present the main design issues and the tests in the setup of a long distance free-space quantum link under develop- ment within the project \u201cQuantumFuture\u201d of the University of Padova. In particular, new achievements in the actual engineering of the link are presented, both for polarization quantum states encoding and for coherent quantum states