Narrow Results

Utilizing Hadamard gates and controlled not gates, maximally entangled states of four, five and six qubits are generated. Then, we propose respectively controlled and assisted cloning schemes of unknown two-, three- and four-qubit states via these constructed states as quantum channels. Each of the three schemes includes two stages: controlled teleportation and assisted cloning. The controlled teleportation stage of our schemes all require a single-qubit $Z$-basis measurement by controller, and need different GHZ-state measurements by senders of three schemes to complete the transmission process of their respective quantum states. In the assisted cloning stage of our schemes, after the state preparers of three schemes perform respectively appropriate GHZ-state measurements, at the senders’ locations, perfect replicas and orthogonal complementary copies of unknown states will be produced through appropriate local quantum operations. Compared with usual assisted cloning schemes, every suggested plan is extremely efficient in terms of quantum resources, as it only requires a small number of GHZ-state measurements, significantly decreasing the need for quantum operations and classical resources. Through discussion, we conclude that our schemes are safe and can be extended to assisted cloning of unknown multi-qubit GHZ-state with multiple controllers.

In the present study, we discuss how to build some partially entangled states of n two-state quantum systems (qubits). The optimal partially entangled state with a high degree of symmetry is considered to be useful for overcoming a shot noise limit of Ramsey spectroscopy under some decoherence. This state is invariant under permutation of any two qubits, and inversion between the ground state |0> and an excited state |1> for each qubit. We show that by using selective phase shifts in certain basis vectors and Grover's inversion about average operations, we can construct this high symmetric entangled state by (polynomial in n) ×2^n/2 successive unitary transformations that are applied on two or three qubits.

According to the thermo field dynamics proposed by Takahashi–Umezawa, the Hamiltonian h = a^†a - ã^†ã and h|n, ñ〉 = 0 of the original optical field excitation state |n〉 and its tilde state |ñ〉 in the Hilbert space cannot embody the eigenvalue spectrum of h. To avoid the weakness, new thermo excitation states are successfully constructed by introducing appropriate thermo excitation operators. It is noted that they are entangled states. Applications of the new thermo excitation state in constructing new thermo squeezed state and phase state are also presented.

In this paper, we generate some new classes of entangled states of a bimodal Bose–Einstein condensate (BEC), a pair of tunnel-coupled BEC, in the presence of two- and three-body elastic as well as mode-exchange collisions. The Hamiltonian of the considered system is very complicated, moreover, it can be fortunately transformed into a simple form using a two-mode displacement operator. After introducing the general form of the time evolved state, various classes of entangled states are generated. Indeed, the influence of different orders of tunneling strengths on the generated entangled states has been studied. Depending on the tunneling strength constants, two-, three- and four-partite entangled states are generated, all of which are superposition states of macroscopic number of BEC atoms. Considering three-particle collision dramatically changes the generated entangled states. Moreover, in particular cases, the resulted states are non-entangled. Also, we show that tunneling and collisional interactions can be manipulated to generate a pair of atomic entangled coherent states (quasi-Bell states). In addition, it is observed that the degree of entanglement for two-partite entangled states can be tuned via the number of BEC atoms, i.e. the corresponding concurrences tend to their maximum value by increasing the atoms in both modes of system.

Adding the maximally mixed state with some weight to the entanglement system leads to disentanglement of the latter. For each predefined entangled state there exists a minimal value of this weight for which the system loses its entanglement properties. These values were proposed to be used as a quantitative measure of entanglement called robustness [G. Vidal and R. Tarrach, Phys. Rev. A 59, 141 (1999)]. Using the concurrence, we propose the derivation of this measure for the system of two-qubit. Namely, for a two-qubit pure state, an exact expression of robustness is obtained. Finally, in the same way, the robustness of special cases of mixed two-qubit states is calculated.

The scheme for controlled unidirectional cyclic remote state preparation of single-qutrit equatorial states is put forward. Alice, Bob, Charlie, and David share a seven-qutrit entangled state as the quantum channel. Under the control of David, Alice can remotely prepare a single-qutrit equatorial state at Bob’s site, Bob can remotely prepare a single-qutrit equatorial state at Charlie’s site, Charlie can remotely prepare a single-qutrit equatorial state at Alice’s site simultaneously. The direction of controlled unidirectional cyclic remote state preparation can be reversed by changing measured qutrits of the quantum channel. The scheme for controlled bidirectional cyclic remote state preparation of single-qutrit equatorial states is also proposed. The schemes can be generalized to controlled unidirectional and bidirectional multi-party cyclic remote state preparation of single-qudit equatorial states.

In this paper, we study Susskind–Glogower phase distribution of entangled state in the framework of interactions involving two modes of interacting Fock space which in its turn provides an interesting insigth into the structure of entangled state.

The coherent entangled state |β, x〉 is proposed in Fock space, which exhibits both the properties of the coherent and entangled states. The |β, x〉 makes up a new quantum mechanical representation, and the completeness relation of |β, x〉 is proved by virtue of the technique of integral within an ordered product of operators. The corresponding Schmidt decomposition of |β, x〉 is investigated. Furthermore, a feasible experimental scheme of |β, x〉 is presented, and generalized P-representation is constructed in the coherent entangled state |β, x〉.

We investigate the lazy states, entangled states and discordant states for 2-qubit systems. We show that many lazy states are discordant, many lazy states are entangled, and many mixed entangled states are not lazy. With these investigations, we provide a laziness-discord-entanglement hierarchy diagram for 2-qubit quantum correlations.

We put forward a scheme on how to generate entangled state of Bose–Einstein condensate (BEC) using electromagnetically induced transparency (EIT). It is shown that we can rapidly generate the entangled state in the dynamical process and the entangled state maintained a long time interval. It is also shown that the better entangled state can be generated by decreasing coupling strengths of two classical laser fields, increasing two-photon detuning and total number of atoms.

We present an alternative scheme to prepare entangled states of multiple trapped ions in thermal motion. In this scheme the vibrational degrees of freedom are only regarded as intermediate states and the ions exchange energy via the mediation of the vibration of the vibrational mode in coupling processes. The scheme is insensitive to both the initial vibrational state and heating if the system remains in the Lamb–Dicke regime. Since the effective Rabi frequency has a small dependence on the vibrational quantum number, the heating will have no direct effect on the internal state evolution. The scheme can also be used for the storage of entangled states.

By virtue of the technique of integration within an ordered product of operators (Fan et al., Ann. Phys.321 (2006) 480) we construct a kind of three-mode entangled squeezed state in the Fock space, which exhibits stronger squeezing in one quadrature than that of the usual two-mode squeezed vacuum state.

By introducing the entangled state representation and following Feynman's assumption that "electron pairs are bosons … A bound pair acts as a Bose particle", we construct a Hamiltonian operator for the Josephson transmission line (JTL) detector, then we use the Heisenberg equation of motion to derive the modified Josephson equation. Based on the modified Josephson equation, the measurement dynamics of JTL detector is investigated, which will be helpful for us to analyze the quantum measurement of JTL detector.

We propose a scheme for concentrating an arbitrary two-particle non-maximally entangled state into a maximally entangled state assisted with three cavities. The scheme involves two interaction–detection cycles and resonant interaction between atom and cavity mode. With the help of the atom trapped in the cavity, the concentration of a two-particle non-maximally entangled state trapped in separate cavities can be realized with a certain probability according to the results of photon detectors. The important feature of our scheme is that we can realize the concentration of an arbitrary two-atom non-maximally entangled state and we do not perform Bell-state measurements.

In this paper, a scheme is presented for generation of W-type entangled states for n atoms trapped in separated cavities connected by optical fibers. The scheme only requires a single atom–cavity–fiber interaction and no classical field is needed. Due to these features, the scheme is simpler and more robust against decoherence than the previous ones. The scheme can also be used to realize quantum state transfer and controlled phase gates between qubits located at distant nodes of a quantum network.

In this paper, we proposed a three-party quantum secret sharing (QSS) scheme using four-dimensional three-particle entangled states. In this QSS scheme, each agent can obtain a shadow of the secret key by performing single-particle measurements. Compared with the existing QSS protocol, this scheme has high efficiency and can resist the eavesdropping attack and entangle-measuring attack, which using three-particle entangled states are based on four-dimensional Hilbert space.

In this paper, via 12-particle entangled states as quantum channels, two schemes for cyclic quantum teleportation are proposed by using two different measurement methods, one is the Bell state measurement, the other is single-particle measurement. Besides, these schemes can be generalized to realize the probabilistic teleportation of an unknown multi-particle high-dimension state. Furthermore, feasibility for the schemes and some conclusions are discussed at the end of this paper.

In 2017, Chen et al. [Quantum Inf. Process.16 (2017) 201] proposed a scheme for cyclic quantum teleportation (CYQT) of three single-qubit information states among three participants by using six-qubit entangled state as a quantum channel. Following the work of Chen et al., we propose a new scheme for CYQT in which four participants cyclically teleport four arbitrary single-qubit information states among themselves by using two $G$-states as a quantum channel. In our scheme, reverse CYQT can also be realized throughout changing the qubit pairs to be measured by each participant. We also generalize our scheme for CYQT of $N$-qubit entangled states.

A gedanken model of an analog optical computer for general purpose Fourier transform tasks is built and studied assuming entangled or non-entangled photons appearing at the input. The performance of the entangled and non-entangled operation modes, regarding information content, power consumption, acquisition time, wavelength and bandwidth limitations are analyzed as function of qubit repetition rate and number of qubits in the entangled state, respectively, based on fundamental physical laws. The total number of input photons is assumed the same for the two operation modes. It is sown that the entangled mode outperforms the non-entangled one, but only above a certain number of qubits in the entangled state. It is argued that, for practical applications to general purpose computing, several limitations should be imposed regarding qubit repetition rate and number of qubits in the entangled state. In addition, it is pointed out that the conclusions that we present are equally valid for general purpose optical quantum computing and for quantum information processing, too.

Two schemes of amplification of two-mode squeezed light in the continuous variable EPR-state are considered. They are based on the integrals of motion, which allow conserving quantum correlations whereas the power of each mode may increase. One of these schemes involves a three-photon parametric process in a nonlinear transparent medium and the other is a Raman type interaction of light with atomic ensemble. A generalization to multimode squeezed light is discussed.