Testing quantum nonlocality for three coupled quantum dots within optical microcavity QED

Bell's inequality for three coupled quantum dots (QDs) within a cavity QED, including Förster and exciton-phonon interactions, is investigated theoretically. For an initially entangled state, Bell's inequality is valid for certain times and violated for some other times. It is shown that the system moves from a product state to an entangled state and back again during its time evolution.     

Alice falls into a black hole: entanglement in noninertial frames

Two observers determine the entanglement between two free bosonic modes by each detecting one of the modes and observing the correlations between their measurements. We show that a state which is maximally entangled in an inertial frame becomes less entangled if the observers are relatively accelerated. This phenomenon, which is a consequence of the Unruh effect, shows that entanglement is an observer-dependent quantity in noninertial frames. In the high acceleration limit, our results can be applied to a nonaccelerated observer falling into a black hole while the accelerated one barely escapes. If the observer escapes with infinite acceleration, the state's distillable entanglement vanishes.     

Violations of Bell's Inequality in Synchronized Hyperchaos

Motivated by a parallel between quantum cryptography and chaos synchronization cryptography, we construct a Bell's inequality for a pair of synchronously coupled variable-order Generalized Rossler Systems, with arbitrarily binarized final states. In the infinite-order limit, although dynamical parameters cannot be extracted from the coupling signal in finite time, the inequality is violated, as with entangled quantum states. The violations are weaker than in quantum theory, vanishing as the differences between corresponding parameters of the coupled systems become small. The fact that Bell's inequality can be violated for a pair of classical systems that are not discernibly connected supports the possibility of a realist interpretation of quantum mechanics.     

Nonlocality and entanglement via the Unruh effect

Modeling the qubit by a two-level semiclassical detector coupled to a massless scalar field, we investigate how the Unruh effect affects the nonlocality and entanglement of two-qubit and three-qubit states when one of the entangled qubits is accelerated. Two distinct differences with the results of free field model in non-inertial frames are (i) for the two-qubit state, the CHSH inequality cannot be violated for sufficiently large but finite acceleration, furthermore, the concurrence will experience “sudden death”; and (ii) for the three-qubit state, not only does the entanglement vanish in the infinite acceleration limit, but also the Svetlichny inequality cannot be violated in the case of large acceleration.     

Testing the nonlocality of entangled states by a new Bell-like inequality

We test a new four-qubit entangled state by the former Bell-like inequalities and find that it violates these inequalities, but not maximally. According to this entangled state, we build a new Bell-like inequality, which is violated by this new state maximally. We also determine the nonlocality of some other four-qubit states by the new inequality. It is found that the new inequality acts as a strong entanglement witness for the new state. This can be used to test new entangled states experimentally.     

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.     

Entanglement from the Vacuum

We explore the entanglement of the vacuum of a relativistic field by letting a pair of causally disconnected probes interact with the field. We find that, even when the probes are initially non-entangled, they can wind up to a final entangled state. This shows that entanglement persists between disconnected regions in the vacuum. However the probe entanglement, unlike correlations, vanishes once the regions become sufficiently separated. The relation between entropy, correlations and entanglement is discussed.     

Classical and quantum communication without a shared reference frame

We show that communication without a shared reference frame is possible using entangled states. Both classical and quantum information can be communicated with perfect fidelity without a shared reference frame at a rate that asymptotically approaches one classical bit or one encoded qubit per transmitted qubit. We present an optical scheme to communicate classical bits without a shared reference frame using entangled photon pairs and linear optical Bell state measurements.     

Frame Independent Nonlocality for Three Qubit State

Bell’s inequality is investigated for the three qubit GHZ state in relativistic regime. Two different relativistic spin operator are considered. One of them is defined by Lee and Ee (New J. Phys. 6:67, 2004). and the other which is the Pauli-Lubanski pseudovector used by Kim and Son (Phys. Rev. A 71:014102, 2005). It is shown that for both spin operator Bell’s inequality is still maximally violated in a Lorentz-boosted frame.     

State-Independent Proofs of Bell's Theorem Without Inequalities and Bell Inequality for Four-Qubit System

A state-dependent proof of Bell's theorem without inequalities using the product state of any two maximally entangled states (Bell states) of two qubits for two observers in an ideal condition, each of which possesses two qubits, is proposed. It is different from the other proofs in which there exists a fundamental requirement that certain specific suitable Bell states have been chosen. Moreover, in any non-ideal situation, a common Bell inequality independent of the choices of the 16-product states is derived, which is used to test the contradiction between quantum mechanics and local reality theory in the reach of current experimental technology.     

Ontological Models,Preparation Contextuality and Nonlocality

The ontological model framework for an operational theory has generated much interest in recent years. The debate concerning reality of quantum states has been made more precise in this framework. With the introduction of generalized notion of contextuality in this framework, it has been shown that completely mixed state of a qubit is preparation contextual. Interestingly, this new idea of preparation contextuality has been used to demonstrate nonlocality of some \(\psi \) -epistemic models without any use of Bell’s inequality. In particular, nonlocality of a non maximally \(\psi \) -epistemic model has been demonstrated from preparation contextuality of a maximally mixed qubit and Schrödinger’s steerability of the maximally entangled state of two qubits (Leifer and Maroney, Phys Rev Lett 110:120401, 2013). In this paper, we, show that any mixed state is preparation contextual. We, then, show that nonlocality of any bipartite pure entangled state, with Schmidt rank two, follows from preparation contextuality and steerability provided we impose certain condition on the epistemicity of the underlying ontological model. More interestingly, if the pure entangled state is of Schmidt rank greater than two, its nonlocality follows without any further condition on the epistemicity. Thus our result establishes a stronger connection between nonlocality and preparation contextuality by revealing nonlocality of any bipartite pure entangled states without any use of Bell-type inequality.     

Generation of Entanglement Between Two Noninteracting Qubits via Interaction with Nonclassical Radiation Field

We study the interaction between a single-mode quantized field and a quantum system composed of two qubits. We suppose that two qubits initially be prepared in the mixed and separable state, and study how entanglement between two qubits arises in the course of evolution according to the Jaynes-Cummings type interaction with nonclassical radiation field. We also investigate the relation between entanglement and purity of qubit subsystem. We show that different photon statistics have different effects on the dynamical behavior of the qubit subsystem. When the qubits are initially prepared in the maximally mixed and separable state, for coherent state field we cannot find entanglement between two qubits; for squeezed state field entanglement between two qubits exists in several short period of time; for even and odd coherent state fields of large photon number, the dynamical behavior of the entanglement between two qubits shows collapse and revival phenomenon. For odd coherent state field of small photon number, the entanglement with both qubits initially prepared in maximally mixed state can be stronger than that with both qubits initially prepared in pure states. For fields of small photon number, the entanglement strongly depends on the states they are initially prepared in. For coherent state field, and odd and even coherent state fields of large photon number, the entanglement only depends on the purity of the initial state of qubit subsystem. We also show that during the evolution the unentangled state may be purer than the entangled state, and the maximum degree of entanglement may not occur at the time when the qubit subsystem is in the purist state.     

A Three-Qubit State Entanglement Concentration Protocol Assisted by Two-Qubit Systems

In this paper we present an entanglement concentration protocol for enhancement of the amount of entanglement maximally in a three qubit non-maximally entangled state. We use a Bell state for this purpose. Here the speciality is that no non-local measurement involving more than one parties is involved in the protocol. It is shown that for obtaining best probability of success a maximally entangled Bell state must be used. The probability of success in our protocol increases with an increase in the amount of entanglement in the assisting Bell state, and is zero when the entanglement vanishes.     

Bidirectional Quantum Controlled Teleportation via a Maximally Seven-qubit Entangled State

A bidirectional quantum controlled teleportation scheme using a seven-qubit maximally entangled state as quantum channel is proposed. This means that Alice can transmit an arbitrary single qubit state of qubit a to Bob and Bob can transmit an arbitrary single qubit state of qubit b to Alice via the control of the supervisor Charlie.     

Violations of local realism by two entangled N-dimensional systems are stronger than for two qubits

Tests of local realism versus quantum mechanics based on Bell's inequality employ two entangled qubits. We investigate the general case of two entangled quantum systems defined in N-dimensional Hilbert spaces, or " quNits." Via a numerical linear optimization method we show that violations of local realism are stronger for two maximally entangled quNits ( 3相似文献   

Generation of Entanglement Between Two Noninteracting Qubits via Interaction with Nonclassical Radiation Field

We study the interaction between a single-mode quantized field and a quantum system composed of two qubits. We suppose that two qubits initially be prepared in the mixed and separable state, and study how entanglement between two qubits arises in the course of evolution according to the Jaynes-Cummings type interaction with nonclassical radiation field. We also investigate the relation between entanglement and purity of qubit subsystem. We show that different photon statistics have different effects on the dynamical behavior of the qubit subsystem. When the qubits are initially prepared in the maximally mixed and separable state, for coherent state field we cannot find entanglement between two qubits; for squeezed state field entanglement between two qubits exists in several short period of time; for even and odd coherent state fields of large photon number, the dynamical behavior of the entanglement between two qubits shows collapse and revival phenomenon. For odd coherent state field of small photon number, the entanglement with both qubits initially prepared in maximally mixed state can be stronger than that with both qubits initially prepared in pure states. For fields of small photon number, the entanglement strongly depends on the states they are initially prepared in. For coherent state field, and odd and even coherent state fields of large photon number, the entanglement only depends on the purity of the initial state of qubit subsystem. We also show that during the evolution the unentangled state may be purer than the entangled state, and the maximum degree of entanglement may not occur at the time when the qubit subsystem is in the purist state.     

利用非局域控制非门操作纯化三粒子纠缠态

提出了两套三粒子纠缠态的纯化方案.第一个方案选择部分纠缠GHZ态作为量子通道,利用具有一个控制位和一个靶位的非局域控制非门操作和采用集体么正操作及适当地制备三粒子A,B和C的初始态,可以以最佳几率2|β|2获得最大三粒子纠缠态.第二个方案选择EPR对作为量子通道,通过利用具有一个控制位和两个靶位的非局域控制非门操作和采用集体么正操作及适当地制备三粒子A,B和C的初始态,可以以与第一个方案相同的几率获得最大三粒子纠缠态.两个方案都可以推广到N粒子纠缠态的纯化.     

从贝尔不等式到量子信息"热"

介绍了贝尔不等式的建立及20世纪70年代以来对它的实验验证．对贝尔不等式与量子力学基本解释的关系的深入研究,加速了“纠缠态”及其品性的发现,推动了量子信息“热”的兴起。     

Non-Maximal Tripartite Entanglement Degradation of Dirac and Scalar Fields in Non-Inertial Frames

The π-tangle is used to study the behavior of entanglement of a nonmaximal tripartite state of both Dirac and scMar fields in accelerated frame. For Dirac fields, the degree of degradation with acceleration of both one-tangle of accelerated observer and π-tangle, for the same initial entanglement, is different by just interchanging the values of probability amplitudes. A fraction of both one-tangles and the π-tangle always survives for any choice of acceleration and the degree of initial entanglement. For scalar field, the one-tangle of accelerated observer depends on the choice of values of probability amplitudes and it vanishes in the range of infinite acceleration, whereas for 1r-tangle this is not always true. The dependence of π-tangle on probability amplitudes varies with acceleration. In the lower range of acceleration, its behavior changes by switching between the values of probability amplitudes and for larger values of acceleration this dependence on probability amplitudes vanishes. Interestingly, unlike bipartite entanglement, the degradation of π-tangle against acceleration in the case of sca/ar fields is slower than for Dirac fields.     

High efficient scheme for remote state preparation with cavity QED

In this paper, a scheme is proposed for remote state preparation （RSP） with cavity quantum electrodynamics （QED）. In our scheme, two observers share two-atom nonmaximally entangled state as quantum channels and can realize remote preparation of state of an atom. We also propose a generalization for remote preparation of N-atom entangled state by （N＋1）-atom GHZ-like state （N ≥ 2）. By this scheme, one single-atom projective measurement is enough for the RSP of a qubit or N-atom entangled state, and the probability of success for RSP is unity. Furthermore, we have considered the case where observers use W-like state as quantum channels to realize RSP of a qubit. We compare our scheme with existing ones.