Time-reversal and super-resolving phase measurements

We demonstrate phase super-resolution in the absence of entangled states. The key insight is to use the inherent time-reversal symmetry of quantum mechanics: our theory shows that it is possible to \emph{measure}, as opposed to prepare, entangled states. Our approach is robust, requiring only photons that exhibit classical interference: we experimentally demonstrate high-visibility phase super-resolution with three, four, and six photons using a standard laser and photon counters. Our six-photon experiment demonstrates the best phase super-resolution yet reported with high visibility and resolution.Comment: 4 pages, 3 figure

Quantum computing on encrypted data

The ability to perform computations on encrypted data is a powerful tool for protecting privacy. Recently, protocols to achieve this on classical computing systems have been found. Here we present an efficient solution to the quantum analogue of this problem that enables arbitrary quantum computations to be carried out on encrypted quantum data. We prove that an untrusted server can implement a universal set of quantum gates on encrypted quantum bits (qubits) without learning any information about the inputs, while the client, knowing the decryption key, can easily decrypt the results of the computation. We experimentally demonstrate, using single photons and linear optics, the encryption and decryption scheme on a set of gates sufficient for arbitrary quantum computations. Because our protocol requires few extra resources compared to other schemes it can be easily incorporated into the design of future quantum servers. These results will play a key role in enabling the development of secure distributed quantum systems

Optical Quantum Computing

In 2001 all-optical quantum computing became feasible with the discovery that scalable quantum computing is possible using only single photon sources, linear optical elements, and single photon detectors. Although it was in principle scalable, the massive resource overhead made the scheme practically daunting. However, several simplifications were followed by proof-of-principle demonstrations, and recent approaches based on cluster states or error encoding have dramatically reduced this worrying resource overhead, making an all-optical architecture a serious contender for the ultimate goal of a large-scale quantum computer. Key challenges will be the realization of high-efficiency sources of indistinguishable single photons, low-loss, scalable optical circuits, high efficiency single photon detectors, and low-loss interfacing of these components.Comment: 5 pages, 4 figure

Demonstration of a simple entangling optical gate and its use in Bell-state analysis

We demonstrate a new architecture for an optical entangling gate that is significantly simpler than previous realisations, using partially-polarising beamsplitters so that only a single optical mode-matching condition is required. We demonstrate operation of a controlled-Z gate in both continuous-wave and pulsed regimes of operation, fully characterising it in each case using quantum process tomography. We also demonstrate a fully-resolving, nondeterministic optical Bell-state analyser based on this controlled-Z gate. This new architecture is ideally suited to guided optics implementations of optical gates.Comment: 4 pages, 3 figures. v2: additional author, improved data and figures (low res), some other minor changes. Accepted for publication in PR

A simple scheme for expanding photonic cluster states for quantum information

We show how an entangled cluster state encoded in the polarization of single photons can be straightforwardly expanded by deterministically entangling additional qubits encoded in the path degree of freedom of the constituent photons. This can be achieved using a polarization--path controlled-phase gate. We experimentally demonstrate a practical and stable realization of this approach by using a Sagnac interferometer to entangle a path qubit and polarization qubit on a single photon. We demonstrate precise control over phase of the path qubit to change the measurement basis and experimentally demonstrate properties of measurement-based quantum computing using a 2 photon, 3 qubit cluster state

Bayesian Nash Equilibria and Bell Inequalities

Games with incomplete information are formulated in a multi-sector probability matrix formalism that can cope with quantum as well as classical strategies. An analysis of classical and quantum strategy in a multi-sector extension of the game of Battle of Sexes clarifies the two distinct roles of nonlocal strategies, and establish the direct link between the true quantum gain of game's payoff and the breaking of Bell inequalities.Comment: 6 pages, LaTeX JPSJ 2 column format, changes in sections 1, 3 and 4, added reference

Demonstration of Controllable Temporal Distinguishability in a Three-Photon State

Multi-photon interference is at the heart of the recently proposed linear optical quantum computing scheme and plays an essential role in many protocols in quantum information. Indistinguishability is what leads to the effect of quantum interference. Optical interferometers such as Michaelson interferometer provide a measure for second-order coherence at one-photon level and Hong-Ou-Mandel interferometer was widely employed to describe two-photon entanglement and indistinguishability. However, there is not an effective way for a system of more than two photons. Recently, a new interferometric scheme was proposed to quantify the degree of multi-photon distinguishability. Here we report an experiment to implement the scheme for three-photon case. We are able to generate three photons with different degrees of temporal distinguishability and demonstrate how to characterize them by the visibility of three-photon interference. This method of quantitative description of multi-photon indistinguishability will have practical implications in the implementation of quantum information protocols

Seleção de uma espécie embaixadora para educação ambiental e conservação da conectividade da paisagem

In this article we review the role of Chrysocyon brachyurus (Class Mammalia; Order Carnivora; Family Canidae) as an "ambassador species" and we propose the possibility of its implementation to promote environmental education and manage the conservation of cultural landscapes in Argentina. The maned wolf is a native vertebrate of South America; it present biological characteristics (large body size), ecological (occupy a significant part of the national territory, inhabit in largely of the ecoregions and overlap their distribution with another 18 medium and large threatened mammals) and sociocultural (charismatic and not representing a threat for the inhabitants), allow to appoint it an ambassador species. This species, categorized since 2019 at the national level as Vulnerable, is the wild canid that has led the largest number of educational campaigns with multiple strategies implemented in the last 20 years. The ambassador figure could become a very powerful tool in large-scale environmental education proposals.  En el presente artículo revisamos el rol de Chrysocyon brachyurus (Clase Mammalia; Orden Carnivora; Familia Canidae) como “especie embajadora” y proponemos la posibilidad de su implementación para impulsar la educación ambiental y gestionar la conservación de los paisajes culturales de la Argentina. El aguará guazú es un vertebrado nativo de América del Sur; presenta características biológicas (tamaño corporal grande), ecológicas (ocupa una parte importante del territorio nacional, habita en gran parte de las ecorregiones y superpone su distribución con otros 18 mamíferos medianos y grandes amenazados) y socioculturales (carismático y no representa una amenaza para los habitantes), permite designarla especie embajadora. Esta especie, categorizada desde el 2019 a nivel nacional como Vulnerable, es el cánido silvestre que ha encabezado el mayor número de campañas educativas con múltiples estrategias, implementadas en los últimos 20 años. La figura de embajador podría constituirse en una herramienta muy poderosa en propuestas de educación ambiental a gran escala.Neste artigo revisamos o papel de Chrysocyon brachyurus (Classe Mamífero; Ordem Carnívora; Família Canídeo) como “espécie embaixadora” e propomos a possibilidade de sua implementação para promover a educação ambiental e gerenciar a conservação de paisagens culturais na Argentina. O lobo-guará é um vertebrado autóctone da América do Sul; devido às suas características biológicas (grande porte corporal), características ecológicas (ocupando grande parte do território nacional, habita a maioria das ecorregiões, sobrepondo sua distribuição com 18 mamíferos de médio e grande porte ameaçados) e socioculturais (ser carismático e não representa uma ameaça para os habitantes), que favorecem propô-la como espécie embaixadora da gestão ambiental. Este canídeo, categorizado desde 2019 a nível nacional como vulnerável, é o canídeo selvagem que liderou o maior número de campanhas educativas com múltiplas estratégias, implementadas nos últimos 20 anos. Essa figura do embaixador pode se tornar uma ferramenta muito poderosa em propostas de educação ambiental em grande escala. &nbsp

A Novel Clustering Algorithm Based on Quantum Games

Enormous successes have been made by quantum algorithms during the last decade. In this paper, we combine the quantum game with the problem of data clustering, and then develop a quantum-game-based clustering algorithm, in which data points in a dataset are considered as players who can make decisions and implement quantum strategies in quantum games. After each round of a quantum game, each player's expected payoff is calculated. Later, he uses a link-removing-and-rewiring (LRR) function to change his neighbors and adjust the strength of links connecting to them in order to maximize his payoff. Further, algorithms are discussed and analyzed in two cases of strategies, two payoff matrixes and two LRR functions. Consequently, the simulation results have demonstrated that data points in datasets are clustered reasonably and efficiently, and the clustering algorithms have fast rates of convergence. Moreover, the comparison with other algorithms also provides an indication of the effectiveness of the proposed approach.Comment: 19 pages, 5 figures, 5 table

Entanglement can increase asymptotic rates of zero-error classical communication over classical channels

It is known that the number of different classical messages which can be communicated with a single use of a classical channel with zero probability of decoding error can sometimes be increased by using entanglement shared between sender and receiver. It has been an open question to determine whether entanglement can ever increase the zero-error communication rates achievable in the limit of many channel uses. In this paper we show, by explicit examples, that entanglement can indeed increase asymptotic zero-error capacity, even to the extent that it is equal to the normal capacity of the channel. Interestingly, our examples are based on the exceptional simple root systems E7 and E8.Comment: 14 pages, 2 figur