Optimal randomness certification in the quantum steering and prepare-and-measure scenarios

Quantum mechanics predicts the existence of intrinsically random processes. Contrary to classical randomness, this lack of predictability can not be attributed to ignorance or lack of control. Here we find the optimal method to quantify the amount of local or global randomness that can be extracted in two scenarios: (i) the quantum steering scenario, where two parties measure a bipartite system in an unknown state but one of them does not trust his measurement apparatus, and (ii) the prepare-and-measure scenario, where additionally the quantum state is known. We use our methods to compute the maximal amount of local and global randomness that can be certified by measuring systems subject to noise and losses and show that local randomness can be certified from a single measurement if and only if the detectors used in the test have detection efficiency higher than 50%.Comment: 11 pages, 6 figures. v2: Published versio

Necessary detection efficiencies for secure quantum key distribution and bound randomness

In recent years, several hacking attacks have broken the security of quantum cryptography implementations by exploiting the presence of losses and the ability of the eavesdropper to tune detection efficiencies. We present a simple attack of this form that applies to any protocol in which the key is constructed from the results of untrusted measurements performed on particles coming from an insecure source or channel. Because of its generality, the attack applies to a large class of protocols, from standard prepare-and-measure to device-independent schemes. Our attack gives bounds on the critical detection efficiencies necessary for secure quantum distribution, which show that the implementation of most partly device independent solutions is, from the point of view of detection efficiency, almost as demanding as fully device-independent ones. We also show how our attack implies the existence of a form of bound randomness, namely non-local correlations in which a non-signalling eavesdropper can find out a posteriori the result of any implemented measurement.Comment: 5 pages. v2: new title, published versio

Detection loophole attacks on semi-device-independent quantum and classical protocols

Semi-device-independent quantum protocols realize information tasks - e.g. secure key distribution, random access coding, and randomness generation - in a scenario where no assumption on the internal working of the devices used in the protocol is made, except their dimension. These protocols offer two main advantages: first, their implementation is often less demanding than fully-device-independent protocols. Second, they are more secure than their device-dependent counterparts. Their classical analogous is represented by random access codes, which provide a general framework for describing one-sided classical communication tasks. We discuss conditions under which detection inefficiencies can be exploited by a malicious provider to fake the performance of semi-device-independent quantum and classical protocols - and how to prevent it.Comment: 13 pages, 1 figure, published versio

Robustness of Device Independent Dimension Witnesses

Device independent dimension witnesses provide a lower bound on the dimensionality of classical and quantum systems in a "black box" scenario where only correlations between preparations, measurements and outcomes are considered. We address the problem of the robustness of dimension witnesses, namely that to witness the dimension of a system or to discriminate between its quantum or classical nature, even in the presence of loss. We consider the case when shared randomness is allowed between preparations and measurements and we provide a threshold in the detection efficiency such that dimension witnessing can still be performed.Comment: 8 pages, 5 figures, published versio

Materials Cloud, a platform for open computational science

Materials Cloud is a platform designed to enable open and seamless sharing of resources for computational science, driven by applications in materials modelling. It hosts 1) archival and dissemination services for raw and curated data, together with their provenance graph, 2) modelling services and virtual machines, 3) tools for data analytics, and pre-/post-processing, and 4) educational materials. Data is citable and archived persistently, providing a comprehensive embodiment of the FAIR principles that extends to computational workflows. Materials Cloud leverages the AiiDA framework to record the provenance of entire simulation pipelines (calculations performed, codes used, data generated) in the form of graphs that allow to retrace and reproduce any computed result. When an AiiDA database is shared on Materials Cloud, peers can browse the interconnected record of simulations, download individual files or the full database, and start their research from the results of the original authors. The infrastructure is agnostic to the specific simulation codes used and can support diverse applications in computational science that transcend its initial materials domain.Comment: 19 pages, 8 figure

Impact of imperfections on correlation-based quantum information protocols

Quantum information science is a rapidly evolving field both from the theoretical and the experimental viewpoint, motivated by the fact that protocols exploiting quantum resources can perform tasks that are unfeasible in classical information theory. Interestingly, the trustworthiness of quantum information protocols can be certified relying upon as few assumptions as possible adopting the "device-independent" (DI) framework. In this scenario no assumption is made on the internal working of the involved devices, which are treated as black boxes. The quantum certification of DI protocols is guaranteed by the nonlocal character of the correlations between the inputs and outputs of those boxes. Unfortunately, demonstrating nonlocality is highly demanding from the implementation point of view, since low levels of experimental imperfections are tolerated. Those imperfections (e.g.noise and losses) may alter the input/output statistics, thus undermining the reliability of DI protocols. The experimental requirements for the security of DI protocols can be relaxed considering partly-DI scenarios, in which additional assumptions on the devices or the systems used in the protocols are made. Indeed, partly-DI protocols offer two main advantages: First, they are more secure than standard device-dependent protocols; second, they are more robust to experimental imperfections than their fully-DI counterparts. The general aim of this Thesis is to provide bounds on imperfections and losses arising in experimental implementations of DI and partly-DI protocols that are necessary or sufficient for security. In the first part, we tackle the problem of secure implementation of quantum key distribution protocols in the DI and partly-DI scenarios. The goal is to establish conditions on the detection efficiency necessary for the security of those protocols. To this aim, we present a general attack on the detectors from which we derive bounds on the critical detection efficiency that do not depend on the number of measurements applied nor on the number of outcomes. In the second part, we study randomness certification in the steering and the prepare-and-measure scenarios. We devise an optimal method for quantifying the local and global randomness that can be extracted in both scenarios. Applying this method we provide sufficient conditions for randomness certification in the presence of noise and losses. Moreover, we present a method that for any fixed state gives the optimal measurements and steering inequality that certify the most randomness. The next question we address is the secure implementation of semi-device-independent (SDI) protocols, whose quantum certification is provided by dimension witnesses. We study the problem of the robustness of DI dimension witnesses to loss, in the case in which shared randomness is allowed between the preparing and measuring devices. The main result in this part is to provide thresholds for the critical detection efficiency necessary to perform reliable dimension witnessing. Furthermore, we study detection loophole attacks on SDI quantum and classical protocols in the case in which the preparing and measuring devices do not share correlations. We determine general conditions under which a potential eavesdropper cannot exploit the experimental losses to hack such protocols. Finally, we focus on a recently demonstrated quantum process and its inverse, namely the quantum state joining and splitting processes. We prove that a linear-optical realization of the quantum state joining of two photons relying only on postselection - and thus simpler than the implementation originally proposed - is not possible, implying that it requires at least one ancilla photon. Furthermore, we demonstrate that the quantum joining process is equivalent to the preparation of a particular class of three-qubit entangled states, showing that this process can also find application for generating complex cluster states of entangled photons.En las últimas décadas, el campo de estudio de la información cuántica está tomando especial relevancia tanto desde el punto de vista teórico como experimental, debido a que los protocolos basados en la física cuántica pueden desempeñar acciones que son prohibidas en los protocolos basados en la física clásica. Especialmente, se ha demostrado que se puede garantizar la fiabilidad de protocolos cuánticos basandose en las mínimas suposiciones posibles, adoptando el escenario denominado 'device-independent' (DI). En este caso, no se hace ninguna suposición sobre el funcionamento de los sistemas e instrumentación usada, siendo tratados como cajas negras. La certificación cuántica de los protocolos DI está basada en la nonlocalidad de las correlaciones entre inputs y outputs de estas cajas. Desafortunadamente, demostrar experimentalmente esta nonlocalidad es un reto actual muy exigente debido a que se requiere un nivel muy bajo de imperfecciones (por ej. ruido y pérdidas). Este requisito se puede relajar considerando escenarios parcialmente DI, en los que se hacen suposiciones adicionales sobre los dispositivos usados. Por un lado, estos protocolos son generalmente menos exigentes desde el punto de vista de la implementación; por otro lado, son más seguros que los 'device-dependent'. El objetivo general de esta Tesis es establecer bajo qué condiciones las imperfecciones experimentales no comprometen la seguridad de protocolos DI y parcialmente DI. Para desarrollar este objetivo, esta Tesis se divide en diferentes apartados. En la primera parte, se consideran protocolos de quantum key distribution en escenarios DI y parcialmente DI, presentando un ataque general a los detectores. De este estudio se derivan los límites para la eficiencia de detección crítica necesaria para una implementación segura de estos protocolos, obteniéndose que no dependen ni del número de medidas aplicadas ni del número de outcomes. En la segunda parte, se estudia la certificación de aleatoriedad en los escenarios de steering y prepare-and-measure. Se introduce un método óptimo para cuantificar la aleatoriedad local y global que se pueden extraer en ambos escenarios y se derivan las condiciones suficientes para certificar la aleatoriedad en presencia de ruido y pérdidas. Además, se presenta un método que obtiene para cada estado las medidas y la desigualdad de steering óptimas que certifican la máxima aleatoriedad. En la tercera parte, se considera la implementación de protocolos semi-device-independent (SDI), cuya certificación cuántica es provista por las dimension witnesses. Se estudia el problema de la robustez a las pérdidas de las dimension witnesses en el escenario DI (DIDWs) cuando el instrumento de preparación y el de medida comparten correlaciones preestablecidas. En ese contexto se determinan los umbrales para la eficiencia de detección crítica necesaria para la fiabilidad de DIDWs. Además, se estudian ataques a detectores en protocolos SDI cuánticos y clásicos, en el caso en que los aparatos de preparación y de medida no estén correlacionados, y se analizan las condiciones para que un espía potencial no pueda hackear estos protocolos usando las pérdidas experimentales. Por último, se estudian los procesos cuánticos demostrados recientemente de quantum state joining/splitting. Se prueba que una realización con óptica lineal del quantum state joining de dos fotones usando solo postselección (por tanto más simple de la demonstrada originalmente) no es posible, sino que este tipo de implementación requiere al menos un fotón auxiliar. Además, se demuestra que el quantum state joining es equivalente a preparar una clase particular de estados entrelazados de 3 qubits, mostrándose una posible aplicación del quantum joining de estados fotónicos para generar estados cluster complejos de fotones entrelazadosPostprint (published version

Impact of imperfections on correlation-based quantum information protocols

Quantum information science is a rapidly evolving field both from the theoretical and the experimental viewpoint, motivated by the fact that protocols exploiting quantum resources can perform tasks that are unfeasible in classical information theory. Interestingly, the trustworthiness of quantum information protocols can be certified relying upon as few assumptions as possible adopting the "device-independent" (DI) framework. In this scenario no assumption is made on the internal working of the involved devices, which are treated as black boxes. The quantum certification of DI protocols is guaranteed by the nonlocal character of the correlations between the inputs and outputs of those boxes. Unfortunately, demonstrating nonlocality is highly demanding from the implementation point of view, since low levels of experimental imperfections are tolerated. Those imperfections (e.g.noise and losses) may alter the input/output statistics, thus undermining the reliability of DI protocols. The experimental requirements for the security of DI protocols can be relaxed considering partly-DI scenarios, in which additional assumptions on the devices or the systems used in the protocols are made. Indeed, partly-DI protocols offer two main advantages: First, they are more secure than standard device-dependent protocols; second, they are more robust to experimental imperfections than their fully-DI counterparts. The general aim of this Thesis is to provide bounds on imperfections and losses arising in experimental implementations of DI and partly-DI protocols that are necessary or sufficient for security. In the first part, we tackle the problem of secure implementation of quantum key distribution protocols in the DI and partly-DI scenarios. The goal is to establish conditions on the detection efficiency necessary for the security of those protocols. To this aim, we present a general attack on the detectors from which we derive bounds on the critical detection efficiency that do not depend on the number of measurements applied nor on the number of outcomes. In the second part, we study randomness certification in the steering and the prepare-and-measure scenarios. We devise an optimal method for quantifying the local and global randomness that can be extracted in both scenarios. Applying this method we provide sufficient conditions for randomness certification in the presence of noise and losses. Moreover, we present a method that for any fixed state gives the optimal measurements and steering inequality that certify the most randomness. The next question we address is the secure implementation of semi-device-independent (SDI) protocols, whose quantum certification is provided by dimension witnesses. We study the problem of the robustness of DI dimension witnesses to loss, in the case in which shared randomness is allowed between the preparing and measuring devices. The main result in this part is to provide thresholds for the critical detection efficiency necessary to perform reliable dimension witnessing. Furthermore, we study detection loophole attacks on SDI quantum and classical protocols in the case in which the preparing and measuring devices do not share correlations. We determine general conditions under which a potential eavesdropper cannot exploit the experimental losses to hack such protocols. Finally, we focus on a recently demonstrated quantum process and its inverse, namely the quantum state joining and splitting processes. We prove that a linear-optical realization of the quantum state joining of two photons relying only on postselection - and thus simpler than the implementation originally proposed - is not possible, implying that it requires at least one ancilla photon. Furthermore, we demonstrate that the quantum joining process is equivalent to the preparation of a particular class of three-qubit entangled states, showing that this process can also find application for generating complex cluster states of entangled photons.En las últimas décadas, el campo de estudio de la información cuántica está tomando especial relevancia tanto desde el punto de vista teórico como experimental, debido a que los protocolos basados en la física cuántica pueden desempeñar acciones que son prohibidas en los protocolos basados en la física clásica. Especialmente, se ha demostrado que se puede garantizar la fiabilidad de protocolos cuánticos basandose en las mínimas suposiciones posibles, adoptando el escenario denominado 'device-independent' (DI). En este caso, no se hace ninguna suposición sobre el funcionamento de los sistemas e instrumentación usada, siendo tratados como cajas negras. La certificación cuántica de los protocolos DI está basada en la nonlocalidad de las correlaciones entre inputs y outputs de estas cajas. Desafortunadamente, demostrar experimentalmente esta nonlocalidad es un reto actual muy exigente debido a que se requiere un nivel muy bajo de imperfecciones (por ej. ruido y pérdidas). Este requisito se puede relajar considerando escenarios parcialmente DI, en los que se hacen suposiciones adicionales sobre los dispositivos usados. Por un lado, estos protocolos son generalmente menos exigentes desde el punto de vista de la implementación; por otro lado, son más seguros que los 'device-dependent'. El objetivo general de esta Tesis es establecer bajo qué condiciones las imperfecciones experimentales no comprometen la seguridad de protocolos DI y parcialmente DI. Para desarrollar este objetivo, esta Tesis se divide en diferentes apartados. En la primera parte, se consideran protocolos de quantum key distribution en escenarios DI y parcialmente DI, presentando un ataque general a los detectores. De este estudio se derivan los límites para la eficiencia de detección crítica necesaria para una implementación segura de estos protocolos, obteniéndose que no dependen ni del número de medidas aplicadas ni del número de outcomes. En la segunda parte, se estudia la certificación de aleatoriedad en los escenarios de steering y prepare-and-measure. Se introduce un método óptimo para cuantificar la aleatoriedad local y global que se pueden extraer en ambos escenarios y se derivan las condiciones suficientes para certificar la aleatoriedad en presencia de ruido y pérdidas. Además, se presenta un método que obtiene para cada estado las medidas y la desigualdad de steering óptimas que certifican la máxima aleatoriedad. En la tercera parte, se considera la implementación de protocolos semi-device-independent (SDI), cuya certificación cuántica es provista por las dimension witnesses. Se estudia el problema de la robustez a las pérdidas de las dimension witnesses en el escenario DI (DIDWs) cuando el instrumento de preparación y el de medida comparten correlaciones preestablecidas. En ese contexto se determinan los umbrales para la eficiencia de detección crítica necesaria para la fiabilidad de DIDWs. Además, se estudian ataques a detectores en protocolos SDI cuánticos y clásicos, en el caso en que los aparatos de preparación y de medida no estén correlacionados, y se analizan las condiciones para que un espía potencial no pueda hackear estos protocolos usando las pérdidas experimentales. Por último, se estudian los procesos cuánticos demostrados recientemente de quantum state joining/splitting. Se prueba que una realización con óptica lineal del quantum state joining de dos fotones usando solo postselección (por tanto más simple de la demonstrada originalmente) no es posible, sino que este tipo de implementación requiere al menos un fotón auxiliar. Además, se demuestra que el quantum state joining es equivalente a preparar una clase particular de estados entrelazados de 3 qubits, mostrándose una posible aplicación del quantum joining de estados fotónicos para generar estados cluster complejos de fotones entrelazado

Impact of imperfections on correlation-based quantum information protocols

Quantum information science is a rapidly evolving field both from the theoretical and the experimental viewpoint, motivated by the fact that protocols exploiting quantum resources can perform tasks that are unfeasible in classical information theory. Interestingly, the trustworthiness of quantum information protocols can be certified relying upon as few assumptions as possible adopting the "device-independent" (DI) framework. In this scenario no assumption is made on the internal working of the involved devices, which are treated as black boxes. The quantum certification of DI protocols is guaranteed by the nonlocal character of the correlations between the inputs and outputs of those boxes. Unfortunately, demonstrating nonlocality is highly demanding from the implementation point of view, since low levels of experimental imperfections are tolerated. Those imperfections (e.g.noise and losses) may alter the input/output statistics, thus undermining the reliability of DI protocols. The experimental requirements for the security of DI protocols can be relaxed considering partly-DI scenarios, in which additional assumptions on the devices or the systems used in the protocols are made. Indeed, partly-DI protocols offer two main advantages: First, they are more secure than standard device-dependent protocols; second, they are more robust to experimental imperfections than their fully-DI counterparts. The general aim of this Thesis is to provide bounds on imperfections and losses arising in experimental implementations of DI and partly-DI protocols that are necessary or sufficient for security. In the first part, we tackle the problem of secure implementation of quantum key distribution protocols in the DI and partly-DI scenarios. The goal is to establish conditions on the detection efficiency necessary for the security of those protocols. To this aim, we present a general attack on the detectors from which we derive bounds on the critical detection efficiency that do not depend on the number of measurements applied nor on the number of outcomes. In the second part, we study randomness certification in the steering and the prepare-and-measure scenarios. We devise an optimal method for quantifying the local and global randomness that can be extracted in both scenarios. Applying this method we provide sufficient conditions for randomness certification in the presence of noise and losses. Moreover, we present a method that for any fixed state gives the optimal measurements and steering inequality that certify the most randomness. The next question we address is the secure implementation of semi-device-independent (SDI) protocols, whose quantum certification is provided by dimension witnesses. We study the problem of the robustness of DI dimension witnesses to loss, in the case in which shared randomness is allowed between the preparing and measuring devices. The main result in this part is to provide thresholds for the critical detection efficiency necessary to perform reliable dimension witnessing. Furthermore, we study detection loophole attacks on SDI quantum and classical protocols in the case in which the preparing and measuring devices do not share correlations. We determine general conditions under which a potential eavesdropper cannot exploit the experimental losses to hack such protocols. Finally, we focus on a recently demonstrated quantum process and its inverse, namely the quantum state joining and splitting processes. We prove that a linear-optical realization of the quantum state joining of two photons relying only on postselection - and thus simpler than the implementation originally proposed - is not possible, implying that it requires at least one ancilla photon. Furthermore, we demonstrate that the quantum joining process is equivalent to the preparation of a particular class of three-qubit entangled states, showing that this process can also find application for generating complex cluster states of entangled photons.En las últimas décadas, el campo de estudio de la información cuántica está tomando especial relevancia tanto desde el punto de vista teórico como experimental, debido a que los protocolos basados en la física cuántica pueden desempeñar acciones que son prohibidas en los protocolos basados en la física clásica. Especialmente, se ha demostrado que se puede garantizar la fiabilidad de protocolos cuánticos basandose en las mínimas suposiciones posibles, adoptando el escenario denominado 'device-independent' (DI). En este caso, no se hace ninguna suposición sobre el funcionamento de los sistemas e instrumentación usada, siendo tratados como cajas negras. La certificación cuántica de los protocolos DI está basada en la nonlocalidad de las correlaciones entre inputs y outputs de estas cajas. Desafortunadamente, demostrar experimentalmente esta nonlocalidad es un reto actual muy exigente debido a que se requiere un nivel muy bajo de imperfecciones (por ej. ruido y pérdidas). Este requisito se puede relajar considerando escenarios parcialmente DI, en los que se hacen suposiciones adicionales sobre los dispositivos usados. Por un lado, estos protocolos son generalmente menos exigentes desde el punto de vista de la implementación; por otro lado, son más seguros que los 'device-dependent'. El objetivo general de esta Tesis es establecer bajo qué condiciones las imperfecciones experimentales no comprometen la seguridad de protocolos DI y parcialmente DI. Para desarrollar este objetivo, esta Tesis se divide en diferentes apartados. En la primera parte, se consideran protocolos de quantum key distribution en escenarios DI y parcialmente DI, presentando un ataque general a los detectores. De este estudio se derivan los límites para la eficiencia de detección crítica necesaria para una implementación segura de estos protocolos, obteniéndose que no dependen ni del número de medidas aplicadas ni del número de outcomes. En la segunda parte, se estudia la certificación de aleatoriedad en los escenarios de steering y prepare-and-measure. Se introduce un método óptimo para cuantificar la aleatoriedad local y global que se pueden extraer en ambos escenarios y se derivan las condiciones suficientes para certificar la aleatoriedad en presencia de ruido y pérdidas. Además, se presenta un método que obtiene para cada estado las medidas y la desigualdad de steering óptimas que certifican la máxima aleatoriedad. En la tercera parte, se considera la implementación de protocolos semi-device-independent (SDI), cuya certificación cuántica es provista por las dimension witnesses. Se estudia el problema de la robustez a las pérdidas de las dimension witnesses en el escenario DI (DIDWs) cuando el instrumento de preparación y el de medida comparten correlaciones preestablecidas. En ese contexto se determinan los umbrales para la eficiencia de detección crítica necesaria para la fiabilidad de DIDWs. Además, se estudian ataques a detectores en protocolos SDI cuánticos y clásicos, en el caso en que los aparatos de preparación y de medida no estén correlacionados, y se analizan las condiciones para que un espía potencial no pueda hackear estos protocolos usando las pérdidas experimentales. Por último, se estudian los procesos cuánticos demostrados recientemente de quantum state joining/splitting. Se prueba que una realización con óptica lineal del quantum state joining de dos fotones usando solo postselección (por tanto más simple de la demonstrada originalmente) no es posible, sino que este tipo de implementación requiere al menos un fotón auxiliar. Además, se demuestra que el quantum state joining es equivalente a preparar una clase particular de estados entrelazados de 3 qubits, mostrándose una posible aplicación del quantum joining de estados fotónicos para generar estados cluster complejos de fotones entrelazado

Detection loophole attacks on semi-device-independent quantum and classical protocols

Semi-device-independent quantum protocols realize information tasks – e.g. secure key distribution, random access coding, and randomness generation – in a scenario where no assumption on the internal working of the devices used in the protocol is made, except their dimension. These protocols offer two main advantages: first, their implementation is often less demanding than fully-device-independent protocols. Second, they are more secure than their device-dependent counterparts. Their classical analogous is represented by random access codes, which provide a general framework for describing one-sided classical communication tasks. We discuss conditions under which detection inefficiencies can be exploited by a malicious provider to fake the performance of semi-device-independent quantum and classical protocols – and how to prevent it

Joining and splitting the quantum states of photons

A photonic process called quantum-state joining has been recently experimentally demonstrated [Vitelli et al., Nat. Photonics 7, 521 (2013)] that corresponds to the transfer of the internal two-dimensional quantum states of two input photons, i.e., two photonic qubits, into the four-dimensional quantum state of a single photon, i.e., a photonic ququart. A scheme for the inverse process, namely, quantum state splitting, has also been proposed. Both processes can be iterated in a cascaded layout, to obtain the joining and/or splitting of more than two qubits, thus leading to a general scheme for varying the number of photons in the system while preserving its total quantum state, or quantum information content. Here, we revisit these processes from a theoretical point of view. After casting the theory of the joining and splitting processes in the more general occupation number notation, we introduce some modified schemes that are in principle unitary [not considering the implementation of the controlled-NOT (CNOT) gates] and do not require projection and feed-forward steps. This can be particularly important in the quantum state splitting case, to obtain a scheme that does not rely on postselection. Moreover, we formally prove that the quantum joining of two photon states with linear optics requires the use of at least one ancilla photon. This is somewhat unexpected, given that the demonstrated joining scheme involves the sequential application of two CNOT quantum gates, for which a linear optical scheme with just two photons and postselection is known to exist. Finally we explore the relationship between the joining scheme and the generation of clusters of multiparticle entangled states involving more than one qubit per particle