Archive ouverte UNIGE | last documents for author 'Nicolas Gisin'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Nicolas Gisin'engConstraints on nonlocality in networks from no-signaling and independencehttps://archive-ouverte.unige.ch/unige:138224https://archive-ouverte.unige.ch/unige:138224The possibility of Bell inequality violations in quantum theory had a profound impact on our understanding of the correlations that can be shared by distant parties. Generalizing the concept of Bell nonlocality to networks leads to novel forms of correlations, the characterization of which is, however, challenging. Here, we investigate constraints on correlations in networks under the natural assumptions of no-signaling and independence of the sources. We consider the triangle network with binary outputs, and derive strong constraints on correlations even though the parties receive no input, i.e., each party performs a fixed measurement. We show that some of these constraints are tight, by constructing explicit local models (i.e. where sources distribute classical variables) that can saturate them. However, we also observe that other constraints can apparently not be saturated by local models, which opens the possibility of having nonlocal (but non-signaling) correlations in the triangle network with binary outputs.Thu, 02 Jul 2020 14:55:15 +0200Optical storage for 0.53 s in a solid-state atomic frequency comb memory using dynamical decouplinghttps://archive-ouverte.unige.ch/unige:137309https://archive-ouverte.unige.ch/unige:137309Quantum memories with long storage times are key elements in long-distance quantum networks. The atomic frequency comb (AFC) memory in particular has shown great promise to fulfill this role, having demonstrated multimode capacity and spin–photon quantum correlations. However, the memory storage times have so-far been limited to about 1 ms, realized in a Eu3+ doped Y2SiO5 crystal at zero applied magnetic field. Motivated by studies showing increased spin coherence times under applied magnetic field, we developed an AFC spin-wave memory utilizing a weak 15 mT magnetic field in a specific direction that allows efficient optical and spin manipulation for AFC memory operations. With this field configuration the AFC spin-wave storage time increased to 40 ms using a simple spin-echo sequence. Furthermore, by applying dynamical decoupling techniques the spin-wave coherence time reaches 530 ms, a 300-fold increase with respect to previous AFC spin-wave storage experiments. This result paves the way towards long duration storage of quantum information in solid-state ensemble memories.Wed, 17 Jun 2020 13:13:51 +0200Mathematical languages shape our understanding of time in physicshttps://archive-ouverte.unige.ch/unige:131924https://archive-ouverte.unige.ch/unige:131924Physics is formulated in terms of timeless, axiomatic mathematics. A formulation on the basis of intuitionist mathematics, built on time-evolving processes, would offer a perspective that is closer to our experience of physical reality.Mon, 09 Mar 2020 14:37:45 +0100Physics without determinism: Alternative interpretations of classical physicshttps://archive-ouverte.unige.ch/unige:131922https://archive-ouverte.unige.ch/unige:131922Classical physics is generally regarded as deterministic, as opposed to quantum mechanics that is considered the first theory to have introduced genuine indeterminism into physics. We challenge this view by arguing that the alleged determinism of classical physics relies on the tacit, metaphysical assumption that there exists an actual value of every physical quantity, with its infinite predetermined digits (which we name principle of infinite precision). Building on recent information-theoretic arguments showing that the principle of infinite precision (which translates into the attribution of a physical meaning to mathematical real numbers) leads to unphysical consequences, we consider possible alternative indeterministic interpretations of classical physics. We also link those to well-known interpretations of quantum mechanics. In particular, we propose a model of classical indeterminism based on finite information quantities (FIQs).Moreover, we discuss the perspectives that an indeterministic physics could open (such as strong emergence), as well as some potential problematic issues. Finally, we make evident that any indeterministic interpretation of physics would have to deal with the problem of explaining how the indeterminate values become determinate, a problem known in the context of quantum mechanics as (part of) the “quantum measurement problem.” We discuss some similarities between the classical and the quantum measurement problems, and propose ideas for possible solutions (e.g., “collapse models” and “top-down causation”).Mon, 09 Mar 2020 14:35:34 +0100Indeterminism in Physics, Classical Chaos and Bohmian Mechanics: Are Real Numbers Really Real?https://archive-ouverte.unige.ch/unige:131921https://archive-ouverte.unige.ch/unige:131921It is usual to identify initial conditions of classical dynamical systems with mathematical real numbers. However, almost all real numbers contain an infinite amount of information. I argue that a finite volume of space can’t contain more than a finite amount of information, hence that the mathematical real numbers are not physically relevant. Moreover, a better terminology for the so-called real numbers is “random numbers”, as their series of bits are truly random. I propose an alternative classical mechanics, which is empirically equivalent to classical mechanics, but uses only finite-information numbers. This alternative classical mechanics is non-deterministic, despite the use of deterministic equations, in a way similar to quantum theory. Interestingly, both alternative classical mechanics and quantum theories can be supplemented by additional variables in such a way that the supplemented theory is deterministic. Most physicists straightforwardly supplement classical theory with real numbers to which they attribute physical existence, while most physicists reject Bohmian mechanics as supplemented quantum theory, arguing that Bohmian positions have no physical reality.Mon, 09 Mar 2020 14:34:06 +0100Entanglement 25 Years after Quantum Teleportation: Testing Joint Measurements in Quantum Networkshttps://archive-ouverte.unige.ch/unige:131920https://archive-ouverte.unige.ch/unige:131920Twenty-five years after the invention of quantum teleportation, the concept of entanglement gained enormous popularity. This is especially nice to those who remember that entanglement was not even taught at universities until the 1990s. Today, entanglement is often presented as a resource, the resource of quantum information science and technology. However, entanglement is exploited twice in quantum teleportation. Firstly, entanglement is the “quantum teleportation channel”, i.e., entanglement between distant systems. Second, entanglement appears in the eigenvectors of the joint measurement that Alice, the sender, has to perform jointly on the quantum state to be teleported and her half of the “quantum teleportation channel”, i.e., entanglement enabling entirely new kinds of quantum measurements. I emphasize how poorly this second kind of entanglement is understood. In particular, I use quantum networks in which each party connected to several nodes performs a joint measurement to illustrate that the quantumness of such joint measurements remains elusive, escaping today’s available tools to detect and quantify it.Mon, 09 Mar 2020 14:32:51 +0100Bell Inequalities with One Bit of Communicationhttps://archive-ouverte.unige.ch/unige:131919https://archive-ouverte.unige.ch/unige:131919We study Bell scenarios with binary outcomes supplemented by one bit of classical communication. We developed a method to find facet inequalities for such scenarios even when direct facet enumeration is not possible, or at least difficult. Using this method, we partially solved the scenario where Alice and Bob choose between three inputs, finding a total of 668 inequivalent facet inequalities (with respect to relabelings of inputs and outputs). We also show that some of these inequalities are constructed from facet inequalities found in scenarios without communication, that is, the well-known Bell inequalities.Mon, 09 Mar 2020 14:31:39 +0100Complete list of tight Bell inequalities for two parties with four binary settingshttps://archive-ouverte.unige.ch/unige:131915https://archive-ouverte.unige.ch/unige:131915We give the complete list of 175 facets of the local polytope for the case where Alice and Bob each choose their measurements from a set of four binary outcome measurements. For each inequality we compute the maximum quantum violation for qubits, the resistance to noise, and the minimal detection efficiency required for closing the detection loophole with maximally entangled qubit states, in the case where both detectors have the same efficiency (symmetric case).Mon, 09 Mar 2020 14:06:05 +0100From quantum foundations to applications and backhttps://archive-ouverte.unige.ch/unige:131912https://archive-ouverte.unige.ch/unige:131912Quantum non-locality has been an extremely fruitful subject of research, leading the scientific revolution towards quantum information science, in particular, to device-independent quantum information processing. We argue that the time is ripe to work on another basic problem in the foundations of quantum physics, the quantum measurement problem, which should produce good physics in theoretical, mathematical, experimental and applied physics. We briefly review how quantum non-locality contributed to physics (including some outstanding open problems) and suggest ways in which questions around macroscopic quantumness could equally contribute to all aspects of physics. This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’.Mon, 09 Mar 2020 13:56:17 +0100Quantum Measurements, Energy Conservation and Quantum Clockshttps://archive-ouverte.unige.ch/unige:131906https://archive-ouverte.unige.ch/unige:131906A spin chain extending from Alice to Bob with nearest neighbors interactions, initially in its ground state, is considered. Assuming that Bob measures the last spin of the chain, the energy of the spin chain has to increase, at least on average, due to the measurement disturbance. Presumably, the energy is provided by Bob’s measurement apparatus. Assuming that, simultaneously to Bob’s measurement, Alice measures the first spin, it is shown that either energy is not conserved, – implausible – or the projection postulate doesn’t apply, and that there is signalling. An explicit measurement model shows that energy is conserved (as expected), but that the spin chain energy increase is not provided by the measurement apparatus(es), that the projection postulate is not always valid – illustrating the Wigner–Araki–Yanase (WAY) theorem – and that there is signalling, indeed. The signalling is due to the non-local interaction Hamiltonian. This raises the question of whether a suitable quantum-information-inspired model of such non-local Hamiltonians can be developed.Mon, 09 Mar 2020 13:48:42 +0100Macroscopic quantum states: Measures, fragility, and implementationshttps://archive-ouverte.unige.ch/unige:131905https://archive-ouverte.unige.ch/unige:131905Large-scale quantum effects have always played an important role in the foundations of quantum theory. With recent experimental progress and the aspiration for quantum enhanced applications, the interest in macroscopic quantum effects has been reinforced. In this review, measures aiming to quantify various aspects of macroscopic quantumness are critically analyzed and discussed. Recent results on the difficulties and prospects to create, maintain, and detect macroscopic quantum states are surveyed. The role of macroscopic quantum states in foundational questions as well as practical applications is outlined. Finally, past and ongoing experimental advances aiming to generate and observe macroscopic quantum states are presented.Mon, 09 Mar 2020 13:46:35 +0100Why Bohmian Mechanics? One- and Two-Time Position Measurements, Bell Inequalities, Philosophy, and Physicshttps://archive-ouverte.unige.ch/unige:131904https://archive-ouverte.unige.ch/unige:131904In Bohmian mechanics, particles follow continuous trajectories, so two-time position correlations have been well defined. However, Bohmian mechanics predicts the violation of Bell inequalities. Motivated by this fact, we investigate position measurements in Bohmian mechanics by coupling the particles to macroscopic pointers. This explains the violation of Bell inequalities despite two-time position correlations. We relate this fact to so-called surrealistic trajectories that, in our model, correspond to slowly moving pointers. Next, we emphasize that Bohmian mechanics, which does not distinguish between microscopic and macroscopic systems, implies that the quantum weirdness of quantum physics also shows up at the macro-scale. Finally, we discuss the fact that Bohmian mechanics is attractive to philosophers but not so much to physicists and argue that the Bohmian community is responsible for the latter.Mon, 09 Mar 2020 13:45:07 +0100Quantum-teleportation experiments turn 20https://archive-ouverte.unige.ch/unige:131903https://archive-ouverte.unige.ch/unige:131903In 1997, it was demonstrated that quantum states can be teleported from one location to a distant one. The discovery had huge consequences for the development of quantum communication and computing.Mon, 09 Mar 2020 13:43:24 +0100Real numbers are the hidden variables of classical mechanicshttps://archive-ouverte.unige.ch/unige:131902https://archive-ouverte.unige.ch/unige:131902Do scientific theories limit human knowledge? In other words, are there physical variables hidden by essence forever? We argue for negative answers and illustrate our point on chaotic classical dynamical systems. We emphasize parallels with quantum theory and conclude that the common real numbers are, de facto, the hidden variables of classical physics. Consequently, real numbers should not be considered as “physically real” and classical mechanics, like quantum physics, is indeterministic.Mon, 09 Mar 2020 13:41:00 +0100Universal bound on the cardinality of local hidden variables in networkshttps://archive-ouverte.unige.ch/unige:131901https://archive-ouverte.unige.ch/unige:131901We present an algebraic description of the sets of local correlations in arbitrary networks, when the parties have finite inputs and outputs. We consider networks generalizing the usual Bell scenarios by the presence of multiple uncorrelated sources. We prove a finite upper bound on the cardinality of the value sets of the local hidden variables. Conse- quently, we fond that the sets of local correlations are connected, closed and semialgebraic, and bounded by tight polynomial Bell-like inequalities.Mon, 09 Mar 2020 13:37:24 +0100Quantum nonlocality network structures, applications and symmetries of scenarioshttps://archive-ouverte.unige.ch/unige:131079https://archive-ouverte.unige.ch/unige:131079Cette thèse a pour objet les corrélations quantiques et leurs conséquences. Un premier chapitre introductif présente l'une des premières preuves du caractère nonlocale des corrélations quantiques, le théorème de Bell. Ensuite nous introduisons les scénarios en réseaux, généralisation des premiers scénarios dans lesquels la nonlocalité a été révélé, comportant plusieurs sources d'aléatoire indépendentes. Nous présentons l'inégalité de bilocalité, exemple fondateur, puis passons au scénario en triangle. Ici, nous donnons le premier authentique exemple de corrélations quantiques non triangle-local, et montrons que l'inégalité de Finner est satisfaite dans le cas quantique et dans la théorie des boites ne permettant pas de transfert instantané d'informations. Nous expliquons également de quelle manière les corrélations quantiques peuvent être utilisées pour certifier des états et mesures intriqués entre plusieurs agents, dans un contexte de confiance limitée dans le fonctionnement des appareils physiques, et comment celles-ci peuvent être utilisées pour certifier la mesure dans la base de Bell, dans un contexte de confiance nulle dans les appareils physiques utilisés. Ensuite, nous étudions les symétries de divers scénarios, montrant comment le scénario de Bell peut être décomposé grâce à ses symétries et comment celles-ci peuvent également être exploitées pour réduire drastiquement la complexité de certains programmes d'optimisation semi-définis en information quantique. Enfin, nous abordons le problème de trouver des mesures quantiques macroscopiques, i.e. une description quantique de nos mesures macroscopiques intuitives.Thu, 20 Feb 2020 11:39:52 +0100Unbounded sequence of observers exhibiting Einstein-Podolsky-Rosen steeringhttps://archive-ouverte.unige.ch/unige:127284https://archive-ouverte.unige.ch/unige:127284A sequential steering scenario is investigated, where multiple Bobs aim at demonstrating steering using successively the same half of an entangled quantum state. With isotropic entangled states of local dimension d, the number of Bobs that can steer Alice is found to be NBob∼d/logd, thus leading to an arbitrary large number of successive instances of steering with independently chosen and unbiased inputs. This scaling is achieved when considering a general class of measurements along orthonormal bases, as well as complete sets of mutually unbiased bases. Finally, we show that similar results can be obtained in an anonymous sequential scenario, where none of the Bobs know their position in the sequence.Tue, 03 Dec 2019 14:42:05 +0100Does large quantum Fisher information imply Bell correlations?https://archive-ouverte.unige.ch/unige:127280https://archive-ouverte.unige.ch/unige:127280The quantum Fisher information (QFI) of certain multipartite entangled quantum states is larger than what is reachable by separable states, providing a metrological advantage. Are these nonclassical correlations strong enough to potentially violate a Bell inequality? Here, we present evidence from two examples. First, we discuss a Bell inequality designed for spin-squeezed states which is violated only by quantum states with a large QFI. Second, we relax a well-known lower bound on the QFI to find the Mermin Bell inequality as a special case. However, a fully general link between QFI and Bell correlations is still open.Tue, 03 Dec 2019 14:32:55 +0100Limits on correlations in networks for quantum and no-signaling resourceshttps://archive-ouverte.unige.ch/unige:124651https://archive-ouverte.unige.ch/unige:124651A quantum network consists of independent sources distributing entangled states to distant nodes which can then perform entangled measurements, thus establishing correlations across the entire network. But how strong can these correlations be? Here we address this question, by deriving bounds on possible quantum correlations in a given network. These bounds are nonlinear inequalities that depend only on the topology of the network. We discuss in detail the notably challenging case of the triangle network. Moreover, we conjecture that our bounds hold in general no-signaling theories. In particular, we prove that our inequalities for the triangle network hold when the sources are arbitrary no-signaling boxes which can be wired together. Finally, we discuss an application of our results for the device-independent characterization of the topology of a quantum network.Mon, 21 Oct 2019 11:42:02 +0200Semi-device-independent characterisation of multipartite entangled states and measurementshttps://archive-ouverte.unige.ch/unige:124648https://archive-ouverte.unige.ch/unige:124648The semi-device-independent framework allows one to draw conclusions about properties of an unknown quantum system under weak assumptions. Here we present a semi-device-independent scheme for the characterisation of multipartite entanglement based around a game played by several isolated parties whose devices are uncharacterised beyond an assumption about the dimension of their Hilbert spaces. Our scheme can certify that an $n$-partite high-dimensional quantum state features genuine multipartite entanglement. Moreover, the scheme can certify that a joint measurement on $n$ subsystems is entangled, and provides a lower bound on the number of entangled measurement operators. These tests are strongly robust to noise, and even optimal for certain classes of states and measurements, as we demonstrate with illustrative examples. Notably, our scheme allows for the certification of many entangled states admitting a local model, which therefore cannot violate any Bell inequality.Mon, 21 Oct 2019 11:37:08 +0200Robust Macroscopic Quantum Measurements in the presence of limited control and knowledgehttps://archive-ouverte.unige.ch/unige:124647https://archive-ouverte.unige.ch/unige:124647Quantum measurements have intrinsic properties which seem incompatible with our everyday-life macroscopic measurements. Macroscopic Quantum Measurement (MQM) is a concept that aims at bridging the gap between well understood microscopic quantum measurements and macroscopic classical measurements. In this paper, we focus on the task of the polarization direction estimation of a system of $N$ spins $1/2$ particles and investigate the model some of us proposed in Barnea et al., 2017. This model is based on a von Neumann pointer measurement, where each spin component of the system is coupled to one of the three spatial components direction of a pointer. It shows traits of a classical measurement for an intermediate coupling strength. We investigate relaxations of the assumptions on the initial knowledge about the state and on the control over the MQM. We show that the model is robust with regard to these relaxations. It performs well for thermal states and a lack of knowledge about the size of the system. Furthermore, a lack of control on the MQM can be compensated by repeated "ultra-weak" measurements.Mon, 21 Oct 2019 11:35:49 +0200General measure for macroscopic quantum states beyond "dead and alive"https://archive-ouverte.unige.ch/unige:124646https://archive-ouverte.unige.ch/unige:124646We consider the characterization of quantum superposition states beyond the pattern "dead and alive". We propose a measure that is applicable to superpositions of multiple macroscopically distinct states, superpositions with different weights as well as mixed states. The measure is based on the mutual information to characterize the distinguishability between multiple superposition states. This allows us to overcome limitations of previous proposals, and to bridge the gap between general measures for macroscopic quantumness and measures for Schr"odinger-cat type superpositions. We discuss a number of relevant examples, provide an alternative definition using basis-dependent quantum discord and reveal connections to other proposals in the literature. Finally, we also show the connection between the size of quantum states as quantified by our measure and their vulnerability to noise.Mon, 21 Oct 2019 11:34:21 +0200Correlations in star networks: from Bell inequalities to network inequalitieshttps://archive-ouverte.unige.ch/unige:124645https://archive-ouverte.unige.ch/unige:124645The problem of characterizing classical and quantum correlations in networks is considered. Contrary to the usual Bell scenario, where distant observers share a physical system emitted by one common source, a network features several independent sources, each distributing a physical system to a subset of observers. In the quantum setting, the observers can perform joint measurements on initially independent systems, which may lead to strong correlations across the whole network. In this work, we introduce a technique to systematically map a Bell inequality to a family of Bell-type inequalities bounding classical correlations on networks in a star-configuration. Also, we show that whenever a given Bell inequality can be violated by some entangled state $ ho$, then all the corresponding network inequalities can be violated by considering many copies of $ ho$ distributed in the star network. The relevance of these ideas is illustrated by applying our method to a specific multi-setting Bell inequality. We derive the corresponding network inequalities, and study their quantum violations.Mon, 21 Oct 2019 11:32:39 +0200All entangled pure quantum states violate the bilocality inequalityhttps://archive-ouverte.unige.ch/unige:124644https://archive-ouverte.unige.ch/unige:124644The nature of quantum correlations in networks featuring independent sources of entanglement remains poorly understood. Here, focusing on the simplest network of entanglement swapping, we start a systematic characterization of the set of quantum states leading to violation of the so-called "bilocality" inequality. First, we show that all possible pairs of entangled pure states can violate the inequality. Next, we derive a general criterion for violation for arbitrary pairs of mixed two-qubit states. Notably, this reveals a strong connection between the CHSH Bell inequality and the bilocality inequality, namely that any entangled state violating CHSH also violates the bilocality inequality. We conclude with a list of open questions.Mon, 21 Oct 2019 11:30:07 +0200Genuine quantum nonlocality in the triangle networkhttps://archive-ouverte.unige.ch/unige:124643https://archive-ouverte.unige.ch/unige:124643Quantum networks allow in principle for completely novel forms of quantum correlations. In particular, quantum nonlocality can be demonstrated here without the need of having various input settings, but only by considering the joint statistics of fixed local measurement outputs. However, previous examples of this intriguing phenomenon all appear to stem directly from the usual form of quantum nonlocality, namely via the violation of a standard Bell inequality. Here we present novel examples of 'quantum nonlocality without inputs', which we believe represent a new form of quantum nonlocality, genuine to networks. Our simplest examples, for the triangle network, involve both entangled states and joint entangled measurements. A generalization to any odd-cycle network is also presented. Finally, we conclude with some open questions.Mon, 21 Oct 2019 11:27:56 +0200Solid-state optical quantum memory: from large entanglement to telecom wavelengthshttps://archive-ouverte.unige.ch/unige:123606https://archive-ouverte.unige.ch/unige:123606Quantum mechanics is leading to many modern technologies, e.g. based on laser physics. Emerging fields are for example nonlinear optics, hybrid systems, or quantum communication. Despite these applications, there are still limitations of our understanding of quantum mechanics. Even though it is broadly accepted that decoherence limits the size of entangled states, we currently do not know how to overcome these limitations. How large can commonly entangled states be? Can we observe macroscopic quantum phenomena with coarse-grained detectors or even with our bare eyes? This thesis contributes towards answering these fundamental questions and to technological advances. The experiments are based on a light-matter platform of quantum optical states and atomic ensembles. The ensembles consist of rare-earth ion-doped crystals. Additionally, the atomic frequency comb protocol turns the ensemble into a quantum memory. In the scope of quantum communication we work on a quantum repeater. First, we investigate fundamental quantum mechanical states and their validity at macroscopic scales. We show that the detection of a collectively emitted excitation can be used to measure the size of the entangled ensemble. The entangled ensemble shared this excitation in a coherent superposition. This superposition of an atomic excitation in an ensemble is called W-state. We certify a new lower bound to the possible size of an entangled W-state between at least 16 million atoms. The proof of entanglement of this size relies on two measures: one determines the number of atoms involved in the storage process. The other one determines the state "quantumness" by examining the re-emitted light. Another experiment studies a macroscopically distinguishable superposition state. The superposition survives the storage in an atomic ensemble. Second, we perform two more experiments that contribute to a quantum repeater. One experiment probes the quantum states simultaneously stored in a multi-mode quantum memory. In this work we also develop the concept of indirect entanglement witness. For the other experiment, a frequency down-conversion platform is set up. The conversion overcomes the wavelength compatibility issue between some quantum memories and the telecommunication C-band. The narrow spectral filtering of the conversion performs better than expected. In summary, our work analyzes large entangled states and building blocks for a quantum repeater. The presented quantum memories are capable of temporal multi-mode storage. Its temporally precise re-emission is due to the re-phasing of the atoms of the W-state. We also show a good way to set up frequency down-conversion for the sake of quantum repeaters. This work emphasizes the atomic frequency comb protocol as a viable and reliable platform for quantum repeaters.Wed, 25 Sep 2019 10:54:44 +0200Spectral noise in frequency conversion from the visible to the telecommunication C-bandhttps://archive-ouverte.unige.ch/unige:121974https://archive-ouverte.unige.ch/unige:121974We report a detailed study of the noise properties of a visible-to-telecom photon frequency converter based on difference frequency generation (DFG). The device converts 580 nm photons to 1541 nm using a strong pump laser at 930 nm, in a periodically poled lithium niobate ridge waveguide. The converter reaches a maximum device efficiency of 46 % (internal efficiency of 67%) at a pump power of 250 mW. The noise produced by the pump laser is investigated in detail by recording the noise spectra both in the telecom and visible regimes and measuring the power dependence of the noise rates. The noise spectrum in the telecom is very broadband, as expected from previous work on similar DFG converters. However, we also observe several narrow dips in the telecom spectrum, with corresponding peaks appearing in the 580 nm noise spectrum. These features are explained by sum frequency generation of the telecom noise at wavelengths given by the phase-matching condition of different spatial modes in the waveguide. The proposed noise model is in good agreement with all the measured data, including the power dependence of the noise rates, both in the visible and telecom regimes. These results are applicable to the class of DFG converters where the pump laser wavelength is in between the input and target wavelength.Tue, 27 Aug 2019 14:53:51 +0200Long range coherent OFDR with DFB laser diode coupled to an external cavityhttps://archive-ouverte.unige.ch/unige:119374https://archive-ouverte.unige.ch/unige:119374A long range coherent OFDR is demonstrated by using a standard DFB laser diode coupled to an all fiber external double cavity. The optical frequency sweep was obtained by runing the phase of the feedback light. The laser linewidth reduction, tunability and mode hopping are also discussed.Fri, 14 Jun 2019 15:07:30 +0200Underwater quantum codinghttps://archive-ouverte.unige.ch/unige:118545https://archive-ouverte.unige.ch/unige:118545abstract not availableFri, 31 May 2019 12:09:38 +0200Mode hopping noise in coherent FMCW reflectometryhttps://archive-ouverte.unige.ch/unige:117884https://archive-ouverte.unige.ch/unige:117884The effect of mode hopping in FMCW reflectometry is discussed. Numerical simulations and experimental results are compared and are shown to have good agreement. A simple formula to predict and position of spurious noise peaks is presented.Tue, 21 May 2019 14:24:44 +0200Experimental and Theoretical Investigations of Coherent OFDR with Semiconductor Laser Sourceshttps://archive-ouverte.unige.ch/unige:117883https://archive-ouverte.unige.ch/unige:117883Experimental and theoretical investigations of coherent optical-frequency-domain reflectometry using semiconductor laser sources are presented. Good agreement was found between the analysis of the signal-to-noise ratio due to the phase noise and the experimental results. The sensitivity limit due to the quantum noise is also described. Limitations due to the nonlinearity in the optical frequency sweep produced by the thermal-response time of the laser and mode hopping are investigated and compared with experimental results. Two interferometric methods to characterize the thermal-response time of the laser and their implementations are described. The effects of mode hopping in the optical-frequency sweep are compared to numerical simulations. A simple formula to predict the position of spurious peaks due to mode hopping are presented. A spatial resolution of 400 /spl mu/m over 10 cm was obtained by correcting the nonlinearity in the optical-frequency sweep by using an auxiliary interferometer. The Rayleigh backscattering was observed for the first time over more than 400 m of fiber using a DFB laser coupled to an external cavity.Tue, 21 May 2019 14:23:16 +0200Pressure Dependence of Polarization Mode Dispersion in HiBi Fibershttps://archive-ouverte.unige.ch/unige:117881https://archive-ouverte.unige.ch/unige:117881Pressure effects in polarization maintaining fibers are investigated both experimentally and theoretically. Interferometric measurements of the propagation times and polarization mode dispersion as a function of hydrostatic pressure over the range 0 to 40 MPa are presented. The finite-element method is used to calculate the pressure-induced variations of the asymmetrical stress in the core. It is observed and comprehensively explained that the birefringence increases with external applied hydrostatic pressure.Tue, 21 May 2019 14:20:34 +0200High-sensitivity-coherent optical frequency-domain reflectometry for characterization of fiber-optic network componentshttps://archive-ouverte.unige.ch/unige:117879https://archive-ouverte.unige.ch/unige:117879The authors have performed high-sensitivity-coherent optical frequency-domain reflectometry (OFDR) measurements of pig-tailed optical devices with centimeter resolution. The oscillations of the Rayleigh backscattering level produced by coherent fading noise was eliminated, strongly improving the capability of OFDR measurements of low level reflections and losses. Sensitivities down to -105 dB were achieved with minute measurement times.Tue, 21 May 2019 14:17:38 +0200Low coherence multiplexed optical sensorshttps://archive-ouverte.unige.ch/unige:117480https://archive-ouverte.unige.ch/unige:117480A novel multiplexing scheme for optical sensors, based on position transducers and low coherence reflectometry is presented. The multiplexing of three sensors was demonstrated, and it is shown that up to ten sensor heads with 1% accuracy can be multiplexed.Wed, 15 May 2019 08:17:48 +0200Spin dynamics in rare-earth-ion-doped crystals for optical quantum memorieshttps://archive-ouverte.unige.ch/unige:115799https://archive-ouverte.unige.ch/unige:115799Spin dynamics are investigated in several rare-earth-ion-doped crystals, both for so-called Kramers ions and non-Kramers ions. I have studied two illustrative examples of such systems, Neodymium (Kramers) and Europium (non-Kramers). The main motivation behind these studies is the search for long relaxation and coherence times in solid state atomic ensembles. To this end I employ spectroscopic techniques to probe different kinds of interactions and relaxation mechanisms taking place in the samples. I perform spectroscopy of Neodymium (Kramers) in two different crystal lattices, yttrium orthosilicate and yttrium orthovanadate. I propose new methods to improve the relaxation lifetimes, and make precise characterizations of the interactions taking place within the atomic ensemble. In particular we achieve the most precise measurement of a kind of interaction which has received considerable attention recently in other areas of physics, the Dzyaloshinskii-Moriya interaction. These spectroscopic studies are of course of fundamental interest, but they can also be applied to quantum communication and computation. In this context, we were able to increase the efficiency of a quantum memory based on a Kramers ion (Nd) to be on par with the best non-Kramers (Eu,Pr) memories. Using Neodymium ions, I have certified that multi-dimensional entanglement can be stored in a quantum memory. The second part of this thesis concerns the study of Europium (non-Kramers), starting with a characterization of the Zeeman and quadrupole interactions. I believe these studies will pave the way for long duration (100 ms or more) quantum storage in a rare-earth-ion-doped crystal.Mon, 08 Apr 2019 13:56:43 +0200Polarization mode dispersion measurements on long buried cableshttps://archive-ouverte.unige.ch/unige:115088https://archive-ouverte.unige.ch/unige:115088This paper describes the polarization mode dispersion (PMD) measurements of long standard fibers, dispersion-shifted (DS-) and dispersion-flattened (DF-) fibers uncabled on bobbins and in cables installed in the test links Berlin IV (18 km long) and Berlin VI (22.6 km). The PMD is measured with the aid of the interferometric technique, using a Michelson-interferometer. This method is not restricted to long fibers with low birefringence, it can be used to measuring the PMD and group beatlength of short high-birefringent fibers with a high accuracy. It will be shown that the cabling procedure increases the PMD values considerably, up to one order of magnitude.Mon, 18 Mar 2019 16:23:41 +0100OFDR measurements of insertion and return losses of integrated optic deviceshttps://archive-ouverte.unige.ch/unige:114599https://archive-ouverte.unige.ch/unige:114599The insertion loss of an integrated optic device is measured by means of the OFDR teclmique. Measurements of Rayleigh BackScattered intensity in the input and output fibers of the device allow direct detection of light power guided in the fiber before and after the component. The insertion and return losses of the component can thus be directly evaluated. Experimental results are in good agreement with measurements performed by the cut-back method.Tue, 26 Feb 2019 11:29:40 +0100Self-heterodyne coherent optical frequency-domain reflectometerhttps://archive-ouverte.unige.ch/unige:114595https://archive-ouverte.unige.ch/unige:114595A high sensitivity coherent optical frequency-domain reflectometer (OFDR) is presented, for measurements of pigtailed optical devices with centimetre resolution. Unique interpretation of the reflectivity data is achieved by frequency shifting the reference reflection and using optical heterodyne detection.Tue, 26 Feb 2019 11:24:19 +0100COST 217 intercomparison of measurements on Er-doped fibreshttps://archive-ouverte.unige.ch/unige:113758https://archive-ouverte.unige.ch/unige:113758An intercomparison of measurements on three Er-doped fibres has been organised in the working group 3 of the European COST 217 project (optical measurement techniques for advanced optical fibre systems and devices). A set of fibre parameters, including core diameter, refractive index difference, mode field diameter, cut-off, absorption, fluorescence peak wavelength, time constant, doping concentration and gain, have been measured using samples from three fibres made by three participating laboratories. The measurement results and the properties of the three fibres with different core compositions are compared.Fri, 01 Feb 2019 14:08:30 +0100Photoacoustical detection of trace gases with an optical microphonehttps://archive-ouverte.unige.ch/unige:113327https://archive-ouverte.unige.ch/unige:113327An alternative to the electrical microphone for photoacoustical detection of trace gases is presented. This microphone is made of a monomode optical fiber wound and glued on a thin plate. This fiber is one arm of a Michelson interferometer. A Sagnac interferometer has also been used and has given better results.Fri, 25 Jan 2019 13:40:38 +0100Automated 'plug & play' quantum key distributionhttps://archive-ouverte.unige.ch/unige:112835https://archive-ouverte.unige.ch/unige:112835An improved 'plug & play' interferometric system for quantum key distribution is presented. Self-alignment and compensation of birefringence remain, while limitations due to reflections are overcome. Original electronics implementing the BB84 protocol makes adjustment simple. Key creation with 0.1 photon per pulse at a rate of 325Hz with a 2.9% QBER, corresponding to a net rate of 210Hz, over a 23km installed cable was realised.Fri, 11 Jan 2019 11:47:53 +0100Quantification of multidimensional entanglement stored in a crystalhttps://archive-ouverte.unige.ch/unige:112336https://archive-ouverte.unige.ch/unige:112336The use of multidimensional entanglement opens new perspectives for quantum information processing. However, an important challenge in practice is to certify and characterize multidimensional entanglement from measurement data that are typically limited. Here, we report the certification and quantification of two-photon multidimensional energy-time entanglement between many temporal modes, after one photon has been stored in a crystal. We develop a method for entanglement quantification which makes use of only sparse data obtained with limited resources. This allows us to efficiently certify an entanglement of formation of 1.18 ebits after performing quantum storage. The theoretical methods we develop can be readily extended to a wide range of experimental platforms, while our experimental results demonstrate the suitability of energy-time multidimensional entanglement for a quantum repeater architecture.Fri, 14 Dec 2018 15:19:28 +0100Characterization of the hyperfine interaction of the excited 5D0 state of Eu3+:Y2SiO5https://archive-ouverte.unige.ch/unige:112332https://archive-ouverte.unige.ch/unige:112332We characterize the europium (Eu3+) hyperfine interaction of the excited state (5D0) and determine its effective spin Hamiltonian parameters for the Zeeman and quadrupole tensors. An optical free induction decay method is used to measure all hyperfine splittings under a weak external magnetic field (up to 10 mT) for various field orientations. On the basis of the determined Hamiltonian, we discuss the possibility to predict optical transition probabilities between hyperfine levels for the 7F0⟷5D0 transition. The obtained results provide necessary information to realize an optical quantum memory scheme which utilizes long spin coherence properties of 151Eu3+:Y2SiO5 material under external magnetic fields.Fri, 14 Dec 2018 15:16:09 +0100Efficient optical pumping using hyperfine levels in 145Nd3+:Y2SiO5 and its application to optical storagehttps://archive-ouverte.unige.ch/unige:112330https://archive-ouverte.unige.ch/unige:112330Efficient optical pumping is an important tool for state initialization in quantum technologies, such as optical quantum memories. In crystals doped with Kramers rare-earth ions, such as erbium and neodymium, efficient optical pumping is challenging due to the relatively short population lifetimes of the electronic Zeeman levels, of the order of 100 ms at around 4 K. In this article we show that optical pumping of the hyperfine levels in isotopically enriched 145Nd3+:Y2SiO5 crystals is more efficient, owing to the longer population relaxation times of hyperfine levels. By optically cycling the population many times through the excited state a nuclear spin flip can be forced in the ground state hyperfine manifold, in which case the population is trapped for several seconds before relaxing back to the pumped hyperfine level. To demonstrate the effectiveness of this approach in applications we perform an atomic frequency comb memory experiment with 33% storage efficiency in 145Nd3+:Y2SiO5 , which is on a par with results obtained in non-Kramers ions, e.g. europium and praseodymium, where optical pumping is generally efficient due to the quenched electronic spin. Efficient optical pumping in neodymium-doped crystals is also of interest for spectral filtering in biomedical imaging, as neodymium has an absorption wavelength compatible with tissue imaging. In addition to these applications, our study is of interest for understanding spin dynamics in Kramers ions with nuclear spin.Fri, 14 Dec 2018 15:11:24 +0100Simultaneous coherence enhancement of optical and microwave transitions in solid-state electronic spinshttps://archive-ouverte.unige.ch/unige:112325https://archive-ouverte.unige.ch/unige:112325Solid-state electronic spins are extensively studied in quantum information science, as their large magnetic moments offer fast operations for computing and communication and high sensitivity for sensing. However, electronic spins are more sensitive to magnetic noise, but engineering of their spectroscopic properties, for example, using clock transitions and isotopic engineering, can yield remarkable spin coherence times, as for electronic spins in GaAs, donors in silicon and vacancy centres in diamond. Here we demonstrate simultaneously induced clock transitions for both microwave and optical domains in an isotopically purified 171Yb3+:Y2SiO5 crystal, reaching coherence times of greater than 100 μs and 1 ms in the optical and microwave domains, respectively. This effect is due to the highly anisotropic hyperfine interaction, which makes each electronic–nuclear state an entangled Bell state. Our results underline the potential of 171Yb3+:Y2SiO5 for quantum processing applications relying on both optical and spin manipulation, such as optical quantum memories, microwave-to-optical quantum transducers, and single-spin detection, while they should also be observable in a range of different materials with anisotropic hyperfine interactions.Fri, 14 Dec 2018 15:04:12 +0100Spectroscopic study of hyperfine properties in 171Yb3+:Y2SiO5https://archive-ouverte.unige.ch/unige:112322https://archive-ouverte.unige.ch/unige:112322Rare-earth ion-doped crystals are promising systems for quantum communication and quantum information processing. In particular, paramagnetic rare-earth centers can be utilized to realize quantum coherent interfaces simultaneously for optical and microwave photons. In this paper, we study hyperfine and magnetic properties of a Y2SiO5 crystal doped with 171Yb3+ ions. This isotope is particularly interesting since it is the only rare–earth ion having electronic spin S=1/2 and nuclear spin I=1/2, which results in the simplest possible hyperfine level structure. In this work, we determine the hyperfine tensors for the ground and excited states on the optical 2F7/2(0)⟷2F5/2(0) transition by combining spectral hole burning and optically detected magnetic resonance techniques. The resulting spin Hamiltonians correctly predict the magnetic-field dependence of all observed optical-hyperfine transitions, from zero applied field up to fields where the Zeeman interaction is dominating the hyperfine interaction. Using the optical absorption spectrum, we can also determine the order of the hyperfine levels in both states. These results pave the way for realizing solid-state optical and microwave quantum memories based on a 171Yb3+:Y2SiO5 crystal.Fri, 14 Dec 2018 14:55:13 +0100Non-Markovian quantum state diffusionhttps://archive-ouverte.unige.ch/unige:112220https://archive-ouverte.unige.ch/unige:112220A nonlinear stochastic Schrödinger equation for pure states describing non-Markovian diffusion of quantum trajectories and compatible with non-Markovian master equations is presented. This provides an unraveling of the evolution of any quantum system coupled to a finite or infinite number of harmonic oscillators without any approximation. Its power is illustrated by several examples, including measurementlike situations, dissipation, and quantum Brownian motion. Some examples treat this environment phenomenologically as an infinite reservoir with fluctuations of arbitrary correlation. In other examples the environment consists of a finite number of oscillators. In such a quasiperiodic case we see the reversible decay of a macroscopic quantum-superposition (“Schrödinger cat”). Finally, our description of open systems is compatible with different positions of the “Heisenberg cut” between system and environment.Wed, 12 Dec 2018 12:17:30 +0100Violation of Bell Inequalities by Photons More Than 10 km Aparthttps://archive-ouverte.unige.ch/unige:112219https://archive-ouverte.unige.ch/unige:112219A Franson-type test of Bell inequalities by photons 10.9 km apart is presented. Energy-time entangled photon pairs are measured using two-channel analyzers, leading to a violation of the inequalities by 16 standard deviations without subtracting accidental coincidences. Subtracting them, a two-photon interference visibility of 95.5% is observed, demonstrating that distances up to 10 km have no significant effect on entanglement. This sets quantum cryptography with photon pairs as a practical competitor to the schemes based on weak pulses.Wed, 12 Dec 2018 12:16:15 +0100Bell inequality, Bell states and maximally entangled states for n qubitshttps://archive-ouverte.unige.ch/unige:112218https://archive-ouverte.unige.ch/unige:112218First, we present a Bell-type inequality for n qubits, assuming that m out of the n qubits are independent. Quantum mechanics violates this inequality by a ratio that increases exponentially with m. Hence an experiment on n qubits violating this inequality sets a lower bound on the number m of entangled qubits. Next, we propose a definition of maximally entangled states of n qubits. For this purpose we study five different criteria. Four of these criteria are found compatible. For any number n of qubits, they determine an orthogonal basis consisting of maximally entangled states generalizing the Bell states.Wed, 12 Dec 2018 12:14:54 +0100Measuring refractive index profiles of integrated LiNbO3 waveguideshttps://archive-ouverte.unige.ch/unige:109633https://archive-ouverte.unige.ch/unige:109633A nondestructive technique that provides near-field measurements as well as refractive index profiles and geometry of waveguides in LiNbO3 within a few minutes is demonstrated. It is a modification of the refracted-near-field (RNF) method well known for optical fiber characterization. Spatial resolution below 1 μm is achieved. The index resolution is about 2 x 10-4, the reproducibility better than 7%. A new calibration method for RNF measurement is introduced.Tue, 23 Oct 2018 12:25:38 +0200Experimental certification of millions of genuinely entangled atoms in a solidhttps://archive-ouverte.unige.ch/unige:108691https://archive-ouverte.unige.ch/unige:108691abstract not availableTue, 02 Oct 2018 09:54:38 +0200The quantum technologies roadmap: a European community viewhttps://archive-ouverte.unige.ch/unige:107161https://archive-ouverte.unige.ch/unige:107161Within the last two decades, quantum technologies (QT) have made tremendous progress, moving from Nobel Prize award-winning experiments on quantum physics (1997: Chu, Cohen-Tanoudji, Phillips; 2001: Cornell, Ketterle, Wieman; 2005: Hall, Hänsch-, Glauber; 2012: Haroche, Wineland) into a cross-disciplinary field of applied research. Technologies are being developed now that explicitly address individual quantum states and make use of the 'strange' quantum properties, such as superposition and entanglement. The field comprises four domains: quantum communication, where individual or entangled photons are used to transmit data in a provably secure way; quantum simulation, where well-controlled quantum systems are used to reproduce the behaviour of other, less accessible quantum systems; quantum computation, which employs quantum effects to dramatically speed up certain calculations, such as number factoring; and quantum sensing and metrology, where the high sensitivity of coherent quantum systems to external perturbations is exploited to enhance the performance of measurements of physical quantities. In Europe, the QT community has profited from several EC funded coordination projects, which, among other things, have coordinated the creation of a 150-page QT Roadmap (http://qurope.eu/h2020/qtflagship/roadmap2016). This article presents an updated summary of this roadmap.Mon, 20 Aug 2018 11:10:40 +0200Alternative configuration for refracted near-field measurements of refractive index on glass-integrated-optics waveguideshttps://archive-ouverte.unige.ch/unige:106826https://archive-ouverte.unige.ch/unige:106826We present experimental investigations of two configurations for refractive-index profiling and for geometry measurements of glass-integrated waveguides with the refracted near-field technique. An original configuration is proposed that simplifies the measurement cell and the handling.Mon, 06 Aug 2018 09:31:48 +0200Experimental investigations of the statistical properties of polarization mode dispersion in single mode fibershttps://archive-ouverte.unige.ch/unige:104528https://archive-ouverte.unige.ch/unige:104528Polarization mode dispersion of installed optical cables and of short concatenations of hi-bi fibers have been measured with polarimetric and interferometric instruments. The results confirm the theoretical models, in particular the predicted relations between the two measurement methods. The importance of a statistical treatment of polarization mode dispersion is underscored by the observed instability of the principal states and the remarkable long-term stability of their statistics.Wed, 16 May 2018 16:10:51 +0200Definitions and measurements of polarization mode dispersion: interferometric versus fixed analyzer methodshttps://archive-ouverte.unige.ch/unige:104527https://archive-ouverte.unige.ch/unige:104527We compare, from a theoretical and an experimental point of view, the interferometric and the fixed analyzer (also known as the wavelength scanning) techniques for measuring polarization mode dispersion. The information provided by both techniques is shown to be identical, up to a Fourier transform. This information is related to a natural definition of polarization mode delay, /spl Delta//spl tau/. For standard communication fibers, /spl Delta//spl tau/ is itself related to the mean delay between the principal polarization modes, /sub /spl omega//, by a simple numerical factor.Wed, 16 May 2018 16:10:25 +0200Measurements on birefringent fibres : COST 217 interlaboratory measurement campaignhttps://archive-ouverte.unige.ch/unige:104497https://archive-ouverte.unige.ch/unige:104497An interlaboratory comparison of measurements on three types of commercially available polarisation maintaining fibres was performed in the framework of COST 217. The obtained accuracy and reproducibility for the parameters as the mode field diameter, cutoff wavelength and chromatic dispersion was in the same range for these high birefringent fibres as for the conventional circular single mode fibres. Two setups were tested for the simultaneous determination of the birefringence and modal birefringence. The results characterising the polarisation properties of the fibres are presented in a 6-term Sellmeier expansion of the refractive index.Tue, 15 May 2018 11:58:31 +0200The quantum-state diffusion model applied to open systemshttps://archive-ouverte.unige.ch/unige:103121https://archive-ouverte.unige.ch/unige:103121A model of a quantum system interacting with its environment is proposed in which the system is represented by a state vector that satisfies a stochastic differential equation, derived from a density operator equation such as the Bloch equation, and consistent with it. The advantages of the numerical solution of these equations over the direct numerical solution of the density operator equations are described. The method is applied to the nonlinear absorber, cascades of quantum transitions, second-harmonic generation and a measurement reduction process. The model provides graphic illustrations of these processes, with statistical fluctuations that mimic those of experiments. The stochastic differential equations originated from studies of the measurement problem in the foundations of quantum mechanics. The model is compared with the quantum-jump model of Dalibard (1992), Carmichael and others, which originated among experimenters looking for intuitive pictures and rules of computation.Wed, 21 Mar 2018 15:11:49 +0100Quantum diffusions, quantum dissipation and spin relaxationhttps://archive-ouverte.unige.ch/unige:103120https://archive-ouverte.unige.ch/unige:103120The authors develop the tool of quantum diffusion (i.e. Hilbert-space-valued stochastic differential equations) for dissipative quantum systems. The aims are to find possible limitations to this approach and to investigate new pictures of open quantum systems. They are guided by the relaxation process for arbitrary spin and the associated natural rotational symmetry. They also impose the condition that the spin-coherent states remain coherent during the dissipative evolution. They present a new quantum diffusion equation that satisfies the above conditions and that is the unique quantum diffusion satisfying Percival's condition (d psi )2=0.Wed, 21 Mar 2018 15:10:56 +0100Quantum State Diffusion Theory and a Quantum Jump Experimenthttps://archive-ouverte.unige.ch/unige:103119https://archive-ouverte.unige.ch/unige:103119We use a recent stochastical diffusion model of quantum evolution to represent the evolution of a three-level quantum system undergoing quantum jumps. This is possible because the continuous change in the quantum state in this diffusion model is so rapid that it appears to be instantaneous in comparison with the time between transitions. Experimental data from a study of the intermittent fluorescence of a single trapped 24Mg+ ion and equivalent theoretical data are shown to be strikingly similar. Statistical comparisons of the data are also made.Wed, 21 Mar 2018 15:10:06 +0100Optical fiber characterization by simultaneous measurement of the transmitted and refracted near fieldhttps://archive-ouverte.unige.ch/unige:103117https://archive-ouverte.unige.ch/unige:103117An experimental setup is presented which permits, in a routine way for R&D purposes, simultaneous measurement of the transmitted near field at 1300 nm and 1550 nm, and the refracted near field at 820 nm. A new method for the calibration of the refractive index is proposed. The obtained accuracy for the refractive index is dn+or-0.0002. The reproducibility of measurements of geometrical parameters like the mode field diameter, the core and cladding diameters, and concentricity error, is +or-0.1 mu m. Measurements of the mode field as a function of polarization state for four different hi-bi fiber are presented.Wed, 21 Mar 2018 15:07:35 +0100Definition of polarization mode dispersion and first results of the COST 241 round-robin measurementshttps://archive-ouverte.unige.ch/unige:102535https://archive-ouverte.unige.ch/unige:102535Two definitions and three measurement schemes for polarization mode dispersion are presented. Their relations, advantages and drawbacks are discussed. A connection with the dynamical equation for the principal polarization states is made. First results from the COST 241 interlaboratory measurement campaign are also presented.Mon, 26 Feb 2018 10:38:04 +0100Spectral hole lifetimes and spin population relaxation dynamics in neodymium-doped yttrium orthosilicatehttps://archive-ouverte.unige.ch/unige:100454https://archive-ouverte.unige.ch/unige:100454We present a detailed study of the lifetime of optical spectral holes due to population storage in Zeeman sublevels of Nd3+:Y2SiO5. The lifetime is measured as a function of magnetic field strength and orientation, temperature, and Nd3+ doping concentration. At the lowest temperature of 3 K we find a general trend where the lifetime is short at low field strengths, then increases to a maximum lifetime at a few hundred mT, and then finally decays rapidly for high field strengths. This behavior can be modeled with a relaxation rate dominated by Nd3+−Nd3+ cross relaxation at low fields and spin lattice relaxation at high magnetic fields. The maximum lifetime depends strongly on both the field strength and orientation, due to the competition between these processes and their different angular dependencies. The cross relaxation limits the maximum lifetime for concentrations as low as 30 ppm of Nd3+ ions. By decreasing the concentration to less than 1 ppm we could completely eliminate the cross relaxation, reaching a lifetime of 3.8 s at 3 K. At higher temperatures the spectral hole lifetime is limited by the magnetic-field-independent Raman and Orbach processes. In addition we show that the cross relaxation rate can be strongly reduced by creating spectrally large holes of the order of the optical inhomogeneous broadening. Our results are important for the development and design of new rare-earth-ion doped crystals for quantum information processing and narrow-band spectral filtering for biological tissue imaging.Mon, 18 Dec 2017 11:09:34 +0100Multimode and Long-Lived Quantum Correlations Between Photons and Spins in a Crystalhttps://archive-ouverte.unige.ch/unige:100453https://archive-ouverte.unige.ch/unige:100453The realization of quantum networks and quantum repeaters remains an outstanding challenge in quantum communication. These rely on the entanglement of remote matter systems, which in turn requires the creation of quantum correlations between a single photon and a matter system. A practical way to establish such correlations is via spontaneous Raman scattering in atomic ensembles, known as the Duan-Lukin-Cirac-Zoller (DLCZ) scheme. However, time multiplexing is inherently difficult using this method, which leads to low communication rates even in theory. Moreover, it is desirable to find solid-state ensembles where such matter-photon correlations could be generated. Here we demonstrate quantum correlations between a single photon and a spin excitation in up to 12 temporal modes, in a 151Eu3+-doped Y2SiO5 crystal, using a novel DLCZ approach that is inherently multimode. After a storage time of 1 ms, the spin excitation is converted into a second photon. The quantum correlation of the generated photon pair is verified by violating a Cauchy-Schwarz inequality. Our results show that solid-state rare-earth-ion-doped crystals could be used to generate remote multimode entanglement, an important resource for future quantum networks.Mon, 18 Dec 2017 11:06:10 +0100Wave-function approach to dissipative processes: are there quantum jumps?https://archive-ouverte.unige.ch/unige:99788https://archive-ouverte.unige.ch/unige:99788In a recent Letter Dalibard and coworkers have presented an efficient method of computing the development of an open quantum system based on stochastic evolution of the state vector, in which quantum jumps are represented explicitly. Independently of this pragmatic approach, physicists interested in the “quantum measurement problem” have been led to consider continuous stochastic diffusion equations for the state vector associated to any density operator evolution. We underline the remarkable convergence of these two trends in physics, and argue that these recent developments may lead to new results and insights into quantum phenomena.Wed, 29 Nov 2017 13:55:59 +0100New optical configuration for nondestructive measurements of refractive index profiles of LiNbO3 waveguideshttps://archive-ouverte.unige.ch/unige:99782https://archive-ouverte.unige.ch/unige:99782A direct and nondestructive measurement technique for the determination of the refractive index profiles of one- and two-dimensional LiNbO3 waveguides is presented for the first time to our knowledge. The technique generalizes the refracted near-field method, which is well known for optical fiber characterization. The spatial resolution and accuracy are 0.1 and 0.4 μm, respectively. The refractive-index calibration is done by an analysis of the near-field light power distribution, and its resolution is approximately 2 × 10−4. The proposed experimental setup permits sample installation and data acquisition in a few minutes.Wed, 29 Nov 2017 13:42:02 +0100Time Correlations and Heisenberg Picture in the Quantum State Diffusion Model of Open Systemshttps://archive-ouverte.unige.ch/unige:99712https://archive-ouverte.unige.ch/unige:99712The quantum state diffusion model of open systems provides a practical tool for the description of the evolution of individual systems. In this letter we present the Heisenberg picture of this model and use it to compute arbitrary multi-time expectation values. In this way we extend the computational power of the model.Mon, 27 Nov 2017 12:10:47 +0100Relevant and irrelevant nonlinear Schrödinger equationshttps://archive-ouverte.unige.ch/unige:99710https://archive-ouverte.unige.ch/unige:99710First, we summarize the argument against deterministic nonlinear Schrodinger equations. We recall that any such equation activates quantum nonlocality in the sense that that information could be signalled in a finite time over arbitrarily large distances. Next we introduce a deterministic nonlinear Schrodinger equation. We justify it by showing that it is closest, in a precise sense, to the master equations for mixed states used to describe the evolution of open quantum systems. We also illustrate some interesting properties of this equation. Finally, we show that this equation can avoid the signalling problem if one adds noise to it in a precise way. Cases of both discrete and continuous noise are introduced explicitly and related to the density operator evolution. The relevance for the classical limit of the obtained stochastic equations is illustrated on a classically chaotic kicked anharmonic oscillator.Mon, 27 Nov 2017 12:09:08 +0100Low-coherence deformation sensors for the monitoring of civil-engineering structureshttps://archive-ouverte.unige.ch/unige:99455https://archive-ouverte.unige.ch/unige:99455An optical-fiber deformation sensor with a resolution of 10 μm and an operational range of 60 mm has been realized. The system is based on low-coherence interferometry in standard single-mode telecommunication fibers. It allows the monitoring of large structures over several months without noticeable drift. No continuous measurement is needed and the system is insensitive to variations of the fiber losses. This technique has been applied to the monitoring of a 20 m × 5 m × 0.5 m, 120 ton concrete slab over six months. It is possible to measure the shrinkage of concrete and its elastic coefficient during pre-straining, giving reproducible results in good agreement with theoretical calculations and measurements performed on small concrete samples. This paper describes the optical arrangement and the procedures used to install optical fibers in concrete.Wed, 22 Nov 2017 13:29:54 +0100Demonstration of Einstein-Podolsky-Rosen Steering Using Single-Photon Path Entanglement and Displacement-Based Detectionhttps://archive-ouverte.unige.ch/unige:96472https://archive-ouverte.unige.ch/unige:96472We demonstrate the violation of an EPR steering inequality developed for single photon path entanglement with displacement-based detection. We use a high-rate source of heralded single-photon path-entangled states, combined with high-efficiency superconducting-based detectors, in a scheme that is free of any post-selection and thus immune to the detection loophole. This result conclusively demonstrates single-photon entanglement in a one-sided device-independent scenario, and opens the way towards implementations of device-independent quantum technologies within the paradigm of path entanglement.Mon, 04 Sep 2017 12:00:52 +0200Heralded amplification of path entangled quantum stateshttps://archive-ouverte.unige.ch/unige:94656https://archive-ouverte.unige.ch/unige:94656Device-independent quantum key distribution (DI-QKD) represents one of the most fascinating challenges in quantum communication, exploiting concepts of fundamental physics, namely Bell tests of nonlocality, to ensure the security of a communication link. This requires the loophole-free violation of a Bell inequality, which is intrinsically difficult due to losses in fibre optic transmission channels. Heralded photon amplification (HPA) is a teleportation-based protocol that has been proposed as a means to overcome transmission loss for DI-QKD. Here we demonstrate HPA for path entangled states and characterise the entanglement before and after loss by exploiting a recently developed displacement-based detection scheme. We demonstrate that by exploiting HPA we are able to reliably maintain high fidelity entangled states over loss-equivalent distances of more than 50 km.Tue, 06 Jun 2017 11:26:03 +0200On the inequivalence of the CH and CHSH inequalities due to finite statisticshttps://archive-ouverte.unige.ch/unige:94565https://archive-ouverte.unige.ch/unige:94565Different variants of a Bell inequality, such as CHSH and CH, are known to be equivalent when evaluated on nonsignaling outcome probability distributions. However, in experimental setups, the outcome probability distributions are estimated using a finite number of samples. Therefore the nonsignaling conditions are only approximately satisfied and the robustness of the violation depends on the chosen inequality variant. We explain that phenomenon using the decomposition of the space of outcome probability distributions under the action of the symmetry group of the scenario, and propose a method to optimize the statistical robustness of a Bell inequality. In the process, we describe the finite group composed of relabeling of parties, measurement settings and outcomes, and identify correspondences between the irreducible representations of this group and properties of outcome probability distributions such as normalization, signaling or having uniform marginals.Wed, 31 May 2017 14:30:44 +0200Maximal violation of Bell's inequality for arbitrarily large spinhttps://archive-ouverte.unige.ch/unige:93838https://archive-ouverte.unige.ch/unige:93838For any nonfactorable state of two quantum systems, it is possible to find pairs of observables whose correlations violate Bell's inequality. In the case of two particles of spin j prepared in a single state, the violation of Bell's inequality remains maximal for arbitrary large j. It is thus seen that large quantum numbers are no guarantee of classical behaviour.Tue, 25 Apr 2017 11:28:55 +0200Polarization mode dispersion for single-mode fibers with polarization dependent losseshttps://archive-ouverte.unige.ch/unige:93837https://archive-ouverte.unige.ch/unige:93837Birefringent single-mode fibers with random polarization mode coupling and polarization dependent losses are considered. The effect of these losses on polarization mode dispersion (PMD) is computed. The relevant number is found to be the differential loss between both polarization modes over one coupling length. For polarizing fibers, the coupling length is of several kilometers, the relevant number is thus high, and the PMD is significantly reduced by the differential loss. For standard fibers, on the other hand, the coupling length is of several tens of meters, the relevant number is thus small and polarization dependent losses have a negligible effect on the PMD.Tue, 25 Apr 2017 11:28:02 +0200Polarization mode dispersion: time versus frequency domainshttps://archive-ouverte.unige.ch/unige:93833https://archive-ouverte.unige.ch/unige:93833We present two ways of analyzing the polarization mode dispersion with random coupling in standard single-mode fibers. The first one is based on the concept of principal polarization states and is valid for highly coherent propagating light (analysis in the frequency domain). The second one is based on unpolarized short pulses split by the local birefringence (analysis in the time domain). The two approaches lead to different definitions of polarization mode dispersion, but we prove that the two definitions lead to the same values. For this purpose we derive the evolution equations for the principal states and for the pulse distribution for arbitrary concatenations of polarization maintaining fibers. Both models have a continuous limit.Tue, 25 Apr 2017 11:15:40 +0200Quantifying Photonic High-Dimensional Entanglementhttps://archive-ouverte.unige.ch/unige:93007https://archive-ouverte.unige.ch/unige:93007High-dimensional entanglement offers promising perspectives in quantum information science. In practice, however, the main challenge is to devise efficient methods to characterize high-dimensional entanglement, based on the available experimental data which is usually rather limited. Here we report the characterization and certification of high-dimensional entanglement in photon pairs, encoded in temporal modes. Building upon recently developed theoretical methods, we certify an entanglement of formation of 2.09(7) ebits in a time-bin implementation, and 4.1(1) ebits in an energy-time implementation. These results are based on very limited sets of local measurements, which illustrates the practical relevance of these methods.Mon, 03 Apr 2017 11:15:07 +0200Low Coherence Fiber Optic Sensors for Structural Monitoringhttps://archive-ouverte.unige.ch/unige:92702https://archive-ouverte.unige.ch/unige:92702This paper reports on experimental and theoretical investigations into the use of optical fiber sensors in the monitoring of civil engineering structures. For this purpose a highly accurate optical fiber deformation sensor based on low coherence interferometry has been developed. This system was conceived in order to monitor a test concrete slab over a period of years. It enabled the measurement of both concrete shrinkage and elastic coefficient in the whole slab with good reproducibility, in agreement with measurements performed on small concrete samples and with theoretical calculations. The thermal effects on the concrete structures are also presented.Tue, 21 Mar 2017 11:01:05 +0100Generation of non-classical correlations between spins and photons using rare-earth-ion doped crystalshttps://archive-ouverte.unige.ch/unige:92034https://archive-ouverte.unige.ch/unige:92034Au cours de ces dernières années, les technologies de l'information se sont développées de manière fulgurante, l'augmentation de la vitesse de connexion `a Internet augmente de manière exponentielle chaque année. Il en résulte que de plus en plus de données personnelles ou confidentielles sont échangées. Il devient donc urgent de sécuriser ces lignes de communications utilisés quotidiennement. La communication quantique permet l'échange de clés de cryptage secrètes dont la sécurité est basée sur les lois de la physique quantique. En effet en encodant de l'information sur des photons uniques, il est possible de détecter l'interception ou la modification de cette information. Cette communication quantique est néanmoins sujette aux pertes intrinsèques au réseau de télécommunication par fibre optique, ainsi la distance de communication est limitée `a une centaine de kilomètres. Il est donc nécessaire de développer un protocole analogue au répéteur utilisé pour les communication classiques. Ce répéteur quantique permettrait la distribution d'états quantiques sur de très longues distances. L'implémentation de ce protocole nécessite l'utilisation d'un dispositif capable de stocker et libérer la lumière quantique sur demande. Les cristaux dopés aux ions terres rares font partie des candidats prometteurs pour la réalisation de telles mémoires quantiques. En utilisant ces cristaux il est possible de transférer le photon en une excitation de spin et vice-versa ce qui permet un stockage de longue durée et sur demande. Au cours de ce doctorat, j'ai étudié différents cristaux dopés aux ions terres rares et en particulier les propriétés de cohérence de leurs états de spin. J'ai utilisé et développé des nouvelles techniques de spectroscopies pour caractériser ces cristaux et leur viabilité pour le stockage d'information quantique. Cela permis de développer les compétences nécessaires au contrôle cohérent de spin dans ces cristaux. Grâce à ce contrôle nous avons pu étendre la durée du temps de stockage de plusieurs ordres de grandeurs de la microseconde jusqu'à la milliseconde. De plus cette expérience a démontré qu'il est possible de manipuler une excitation unique délocalisé sur un grand ensemble d'ions (plus de mille milliards) sans dégrader le signal en sortie. Suite à cela j'ai caractérisé la capacité multimode et la fidélité de notre mémoire ainsi que son caractère quantique. J'ai ensuite développé et implémenté un protocole permettant d'utiliser le cristal en tant que source et mémoire sur demande de photons uniques. Cette expérience est la première réalisation d'un tel protocole dans les solides en utilisant des photons uniques. Finalement j'ai étudié le potentiel d'un cristal fortement dopé pour le stockage d'information quantique. J'ai identifié et mesuré une interaction de type Dzyaloshinskii-Moriya entre deux ions voisins dans le cristal. Ce couple d'ions, analogue à une molécule piégé dans la maille cristalline, pourrait présenté un potentiel avantage comparés à l'ion isolé.Wed, 22 Feb 2017 11:08:45 +0100Interlaboratory evaluation of theoretical modelling techniques for single-mode optical fibreshttps://archive-ouverte.unige.ch/unige:91474https://archive-ouverte.unige.ch/unige:91474The results obtained in an interlaboratory evaluation of theoretical modelling methods for single-mode optical fibres, developed in the framework of the COST 217 project, are presented. Values of transmission parameters computed from four different fibre designs are compared with the experimental values obtained in a previous interlaboratory measurement campaign. The consistency of the results obtained with different calculation methods and the accuracy of theoretical predictions are analysed for every one of the usual characterisation parameters of the fibres. The accuracy of the refractive index profile data appears to be the main factor affecting the agreement between theoretically predicted and measured values of the transmission parameters.Mon, 30 Jan 2017 14:19:41 +0100Second order polarization mode dispersionhttps://archive-ouverte.unige.ch/unige:91325https://archive-ouverte.unige.ch/unige:91325abstract not availableWed, 25 Jan 2017 15:02:45 +0100Polarization mode dispersion analyses with polarization OTDRshttps://archive-ouverte.unige.ch/unige:91292https://archive-ouverte.unige.ch/unige:91292abstract not availableTue, 24 Jan 2017 12:55:29 +0100POTDR, depolarization and detection of sections with large PMDhttps://archive-ouverte.unige.ch/unige:91254https://archive-ouverte.unige.ch/unige:91254abstract not availableMon, 23 Jan 2017 14:25:55 +0100Multiplexed on-demand storage of polarization qubits in a crystalhttps://archive-ouverte.unige.ch/unige:90519https://archive-ouverte.unige.ch/unige:90519A long-lived and multimode quantum memory is a key component needed for the development of quantum communication. Here we present temporally multiplexed storage of five photonic polarization qubits encoded onto weak coherent states in a rare-earth-ion doped crystal. Using spin refocusing techniques we can preserve the qubits for more than half a millisecond. The temporal multiplexing allows us to increase the effective rate of the experiment by a factor of five, which emphasizes the importance of multimode storage for quantum communication. The fidelity upon retrieval is higher than the maximum classical fidelity achievable with qubits encoded onto single photons and we show that the memory fidelity is mainly limited by the memory signal-to-noise ratio. These results show the viability and versatility of long-lived, multimode quantum memories based on rare-earth-ion doped crystals.Mon, 19 Dec 2016 11:53:56 +0100Coherent Spin Control at the Quantum Level in an Ensemble-Based Optical Memoryhttps://archive-ouverte.unige.ch/unige:90518https://archive-ouverte.unige.ch/unige:90518Long-lived quantum memories are essential components of a long-standing goal of remote distribution of entanglement in quantum networks. These can be realized by storing the quantum states of light as single-spin excitations in atomic ensembles. However, spin states are often subjected to different dephasing processes that limit the storage time, which in principle could be overcome using spin-echo techniques. Theoretical studies suggest this to be challenging due to unavoidable spontaneous emission noise in ensemble-based quantum memories. Here, we demonstrate spin-echo manipulation of a mean spin excitation of 1 in a large solid-state ensemble, generated through storage of a weak optical pulse. After a storage time of about 1 ms we optically read-out the spin excitation with a high signal-to-noise ratio. Our results pave the way for long-duration optical quantum storage using spin-echo techniques for any ensemble-based memory.Mon, 19 Dec 2016 11:51:42 +0100Temporal Multimode Storage of Entangled Photon Pairshttps://archive-ouverte.unige.ch/unige:90247https://archive-ouverte.unige.ch/unige:90247Multiplexed quantum memories capable of storing and processing entangled photons are essential for the development of quantum networks. In this context, we demonstrate and certify the simultaneous storage and retrieval of two entangled photons inside a solid-state quantum memory and measure a temporal multimode capacity of ten modes. This is achieved by producing two polarization-entangled pairs from parametric down-conversion and mapping one photon of each pair onto a rare-earth-ion-doped (REID) crystal using the atomic frequency comb (AFC) protocol. We develop a concept of indirect entanglement witnesses, which can be used as Schmidt number witnesses, and we use it to experimentally certify the presence of more than one entangled pair retrieved from the quantum memory. Our work puts forward REID-AFC as a platform compatible with temporal multiplexing of several entangled photon pairs along with a new entanglement certification method, useful for the characterization of multiplexed quantum memories.Mon, 12 Dec 2016 12:03:46 +0100Demonstration of Light-Matter Micro-Macro Quantum Correlationshttps://archive-ouverte.unige.ch/unige:89853https://archive-ouverte.unige.ch/unige:89853Quantum mechanics predicts microscopic phenomena with undeniable success. Nevertheless, current theoretical and experimental efforts still do not yield conclusive evidence that there is or is not a fundamental limitation on the possibility to observe quantum phenomena at the macroscopic scale. This question prompted several experimental efforts producing quantum superpositions of large quantum states in light or matter.We report on the observation of quantum correlations, revealed using an entanglement witness, between a single photon and an atomic ensemble of billions of ions frozen in a crystal. The matter part of the state involves the superposition of two macroscopically distinguishable solid-state components composed of several tens of atomic excitations. Assuming the insignificance of the time ordering our experiment indirectly shows lightmatter micro-macro entanglement. Our approach leverages from quantum memory techniques and could be used in other systems to expand the size of quantum superpositions in matter.Mon, 05 Dec 2016 11:36:19 +0100Storage of hyperentanglement in a solid-state quantum memoryhttps://archive-ouverte.unige.ch/unige:89846https://archive-ouverte.unige.ch/unige:89846Two photons can simultaneously share entanglement between several degrees of freedom such as polarization, energy-time, spatial mode and orbital angular momentum. This resource is known as hyperentanglement, and it has been shown to be an important tool for optical quantum information processing. Here we demonstrate the quantum storage and retrieval of photonic hyperentanglement in a solid-state quantum memory. A pair of photons entangled in polarization and energy-time is generated such that one photon is stored in the quantum memory, while the other photon has a telecommunication wavelength suitable for transmission in optical fibre. We measured violations of a Clauser-Horne-Shimony-Holt (CHSH) Bell inequality for each degree of freedom, independently of the other one, which proves the successful storage and retrieval of the two bits of entanglement shared by the photons. Our scheme is compatible with long-distance quantum communication in optical fibre, and is in particular suitable for linear-optical entanglement purification for quantum repeaters.Mon, 05 Dec 2016 11:14:24 +0100Exploring storage capability of a solid-state quantum memory for lighthttps://archive-ouverte.unige.ch/unige:89502https://archive-ouverte.unige.ch/unige:89502The present thesis includes a series of experiments exploring the potential of using rare-earth ion-doped crystals as a light-matter quantum interface using different quantum states of light as a probe resource. We start with the realization of an elementary quantum repeater block, implementing quantum teleportation of a polarization qubit into a solid-state quantum memory based on a yttrium-orthosilicate crystal doped with neodymium ions. The storage of an hyperentangled state of light compatible with long-distance quantum communication was demonstrated to show the potential of using this quantum memory for quantum purification techniques. To explore the temporal multiplexing capability of atomic frequency combs in atomic ensembles we performed temporal multimode storage of multi-photon entangled state of light. Such a temporal multimode capacity together with the developed novel theoretical techniques allowed us to show the storage of more than one bit of entanglement (ebit) using multi-photon and multi-dimensional entangled photon states.Mon, 28 Nov 2016 13:50:17 +0100Towards an understanding of correlations and measurements in quantum theoryhttps://archive-ouverte.unige.ch/unige:88734https://archive-ouverte.unige.ch/unige:88734Cette thèse porte sur des aspects fondamentaux de la mécanique quantique, dont le plus bouleversant est l'existence des corrélations non locales montrant que la nature défie le principe de localité. Un aspect moins étonnant mais tout aussi fascinant aborde la transition entre le monde classique et le monde quantique ou plus précisément la réconciliation de notre intuition avec la mécanique quantique. La première partie de ce travail est dédiée au phénomène de la non localité. Dans toute expérience physique, nous sommes confrontés à des pertes. Nous caractérisons ici la robustesse de la non localité des états de Dicke soumis aux pertes. Dans la deuxième partie, notre attention porte sur le processus de mesure pour de grands systèmes et sa description au sein du formalisme de la mécanique quantique.Mon, 07 Nov 2016 15:45:30 +0100La téléportation débarque en villehttps://archive-ouverte.unige.ch/unige:87758https://archive-ouverte.unige.ch/unige:87758Cryptographie. Deux expériences confirment l'intérêt grandissant de la téléportation quantique pour échanger des informations à distance en toute sécuritéMon, 26 Sep 2016 12:21:57 +0200Measurement dependence, limited detection and more: problems and applications of quantum nonlocalityhttps://archive-ouverte.unige.ch/unige:85244https://archive-ouverte.unige.ch/unige:85244The work in this thesis focuses on the subject of quantum nonlocality. The fact that the principle of locality is violated in nature is among the most counter-intuitive revelations of the past century. The original proof of the nonlocal nature of quantum mechanics relies on the so-called measurement independence assumption. This assumption has come under scrutiny in recent years, mostly because nonlocality and its implications have found their way into the realm of potential applications. In this thesis, the measurement independence assumption is relaxed and replaced by a form of limited measurement dependence. The resulting set is analyzed and it is shown that the nonlocal features of quantum mechanics resist arbitrary lack of free choice. Additionally, a new perspective on how to deal with the so-called detection loophole is provided by the definition of limited detection locality. Finally, work on applications of quantum nonlocality, specifically entanglement certification and quantification is presented.Wed, 13 Jul 2016 11:01:04 +0200Single photon interaction and entanglementhttps://archive-ouverte.unige.ch/unige:84495https://archive-ouverte.unige.ch/unige:84495Quantum mechanics not only revolutionized physics, but also provided an important tool allowing the development of unprecedented technology in the past century. Today, the quantum revolution continues to shape the development of new technologies through the fields of quantum information and communication. This thesis deals with the generation, manipulation and detection of quantum information using single photons. Creating single photons with precision, interacting individual photons and generating quantum entanglement are some of the questions we will deal with in detail.Mon, 13 Jun 2016 14:02:18 +0200Random Variation of Detector Efficiency: A Countermeasure Against Detector Blinding Attacks for Quantum Key Distributionhttps://archive-ouverte.unige.ch/unige:84117https://archive-ouverte.unige.ch/unige:84117In the recent decade, it has been discovered that QKD systems are extremely vulnerable to side-channel attacks. In particular, by exploiting the internal working knowledge of practical detectors, it is possible to bring them to an operating region whereby only certain target detectors are sensitive to detections. Crucially, the adversary can use this loophole to learn everything about the secret key without introducing any error to the quantum channel. In this paper, as a step toward overcoming detector blinding attacks, we focus on an experimentally convenient countermeasure, where the efficiency of the detectors is randomly varied.Wed, 01 Jun 2016 13:37:51 +0200Provably secure and practical quantum key distribution over 307 km of optical fibrehttps://archive-ouverte.unige.ch/unige:84116https://archive-ouverte.unige.ch/unige:84116Proposed in 1984, quantum key distribution (QKD) allows two users to exchange provably secure keys via a potentially insecure quantum channel1. Since then, QKD has attracted much attention and significant progress has been made both in theory and practice2, 3. On the application front, however, the operating distance of practical fibre-based QKD systems is limited to about 150 km (ref. 4), mainly due to the high background noise of practical single-photon detectors5, 6 and inefficient finite-key security analysis7, 8, 9. Here, we present, for the first time, a compact and autonomous QKD system that is capable of distributing provably secure cryptographic keys over 307 km of optical fibre. This is achieved by using semiconductor single-photon detectors with record low background noise10 and a novel finite-key security analysis, which is efficient even for short key lengths. This demonstrates the feasibility of practical long-distance QKD based on standard fibre-optic telecom components.Wed, 01 Jun 2016 13:30:17 +0200Quantum light : The puzzling predictions of entanglement are coming of agehttps://archive-ouverte.unige.ch/unige:84115https://archive-ouverte.unige.ch/unige:84115Entanglement is the physical property that marks the most striking deviation of the quantum from the classical world. It has been mentioned first by the great Austrian Physicist Erwin Schr¨odinger in 1935 (an introduction to this and other quantum phenomena is given in [1]). Yet, despite this theoretical prediction now being 80 years old, and the famous experimental verifications by Alain Aspect dating back to the early eighties [2], entanglement and its use entered mainstream physics as a key element of quantum information science [3] only in the 1990’s.Wed, 01 Jun 2016 13:27:33 +0200Nonlocality of W and Dicke states subject to losseshttps://archive-ouverte.unige.ch/unige:84114https://archive-ouverte.unige.ch/unige:84114We discuss the nonlocality of W and Dicke states subject to losses. We consider two noise models, namely, loss of excitations and loss of particles, and investigate how much loss can be tolerated such that the final state remains nonlocal. This leads to a measure of robustness of the nonlocality of Dicke states, with a clear physical interpretation. Our results suggest that the relation between nonlocality and entanglement of Dicke states is not monotonic.Wed, 01 Jun 2016 13:24:24 +0200How far can one send a photon?https://archive-ouverte.unige.ch/unige:84113https://archive-ouverte.unige.ch/unige:84113The answer to the question How far can one send a photon? depends heavily on what one means by a photon and on what one intends to do with that photon. For direct quantum communication, the limit is approximately 500 km. For terrestrial quantum communication, near-future technologies based on quantum teleportation and quantum memories will soon enable quantum repeaters that will turn the development of a world-wide-quantum-web (WWQW) into a highly non-trivial engineering problem. For Device-Independent Quantum Information Processing, near-future qubit amplifiers (i.e., probabilistic heralded amplification of the probability amplitude of the presence of photonic qubits) will soon allow demonstrations over a few tens of kilometers.Wed, 01 Jun 2016 13:21:50 +0200Quantum memory for photonshttps://archive-ouverte.unige.ch/unige:84112https://archive-ouverte.unige.ch/unige:84112The quantum state of a photon can be transferred to a single trapped atom or to a bunch of atoms in a gas or solid and be stored for later release on demand.Wed, 01 Jun 2016 13:20:17 +0200Revealing entanglement and nonlocality in few photon stateshttps://archive-ouverte.unige.ch/unige:83979https://archive-ouverte.unige.ch/unige:83979In the past two decades, the union of quantum mechanics and information science has led to significant progress in our understanding of the quantum world and in our ability to coherently control individual quantum systems. New technologies have appeared, and some of them, such as quantum key distribution (QKD), are now commercially available. We are now envisioning quantum networks that could allow us to communicate over intercontinental scales, as the internet does, but with a provable security. An open question in this framework is how to certify that a given quantum network performs well, i.e. how to detect entanglement between 2, 5, 10 or 100 nodes located far away from each other. Bell tests are appealing for such a task as they allow one to certify the presence of entanglement deviceindependently, i.e. without assumptions on the Hilbert space dimension or on the proper alignment of the measurement devices. In the first chapter, I start by examining in depth the most standard approach to perform a Bell test in which a source based on spontaneous parametric down conversion (SPDC) is used to create photon pairs entangled in polarization. I show, in particular, that in this case the Bell inequality violation is fundamentally limited by vacuum and multi-photon emissions. In the second part of this thesis, I show how to overcome this limitation by using the SPDC source to create single photon entanglement, i.e. entanglement between two paths sharing a single photon, or two-mode squeezed states. Here, the non local content of the states is revealed using photon counting techniques preceded by small displacements in phase space. In the third part, I show how these measurements can be used for an optomechanical Bell test. This opens the way for device-independent quantum information processing with optomechanical systems and might be relevant to test collapse models in a theory independent manner. In the fourth chapter, the same measurements are used to build up a witness that can reveal genuine path entanglement. This witness has triggered an experiment which highlights its potential for network certification. I conclude the thesis by showing how path entanglement can be revealed with the human eye by exploiting these displacement operations.Mon, 30 May 2016 13:34:26 +0200Quantum Nonlocality with Arbitrary Limited Detection Efficiencyhttps://archive-ouverte.unige.ch/unige:83949https://archive-ouverte.unige.ch/unige:83949The demonstration and use of nonlocality, as defined by Bell's theorem, rely strongly on dealing with non-detection events due to losses and detectors' inefficiencies. Otherwise, the so-called detection loophole could be exploited. The only way to avoid this is to have detection efficiencies that are above a certain threshold. We introduce the intermediate assumption of limited detection efficiency, that is, in each run of the experiment, the overall detection efficiency is lower bounded by $eta_{min} > 0$. Hence, in an adversarial scenario, the adversaries have arbitrary large but not full control over the inefficiencies. We analyse the set of possible correlations that fulfill Limited Detection Locality (LDL) and show that they necessarily satisfy some linear Bell-like inequalities. We prove that quantum theory predicts the violation of one of these inequalities for all $eta_{min} > 0$. Hence, nonlocality can be demonstrated with arbitrarily small limited detection efficiencies. We validate this assumption experimentally via a twin-photon implementation in which two users are provided with one photon each out of a partially entangled pair. We exploit on each side a passive switch followed by two measurement devices with fixed settings. Assuming the switches are not fully controlled by an adversary, nor by hypothetical local variables, we reveal the nonlocality of the established correlations despite a low overall detection efficiency.Mon, 30 May 2016 10:03:59 +0200Comparing different approaches for generating random numbers device-independently using a photon pair sourcehttps://archive-ouverte.unige.ch/unige:83946https://archive-ouverte.unige.ch/unige:83946What is the most efficient way to generate random numbers device-independently using a photon pair source based on spontaneous parametric down conversion? We consider this question by comparing two implementations of a detection-loophole-free Bell test. In particular, we study in detail a scenario where a source is used to herald path-entangled states, i.e. entanglement between two spatial modes sharing a single photon and where non-locality is revealed using photon counting preceded by small displacement operations. We start by giving a theoretical description of such a measurement. We then show how to optimize the Bell–CHSH violation through a non-perturbative calculation, taking the main experimental imperfections into account. We finally bound the amount of randomness that can be extracted and compare it to the one obtained with the conventional scenario using photon pairs entangled e.g. in polarization and analyzed through photon counting. While the former requires higher overall detection efficiencies, it is far more efficient in terms of the entropy per experimental run and under reasonable assumptions, it provides higher random bit rates.Mon, 30 May 2016 09:58:38 +0200