Quantum nonlocality in weak-thermal-light interferometry

In astronomy, interferometry of light collected by separate telescopes is often performed by physically bringing the optical paths together in the form of Young's double-slit experiment. Optical loss severely limits the efficiency of this so-called direct detection method, motivating the fundamental question of whether one can achieve a comparable performance using separate optical measurements at the two telescopes before combining the measurement results. Using quantum mechanics and estimation theory, here I show that any such spatially local measurement scheme, such as heterodyne detection, is fundamentally inferior to coherently nonlocal measurements, such as direct detection, for estimating the mutual coherence of bipartite thermal light when the average photon flux is low. This surprising result reveals an overlooked signature of quantum nonlocality in a classic optics experiment.     

Quantum nonlocality as an axiom

In the conventional approach to quantum mechanics, indeterminism is an axiom and nonlocality is a theorem. We consider inverting the logical order, making nonlocality an axiom and indeterminism a theorem. Nonlocal superquantum correlations, preserving relativistic causality, can violate the CHSH inequality more strongly than any quantum correlations.     

Quantum nonlocality without counterfactual definiteness?

Stapp's recent proof of the nonlocality of quantum mechanics is critically discussed. It is demonstrated that in his derivation of the Bell inequalities an extra requirement, over locality, is used, which is tantamount to counterfactual definiteness, and that is not a consequence of locality.1. Research associate N.F.W.O. (Belgium).     

Quantum nonlocality does not imply entanglement distillability

Entanglement and nonlocality are both fundamental aspects of quantum theory, and play a prominent role in quantum information science. The exact relation between entanglement and nonlocality is, however, still poorly understood. Here we make progress in this direction by showing that, contrary to what previous work suggested, quantum nonlocality does not imply entanglement distillability. Specifically, we present analytically a 3-qubit entangled state that is separable along any bipartition. This implies that no bipartite entanglement can be distilled from this state, which is thus fully bound entangled. Then we show that this state nevertheless violates a Bell inequality. Our result also disproves the multipartite version of a long-standing conjecture made by Peres.     

Quantum nonlocality and interference in frequency domain

The experiment is proposed which should demonstrate that if the filter providing the spectral selection is placed in the route of one photon of the entangled pair produced by spontaneous down-conversion and the photon is detected behind it then the interference appears in the (distant) Mach-Zehnder interferometer placed in the route of the other photon of the pair even if the optical path difference through the interferometer greatly exceeds the coherence length of the light and if the spectra of these two photons do not overlap. The theoretical analysis is carried out and physical interpretation is discussed.     

Quantum nonlocality can be distributed via separable states

正Quantum nonlocality is one of the most astonishing features in quantum physics.It is of great importance in understanding the conceptual foundations of quantum theory and is closely related to certain quantum information processing such as quantum protocols for decreasing communication complexity[1]and secure quantum communication[2,3],see refs.[4-9]for more details.     

Quantum transfer functions,weak nonlocality and relativity

The method of transfer functions is developed as a tool for studying Bell inequalities, alternative quantum theories and the associated physical properties of quantum systems. Non-negative probabilities for transfer functions result in Bell-type inequalities. The method is used to show that all realistic Lorentz-invariant quantum theories, which give unique results and have no preferred frame, can be ruled out on the grounds that they lead to weak backward causality.     

Quantum nonlocality and partial transposition for continuous-variable systems

A continuous-variable Bell inequality, valid for an arbitrary number of observers measuring observables with an arbitrary number of outcomes, was recently introduced [Cavalcanti, Phys. Rev. Lett. 99, 210405 (2007)10.1103/PhysRevLett.99.210405]. We prove that any n-mode quantum state violating this inequality with quadrature measurements necessarily has a negative partial transposition. Our results thus establish the first link between nonlocality and bound entanglement for continuous-variable systems.     

Quantum nonlocality and beyond: limits from nonlocal computation

We address the problem of "nonlocal computation," in which separated parties must compute a function without any individual learning anything about the inputs. Surprisingly, entanglement provides no benefit over local classical strategies for such tasks, yet stronger nonlocal correlations allow perfect success. This provides intriguing insights into the limits of quantum information processing, the nature of quantum nonlocality, and the differences between quantum and stronger-than-quantum nonlocal correlations.     

Quantum nonlocality effect and radiative damping of the Dirac electron

Quantum spacetime nonlocality, i.e., retardation of the interaction between an electron and its own radiation field at distances of about the Compton wavelength, is established. By taking into account a finite variance of the electron-coordinate increment in the intrinsic coordinate system, the radiative damping coefficient is obtained as a divergence-free function of frequency not subject to the well-known paradoxes of the classical theory of radiative damping. A relation between radiative damping, the Lamb shift, and the electromagnetic mass of the electron is found.     

Quantum entanglement and nonlocality properties of two-mode Gaussian squeezed states

Quantum entanglement and nonlocality properties of a family of two-mode Gaussian pure states have been investigated. The results show that the entanglement of these states is determined by both the two-mode squeezing parameter and the difference of the two single-mode squeezing parameters. For the same two-mode squeezing parameter, these states show larger entanglement than the usual two-mode squeezed vacuum state. The violation of Bell inequality depends strongly on all the squeezing parameters of these states and disappears completely in the limit of large squeezing. In particular, these states can exhibit much stronger violation of local realism than two-mode squeezed vacuum state in the range of experimentally available squeezing values.     

Quantum nonlocality of generic family of four-qubit entangled pure states

We directly introduce a Bell-type inequality for four-qubit systems. Using the inequality we investigate quantum nonlocality of a generic family of states |Gabcd[Phys. Rev. A 65 052112(2002)] and several canonical four-qubit entangled states. It has been demonstrated that the inequality is maximally violated by the so called "four-qubit the maximally entangled state |Gm" and it is also violated by four-qubit W state and a special family of states |Gab00. Moreover, a useful entanglement-nonlocality relationship for the family of states |Gab00is derived. Finally, we present a scheme of preparation of the state |Gmwith linear optics and cross-Kerr nonlinearities.     

Quantum nonlocality for two-mode correlated states based on generalized pseudospin operators

In this Letter, we investigate the quantum nonlocality of two-mode correlated states. We find that the pseudospin formalism [Z.B. Chen, J.W. Pan, G. Hou, Y.D. Zhang, Phys. Rev. Lett. 88 (2002) 040406] generally fails to depict the nonlocality of the states when the photon number difference between the two modes is odd. The formalism is then generalized such that the nonlocality of a two-mode correlated state can be well revealed without regard to the difference. Later we consider the nonlocality of the two-mode intelligent SU(1,1) states in the generalized formalism and compare our results with the entanglement of the corresponding states.     

Quantum entanglement and quantum nonlocality for N-photon entangled states

Quantum entanglement and quantum nonlocality of N-photon entangled states |\psi_{N m}\rangle =C_m[\cos\gamma|N-m\rangle₁|m\rangle₂ +\e^{\i\θ_m}\sin\gamma|m\rangle₁|N-m\rangle₂] and their superpositions are studied. We point out that the relative phase θ_m affects the quantum nonlocality but not the quantum entanglement for the state |\psi_{N m}\rangle. We show that quantum nonlocality can be controlled and manipulated by adjusting the state parameters of |\psi_{N m}\rangle, superposition coefficients, and the azimuthal angles of the Bell operator. We also show that the violation of the Bell inequality can reach its maximal value under certain conditions. It is found that quantum superpositions based on |\psi_{N m}\rangle can increase the amount of entanglement, and give more ways to reach the maximal violation of the Bell inequality.     

Quantum nonlocality in the presence of superselection rules and data hiding protocols

We consider a quantum system subject to superselection rules, for which certain restrictions apply to the quantum operations that can be implemented. It is shown how the notion of quantum nonlocality has to be redefined in the presence of superselection rules: there exist separable states that cannot be prepared locally and exhibit some form of nonlocality. Moreover, the notion of local distinguishability in the presence of classical communication has to be altered. This can be used to perform quantum information tasks that are otherwise impossible. In particular, this leads to the introduction of perfect quantum data hiding protocols, for which quantum communication (eventually in the form of a separable but nonlocal state) is needed to unlock the secret.     

Quantum correlations from local amplitudes and the resolution of the Einstein-Podolsky-Rosen nonlocality puzzle

The Einstein-Podolsky-Rosen (EPR) nonlocality puzzle has been recognized as one of the most important unresolved issues in the foundational aspects of quantum mechanics. We show that the problem is more or less entirely resolved, if the quantum correlations are calculated directly from local quantities, which preserve the phase information in the quantum system. We assume strict locality for the probability amplitudes instead of local realism for the outcomes and calculate an amplitude correlation function. Then the experimentally observed correlation of outcomes is calculated from the square of the amplitude correlation function. Locality of amplitudes implies that measurement on one particle does not collapse the companion particle to a definite state. Apart from resolving the EPR puzzle, this approach shows that the physical interpretation of apparently “nonlocal” effects, such as quantum teleportation and entanglement swapping, are different from what is usually assumed. Bell-type measurements do not change distant states. Yet the correlations are correctly reproduced, when measured, if complex probability amplitudes are treated as the basic local quantities. As examples, we derive the quantum correlations of two-particle maximally entangled states and the three-particle Greenberger-Horne-Zeilinger entangled state.     

Measurement-induced nonlocality

We interpret the maximum global effect caused by locally invariant measurements as measurement-induced nonlocality, which is in some sense dual to the geometric measure of quantum discord [Dakic, Vedral, and Brukner, Phys. Rev. Lett. 105, 190502 (2010)]. We quantify measurement-induced nonlocality from a geometric perspective in terms of measurements, and obtain analytical formulas for any dimensional pure states and 2 × n dimensional mixed states. We further derive a tight upper bound to measurement-induced nonlocality in general case. The physical significance of measurement-induced nonlocality is discussed in the context of correlations, entanglement, quantumness, and cryptographic communication.     

Irreversibility and nonlocality

We discuss and illustrate the fact that the second law of thermodynamics, when formulated as a dynamical principle, implies a departure from locality. This discussion involves an extension of our theory to singular distribution functions. Certain analogies between our considerations and the conclusion of Gödel's theorem are briefly mentioned.