Integrated photonic transmitters for secure space quantum communication

An important issue in today's information society is the security of data transmission against potential intruders, which always put at risk the confidentiality. Current methods to increase security require that the two parties wishing to transmit information, exchange or share one or more security keys. Once the key has been identified, the information can be transferred in a provable secure way using a one-time pad, i. e. the key length is as long as the plaintext. Therefore, the security of the information transmission is based exclusively on the security of the key exchange. Quantum cryptography, or more precisely quantum key distribution (QKD), guarantees absolutely secure key distribution based on the principles of quantum physics, according to which it is not possible to measure or reproduce a state (e.g. polarization or phase of a photon) without being detected. The key is generated out from the measurement of the information encoded into specific quantum states of a photon, named qubits. For example, a qubit can be created using properties such as the polarization or the phase of a photon. Achieved goals of this thesis are the development of a new class of high speed integrated photonic sources for applications in quantum key distribution systems, capable of producing unprecedented qubit rates (100 Mbps - 1 Gbps) and transmitting those over larger distances than those achieved so far (>200 km). More specifically the work has been focused on developing faint pulse sources which can be used in very demanding environmental conditions, such as those in Space. For the development of these sources, apart from the optical design, essential is the opto-mechanical engineering as well as the integration with the electronics. One of the objectives was to achieve very high level of integration and power efficiency, e.g. volumes and power consumption between 10 and 100 times smaller than those typical of a laboratory experiment. Moreover, work in related parts of a whole QKD transmission system has been carried out. In particular, a new scheme for a compact, fast and simple random number generator has been demonstrated successfully achieving a random number generation rate of 1.1 Gbps. Also, during the course of this thesis, the development and engineering of a free-space QKD optical link has been initiated. This thesis makes use of novel ideas to alternatively demonstrate proof-of-concept experiments, which could then further develop into commercial products. To this end, close collaborations with world-wide leading companies in the field have been established. The Optoelectronics Group at ICFO has been involved in current European Space Agency (ESA) projects to develop a small footprint and low power consumption quantum transceiver and a high-flux entangled photon source.En l’actual societat del coneixement és important la seguretat en la transmissió de dades contra potencial intrusos, els quals sempre posen en risc la confidencialitat. Mètodes actuals per incrementar la seguretat requereixen que les dos parts que volen transmetre informació, intercanviïn o comparteixin una o més claus. Una vegada la clau ha estat identificada, la informació pot ser transferida de forma provadament segura utilitzant ”‘one-time pad”’. Per tant, la seguretat en la transmissió de la informació es basa exclusivament en la seguretat en l’intercanvi de la clau. La criptografia quàntica, o més precisament distribució de clau quàntica (QKD), garanteix absolutament la seguretat de la distribució de la clau basant-se en els principis de la física quàntica, segons la qual no és possible mesurar o reproduir un estat (p. e. la polarització o fase d’un fotó) sense ser detectat. La clau es genera a partir de les mesures de la informació codificada en estat quàntics del fotó, anomenats qubits. Per exemple, un qubit pot ser creat utilitzant propietats com la polarització o fase d’un fotó. Els objectius aconseguits d’aquesta tesis són el desenvolupament d’una nova classe d’emissors fotònics d’alta velocitat per a aplicacions en sistemes de distribució de clau quàntica, capaç¸os de produir velocitats de qubit sense precedents (100 Mbps - 1 Gbps) i transmetre’ls a través de distàncies més llunyanes que les aconseguides fins ara (> 200 Km). Més en concret el treball s’ha centrat en el desenvolupament de fonts de pulsos atenuats que poden ser usades en condicions ambientals molt extremes, com les presents a l’Espai. Per al desenvolupament d’aquestes fonts, apart del disseny òptic, importantíssim es l’enginyeria optomecànica com també la integració amb la electrònica. Un dels objectius ha estat aconseguir un molt alt nivell de integració i eficiència de potència, p. e. volums i consums de potència entre 10 i 100 vegades més petits que els típics en experiments de laboratori. Ademés, s’ha realitzat treball en altres parts relacionades amb un sistema de transmissió QKD. En particular, un nou esquema per a un generador de números aleatori compacte, ràpid i simple ha estat positivament demostrat aconseguint velocitats de generació de números aleatoris de 1:1 Gbps. També, el desenvolupament i enginyeria d’un enllaç òptic per a QKD en espai lliure ha estat iniciat durant aquesta tesis. Aquesta tesis utilitza idees novedoses per a demostrar experiments de prova de concepte, els quals poden esdevenir en productes comercials. Per a aquest fi, s’han establert col•laboracions amb empreses internacionals líders del sector. A més a més, el Grup d’Optoelectrònica de ICFO ha estat involucrat en projectes de la Agència Espacial Europea (ESA) per a desenvolupar un transceptor quàntic de tamany reduït i baix consum de potència, el qual també conté una font de fotons entrellaçts d’alt flux

Superheterodyne Microwave System for the Detection of Bioparticles with Coplanar Electrodes on a Microfluidic Platform

The combination of microwave and microfluidic technologies has the potential to enable wireless monitoring and interaction with bioparticles, facilitating in this way the exploration of a still largely uncharted territory at the intersection of biology, communication engineering and microscale physics. Opportunely, the scientific and technical requirements of microfluidics and microwave techniques converge to the need of system miniaturization to achieve the required sensitivity levels. This work, therefore, presents the design and optimization of a measurement system for the detection of bioparticles over the frequency range $0.01$ to $10$ $\mathrm{GHz}$, with different coplanar electrodes configurations on a microfluidic platform. The design of the measurement signal-chain setup is optimized for a novel real-time superheterodyne microwave detection system. In particular, signal integrity is achieved by means of a microwave-shielded chamber, which is protected from external electromagnetic interference that may potentially impact the coplanar electrodes mounted on the microfluidic device. Additionally, analytical expressions and experimental validation of the system-level performance are provided and discussed for the different designs of the coplanar electrodes. This technique is applied to measure the electrical field perturbation produced by $10$ $\mathrm{\mu m}$ polystyrene beads with a concentration of $10^5$ $\mathrm{beads/mL}$, and flowing at a rate of $10$ $\mathrm{\mu L/m}$. The achieved SNR is in the order of $40$ $\mathrm{dB}$ for the three coplanar electrodes considered

Thermophysical analysis of microconfined turbulent flow regimes at supercritical fluid conditions in heat transfer applications

The technological opportunities enabled by understanding and controlling microscale systems have not yet been capitalized to disruptively improve energy processes. The main limitation corresponds to the laminar flows typically encountered in microdevices, which result in small mixing and transfer rates. This is a central unsolved problem in the thermal-fluid sciences, in what some researchers refer to as “quot;ab-on-a-chip and energy - microfluidic frontier”. Therefore, this work focuses on analyzing the potential of supercritical fluids to achieve turbulence in microconfined systems by studying their thermophysical properties. In particular, a real-gas thermodynamic model, combined with high-pressure transport coefficients, is utilized to characterize the Reynolds number achieved as a function of supercritical pressures and temperatures. The results indicate that fully-turbulent flows can be attained for a wide range of working fluids related to heat transfer applications, power cycles and energy conversion systems, and presenting increment ratios of O(100) with respect to atmospheric (subcritical) thermodynamic conditions. The underlying physical mechanism to achieve relatively high Reynolds numbers is based on operating within supercritical thermodynamic states (close to the critical point and pseudo-boiling region) in which density is relatively large while dynamic viscosity is similar to that of a gas. In addition, based on the Reynolds numbers achieved and the thermophysical properties of the fluids studied, an assessment of heat transfer at turbulent microfluidic conditions is presented to demonstrate the potential of supercritical fluids to enhance the performances of standard microfluidic systems by factors up to approximately 50x.Peer ReviewedPostprint (author's final draft

Experimental Verification of Dielectric Models with a Capacitive Wheatstone Bridge Biosensor for Living Cells: E. coli

Dielectric spectroscopy; E. coli bacteria; Maxwell–Garnet modelEspectroscopia dieléctrica; Bacterias E. coli; Modelo Maxwell-Garnet;Espectroscòpia dielèctrica; Bacteris E. coli; Model Maxwell-GarnetDetection of bioparticles is of great importance in electrophoresis, identification of biomass sources, food and water safety, and other areas. It requires a proper model to describe bioparticles' electromagnetic characteristics. A numerical study of Escherichia coli bacteria during their functional activity was carried out by using two different geometrical models for the cells that considered the bacteria as layered ellipsoids and layered spheres. It was concluded that during cell duplication, the change in the dielectric permittivity of the cell is high enough to be measured at radio frequencies of the order of 50 kHz. An experimental setup based on the capacitive Wheatstone bridge was designed to measure relative changes in permittivity during cell division. In this way, the theoretical model was validated by measuring the dielectric permittivity changes in a cell culture of Escherichia coli ATTC 8739 from WDCM 00012 Vitroids. The spheroidal model was confirmed to be more accurate

Kinetic-energy- and pressure-equilibrium-preserving schemes for real-gas turbulence in the transcritical regime

Numerical simulations of compressible turbulent flows governed by real-gas equations of state, such as high-pressure transcritical flows, are strongly susceptible to instabilities. In addition to the inherent multi-scale nature of the flow, the presence of a pseudo-interface can generate spurious pressure oscillations when conventional schemes are utilized. This study proposes a general framework to derive and analyze discretization methods that are able to preserve kinetic energy by convection, and simultaneously maintain pressure equilibrium in discontinuity-free compressible real-gas flows. The formal analysis reveals that the discrete pressure-equilibrium condition can be fulfilled at most to second-order accuracy, as it requires the spatial differential operator to satisfy a discrete chain rule when total, or internal energy, are directly discretized. A novel class of schemes based on the solution of a pressure equation is thus proposed, which preserves mass, momentum, kinetic energy and pressure equilibrium, but not total energy. Extensive numerical tests of increasing complexity confirm the theoretical predictions, and show that the proposed scheme is capable of providing non-dissipative, stable and oscillation-free simulations, unlike existing methods tailored for the transcritical regime.This work is funded by the European Union (ERC, SCRAMBLE, 101040379).Peer ReviewedPostprint (author's final draft

Dimensionality reduction of non-buoyant microconfined high-pressure transcritical fluid turbulence

This work utilizes a novel data-driven methodology to reduce the dimensionality of non-buoyant microconfined high-pressure transcritical fluid turbulence. Classical dimensional analysis techniques are limited by the non-uniqueness of scale-free groups and the lack of a general strategy for quantifying their importance. Instead, the data-driven approach utilized is based on augmenting Buckingham’s theorem with ideas from active subspaces to overcome these limitations. Through this methodology, a principal dimensionless group has been identified that efficiently describes the behavior of the system in terms of normalized bulk turbulent kinetic energy. Additionally, a simplified version of the new dimensionless group is proposed, which presents the structure of a Reynolds number augmented with dynamic viscosity, thermal conductivity, or equivalently Prandtl number and isobaric heat capacity, and specific gas constant to account for thermophysical effects. Finally, the results obtained in this study, which is based on a realistic regime inspired by nitrogen at high-pressure microfluidic conditions, can be generalized to other fluids using the principle of corresponding states.Peer ReviewedPostprint (author's final draft

Investigation of a novel numerical scheme for high-pressure supercritical fluids turbulence

High-pressure supercritical turbulence simulations are strongly susceptible to numerical instabilities. The multi-scale nature of the flow, in conjunction with the nonlinear thermodynamics and the strong density gradients across the pseudo-boiling line can trigger spurious pressure oscillations and unbounded amplification of aliasing errors. A wide variety of regularization approaches have been traditionally utilized by the community, including upwind-biased schemes, artificial dissipation, and/or high-order filtering, where stability is achieved at the expense of suppressing part of the turbulent energy spectrum. In this work, a novel numerical scheme based on the paradigm of physics-compatible discretizations is investigated. In particular, the proposed method discretely enforces kinetic-energy conservation (by convection) as well as preservation of pressure equilibrium; the former is achieved using proper splitting of the convective terms, whereas the latter is obtained by directly evolving an equation for pressure. The simultaneous enforcement of these two properties can lead to stable and physically consistent scale-resolving simulations of supercritical turbulence without the need for any form of artificial stabilization. The novel method is preliminarly assessed on two benchmark cases, with numerical results supporting the theoretical findings.Preprin

Microconfined high-pressure transcritical fluid turbulence

Microfluidics technology has grown rapidly over the past decades due to its high surface-to-volume ratios, flow controllability, and length scales efficiently suited for interacting with microscopic elements.However, as a consequence of the small rates of mixing and transfer they achieve due to operating under laminar flow regimes, the utilization of microfluidics for energy applications has long been a key challenge.In this regard, as a result of the hydrodynamic and thermophysical properties they exhibit in the vicinity of the pseudo-boiling region, it has been recently proposed that microconfined turbulence could be achieved by operating at high-pressure transcritical fluid conditions.Nonetheless, the underlying flow mechanisms of such systems are still not well characterized, and, thus, need to be carefully investigated.This work, consequently, analyses supercritical microconfined turbulence by computing DNS of high-pressure ($P/P_c = 2$) N$_2$ at transcritical conditions imposed by a temperature difference between the bottom (${T/T_c}=0.75$) and top (${T/T_c}=1.5$) walls for a friction Reynolds number of $Re_\tau=100$ (bottom wall).The results obtained indicate that microconfined turbulence can be achieved under such conditions, leading to mixing and heat transfer increments up to $100\times$ and $20\times$, respectively, with respect to equivalent low-pressure systems.In addition, it is found that the near-wall flow physics deviates from single-phase boundary layer theory due to the presence of a baroclinic instability in the vicinity of the hot/top wall.This instability strongly modifies the flow behaviour in the vicinity of the wall and renders present 'law of the wall' transformation models not accurate.Peer ReviewedPostprint (published version

Non-dissipative large-eddy simulation of high-pressure transcritical turbulent flows: formulation and a priori analysis

This work presents a filtered set of equations suitable for the large-eddy simulation of high-pressure transcritical turbulent flows. The formulation is derived from a novel kinetic-energy- and pressureequilibrium- preserving numerical framework, recently proposed to provide physics-compatible (stable and non-dissipative) simulations of the problem. In particular, the compressible Navier-Stokes equations are required to describe the evolution of supercritical fluids along with adequate real-gas thermodynamic closures based, for example, on the Peng-Robinson equation of state. The novelty of this work focuses, therefore, on (i) deriving the filtered set of equations based on the kinetic-energy and pressure-equilibriumpreserving framework, (ii) identifying the sub-filtered unclosed terms, and (iii) performing exploratory assessments of the resulting framework. In the future, these results will potentially enable the design of physics-based sub-filter scale models for high-fidelity LES of high-pressure transcritical turbulent flows.This work is funded by the European Union (ERC, SCRAMBLE,101040379).Peer ReviewedPostprint (published version

Initial design of a faint pulse photon source for quantum key distribution

The project aims at developing new photonic transmitters for quantum cryptography applications which could be used to increase the security of communication networks. The transmitter will be designed to generate security keys at a speed up to 100 Mb/s, two orders of magnitude larger than the state-of-the-art sources. The whole transmitter, with the optical and electronic parts integrated, will have a reduced size and power consumption. In addition, space-qualified optoelectronic devices will be used, so that the final prototype will be ready for satellites and future European Space Agency (ESA) missions