Sharing quantum nonlocality in the noisy scenario

It was showed in [Phys. Rev. Lett. 125 090401(2020)] that there exist unbounded number of independent Bobs who can share quantum nonlocality with a single Alice by performing sequentially measurements on the Bob's half of the maximally entangled pure two-qubit state. However, from practical perspectives, errors in entanglement generation and noises in quantum measurements will result in the decay of nonlocality in the scenario. In this paper, we analyze the persistency and termination of sharing nonlocality in the noisy scenario. We first obtain the two sufficient conditions under which there exist n independent Bobs who can share nonlocality with a single Alice under noisy measurements and the noisy initial two qubit entangled state. Analyzing the two conditions, we find that the influences on persistency under different kinds of noises can cancel each other out. Furthermore, we describe the change patterns of the maximal nonlocality-sharing number under the influence of different noises. Finally, we extend our investigation to the case of arbitrary finite-dimensional systems.

Einstein-Podolsky-Rosen Steering and Nonlocality in Open Quantum Systems

We investigate the dynamical behavior of quantum steering (QS), Bell nonlocality, and entanglement in open quantum systems. We focus on a two-qubit system evolving within the framework of Kossakowski-type quantum dynamical semigroups. Our findings reveal that the measures of quantumness for the asymptotic states rely on the primary parameter of the quantum model. Furthermore, control over these measures can be achieved through a careful selection of these parameters. Our analysis encompasses various cases, including Bell states, Werner states, and Horodecki states, demonstrating that the asymptotic states can exhibit steering, entanglement, and Bell nonlocality. Additionally, we find that these three quantum measures of correlations can withstand the influence of the environment, maintaining their properties even over extended periods.

Simplified Scheme for Test of Quantum Nonlocality Without Using Bell Inequality

A simplified scheme is proposed for the test of quantum nonlocality of the type described by Hardy [Phys.Rev.Left.71 (1993) 1665].In the scheme two appropriately prepared atoms are simultaneously sent through a cavity and dispersively interact with the cavity field.Then state-selective measurements are performed on these atoms,which may reveal quantum nonlocality without using Bell inequality.We also propose a simple scheme for the generation of multi-atom entangled states.

Sharing quantum nonlocality in star network scenarios

The Bell nonlocality is closely related to the foundations of quantum physics and has significant applications to security questions in quantum key distributions.In recent years,the sharing ability of the Bell nonlocality has been extensively studied.The nonlocality of quantum network states is more complex.We first discuss the sharing ability of the simplest bilocality under unilateral or bilateral POVM measurements,and show that the nonlocality sharing ability of network quantum states under unilateral measurements is similar to the Bell nonlocality sharing ability,but different under bilateral measurements.For the star network scenarios,we present for the first time comprehensive results on the nonlocality sharing properties of quantum network states,for which the quantum nonlocality of the network quantum states has a stronger sharing ability than the Bell nonlocality.

Inequality relations for the hierarchy of quantum correlations in two-qubit systems

Entanglement,quantum steering and Bell nonlocality can be used to describe the distinct quantum correlations of quantum systems.Because of their different characteristics and application fields,how to divide them quantitatively and accurately becomes particularly important.Based on the sufficient and necessary criterion for quantum steering of an arbitrary two-qubit T-state,we derive the inequality relations between quantum steering and entanglement as well as between quantum steering and Bell nonlocality for the T-state.Additionally,we have verified those relations experimentally.

One-step device-independent quantum secure direct communication

Device-independent quantum secure direct communication(DI-QSDC)can relax the security assumptions about the devices’internal working,and effectively enhance QSDC’s security.In this paper,we put forward the first hyperentanglement-based one-step DI-QSDC protocol.In this protocol,the communication parties adopt the nonlocal hyperentanglement-assisted complete Bell state analysis,which enables the photons to transmit in the quantum channel for only one round.The one-step DI-QSDC can directly transmit 2 bits of messages by a hyperentangled photon pair,and is unconditionally secure in theory.Compared with the original DI-QSDC protocol(Sci.Bull.65,12(2020)),the one-step DI-QSDC protocol can simplify the experiment and reduce the message loss.In particular,with the help of the hyperentanglement heralded amplification and the hyperentanglement purification,the message loss and the message error caused by the channel noise can be completely eliminated,and the communication distance can be largely extended.By using the photon source with a repetition rate of 10 GHz,the one-step DI-QSDC’s secret message capacity under 50 km communication distance achieves about 7 bit/s with the initial fidelity in each degree of freedom of 0.8.Combined with the quantum repeater,it is possible for researchers to realize the one-step DI-QSDC with an arbitrarily long distance.