Probabilistic and Hierarchical Quantum Information Splitting Based on the Non-Maximally Entangled Cluster State

With the emergence of classical communication security problems, quantum communication has been studied more extensively. In this paper, a novel probabilistic hierarchical quantum information splitting protocol is designed by using a non-maximally entangled four-qubit cluster state. Firstly, the sender Alice splits and teleports an arbitrary one-qubit secret state invisibly to three remote agents Bob, Charlie, and David. One agent David is in high grade, the other two agents Bob and Charlie are in low grade. Secondly, the receiver in high grade needs the assistance of one agent in low grade, while the receiver in low grade needs the aid of all agents. While introducing an ancillary qubit, the receiver’s state can be inferred from the POVM measurement result of the ancillary qubit. Finally, with the help of other agents, the receiver can recover the secret state probabilistically by performing certain unitary operation on his own qubit. In addition, the security of the protocol under eavesdropping attacks is analyzed. In this proposed protocol, the agents need only single-qubit measurements to achieve probabilistic hierarchical quantum information splitting, which has appealing advantages in actual experiments. Such a probabilistic hierarchical quantum information splitting protocol hierarchical is expected to be more practical in multipartite quantum cryptography.     

Quantum state measurement of any arbitrary entangled field-state via Ramsey interferometry

Multipartite entangled states are the key resource and play a crucial role in latest applications of quantum mechanics. We propose a scheme for the measurement of quantum state of multimode entangled field state trapped in multiple cavities. The scheme is based on the measurement of photon statistics of the displaced entangled field state in Ramsey type set-up. In this set-up, the atoms undergo a dispersive phase shift when they pass through the off-resonant entangled field in cavities. By measuring the internal states of the atoms, the photon statistics and the Wigner function can be reconstructed.     

Quantum information processing for ions via linear optics

We propose a linear optical scheme for the transfer of unknown ionic states, the entanglement concentration for nonmaximally entangled states for ions via entanglement swapping and the remote preparation for ionic entangled states. The joint Bell state measurement needed in the previous schemes is not needed in the current scheme, i.e. the joint Bell state measurement has been converted into the product of separate measurements on single ions and photons. In addition, the current scheme can realize the quantum information processes for ions by using linear optical elements, which simplify the implementation of quantum information processing for ions.     

Typical entanglement in multiple-qubit systems

Quantum entanglement and its paradoxical properties hold the key to an information processing revolution. Much attention has focused recently on the challenging problem of characterizing entanglement. Entanglement for a two qubit system is reasonably well understood; however, the nature and properties of multiple qubit systems are largely unexplored. Motivated by the importance of such systems in quantum computing, we show that typical pure states of N qubits are highly entangled but have decreasing amounts of pairwise entanglement (measured using the Wootter concurrence formula) as N increases. Above six qubits, very few states have any pairwise entanglement and, generally, for a typical pure state of N qubits there is a sharp cut-off where its subsystems of size m become positive partial transpose (i.e. separable or only bound entangled) around N ? 2m + 3, based on numerical analysis up to N = 13.     

Creating quantum entanglement between multi-atom Dicke states and two cavity modes

We present a scheme to create quantum entanglement between multi-atom Dicke states and two cavity modes by passing N three-level atoms in Λ configuration through a resonant two-mode cavity one by one. We further show that such a scheme can be used to generate arbitrary two-mode N-photon entangled states, arbitrary superposition of Dicke states, and a maximal entangled state of Dicke states. These states may find applications in the demonstration of quantum non-locality, high-precision spectroscopy and quantum information processing.     

Quantum teleportation with a complete Bell state measurement

By using the quantum teleportation protocol, Alice can send an unknown quantum state (e.g. the polarization of a single photon) to Bob without ever knowing about it. This paper discusses a quantum teleportation experiment in which nonlinear interactions are used for the Bell state measurement. Since the Bell state measurement is based on nonlinear interactions, all four Bell states can be distinguished. Therefore, teleportation of a polarization state can occur with certainty, in principle. Details of the theory and the experimental set-up are discussed.     

A monolithic array of three-dimensional ion traps fabricated with conventional semiconductor technology

The coherent control of quantum-entangled states of trapped ions has led to significant advances in quantum information, quantum simulation, quantum metrology and laboratory tests of quantum mechanics and relativity. All of the basic requirements for processing quantum information with arrays of ion-based quantum bits (qubits) have been proven in principle. However, so far, no more than 14 ion-based qubits have been entangled with the ion-trap approach, so there is a clear need for arrays of ion traps that can handle a much larger number of qubits. Traps consisting of a two-dimensional electrode array have undergone significant development, but three-dimensional trap geometries can create a superior confining potential. However, existing three-dimensional approaches, as used in the most advanced experiments with trap arrays, cannot be scaled up to handle greatly increased numbers of ions. Here, we report a monolithic three-dimensional ion microtrap array etched from a silica-on-silicon wafer using conventional semiconductor fabrication technology. We have confined individual (88)Sr(+) ions and strings of up to 14 ions in a single segment of the array. We have measured motional frequencies, ion heating rates and storage times. Our results demonstrate that it should be possible to handle several tens of ion-based qubits with this approach.     

Quantum Secure Direct Communication Protocol with Mutual Authentication Based on Single Photons and Bell States

Quantum secure direct communication (QSDC) can transmit secret messages directly from one user to another without first establishing a shared secret key, which is different from quantum key distribution. In this paper, we propose a novel quantum secure direct communication protocol based on signal photons and Bell states. Before the execution of the proposed protocol, two participants Alice and Bob exchange their corresponding identity IDA and IDB through quantum key distribution and keep them secret, respectively. Then the message sender, Alice, encodes each secret message bit into two single photons (| 01〉or|10〉) or a Bell state , and composes an ordered secret message sequence. To insure the security of communication, Alice also prepares the decoy photons and inserts them into secret message sequence on the basis of the values of IDA and IDB. By the secret identity IDA and IDB, both sides of the communication can check eavesdropping and identify each other. The proposed protocol not only completes secure direct communication, but also realizes the mutual authentication. The security analysis of the proposed protocol is presented in the paper. The analysis results show that this protocol is secure against some common attacks, and no secret message leaks even if the messages are broken. Compared with the two-way QSDC protocols, the presented protocol is a one-way quantum communication protocol which has the immunity to Trojan horse attack. Furthermore, our proposed protocol can be realized without quantum memory.     

Controlled Secure Direct Communication Protocol via the Three-Qubit Partially Entangled Set of States

In this paper, we first re-examine the previous protocol of controlled quantum secure direct communication of Zhang et al.’s scheme, which was found insecure under two kinds of attacks, fake entangled particles attack and disentanglement attack. Then, by changing the party of the preparation of cluster states and using unitary operations, we present an improved protocol which can avoid these two kinds of attacks. Moreover, the protocol is proposed using the three-qubit partially entangled set of states. It is more efficient by only using three particles rather than four or even more to transmit one bit secret information. Given our using state is much easier to prepare for multiqubit states and our protocol needs less measurement resource, it makes this protocol more convenient from an applied point of view.     

Information Transfer with Two-state Two-particle Quantum Systems

Abstract

Any future quantum information machine will contain unitary operators and entangled particle states. The Hilbert space describing the action of the quantum information machine separates into a bosonic and a fermionic sector. Because the bosonic sector is of higher dimension, it is always possible to encode more information into a multiboson state than into a multifermion state, given the same complexity, that is unitary representation, of the quantum information machine. This is explicitly studied for the case of two particles defined in two modes. There the beam splitter is a generic representation of any U(2) matrix, and it has recently been shown that one can realize any N-dimensional unitary operator by successive application of such two-dimensional operators. The two-boson two-mode Hilbert space is of dimension three, and thus one can encode log₂3 = 1·57 bits of information into such an entangled state. Finally, some explicit schemes for creating and detecting the three possible, two-photon, two-mode states spanning the bosonic Bell basis are given.     

Phase auto-compensating high-dimensional quantum cryptography in elliptical-core few-mode fibres

High-dimensional quantum cryptography through optical fibres with several spatial modes requires an efficient quantum key distribution (QKD). However, optical modes acquire different phases and lags due to modal dispersion and random fluctuations, and a modal crosstalk appears under propagation. At present, special optical fibres for spatial multiplexing are being proposed in order to reduce notably the modal crosstalk, however, arbitrary relative phases and lags between modes are always present, which prevents getting an efficient phase encoding QKD. In this work, we take advantage of elliptical-core few-mode optical fibres presenting a very low modal crosstalk and propose an exact phase auto-compensating method by making photons travel several times the path between Alice and Bob (rounds) and by using appropriate modal inversions in each round trip. In order to make clear the proposed phase auto-compensating method, we study in detail a four-dimensional BB84 QKD case with single photon states excited in both polarization and spatial LP modes.     

Non-classical correlations in the general state of two SC-qubit with a phase damping: non-local correlation and geometric discord

A system consists of two charged qubits are initially prepared in a maximum entangled Bell state and having no mutual interaction, where each qubit interacts independently with a superconducting transmission line resonator. An analytical solution of the time evolution of the final state of the system with the effect of a phase decoherence is found. In previous works, quantum correlations are only investigated in X-state for the models which are as our model. In this work, the analytical formulas of the geometric quantum discord (GQD) and measurement-induced non-locality (MIN) are introduced for a general state of two-qubit (non-X-state). Quantum correlations are studied via GQD and MIN with quantum entanglement (QE). It is found that a sudden disappearance only occurs for QE, while MIN and GQD still exist. Due to the increase in the amplitude of the coherent states, the intervals of the sudden disappearance of QE increase and MIN and GQD decrease. It is interesting to note that initial correlations can be lost and they reach their stationary correlations with the increase in the intrinsic decoherence. The stationary correlation of MIN can be destroyed, it reach zero value, when both the decoherence effect and detuning are present simultaneously. By starting with different types of Bell-like states, the stationary correlations as well as the time intervals of sudden disappearance have notable changes. It is possible to control the quantum correlations with certain parameter sets.     

光孤子对量子态的高速调制

从高速调制的角度,研究了量子光通信系统。提出用孤子调制承载量子态的方案。方案着重从调制量子态的通信信道出发,分别论证了孤子在NLSE光纤和SIT媒介中的传输演变;得出量子压缩态和纠缠态可以在二级传输线(由传统光纤和支持自激励透明孤子的二级媒介组成)中,由孤子的传输演变产生;从而实现孤子对量子态的调制。这种调制方案,利用了光的波粒二重性,融合了量子和孤子的特性,是未来量子光通信的一种发展趋势。     

Transferring the spatial state of an entangled photon pair to a single photon by photon detection

We show theoretically that the spatial state of entangled photons generated by parametric down-conversion can be transferred to the spatial state of an idler photon by signal photon detection. This study considered the general condition with an arbitrary pump field profile and the detection of a signal photon at an arbitrary distance from a nonlinear crystal where the entangled photons are generated. Upon the detection of a signal photon, the two-photon state function of the entangled state can be transferred to a single-photon state function of the idler field due to the EPR type correlation between the signal and idler fields. The spatial state of the idler field contains more information on the original two-photon state.     

The Damped-vacuum-field Jaynes-Cummings Model

Abstract

We study the dynamics of a two-level atom interacting with a single mode of a damped cavity at 0 K when the cavity is initially in the vacuum state and the atom enters it in an arbitrary (pure or mixed) state. A complete analytical solution of this simple model is presented. On the basis of this solution we firstly investigate the pseudo-spin dynamics of the atom and the cavity field, secondly give an illustration of the Araki-Lieb theorem concerning the von Neumann entropies of interacting quantum systems and thirdly demonstrate the generation of entangled states of the atom and cavity field that are of interest in connection with the Bell inequalities.     

Preparation of two-mode anticorrelated photon-number state via atom de Broglie wave deflection

Abstract

It is shown that the deflection of an atom de Broglie wave at two adjacent cavities containing non-resonant weak fields can yield a highly entangled quantum state of the atom–field system in which discernible atomic beams are entangled to internal states of the atom and to two-mode photon-number states of the fields. Two-mode anticorrelated entangled photon-number states characterized by the total photon number can be prepared by the detection of the atom in given directions of the propagation.     

Fast preparation of entangled states of two qutrits in cavity or circuit QED

We present a scheme for generating entangled states of two qutrits, by using a 4-level system and a 3-level system coupled to a single cavity. Because of only employing resonant interactions, the entangled state can be fast created. This scheme is easy to implement in experiments since only a single cavity is needed and none of measurement, auxiliary system and identical system-cavity coupling constant is required. Furthermore, two-qutrit partially or maximally entangled states can be created by using this scheme. The proposal is quite general, which can be used to prepare entangled states of two qutrits in various physical systems, such as two natural or artificial atoms of the Λ-type or Δ-type three lowest levels, coupled to an optical or microwave cavity.     

Quantum nonlocality for a mixed entangled coherent state

Quantum nonlocality is tested for an entangled coherent state, interacting with a dissipative environment. A pure entangled coherent state violates Bell's inequality regardless of its coherent amplitude. The higher the initial nonlocality, the more rapidly quantum nonlocality is lost. The entangled coherent state can also be investigated in the framework of 2 X 2 Hilbert space. The quantum nonlocality persists longer in 2 X 2 Hilbert space. When it decoheres it is found that the entangled coherent state fails the nonlocality test, which contrasts with the fact that the decohered entangled state is always entangled.     

Measurement device independent quantum key distribution assisted by hybrid qubit

Abstract

Measurement device independent Quantum Key Distribution (MDI-QKD), is immune to all attacks on detection and achieve immense improvement with respect to quantum key distribution system security. However, Bell state measurement (BSM), the kernel processing in MDI-QKD, can only identify two of the four Bell states, which limits the efficiency of the protocol. In this paper, a modified MDI-QKD with hybrid qubit is proposed to provide a major step towards answering this question. The hybrid qubits, which are composed of single photon qubit qubits and coherent qubit, are sent to the quantum relay to perform parallel BSMs synchronously and bit flip can be easily operated to complete the whole key distribution process. The secure key rate can be improved with our modified protocol owing to the higher success probability of BSM, which is increased by adding the parity check of coherent qubit. Furthermore, though our protocol requires photon number resolving detectors, the BSM of coherent state could be instead implemented using squeezed state which makes our scheme practical with state-of-the-art devices.     

Exact solutions of a linear model of quantum solids

The ground state energyE of a linear quantum crystal can be calculated exactly by the transfer integral method, if the total wave function is assumed to be a product of single-particle functions ? and Jastrow factorsf between nearest neighbors only. The variation ofE with respect tof leads to a Schrödinger equation containing an effective potential, which consists of the bare interaction between two particles and a part due to the influence of all other particles. This equation is coupled to the transfer integral equation. For any given ? the coupled system of equations can be solved in a self-consistent way. The one- and two-particle distribution functions and the effective potential can also be computed exactly. In a similar way for any givenf the optimal ? can be determined.