Entanglement and nonlocality in multi-particle systems

Dirk Bouwmeester, Jian-Wei Pan, Matthew Daniell +2 more

M. D. Reid, Qiongyi He, P. D. Drummond

Entanglement, the Einstein-Podolsky-Rosen (EPR) paradox and Bell’s failure of local-hiddenvariable (LHV) theories are three historically famous forms of “quantum nonlocality”. We give experimental criteria for these three forms of nonlocality in multi-particle systems, with the aim of better understanding the transition from microscopic to macroscopic nonlocality. We examine the nonlocality of N separated spin J systems. First, we obtain multipartite Bell inequalities that address the correlation between spin values measured at each site, and then we review spin squeezing inequalities that address the degree of reduction in the variance of collective spins. The latter have been particularly useful as a tool for investigating entanglement in Bose-Einstein condensates (BEC). We present solutions for two topical quantum states: multi-qubit Greenberger-Horne-Zeilinger (GHZ) states, and the ground state of a two-well BEC.

J. G. Cramer

C. S. Unnikrishnan

I have been arguing that quantum nonlocality, deeply entrenched in the present formalism of quantum mechanics and widely believed as a reality by physicists, is in fact absent. Spooky nonlocal state reduction is the most, and perhaps the only irrational feature of present day physics. There are experimental results that reject nonlocal state reduction at a distance. Also, there are arguments that show that signal locality itself can be violated if there is true nonlocal collapse of the wavefunction. The Bell's inequalities, the violation of which polarized physicists in favour of nonlocality, arise not due to nonlocality, but due to ignoring prior information on correlations encoded in the phase of local probability amplitudes. Here I discuss an experiment involving particles entangled in energy and time variables that shows that there is no nonlocal state reduction during measurements on entangled particles. Quantum mechanics is inconsistent if it includes the concept of wavefunction collapse as a physical process for entangled multi-particle systems.

寿彦 佐々木

Duc Manh Doan, Hung Q. Nguyen

Nonlocality can be studied through different approaches, such as Bell's inequalities, and it can be found in numerous quantum states, including GHZ states or graph states. Hardy's paradox, or Hardy-type nonlocality, provides a way to investigate nonlocality for entangled states of particles without using inequalities. Previous studies of Hardy's nonlocality have mostly focused on the fully entangled systems, while other entanglement configurations remain less explored. In this work, the system under investigation consists of four particles arranged in a cyclic entanglement configuration, where each particle forms entangled pairs with two neighbors, while non-neighboring particles remain unentangled. We found that this entanglement structure offers a larger set of conditions that lead to the contradiction with the LHV model, compared to the fully entangled systems. This enhancement can be attributed to the presence of multiple excluded states and correlations, in which the measurement result of a particle only influences the result of its paired partners. We implement quantum circuits compatible with the cyclic entanglement structure, and through simulation, the correlation patterns and the states of interest are identified. We further execute the proposed circuits on IBM Brisbane, a practical backend; however, the results show considerable deviations from the simulation counterparts.