Multistage entanglement swapping

We report an experimental demonstration of entanglement swapping over two quantum stages. By successful realizations of two cascaded photonic entanglement swapping processes, entanglement is generated and distributed between two photons, that originate from independent sources and do not share any common past. In the experiment we use three pairs of polarization entangled photons and conduct two Bell-state measurements: one between the first and second pair, and one between the second and third pair. This results in projecting the remaining two outgoing photons from pair 1 and 3 into an entangled state, as characterized by an entanglement witness. The experiment represents an important step towards a full quantum repeater where multiple entanglement swapping is a key ingredient.     

Qubit movement-assisted entanglement swapping

We propose a scheme to generate entanglement between two distant qubits(two-level atom) which are separately trapped in their own(in general) non-Markovian dissipative cavities by utilizing entangling swapping, considering the case in which the qubits can move along their cavity axes rather than a static state of motion. We first examine the role of movement of the qubit by studying the entropy evolution for each subsystem. The average entropy over the initial states of the qubit is calculated. Then by performing a Bell state measurement on the fields leaving the cavities, we swap the entanglement between qubit-field in each cavity into qubit-qubit and field-field subsystems. The entangling power is used to measure the average amount of swapped entanglement over all possible pure initial states. Our results are presented in two weak and strong coupling regimes, illustrating the positive role of movement of the qubits on the swapped entanglement.It is revealed that by considering certain conditions for the initial state of qubits, it is possible to achieve a maximally long-leaving stationary entanglement(Bell state) which is entirely independent of the environmental variables as well as the velocity of qubits. This happens when the two qubits have the same velocities.     

Quantum teleportation by entanglement swapping with trapped ions

An effective teleportation scheme for an unknown ionic internal state via trapped ions is proposed without joint Bell-state measurement (BSM). In the constructed quantum channel process, we make use of entanglement swapping to avoid decrease in entanglement during the distributing of particles. Thus our scheme provides new prospects for quantum teleportation in a longer distance. The distinct advantage of our scheme is insensitive to the heating of vibrational mode. Furthermore, our scheme has no any individual optical access, and the successful probability also can reach 1.     

Quantum secure direct communication by entangled qutrits and entanglement swapping

A protocol for quantum secure direct communication by using entangled qutrits and swapping quantum entanglement is proposed. In this protocol, a set of ordered two-qutrit entangled states is used as quantum information channels for sending secret messages directly. During the process of transmission of particles, the transmitted particles do not carry any secret messages and are transmitted only one time. The protocol has higher source capacity than protocols using usual two-dimensional Bell-basis states as quantum channel. The security is ensured by the unitary operations randomly performed on all checking groups before the particle sequence is transmitted and the application of entanglement swapping.     

Faithful entanglement swapping based on sum-frequency generation

We show that an entanglement swapping operation performed with spontaneous parametric down-conversion can be made faithful without postselection using sum-frequency generation. This invites us to revisit the sum-frequency process and from a proof-of-principle experiment, we demonstrate that it provides a realistic solution for nonlinear optics at the single-photon level. This opens the way to attractive alternatives to six-photon protocols based on linear optics used, e.g., for the heralded creation of maximally entangled pairs or for device-independent quantum key distribution.     

Relativity of quantum states in entanglement swapping

The entanglement swapping protocol is analyzed in a relativistic setting, where shortly after the entanglement swapping is performed, a Bell test is performed. For an observer in the laboratory frame, a Bell violation is observed between the qubits with the swapped entanglement. In a moving frame, the order of the measurements is reversed, and a Bell violation is observed even though the particles are not entangled, directly or indirectly, or at any point in time. Although the measurement results are identical, the wavefunctions for the two frames are starkly different — one is entangled and the other is not. Furthermore, for boosts in a perpendicular direction, in the presence of decoherence, we show that the maximum Bell violation can occur across non-simultaneous points in time. This is a signature of entanglement that is spread across both space and time, showing both non-local and non-simultaneous aspects of entanglement.     

基于纠缠交换的仲裁量子签名方案

提出了一种基于量子纠缠交换的仲裁签名协议. 以Bell态为基础,首先将待签消息利用幺正算符序列进行编码,通过算符序列对Bell态进行调制,再通过对量子信息加密产生签名.验证者将签名信息与仲裁者通过纠缠交换所产生的关联态相结合,通过Bell测量来对签名的真实性进行验证.算法利用量子加密保障了真实签名的不可伪造性,同时通过仲裁的参与结合量子密钥有效解决了双方的抵赖问题,方案还能够有效实现对通信双方隐私信息的保护. 关键词： 量子密码 量子签名 纠缠交换     

Secret sharing of quantum information via entanglement swapping

Based on entanglement swapping, a scheme for the secret sharing of an arbitrary two-particle entangled state is proposed. If the controllers do not co-operate with the eavesdropper, the eavesdropper's successful probability decreases with the number of the controllers increasing. In addition, only the Bell-state measurements are required to realize the secret sharing scheme.     

Controlled mutual quantum entity authentication using entanglement swapping

In this paper, we suggest a controlled mutual quantum entity authentication protocol by which two users mutually certify each other on a quantum network using a sequence of Greenberger–Horne–Zeilinger(GHZ)-like states. Unlike existing unidirectional quantum entity authentication, our protocol enables mutual quantum entity authentication utilizing entanglement swapping; moreover, it allows the managing trusted center(TC) or trusted third party(TTP) to effectively control the certification of two users using the nature of the GHZ-like state. We will also analyze the security of the protocol and quantum channel.     

Quantum entanglement swapping of two arbitrary biqubit pure states

In this paper, the issue of swapping quantum entanglements in two arbitrary biqubit pure states via a local bipartite entangledstate projective measure in the middle node is studied in depth, especially with regard to quantitative aspects. Attention is mainly focused on the relation between the measure and the final entanglement obtained via swapping. During the study, the entanglement of formation (EoF) is employed as a quantifier to characterize and quantify the entanglements present in all involved states. All concerned EoFs are expressed analytically; thus, the relation between the final entanglement and the measuring state is established. Through concrete analyses, the measure demands for getting a certain amount of a final entanglement are revealed. It is found that a maximally entangled final state can be obtained from any two given initial entangled states via swapping with a certain probability; however, a peculiar measure should be performed. Moreover, some distinct properties are revealed and analyzed. Such a study will be useful in quantum information processes.     

Experimental demonstration of unconditional entanglement swapping for continuous variables

The unconditional entanglement swapping for continuous variables is experimentally demonstrated. Two initial entangled states are produced from two nondegenerate optical parametric amplifiers operating at de-amplification. Through implementing the direct measurement of the Bell-state between two optical beams from each amplifier the remaining two optical beams, which have never directly interacted with each other, are entangled. The quantum correlation degrees of 1.23 and 1.12 dB below the shot noise limit for the amplitude and phase quadratures resulting from the entanglement swapping are measured straightly.     

Teleportation of N-particle entangled W state via entanglement swapping

A scheme for teleporting an unknown N-particle entangled W state is proposed via entanglement swapping. In this scheme, N maximally entangled particle pairs are used as quantum channel. As a special case, the teleportation of an unknown four-particle entangled W state is studied.     

Macroscopic thermal entanglement due to radiation pressure

Can entanglement and the quantum behavior in physical systems survive at arbitrary high temperatures? In this Letter we show that this is the case for a electromagnetic field mode in an optical cavity with a movable mirror in a thermal state. We also identify two different dynamical regimes of generation of entanglement separated by a critical coupling strength.     

Macroscopic entanglement on a hybrid quantum circuit

A scheme is proposed to deterministically create maximal entanglement between hybrid artificial atoms: superconducting charge and flux qubits. By tuning the circuit, the two qubits are dynamically decoupled and entanglement can be long-lived. This provides a new version of the Einstein-Podolsdy-Rosen (EPR) situation where the components of a macroscopic EPR pair are in opposite regimes.     

基于纠缠交换的多方多级量子密钥分配协议

提出了一种基于纠缠交换的多方多级量子密钥分配协议.构造了一种两方三级系统的完备正交归一化基，利用该正交归一化基和纠缠交换可以实现两方量子密钥分配.同时，三级可以推广到多级以及两方推广到多方，即可以实现基于纠缠交换的多方多级量子密钥分配.这样，利用纠缠交换和多级密钥分配可以极大地提高检测窃听的效率、密钥生成率以及信息容量. 关键词： 量子密钥分配 纠缠交换 多方 多级     

Violation of Clauser-Horne-Shimony-Holt inequality for states resulting from entanglement swapping

We consider violation of CHSH inequality for states before and after entanglement swapping. We present a pair of initial states which do not violate CHSH inequality however the final state violates CHSH inequality for some results of Bell measurement performed in order to swap entanglement.     

Participant attack on quantum secret sharing based on entanglement swapping

The security of quantum secret sharing based on entanglement swapping is revisited and a participant attack is presented. In this attack two dishonest agents together can illegally recover the secret quantum state without the help of any other controller, and it will not be detected by any other users. Furthermore, by modifying the distribution process of particles and adding a detection step after each distribution process, we propose an improved protocol which can resist this kind of attack.     

Teleportation for atomic entangled state by entanglement swapping with separate measurements in cavity QED

Experimentally feasible scheme for teleportation of atomic entangled state via entanglement swapping is proposed in cavity quantum electrodynamics without joint Bell-state measurement. In the teleportation processes the interaction between atoms and a single-mode nonresonant cavity with the assistance of a strong classical driving field substitute the joint measurements. The discussion of the scheme indicates that it can be realized by current technologies.