Optimal continuous-variable teleportation under energy constraint

Quantum teleportation is one of the crucial protocols in quantum information processing. It is important to accomplish an efficient teleportation under practical conditions, aiming at a higher fidelity desirably using fewer resources. The continuous-variable (CV) version of quantum teleportation was first proposed using a Gaussian state as a quantum resource, while other attempts were also made to improve performance by applying non-Gaussian operations. We investigate the CV teleportation to find its ultimate fidelity under energy constraint identifying an optimal quantum state. For this purpose, we present a formalism to evaluate teleportation fidelity as an expectation value of an operator. Using this formalism, we prove that the optimal state must be a form of photon-number entangled states. We further show that Gaussian states are near-optimal while non-Gaussian states make a slight improvement and therefore are rigorously optimal, particularly in the low-energy regime.Comment: 8 pages, 4 figures, published versio

Gaussian benchmark for optical communication aiming towards ultimate capacity

We establish the fundamental limit of communication capacity within Gaussian schemes under phase-insensitive Gaussian channels, which employ multimode Gaussian states for encoding and collective Gaussian operations and measurements for decoding. We prove that this Gaussian capacity is additive, i.e., its upper bound occurs with separable encoding and separable receivers so that a single-mode communication suffices to achieve the largest capacity under Gaussian schemes. This rigorously characterizes the gap between the ultimate Holevo capacity and the capacity within Gaussian communication, showing that Gaussian regime is not sufficient to achieve the Holevo bound particularly in the low-photon regime. Furthermore the Gaussian benchmark established here can be used to critically assess the performance of non-Gaussian protocols for optical communication. We move on to identify non-Gaussian schemes to beat the Gaussian capacity and show that a non-Gaussian receiver recently implemented by Becerra et al. [Nat. Photon. 7, 147 (2013)] can achieve this aim with an appropriately chosen encoding strategy.Comment: 9 pages, 6 figures, with supplemental materia

Continuous-variable dense coding via a general Gaussian state: Monogamy relation

We study a continuous variable (CV) dense-coding protocol, originally proposed to employ a two-mode squeezed state, using a general two-mode Gaussian state as a quantum channel. We particularly obtain conditions to manifest quantum advantage by beating two well-known single-mode schemes, namely, the squeezed-state scheme (best Gaussian scheme) and the number-state scheme (optimal scheme achieving the Holevo bound). We then extend our study to a multipartite Gaussian state and investigate the monogamy of operational entanglement measured by the communication capacity under the dense-coding protocol. We show that this operational entanglement represents a strict monogamy relation, by means of Heisenberg's uncertainty principle among different parties, i.e., the quantum advantage for communication can be possible for only one pair of two-mode systems among many parties

Single-photon quantum nonlocality: Violation of the Clauser-Horne-Shimony-Holt inequality using feasible measurement setups

We investigate quantum nonlocality of a single-photon entangled state under feasible measurement techniques consisting of on-off and homodyne detections along with unitary operations of displacement and squeezing. We test for a potential violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality, in which each of the bipartite party has a freedom to choose between 2 measurement settings, each measurement yielding a binary outcome. We find that single-photon quantum nonlocality can be detected when two or less of the 4 total measurements are carried out by homodyne detection. The largest violation of the CHSH inequality is obtained when all four measurements are squeezed-and-displaced on-off detections. We test robustness of violations against imperfections in on-off detectors and single-photon sources, finding that the squeezed-and-displaced measurement schemes perform better than the displacement-only measurement schemes.Comment: 7+ pages, 7 figures, 1 table, close to published versio

Is the Repeal of Net Neutrality a Necessary Evil? An Empirical Analysis of Net Neutrality and Cybercrime

While net neutrality guarantees equal access to the Internet and online content, it serves as a limiting factor in identifying and tracking criminal activities in cyberspace by ensuring that data packet is transmitted with equal priority, irrespective of its source and content. Exploiting a natural experiment in which net neutrality policies were officially repealed in 2018 in the United States, this study examines the impact of net neutrality on the occurrence of cybercrime. Our findings suggest that the repeal of net neutrality is negatively associated with the occurrence of malicious code and content in an attempt to compromise computer systems (e.g., malware and ransomware). In contrast, we do not find any significant relationship with cybercrime victimization, and cybercrime that may subsequently occur in compromised systems (e.g., data breaches and denial-of-service attacks). This study provides novel insights into the role of net neutrality and open Internet toward the preventive cybersecurity paradigm

Returns to Cloud Computing Investments: The Role of Environmental Uncertainty

Although a substantial portion of IT spending has shifted to the cloud, empirical evidence on economic value of cloud computing is lacking. This study examines the effect of cloud computing on productivity and scrutinizes how its effect differs depending on environmental uncertainty. Using publicly available data on the product sales and the inter-industry purchase flows, we measure purchased cloud services in U.S. industries during 1997-2018 and distinguish between software-as-a-service (SaaS) and infrastructure-as-a-service (IaaS). Employing a production function approach, our findings suggest that cloud computing investments do not always lead to productivity gains, but its effect varies by the level of environmental uncertainty. Specifically, while cloud computing contributes to productivity under high environmental uncertainty, it may have an adverse effect under stable environments. Further, this positive impact under uncertain environments is found to be driven mainly by IaaS, rather than SaaS. This study provides important implications on cloud computing investment strategies

Classical capacity of Gaussian communication under a single noisy channel

A long-standing problem on the classical capacity of bosonic Gaussian channels has recently been resolved by proving the minimum output entropy conjecture. It is also known that the ultimate capacity quantified by the Holevo bound can be achieved asymptotically by using an infinite number of channels. However, it is less understood to what extent the communication capacity can be reached if one uses a finite number of channels, which is a topic of practical importance. In this paper, we study the capacity of Gaussian communication, i.e., employing Gaussian states and Gaussian measurements to encode and decode information under a single-channel use. We prove that the optimal capacity of single-channel Gaussian communication is achieved by one of two well-known protocols, i.e., coherent-state communication or squeezed-state communication, depending on the energy (number of photons) as well as the characteristics of the channel. Our result suggests that the coherent-state scheme known to achieve the ultimate information-theoretic capacity is not a practically optimal scheme for the case of using a finite number of channels. We find that overall the squeezed-state communication is optimal in a small-photon-number regime whereas the coherent-state communication performs better in a large-photon-number regime.Comment: 9 pages, 4 figures, published versio

Steering Criteria via Covariance Matrices of Local Observables in Arbitrary Dimensional Quantum Systems

We derive steerability criteria applicable for both finite and infinite dimensional quantum systems using covariance matrices of local observables. We show that these criteria are useful to detect a wide range of entangled states particularly in high dimensional systems and that the Gaussian steering criteria for general M x N-modes of continuous variables are obtained as a special case. Extending from the approach of entanglement detection via covariance matrices, our criteria are based on the local uncertainty principles incorporating the asymmetric nature of steering scenario. Specifically, we apply the formulation to the case of local orthogonal observables and obtain some useful criteria that can be straightforwardly computable, and testable in experiment, with no need for numerical optimization.Comment: 6 pages with further "Remarks" and "Acknowledgement" adde