Recursive quantum repeater networks

Internet-scale quantum repeater networks will be heterogeneous in physical technology, repeater functionality, and management. The classical control necessary to use the network will therefore face similar issues as Internet data transmission. Many scalability and management problems that arose during the development of the Internet might have been solved in a more uniform fashion, improving flexibility and reducing redundant engineering effort. Quantum repeater network development is currently at the stage where we risk similar duplication when separate systems are combined. We propose a unifying framework that can be used with all existing repeater designs. We introduce the notion of a Quantum Recursive Network Architecture, developed from the emerging classical concept of 'recursive networks', extending recursive mechanisms from a focus on data forwarding to a more general distributed computing request framework. Recursion abstracts independent transit networks as single relay nodes, unifies software layering, and virtualizes the addresses of resources to improve information hiding and resource management. Our architecture is useful for building arbitrary distributed states, including fundamental distributed states such as Bell pairs and GHZ, W, and cluster states.Comment: 14 page

Ultrastrong light-matter interaction in quantum technologies

120 p.En esta Tesis, hemos analizado teóricamente fenómenos cuánticos novedosos emergentes en el régimenultrafuerte (USC) de interacción entre luz y materia. Nos hemos enfocado en procesos que pueden serobservados usando tecnologías cuánticas actuales o que puedan motivar desarrollos tecnológicospróximos. Los resultados presentados en esta Tesis pueden ser agrupados en dos categorías. Por un lado,hemos estudiado modelos que pueden ser implementados usando circuitos superconductores, unaplataforma cuántica donde el régimen USC puede ser alcanzado de forma natural. En especifico, hemosestudiado la transferencia de excitaciones en cadenas de resonadores cuánticos, generación deentrelazamiento cuántico vía efecto Casimir dinámico y ingeniería de estados cuánticos en el régimenUSC. Por otro lado, hemos desarrollado métodos para reproducir la física de la interacción ultrafuerte ensistemas de iones atrapados y de átomos fríos en retículos ópticos. Esas propuestas aprovechan lasventajas de cada plataforma cuántica para alcanzar regímenes de parámetros e implementar medidas queno son accesibles en implementaciones naturales de los procesos considerados.Los resultados presentado en esta Tesis profundizan nuestra comprensión de fenómenos cuánticosrelativos al régimen ultrafuerte de interacción entre luz y materia. Además, nuestras propuestas aspiran apromover el desarrollo conjunto de herramientas teóricas y técnicas experimentales en esta área deinvestigació

Room temperature caesium quantum memory for quantum information applications

Quantum memories are key components in quantum information networks. Their ability to store and retrieve information on demand makes repeat-until-success strategies scalable. Warm alkali-metal vapours are interesting candidates for the implementation of such memories, thanks to their long storage times and experimental simplicity. Operation with the Raman protocol enables high time-bandwidth products, which allows for multiple synchronisation trials of probabilistically operating quantum gates via memory-based temporal multiplexing. This makes the Raman memory a promising tool, whose broad spectral bandwidth facilitates direct interfacing with other photonic primitives, such as single photon sources. Here, such a light-matter interface is implemented in a warm caesium vapour. Firstly, we study the storage of polarisation-encoded information in the memory. High quality polarisation preservation for bright coherent state input signals can be achieved, when operating the Raman memory in a dual-rail configuration inside a polarisation interferometer. Secondly, heralded single photons are stored in the memory. To this end, the memory is operated on-demand by feed-forward of source heralding events, which is a key technological capability. Prior to storage, single photons are produced in a spontaneous parametric down conversion source, whose bespoke design spectrally tailors the photons to the memory acceptance line. The faithful retrieval of stored single photons is found to be currently limited by noise in the memory, with a signal-to-noise ratio of 0.3 in the memory output. Nevertheless, a clear influence of the input's quantum nature is observed in the retrieved light by measuring signal's photon statistics. Finally, the memory noise processes are examined in detail. Four-wave-mixing noise is determined as the sole important noise source for the Raman memory

Quantum NETwork: from theory to practice

The quantum internet is envisioned as the ultimate stage of the quantum revolution, which surpasses its classical counterpart in various aspects, such as the efficiency of data transmission, the security of network services, and the capability of information processing. Given its disruptive impact on the national security and the digital economy, a global race to build scalable quantum networks has already begun. With the joint effort of national governments, industrial participants and research institutes, the development of quantum networks has advanced rapidly in recent years, bringing the first primitive quantum networks within reach. In this work, we aim to provide an up-to-date review of the field of quantum networks from both theoretical and experimental perspectives, contributing to a better understanding of the building blocks required for the establishment of a global quantum internet. We also introduce a newly developed quantum network toolkit to facilitate the exploration and evaluation of innovative ideas. Particularly, it provides dual quantum computing engines, supporting simulations in both the quantum circuit and measurement-based models. It also includes a compilation scheme for mapping quantum network protocols onto quantum circuits, enabling their emulations on real-world quantum hardware devices. We showcase the power of this toolkit with several featured demonstrations, including a simulation of the Micius quantum satellite experiment, a testing of a four-layer quantum network architecture with resource management, and a quantum emulation of the CHSH game. We hope this work can give a better understanding of the state-of-the-art development of quantum networks and provide the necessary tools to make further contributions along the way.Comment: 36 pages, 33 figures; comments are welcom

Architectures for photon-mediated quantum information processing

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 173-186).In this thesis, I present architectures for quantum information processing where photons are used as the quantum bit (qubit) or for mediating entanglement between other qubits. The emphasis of this research is to simplify the basic building blocks required in such processors. The all-photonic repeater and computing architectures do not require material nonlinearities, and their resource requirements are reduced by several orders of magnitude. The photon-mediated atomic memory architecture is designed to work with faulty memories and experimentally demonstrated values of coherence time and photonic coupling efficiency. In the quantum network architecture, the only operation at every node is probabilistic Bell measurement.by Mihir Pant.Ph. D

FGQT Q04 - Standardization Roadmap on Quantum Technologies [written by the CEN-CENELEC Focus Group on Quantum Technologies (FGQT)]

In 2018, the European Commission launched its long term and large scale Quantum Technology FET Flagship Program. The European Commission is also very interested in boosting standards for quantum technologies (QT). The Quantum Flagship has its own cooperation and coordination activities to “coordinate national strategies and activities” and in its “Quantum Manifesto” [1] explicitly advises to form “advisory boards” to promote collaboration in standardization. The CEN/CENELEC Focus Group for Quantum Technologies (FGQT) was formed in June 2020 with the goal to support the plans of the Commission. Currently, a multitude of standardization activities in QT are ongoing worldwide. While there is overlap in certain areas, other areas of this wide technological field are not being addressed at all. A coordinated approach will be highly beneficial to unleash the full potential of standardization for speeding up progress—also because the pool of standardization experts available for quantum technologies is still very limited. Furthermore, not all areas are yet “ready for standardization”, i.e., while in some fields early standardization is capable of boosting progress, it may be a problem in other areas. Thus, an assessment of standardization readiness of the different areas is required, too. The FGQT was established to identify standardization needs and opportunities for the entire field of QT with the final goal to boost the establishment of new industries in Europe and consequently the development and engineering of unprecedented novel devices and infrastructures for the benefit of European citizens. The QT standardization roadmap follows a constructive approach, starting with basic enabling technologies, from which QT components and subsystems are constructed, which again are assembled into QT systems that in turn form composite systems, constituting the building blocks for use cases. Thus, the roadmap is structured approximating very closely the categories of the EC quantum technology FET Flagship Program: quantum communication, quantum computing and simulation, quantum metrology, sensing, and enhanced imaging, while the basic enabling technologies and sub-systems are organized in two pools —thus supporting re-use in the different system categories. The separate types of QT unit systems are then foundations of general QT infrastructures or composite systems. On the level of use cases, the QT standardization roadmap describes basic domains of applicability, so-called “meta use cases”, while the detailed use cases are listed in a separate document of the FGQT: “FGQT Q05 Use Cases”. Finally, the QT standardization roadmap presents an outlook and conclusions, including an actual prioritization of the single identified standardization needs in the form of sequence diagrams (Gantt charts). This approach differs slightly from the QT “Pillar design” of the EU Quantum Flagship but, in our opinion, it extends it and is better adapted to standardization purposes, while the former is optimally suited as a research program design. The FGQT is an open group of European-based experts, working in QT research areas or enabling technologies, and of developers of components, products, or services related to QT. If you are based in Europe, and interested in guidelines and standards to help setting up a research infrastructure, or structuring and boosting your market relevance; if you want to improve coordination with your stakeholders and are interested in coordination and exchange with other experts in the field of QT—please consider to join the CEN/CENELEC FGQT. NOTE 1 European QT standards development in CEN/CENELEC will take place in the new JTC 22 QT (Joint Technical Committee 22 on Quantum Technologies). The work in JTC 22 QT will be guided by the present roadmap doc ument, and it is expected that the FGQT roadmap-development activity will be absorbed/continued by JTC 22 Q

Superconducting flux qubits for high-connectivity quantum annealing without lossy dielectrics

Quantum annealing can potentially be used to find better solutions to hard optimization problems faster than purely classical hardware. To take full advantage of quantum effects such as tunneling, a physical annealer should be comprised of qubits with a sufficient degree of quantum coherence. In addition, to encode useful problems, an annealer should provide a dense physical connectivity graph between qubits. Towards these goals, we develop superconducting ``fluxmon'' flux qubits suitable for high-connectivity quantum annealing without the use of performance-degrading lossy dielectrics. We carry out in-depth studies of noise and dissipation, and of qubit-qubit coupling in the strongly nonlinear regime. We perform the first frequency-resolved measurements extracting both the quantum and classical parts of the 1/f flux noise intrinsic to superconducting devices, and observe the classical-to-quantum crossover of the noise. We also identify atomic hydrogen as a magnetic dissipation source. We then implement tunable inter-qubit coupling compatible with high connectivity, and provide direct spectroscopic measurements of ultra-strong coupling between qubits. Finally, we use our system to explore quantum annealing faster than the system thermalization time, a previously unaccessed regime

Improving Quantum Key Distribution and Quantum Random Number Generation in presence of Noise

The argument of this thesis might be summed up as the exploitation of the noise to generate better noise. More specifically this work is about the possibility of exploiting classic noise to effectively transmit quantum information and measuring quantum noise to generate better quantum randomness. What do i mean by exploiting classical noise to transmit effectively quantum information? In this case I refer to the task of sending quantum bits through the atmosphere in order set up transmissions of quantum key distribution (QKD) and this will be the subject of Chapter 1 and Chapter 2. In the Quantum Communications framework, QKD represents a topic with challenging problems both theoretical and experimental. In principle QKD offers unconditional security, however practical realizations of it must face all the limitations of the real world. One of the main limitation are the losses introduced by real transmission channels. Losses cause errors and errors make the protocol less secure because an eavesdropper could try to hide his activity behind the losses. When this problem is addressed under a full theoretical point of view, one tries to model the effect of losses by means of unitary transforms which affect the qubits in average according a fixed level of link attenuation. However this approach is somehow limiting because if one has a high level of background noise and the losses are assumed in average constant, it could happen that the protocol might abort or not even start, being the predicted QBER to high. To address this problem and generate key when normally it would not be possible, we have proposed an adaptive real time selection (ARTS) scheme where transmissivity peaks are instantaneously detected. In fact, an additional resource may be introduced to estimate the link transmissivity in its intrinsic time scale with the use of an auxiliary classical laser beam co-propagating with the qubits but conveniently interleaved in time. In this way the link scintillation is monitored in real time and the selection of the time intervals of high channel transmissivity corresponding to a viable QBER for a positive key generation is made available. In Chapter 2 we present a demonstration of this protocol in conditions of losses equivalent to long distance and satellite links, and with a range of scintillation corresponding to moderate to severe weather. A useful criterion for the preselection of the low QBER interval is presented that employs a train of intense pulses propagating in the same path as the qubits, with parameters chosen such that its fluctuation in time reproduces that of the quantum communication. For what concern the content of Chapter 3 we describe a novel principle for true random number generator (TRNG) which is based on the observation that a coherent beam of light crossing a very long path with atmospheric turbulence may generate random and rapidly varying images. To implement our method in a proof of concept demonstrator, we have chosen a very long free space channel used in the last years for experiments in Quantum Communications at the Canary Islands. Here, after a propagation of 143 km at an altitude of the terminals of about 2400 m, the turbulence in the path is converted into a dynamical speckle at the receiver. The source of entropy is then the atmospheric turbulence. Indeed, for such a long path, a solution of the Navier-Stokes equations for the {atmospheric flow in which the beam propagates is out of reach. Several models are based on the Kolmogorov statistical theory, which parametrizes the repartition of kinetic energy as the interaction of decreasing size eddies. However, such models only provide a statistical description for the spot of the beam and its wandering and never an instantaneous prediction for the irradiance distribution. These are mainly ruled by temperature variations and by the wind and cause fluctuations in the air refractive index. For such reason, when a laser beam is sent across the atmosphere, this latter may be considered as a dynamic volumetric scatterer which distorts the beam wavefront. We will evaluate the experimental data to ensure that the images are uniform and independent. Moreover, we will assess that our method for the randomness extraction based on the combinatorial analysis is optimal in the context of Information Theory. In Chapter 5 we will present a new approach for what concerns the generation of random bits from quantum physical processes. Quantum Mechanics has been always regarded as a possible and valuable source of randomness, because of its intrinsic probabilistic Nature. However the typical paradigm is employed to extract random number from a quantum system it commonly assumes that the state of said system is pure. Such assumption, only in theory would lead to full and unpredictable randomness. The main issue however it is that in real implementations, such as in a laboratory or in some commercial device, it is hardly possible to forge a pure quantum state. One has then to deal with quantum state featuring some degree of mixedness. A mixed state however might be somehow correlated with some other system which is hold by an adversary, a quantum eavesdropper. In the extreme case of a full mixed state, practically one it is like if he is extracting random numbers from a classical state. In order to do that we will show how it is important to shift from a classical randomness estimator, such as the min-classical entropy H-min(Z) of a random variable Z to quantum ones such as the min-entropy conditioned on quantum side information E. We have devised an effective protocol based on the entropic uncertainty principle for the estimation of the min-conditional entropy. The entropic uncertainty principle lets one to take in account the information which is shared between multiple parties holding a multipartite quantum system and, more importantly, lets one to bound the information a party has on the system state after that it has been measured. We adapted such principle to the bipartite case where an user Alice, A, is supplied with a quantum system prepared by the provider Eve, E, who could be maliciously correlated to it. In principle then Eve might be able to predict all the outcomes of the measurements Alice performs on the basis Z in order to extract random numbers from the system. However we will show that if Alice randomly switches from the measurement basis to a basis X mutually unbiased to Z, she can lower bound the min entropy conditioned to the side information of Eve. In this way for Alice is possible to expand a small initial random seed in a much larger amount of trusted numbers. We present the results of an experimental demonstration of the protocol where random numbers passing the most rigorous classical tests of randomness were produced. In Chapter 6, we will provide a secure generation scheme for a continuos variable (CV) QRNG. Since random true random numbers are an invaluable resource for both the classical Information Technology and the uprising Quantum one, it is clear that to sustain the present and future even growing fluxes of data to encrypt it is necessary to devise quantum random number generators able to generate numbers in the rate of Gigabit or Terabit per second. In the Literature are given several examples of QRNG protocols which in theory could reach such limits. Typically, these are based on the exploitation of the quadratures of the electro-magnetic field, regarded as an infinite bosonic quantum system. The quadratures of the field can be measured with a well known measurement scheme, the so called homodyne detection scheme which, in principle, can yield an infinite band noise. Consequently the band of the random signal is limited only by the passband of the devices used to measure it. Photodiodes detectors work commonly in the GHz band, so if one sample the signal with an ADC enough fast, the Gigabit or Terabit rates can be easily reached. However, as in the case of discrete variable QRNG, the protocols that one can find in the Literature, do not properly consider the purity of the quantum state being measured. The idea has been to extend the discrete variable protocol of the previous Chapter, to the Continuous case. We will show how in the CV framework, not only the problem of the state purity is given but also the problem related to the precision of the measurements used to extract the randomness