arXiv subjects

Yong-De Zhang

Publications and source records attributed to Yong-De Zhang.

14 recordsLinked to original sources

arXiv2003

Unifying Entanglement and Nonlocality as a Single Concept: Quantum Wholeness

Although entanglement is widely recognized as one of the most fascinating characteristics of quantum mechanics, nonlocality remains to be a big labyrinth. The proof of existence of nonlocality is as yet not much convincing because of its strong reliance on Bell's theorem where the assumption of realism weakens the proof. We demonstrate that entanglement and quantum nonlocality are two equivalent aspects of the same quantum wholeness for spacelike separated quantum systems. This result implies that quantum mechanics is indeed a nonlocal theory and lays foundation of understanding quantum nonlocality beyond Bell's theorem.

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arXiv2003

Violations of Locality Beyond Bell's Theorem

Locality and realism are two main assumptions in deriving Bell's inequalities. Though the experimentally demonstrated violations of Bell's inequalities rule out local realism, it is, however, not clear what role each of the two assumptions solely plays in the observed violations. Here we show that two testable inequalities for the statistical predictions of two-qubit systems can be derived by assuming either locality or realism. It turns out that quantum mechanics respects a nonlocal classical realism, and it is locality that is incompatible with experimental observations and quantum mechanics.

quant-ph↗

arXiv2003

Violation of Locality Without Inequalities for Multiparticle Perfect Correlations

We prove that for a three-qubit system in the Greenberger-Horne-Zeilinger (GHZ) state, locality per se is in conflict with the perfect GHZ correlations. The proof does not in any way use the realism assumption and can lead to a refutation of locality. We also provide inequalities that are imposed by locality. The experimental confirmation of the present reasoning may imply a genuine quantum nonlocality and will deepen our understanding of nonlocality of nature.

quant-ph↗

arXiv2003

All-Versus-Nothing Violation of Local Realism for Two Entangled Photons

It is shown that the Greenberger-Horne-Zeilinger theorem can be generalized to the case with only two entangled particles. The reasoning makes use of two photons which are maximally entangled both in polarization and in spatial degrees of freedom. In contrast to Cabello's argument of "all versus nothing" nonlocality with four photons [Phys. Rev. Lett. 87, 010403 (2001)], our proposal to test the theorem can be implemented with linear optics and thus is well within the reach of current experimental technology.

quant-ph↗

arXiv2003

Perfect Test of Entanglement for Two-level Systems

A 3-setting Bell-type inequality enforced by the indeterminacy relation of complementary local observables is proposed as an experimental test of the 2-qubit entanglement. The proposed inequality has an advantage of being a sufficient and necessary criterion of the separability. Therefore any entangled 2-qubit state cannot escape the detection by this kind of tests. It turns out that the orientation of the local testing observables plays a crucial role in our perfect detection of the entanglement.

quant-ph↗

arXiv2002

Classifying N-qubit Entanglement via Bell's Inequalities

All the states of N qubits can be classified into N-1 entanglement classes from 2-entangled to N-entangled (fully entangled) states. Each class of entangled states is characterized by an entanglement index that depends on the partition of N. The larger the entanglement index of an state, the more entangled or the less separable is the state in the sense that a larger maximal violation of Bell's inequality is attainable for this class of state.

quant-ph↗

arXiv2002

Quantum Hall Effect in a Rotating Bose-Einstein Condensate: An Atomic Twin of the Electronic Brother?

We exploit the analogy with the quantum Hall (QH) effect for electrons to study the possible atomic QH states of a rapidly-rotating Bose-Einstein condensate. Actually, there is a nearly perfect map of the present problem in the QH regime to the QH physics for electrons. The profound map enables one to give a physically appealing definitions of the filling fraction and the "atomic Hall conductance" that is quantized for atomic Laughlin states. This quantization might imply an exotic fractionalization of atomic mass. We also briefly discuss an effective Chern-Simons theory for describing the atomic QH liquids where a gravitational-like field naturally emerges.

cond-mat.mes-hall↗

arXiv2002

Calculation of entanglement for continious variable states

In this paper, we present a general formula for obtaining the reduced density opeator for any biparticle pure entangled state. Using this formula, we derive, in a compact form, the explicit formula of the entanglement for any bipartical pure entangled Gaussian state. In the case of Gaussian states, the criteria of separabelity can be naturely obtained by the formula. For non-Gaussian states, we also show the usefulness of the method presented in this paper.

quant-ph↗

arXiv2002

Greenberger-Horne-Zeilinger nonlocality for continuous variable systems

As a development of our previous work, this paper is concerned with the Greenberger-Horne-Zeilinger (GHZ) nonlocality for continuous variable cases. The discussion is based on the introduction of a pseudospin operator, which has the same algebra as the Pauli operator, for each of the $N$ modes of a light field. Then the Bell-CHSH (Clauser, Horne, Shimony and Holt) inequality is presented for the $N$ modes, each of which has a continuous degree of freedom. Following Mermin's argument, it is demonstrated that for $N$-mode parity-entangled GHZ states (in an infinite-dimensional Hilbert space) of the light field, the contradictions between quantum mechanics and local realism grow exponentially with $N$, similarly to the usual $N$-spin cases.

quant-ph↗

arXiv2002

Quantum nonlocality and applications in quantum-information processing of hybrid entangled states

The hybrid entangled states generated, e.g., in a trapped-ion or atom-cavity system, have exactly one ebit of entanglement, but are not maximally entangled. We demonstrate this by showing that they violate, but in general do not maximally violate, Bell's inequality due to Clauser, Horne, Shimony and Holt. These states are interesting in that they exhibit the entanglement between two distinct degrees of freedom (one is discrete and another is continuous). We then demonstrate these entangled states as a valuable resource in quantum information processing including quantum teleportation, entanglement swapping and quantum computation with "parity qubits". Our work establishes an interesting link between quantum information protocols of discrete and continuous variables.

quant-ph↗

arXiv2002

Linear Optics Quantum Communication over Long Distances

We propose a feasible scheme for teleporting an arbitrary polarization state or entanglement of photons by requiring only single-photon (SP) sources, simple linear optical elements and SP quantum non-demolition measurements. An unknown SP polarization state can be faithfully teleported either to a duplicate polarization state or to an entangled state. Our proposal can be used to implement long-distance quantum communication in a simple way. The scheme is within the reach of current technology and significantly simplifies the realistc implementation of long-distance high-fidelity quantum communication with photon qubits.

quant-ph↗

arXiv2002

Possible realization of Josephson charge qubits in two coupled Bose-Einstein condensates

We demonstrate that two coupled Bose-Einstein condensates (BEC) at zero temperature can be used to realize a qubit which is the counterpart of Josephson charge qubits. The two BEC are weakly coupled and confined in an asymmetric double-well trap. When the "charging energy" of the system is much larger than the Josephson energy and the system is biased near a degeneracy point, the two BEC represent a qubit with two states differing only by one atom. The realization of the BEC qubits in realistic BEC experiments is briefly discussed.

cond-mat.soft↗

arXiv2002

Maximal Violation of Bell's Inequalities for Continuous Variable Systems

We generalize Bell's inequalities to biparty systems with continuous quantum variables. This is achieved by introducing the Bell operator in perfect analogy to the usual spin-1/2 systems. It is then demonstrated that two-mode squeezed vacuum states display quantum nonlocality by using the generalized Bell operator. In particular, the original Einstein-Podolsky-Rosen entangled states, which are the limiting case of the two-mode squeezed vacuum states, can maximally violate Bell's inequality due to Clauser, Horne, Shimony and Holt. The experimental aspect of our scheme and nonlocality of arbitrary biparticle entangled pure states of continuous variables are briefly considered.

quant-ph↗

arXiv2001

Feasible Linear-optics Generation of Polarization-entangled Photons Assisted with Single-photon Quantum Non-demolition Measurement

We propose a feasible scheme to create $n$-party ($n\geq 2$) polarization-entangled photon states in a controllable way. The scheme requires only single-photon sources, single-photon quantum non-demolition measurement (SP-QNDM) and simple linear optical elements, usually with high perfections. The SP-QNDM acts as a non-destructive projection measurement onto the wanted entangled states and filters out the unwanted terms. Our scheme in fact realizes entanglement of non-interacting photons; the interaction occurs only implicitly in the optical elements and SP-QNDM. We also briefly consider purification of mixed single-photon states within our scheme.

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