Experimental observation of quantum nonlocality in general networks with different topologies

Nonlocal correlation plays an important role in device independent quantum information processing.The stan-dard Bell nonlocality has been well studied with single local hidden variables,however the nonlocal correlations in general networks with several independent quantum sources and distant observers have been far less explored.Here,by using three independent entangled photon sources and recently constructed nonlinear Bell inequalities,we experimentally test the nonlocal correlations in the network scenario with different topologies.The violation of the inequalities can be obtained simply with separate measurements,which is much more favourable from the practical point of view.Our experiment results show a violation as 0.7779±0.0093 for the star network,and 0.7303±0.0024 for the chain network.Furthermore,we demonstrate that more measurement settings for each observer can bound more information against an eavesdropper.

Direct measurement of nonlocal quantum states without approximation

Efficient acquiring information from a quantum state is important for research in fundamental quantum physics and quantum information applications. Instead of using standard quantum state tomography method with reconstruction algorithm, weak values were proposed to directly measure density matrix elements of quantum state. Recently, similar to the concept of weak value, modular values were introduced to extend the direct measurement scheme to nonlocal quantum wavefunction. However, this method still involves approximations, which leads to inherent low precision. Here, we propose a new scheme which enables direct measurement for ideal value of the nonlocal density matrix element without taking approximations. Our scheme allows more accurate characterization of nonlocal quantum states, and therefore has greater advantages in practical measurement scenarios.

Relativistic motion on Gaussian quantum steering for two-mode localized Gaussian states

Realistic quantum systems always exhibit gravitational and relativistic features.In this paper,we investigate the properties of Gaussian steering and its asymmetry by the localized two-mode Gaussian quantum states,instead of the traditional single-mode approximation method in the relativistic setting.We find that the one-side Gaussian quantum steering will monotonically decrease with increasing observers of acceleration.Meanwhile,our results also reveal the interesting behavior of the Gaussian steering asymmetry,which increases for a specific range of accelerated parameter and then gradually approaches to a finite value.Such finding is well consistent and explained by the well-known Unruh effect,which could significantly destroy the one-side Gaussian quantum steering.Finally,our results could also be applied to the dynamical studies of Gaussian steering between the Earth and satellites,since the effects of acceleration are equal to the effects of gravity according to the equivalence principle.

Steering quantum nonlocalities of quantum dot system suffering from decoherence

The important applications of quantum dot system are to implement logic operations and achieve universal quantum computing based on different quantum nonlocalities.Here,we characterize the quantum steering,Bell nonlocality,and nonlocal advantage of quantum coherence(NAQC)of quantum dot system suffering nonunital and unital channels.The results reveal that quantum steering,Bell nonlocality,and NAQC can display the traits of dissipation,enhancement,and freezing.One can achieve the detections of quantum steering,Bell nonlocality,and NAQC of quantum dot system in different situations.Among these quantum nonlocalities,NAQC is the most fragile,and it is most easily influenced by different system parameters.Furthermore,considering quantum dot system coupling with amplitude damping channel and phase damping channel,these quantum nonlocalities degenerate with the enlargement of the channel parameters t andΓ.Remarkably,measurement reversal can effectively control and enhance quantum steering,Bell nonlocality,and NAQC of quantum dot system suffering from decoherence,especially in the scenarios of the amplitude damping channel and strong operation strength.

Demonstration of controlled high-dimensional quantum teleportation

Controlled quantum teleportation(CQT), which is regarded as the prelude and backbone for a genuine quantum internet, reveals the cooperation, supervision, and control relationship among the sender, receiver, and controller in the quantum network within the simplest unit. Compared with low-dimensional counterparts, high-dimensional CQT can exhibit larger information transmission capacity and higher superiority of the controller's authority. In this article, we report a proof-of-principle experimental realization of three-dimensional(3D) CQT with a fidelity of 97.4% ± 0.2%. To reduce the complexity of the circuit, we simulate a standard 4-qutrit CQT protocol in a 9×9-dimensional two-photon system with high-quality operations. The corresponding control powers are 48.1% ± 0.2% for teleporting a qutrit and 52.8% ± 0.3% for teleporting a qubit in the experiment, which are both higher than the theoretical value of control power in 2-dimensional CQT protocol(33%). The results fully demonstrate the advantages of high-dimensional multi-partite entangled networks and provide new avenues for constructing complex quantum networks.

Device-independent characterization of entanglement based on bell nonlocality

Quantum entanglement,has been acknowledged as a precious resource due to its inherent nonclassical correla-tions between subsystems.These quantum correlations have the potential for many quantum processes,includ-ing canonical ones:quantum cryptography,quantum teleportation,and dense coding.To exploit the advantages of quantum entanglement,two essential premises are required,i.e.,to prepare high-quality entanglement and characterize quality level of prepared entanglement.Thus far,quantum entanglement can be produced in various quantum systems;however,it appears that this new resource is complex and difficult to characterize.The standard methods to characterize multipartite entanglement,e.g.,entanglement witness,state tomography,or quantum state verification,require full knowledge of the Hilbert space dimension and precise calibration of measurement devices,which are usually difficult to acquire in experiment.The most radical way to overcome these problems is to detect entanglement solely based on the Bell-like correlations of measurement outcomes collected in the exper-iment,namely,device-independent characterization of entanglement.This article reviews the recently developed device-independent methods to characterize entanglement,including self-testing and device-independent certi-fication of entanglement.These approaches can be widely applied in kinds of quantum information processing,especially for those with security demands.

A nonlocal quantum simulator and simulation of superluminality

With the support by the National Natural Science Foundation of China and the Chinese Academy of Sciences(CAS),the research team led by Prof.Li Chuanfeng(李传锋)at the CAS Key Lab of Quantum Information,University of Science and Technology of China,developed a nonlocal quantum simulator and simulated the superluminality phenomenon in parity-time(PT)world.This study for the first time exhib-