Quantum information with black boxes : lifting protocols from theory to implementation

According to recent estimates, 10^18 bytes of data are generated on a daily basis around the globe. Our information society urges for radical solutions to treat such data deluge. By exploiting fundamental key elements of quantum theory -arguably the most probed theory of modern physics- quantum information science is nowadays revolutionizing the way in which we acquire, process, store and transmit information. In the midst of the information era, the potential of quantum technologies is being recognized by the industry sector, and in turn, new capabilities for quantum information processing keep driving exciting discoveries related to more fundamental aspects of science. There are several research programs all around the world fostering the development and commercialization of quantum technologies, mostly for cryptographic and randomness generation duties. Thus, the technological limitations that today step us aside from the quantum information era are gradually being overcome. But there is a fundamental issue that still needs to be faced: the impossibility to know what is really going on in quantum experiments, due to their atomic-scale dimensions. Indeed, how will an average user guarantee the proper functioning of a quantum device that has been purchased from an external company? To his eyes, the device will merely look like a black box. Even if the customer holds a PhD in quantum science, the issue will remain fundamentally cumbersome because of the impossibility to fully control, i.e. monitor, all the physical processes occurring in any quantum experiment. Furthermore, the situation turns even more dramatic when considering adversarial applications, where a malicious eavesdropper could break the devices to manipulate their internal working, turning the protocol insecure and hence irrelevant as well. Therefore, it is the purpose of this Thesis to contribute to the experimental development of quantum information protocols with uncharacterized devices, namely, device-independent quantum information protocols. These protocols are naturally immune to any attack or failure related to mismatches between protocol theory and its actual implementation. This is achieved throughout the different Chapters by pursuing the following three overlapping duties: (i) To broaden theoretic capabilities by establishing a richer understanding of relevant fundamental resources lying at the basis of the theory of quantum information with uncharacterized devices. (ii) To develop competitive quantum information protocols by finding an adequate trade-off between high-performance and practicability; between the power of the device-independent framework and its less demanding, so-called semi-device-independent, relaxations. (iii) To analyze and improve experimental conditions of diverse physical setups in order to carry out implementations in proof-of-principle experiments demonstrating quantum information protocols with black boxes. Our objective of turning the theory of quantum information into a graspable technology for our society through the development and implementation of protocols based on the minimalist, user-friendly, black-box paradigm contributes not only to the technological development of these protocols, but it also offers valuable insights on more fundamental aspects of quantum theory. In this sense, we contribute to the characterization and quantification of entanglement -the pivotal quantum resource at the basis of most testable phenomena without classical account- in scenarios of practical interest where uncharacterized devices are used. From the more applied perspective, we contribute to the development of two specific information tasks: the certification of genuinely random numbers in device-independent and semi-device-independent scenarios, and the generation of a shared secret key among two parties in a full device-independent manner.De acuerdo con estimaciones recientes, 10^18 bytes de datos se generan diariamente alrededor del mundo. Nuestra sociedad necesita urgentemente soluciones efectivas para lidiar con este diluvio de datos. Utilizando elementos fundamentales de la teoría cuántica -la teoría más explorada de la física moderna, posiblemente- la información cuántica está revolucionando la forma en la que adquirimos, procesamos, almacenamos y transmitimos información. En plena era de la información, el sector industrial reconoce cada vez más el potencial de las tecnologías cuánticas, y a su vez nuevos desarrollos en el procesamiento de la información cuántica continúan impulsando descubrimientos prominentes relacionados con aspectos científcos de carácter más fundamental. Existen varios programas de investigación alrededor del mundo desarrollando y comercializando tecnologías cuánticas, principalmente para aplicaciones de criptografía y generación de números aleatorios. Así, las limitaciones que hoy nos separan de la era de la información cuántica están siendo gradualmente superadas. Sin embargo, existe un problema fundamental que aún necesita ser enfrentado: la imposibilidad de saber lo que realmente sucede en un experimento cuántico, debido a sus dimensiones de tamaño atómico. En efecto, ¿cómo podrá un usuario garantizar el funcionamiento adecuado de un dispositivo cuántico que ha sido adquirido a través de una compañía externa? A sus ojos el dispositivo será una verdadera caja negra. Incluso si el usuario contara con un Doctorado en ciencia cuántica, el problema prevalecería insoluble debido a la imposibilidad de controlar a la perfección, es decir monitorear, todos los procesos físicos que ocurren en cualquier experimento cuántico. Además, la situación se vuelve aún más dramática si se piensa en aplicaciones en donde un agente maligno pudiese hackear los dispositivos y manipular su funcionamiento interno, volviendo así el protocolo en cuestión inseguro y por ende también irrelevante. El propósito de esta Tesis es entonces contribuir al desarrollo experimental de protocolos de información cuántica con dispositivos sin caracterizar, llamados "device-independent". Estos protocolos son, por naturaleza, immunes a cualquier ataque o falla relacionada con desajustes entre la teoría y la implementación del protocolo. Esto se logra a lo largo de los diferentes Capítulos prosiguiendo las siguientes tres tareas que en ocasiones se traslapan: (i) Ampliar las capacidades teóricas estableciendo un entendimiento mayor de los recursos fundamentales de la teoría de la información cuántica con dispositivos sin caracterizar. (ii) Desarrollar protocolos de información cuántica competitivos, encontrando un intercambio adecuado entre alto rendimiento y practicabilidad; entre el poder del marco de trabajo device-independent y sus menos demandantes versiones, dichas "semi-device-independent". (iii) Analizar y mejorar las condiciones experimentales de diversas plataformas para llevar a cabo implementaciones en experimentos de prueba de principio, demostrando la realización de protocolos de información cuántica con cajas negras. Nuestro objetivo de convertir la teoría de la información cuántica en una tecnología tangible para nuestra sociedad a través del uso de dispositivos sin caracterizar contribuye no solamente al desarrollo tecnológico de estos protocolos, sino que también ofrece una visión valiosa de aspectos más fundamental. En este sentido, contribuimos a la caracterización y cuantificación del entrelazamiento -el recurso cuántico fundamental de muchos fenómenos sin contraparte clásica- en escenarios de interés práctico en dónde se consideran dispositivos sin caracterizar. Desde la perspectiva más aplicada, contribuimos al desarrollo de dos tareas específicas: la certificación de números genuinamente aleatorios en escenarios device-independent y semi-device-independent, y la generación de una llave secreta entre dos partes de manera device-independent.Postprint (published version

Quantum information with black boxes : lifting protocols from theory to implementation

According to recent estimates, 10^18 bytes of data are generated on a daily basis around the globe. Our information society urges for radical solutions to treat such data deluge. By exploiting fundamental key elements of quantum theory -arguably the most probed theory of modern physics- quantum information science is nowadays revolutionizing the way in which we acquire, process, store and transmit information. In the midst of the information era, the potential of quantum technologies is being recognized by the industry sector, and in turn, new capabilities for quantum information processing keep driving exciting discoveries related to more fundamental aspects of science. There are several research programs all around the world fostering the development and commercialization of quantum technologies, mostly for cryptographic and randomness generation duties. Thus, the technological limitations that today step us aside from the quantum information era are gradually being overcome. But there is a fundamental issue that still needs to be faced: the impossibility to know what is really going on in quantum experiments, due to their atomic-scale dimensions. Indeed, how will an average user guarantee the proper functioning of a quantum device that has been purchased from an external company? To his eyes, the device will merely look like a black box. Even if the customer holds a PhD in quantum science, the issue will remain fundamentally cumbersome because of the impossibility to fully control, i.e. monitor, all the physical processes occurring in any quantum experiment. Furthermore, the situation turns even more dramatic when considering adversarial applications, where a malicious eavesdropper could break the devices to manipulate their internal working, turning the protocol insecure and hence irrelevant as well. Therefore, it is the purpose of this Thesis to contribute to the experimental development of quantum information protocols with uncharacterized devices, namely, device-independent quantum information protocols. These protocols are naturally immune to any attack or failure related to mismatches between protocol theory and its actual implementation. This is achieved throughout the different Chapters by pursuing the following three overlapping duties: (i) To broaden theoretic capabilities by establishing a richer understanding of relevant fundamental resources lying at the basis of the theory of quantum information with uncharacterized devices. (ii) To develop competitive quantum information protocols by finding an adequate trade-off between high-performance and practicability; between the power of the device-independent framework and its less demanding, so-called semi-device-independent, relaxations. (iii) To analyze and improve experimental conditions of diverse physical setups in order to carry out implementations in proof-of-principle experiments demonstrating quantum information protocols with black boxes. Our objective of turning the theory of quantum information into a graspable technology for our society through the development and implementation of protocols based on the minimalist, user-friendly, black-box paradigm contributes not only to the technological development of these protocols, but it also offers valuable insights on more fundamental aspects of quantum theory. In this sense, we contribute to the characterization and quantification of entanglement -the pivotal quantum resource at the basis of most testable phenomena without classical account- in scenarios of practical interest where uncharacterized devices are used. From the more applied perspective, we contribute to the development of two specific information tasks: the certification of genuinely random numbers in device-independent and semi-device-independent scenarios, and the generation of a shared secret key among two parties in a full device-independent manner.De acuerdo con estimaciones recientes, 10^18 bytes de datos se generan diariamente alrededor del mundo. Nuestra sociedad necesita urgentemente soluciones efectivas para lidiar con este diluvio de datos. Utilizando elementos fundamentales de la teoría cuántica -la teoría más explorada de la física moderna, posiblemente- la información cuántica está revolucionando la forma en la que adquirimos, procesamos, almacenamos y transmitimos información. En plena era de la información, el sector industrial reconoce cada vez más el potencial de las tecnologías cuánticas, y a su vez nuevos desarrollos en el procesamiento de la información cuántica continúan impulsando descubrimientos prominentes relacionados con aspectos científcos de carácter más fundamental. Existen varios programas de investigación alrededor del mundo desarrollando y comercializando tecnologías cuánticas, principalmente para aplicaciones de criptografía y generación de números aleatorios. Así, las limitaciones que hoy nos separan de la era de la información cuántica están siendo gradualmente superadas. Sin embargo, existe un problema fundamental que aún necesita ser enfrentado: la imposibilidad de saber lo que realmente sucede en un experimento cuántico, debido a sus dimensiones de tamaño atómico. En efecto, ¿cómo podrá un usuario garantizar el funcionamiento adecuado de un dispositivo cuántico que ha sido adquirido a través de una compañía externa? A sus ojos el dispositivo será una verdadera caja negra. Incluso si el usuario contara con un Doctorado en ciencia cuántica, el problema prevalecería insoluble debido a la imposibilidad de controlar a la perfección, es decir monitorear, todos los procesos físicos que ocurren en cualquier experimento cuántico. Además, la situación se vuelve aún más dramática si se piensa en aplicaciones en donde un agente maligno pudiese hackear los dispositivos y manipular su funcionamiento interno, volviendo así el protocolo en cuestión inseguro y por ende también irrelevante. El propósito de esta Tesis es entonces contribuir al desarrollo experimental de protocolos de información cuántica con dispositivos sin caracterizar, llamados "device-independent". Estos protocolos son, por naturaleza, immunes a cualquier ataque o falla relacionada con desajustes entre la teoría y la implementación del protocolo. Esto se logra a lo largo de los diferentes Capítulos prosiguiendo las siguientes tres tareas que en ocasiones se traslapan: (i) Ampliar las capacidades teóricas estableciendo un entendimiento mayor de los recursos fundamentales de la teoría de la información cuántica con dispositivos sin caracterizar. (ii) Desarrollar protocolos de información cuántica competitivos, encontrando un intercambio adecuado entre alto rendimiento y practicabilidad; entre el poder del marco de trabajo device-independent y sus menos demandantes versiones, dichas "semi-device-independent". (iii) Analizar y mejorar las condiciones experimentales de diversas plataformas para llevar a cabo implementaciones en experimentos de prueba de principio, demostrando la realización de protocolos de información cuántica con cajas negras. Nuestro objetivo de convertir la teoría de la información cuántica en una tecnología tangible para nuestra sociedad a través del uso de dispositivos sin caracterizar contribuye no solamente al desarrollo tecnológico de estos protocolos, sino que también ofrece una visión valiosa de aspectos más fundamental. En este sentido, contribuimos a la caracterización y cuantificación del entrelazamiento -el recurso cuántico fundamental de muchos fenómenos sin contraparte clásica- en escenarios de interés práctico en dónde se consideran dispositivos sin caracterizar. Desde la perspectiva más aplicada, contribuimos al desarrollo de dos tareas específicas: la certificación de números genuinamente aleatorios en escenarios device-independent y semi-device-independent, y la generación de una llave secreta entre dos partes de manera device-independent

Optimal randomness generation from optical Bell experiments

Genuine randomness can be certified from Bell tests without any detailed assumptions on the working of the devices with which the test is implemented. An important class of experiments for implementing such tests is optical setups based on polarisation measurements of entangled photons distributed from a spontaneous parametric down conversion source. Here we compute the maximal amount of randomness which can be certified in such setups under realistic conditions. We provide relevant yet unexpected numerical values for the physical parameters and achieve four times more randomness than previous methods.Comment: 15 pages, 4 figure

Device-independent quantum key distribution with single-photon sources

Device-independent quantum key distribution protocols allow two honest users to establish a secret key with minimal levels of trust on the provider, as security is proven without any assumption on the inner working of the devices used for the distribution. Unfortunately, the implementation of these protocols is challenging, as it requires the observation of a large Bell-inequality violation between the two distant users. Here, we introduce novel photonic protocols for device-independent quantum key distribution exploiting single-photon sources and heralding-type architectures. The heralding process is designed so that transmission losses become irrelevant for security. We then show how the use of single-photon sources for entanglement distribution in these architectures, instead of standard entangled-pair generation schemes, provides significant improvements on the attainable key rates and distances over previous proposals. Given the current progress in single-photon sources, our work opens up a promising avenue for device-independent quantum key distribution implementations.Comment: 20 pages (9 + appendices and bibliography), 5 figures, 1 tabl

Frequency-Bin Entanglement of Ultra-Narrow Band Non-Degenerate Photon Pairs

We demonstrate frequency-bin entanglement between ultra-narrowband photons generated by cavity enhanced spontaneous parametric down conversion. Our source generates photon pairs in widely non-degenerate discrete frequency modes, with one photon resonant with a quantum memory material based on praseodymium doped crystals and the other photon at telecom wavelengths. Correlations between the frequency modes are analyzed using phase modulators and narrowband filters before detection. We show high-visibility two photon interference between the frequency modes, allowing us to infer a coherent superposition of the modes. We develop a model describing the state that we create and use it to estimate optimal measurements to achieve a violation of the Clauser-Horne (CH) Bell inequality under realistic assumptions. With these settings we perform a Bell test and show a significant violation of the CH inequality, thus proving the entanglement of the photons. Finally we demonstrate the compatibility with a quantum memory material by using a spectral hole in the praseodymium (Pr) doped crystal as spectral filter for measuring high-visibility two-photon interference. This demonstrates the feasibility of combining frequency-bin entangled photon pairs with Pr-based solid state quantum memories.Comment: 15 pages, 6 figure

Experimental multipartite entanglement and randomness certification of the W state in the quantum steering scenario

Recently [Cavalcanti \textit{et al.} Nat Commun \textbf{6}, 7941 (2015)] proposed a method to certify the presence of entanglement in asymmetric networks, where some users do not have control over the measurements they are performing. Such asymmetry naturally emerges in realistic situtations, such as in cryptographic protocols over quantum networks. Here we implement such "semi-device independent" techniques to experimentally witness all types of entanglement on a three-qubit photonic W state. Furthermore we analise the amount of genuine randomness that can be certified in this scenario from any bipartition of the three-qubit W state.Comment: v2: text improved, steering witness redefined so that their violation provides the steering robustness, experimental data included in an appendi

Towards an equivalence between maximal entanglement and maximal quantum nonlocality

While all bipartite pure entangled states are known to generate correlations violating a Bell inequality, and are therefore nonlocal, the quantitative relation between pure state entanglement and nonlocality is poorly understood. In fact, some Bell inequalities are maximally violated by non-maximally entangled states and this phenomenon is also observed for other operational measures of nonlocality. In this work, we study a recently proposed measure of nonlocality defined as the probability that a pure state displays nonlocal correlations when subjected to random measurements. We first prove that this measure satisfies some natural properties for an operational measure of nonlocality. Then, we show that for pure states of two qubits the measure is monotonic with entanglement for all correlation two-outcome Bell inequalities: for all these inequalities, the more the state is entangled, the larger the probability to violate them when random measurements are performed. Finally, we extend our results to the multipartite setting.QID/Wehner GroupQuTec