Before global-scale quantum networks become operational, it is important to consider how to evaluate their performance so that they can be suitably built to achieve the desired performance. In this work, we consider three figures of merit for the performance of a quantum network: the average global connection time, the average point-to-point connection time, and the average largest entanglement cluster size. These three quantities are based on the generation of elementary links in a quantum network, which is a crucial initial requirement that must be met before any long-range entanglement distribution can be achieved. We evaluate these figures of merit for a particular class of quantum repeater protocols consisting of repeat-until-success elementary link generation along with entanglement swapping at intermediate nodes in order to achieve long-range entanglement. We obtain lower and upper bounds on these three quantities, which lead to requirements on quantum memory coherence times and other aspects of quantum network implementations. Our bounds are based solely on the inherently probabilistic nature of elementary link generation in quantum networks, and they apply to networks with arbitrary topology. CONTENTS

It was suggested in Ref. [Phys. Rev. Lett. 114, 170802] that optical networks with relatively inexpensive overhead-single photon Fock states, passive optical elements, and single photon detection-can show significant improvements over classical strategies for single-parameter estimation, when the number of modes in the network is small (n < 7). A similar case was made in Ref. [Phys. Rev. Lett. 111, 070403] for multi-parameter estimation, where measurement is instead made using photon-number resolving detectors. In this paper, we analytically compute the quantum Cramér-Rao bound to show these networks can have a constant-factor quantum advantage in multi-parameter estimation for even large number of modes. Additionally, we provide a simplified measurement scheme using only single-photon (on-off) detectors that is capable of approximately obtaining this sensitivity for a small number of modes.

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We present aperiodic multilayer structures with ultrabroadband near-perfect absorption in the visible and near-infrared wavelength range. We use a hybrid optimization algorithm coupled with the transfer-matrix method, to optimize both the material composition and the layer thicknesses of the aperiodic multilayer structures that are composed of infinite slabs of material above a semi-infinite substrate. In order to achieve ultrabroadband nearperfect absorption, we consider a broad range of materials including dielectrics, metals, and semiconductors. The optimization algorithms previously used to design ultrabroadband near-perfect absorbers only optimized the layer thicknesses of structures with fixed material composition. In contrast, we find that our approach of simultaneously optimizing the material composition as well as the layer thicknesses leads to structures with broader near-perfect absorption. For an optimized eleven-layer structure the lower and upper absorption band edges are 400 nm and ∼3800 nm, respectively. In addition, we find that, even though the structures are optimized for normally incident light, the absorption is high in a broad angular range within the wavelength range of interest. We also explain the physical origin of ultrabroadband absorption in these structures. Our results will contribute to the development of a new generation of devices for solar photovoltaics, imaging, and photodetection.

It was previously shown that optical networks using only single photon Fock states, passive linear optics, and single photon detection could achieve post-classical sensitivity for singleparameter estimation when the mode number is small. [1] Furthermore, it was shown that multiparameter estimation could achieve similar results with number-resolving detection. [2] Here, we consider an analogous architecture to the Quantum Fourier Transform Interferometer(QuFTI) proposed in [1] for the estimation of multiple phases simultaneously. Thus, we will see that this yields post-classical sensitivity for multiparameter estimation even for an asymptotically large number of modes. This system is also considered for nondeterministic photon sources and a variety of measurement schemes.