Entanglement of two interacting electrons in a double quantum dot

We study the dynamics of two interacting electrons confined in a double quantum dot in the presence of radiation pulses. We show that the strong transition between two localized states is induced at the avoided crossing between the corresponding energy levels, leading to the maximally entangled Bell states. The frequency and duration of selective light pulses for producing maximally entangled states are identified by both analytic and exact numerical solution of the quantum dynamical equations.     

Entangled photon states generation and regeneration using a nonlinear fiber ring resonator

We propose a simple system of the entangled photon states generation and regeneration using a standard diode laser, a Mach Zehnder Interferometer (MZI) and a fiber optic ring resonator (FORR). Light from the diode laser is launched into an MZI and circulated in the FOOR, without any optical pumping components included in the system. The nonlinear light pulses are generated by a Kerr nonlinear effects type, while the resonance peaks are formed by the four-wave mixing of light pulses in the FORR. The entangled photons can be performed by using the polarization control device, and then detected by the avalanche photo-detectors, where the entangled photon visibility is plotted and seen. Similarly, the entangled photon states can be easily formed by using the appropriated coupling ratios into a fiber coupler, then into a ring resonator, i.e. without an MZI. The use of the entangled photons generation based on a fiber optic scheme for quantum teleportation, quantum key distribution via optical wireless link, and the system of the entangled photon states recovery by using a fiber ring resonator incorporating an erbium-doped fiber (EDF) have been investigated and discussed. The feasibility of dense coding using multi-entangled photons generation based on the fiber optic scheme and the effect of the entangled state walk-off along the optical fiber are also discussed, respectively.     

Efficient two-photon light amplification by a coherent biexciton wave

A reversible coupling between photon pair states and a long-lived, highly coherent biexciton wave in CuCl allows efficient phase-sensitive two-photon amplification or attenuation of ultrashort light pulses. We demonstrate a gain of 350 cm(-1) for a pump intensity of 1 MW/cm(2), nearly 2 orders of magnitude higher than that achievable with conventional parametric crystal amplifiers. We develop a theoretical model that describes this new type of parametric converter where the light pump is replaced by a coherent biexciton wave and show that it is well suited for the generation of entangled photons and the squeezing of an optical beam with ultrafast time gating.     

Second-harmonic generation via micro ring resonators for optimum entangled photon visibility

An analysis of a new technique for quantum key distribution (QKD) using the entangled photon within a micro ring resonator is presented. The Kerr nonlinear type of light in the micro ring resonator induces the nonlinear behavior known as chaos within the device, where the superposition of the chaotic signals via a four-wave mixing type introduces the clear second-harmonic pulses. The generation of clear second-harmonic pulses is achieved by controlling the appropriate ring parameters. When the polarization control devices are applied into the system, the optimal entangle photon visibility is obtained. The condition for long-distance link is discussed, where the optimal entangled photon visibility in term of Bell's states is described.     

Entanglement properties in a system of a pairwise entangled state

Based on the quantum information theory, this paper has investigated the entanglement properties of a system which is composed of the two entangled two-level atoms interacting with the two-mode entangled coherent fields. The influences of the strength of light field and the two parameters of entanglement between the two-mode fields on the field entropy and on the negative eigenvalues of partial transposition of density matrix are discussed by using numerical calculations. The result shows that the entanglement properties in a system of a pairwise entangled states can be controlled by appropriately choosing the two parameters of entanglement between the two-mode entangled coherent fields and the strength of two light fields respectively.     

Increasing entanglement between Gaussian states by coherent photon subtraction

We experimentally demonstrate that the entanglement between Gaussian entangled states can be increased by non-Gaussian operations. Coherent subtraction of single photons from Gaussian quadrature-entangled light pulses, created by a nondegenerate parametric amplifier, produces delocalized states with negative Wigner functions and complex structures more entangled than the initial states in terms of negativity. The experimental results are in very good agreement with the theoretical predictions.     

飞秒超短脉冲的双光子干涉

利用钛宝石飞秒激光超短脉冲抽运BBO晶体,产生Ⅰ型自发光参量下转移的双光子对,在实验上得到双光子对的四阶干涉现象,证明了双光子纠缠态的关联特性。     

Entanglement of states and quantum distributions in pulsed optical parametric oscillator

The time-domain properties of Einstein-Podolsky-Rosen (EPR) entangled states of light are studied by means of analysis of both the Wigner functions and variance of quadrature amplitudes of generated modes. As a source of EPR states a non-degenerate optical parametric oscillator is considered driven by a sequence of laser pulses with a Gaussian time envelope.     

Polarized soliton pulses generation using nonlinear micro ring resonators for multi- and long distance links

We propose a new concept of quantum soliton pulses generation using a soliton pulse in the micro ring resonators. Firstly, the chaotic soliton pulses are generated and circulated within the integrated micro ring resonators. Secondly, the specific second harmonic pulses are selected by using the appropriate ring parameters. The superposition of the second harmonic pulses within the micro ring devices introduces the randomly polarized photons within the micro ring device. The entangled photon visibility of the polarized photon is seen after passing the polarization control devices and projecting on the detectors. The optimum entangled photon visibility is obtained. The advantage of such a system is that the quantum repeater unit can be redundant for long distance quantum communication link, whereas the use of the system for multi-entangled photon sources and links is also available. The system degradation via the entangled photon states timing walk-off is also discussed.     

Experimental unconditional preparation and detection of a continuous bound entangled state of light

Among the possibly most intriguing aspects of quantum entanglement is that it comes in free and bound instances. The existence of bound entangled states certifies an intrinsic irreversibility of entanglement in nature and suggests a connection with thermodynamics. In this Letter, we present a first unconditional, continuous-variable preparation and detection of a bound entangled state of light. We use convex optimization to identify regimes rendering its bound character well certifiable, and continuously produce a distributed bound entangled state with an extraordinary and unprecedented significance of more than 10 standard deviations away from both separability and distillability. Our results show that the approach chosen allows for the efficient and precise preparation of multimode entangled states of light with various applications in quantum information, quantum state engineering, and high precision metrology.     

Correlations of Multiplexed Quantum Ghost Images and Improvement of the Quality of Restored Image

The currently used ghost-image schemes traditionally involve two-mode entangled light states or incoherent radiation. Here, we consider the application of four-mode entangled light states and show that multiplexed ghost images (MGI) formed by four-mode entangled quantum light states have mutual spatial correlations determined by the eighth-order field correlation functions. We develop a special algorithm to calculate high-order correlations of Bose operators. We also demonstrate that accounting of the MGI correlations allows us to improve the quality of the restored image of an object while processing the MGI by the measurement reduction method. We carry out computer modeling of the image recovery from the MGI. We establish that in the considered example the signal-to-noise ratio of the reduced ghost image is 4.6 times higher than the best signal-to-noise ratio for the ghost images themselves.     

Measurement induced entanglement and quantum computation with atoms in optical cavities

We propose a method to prepare entangled states and implement quantum computation with atoms in optical cavities. The internal states of the atoms are entangled by a measurement of the phase of light transmitted through the cavity. By repeated measurements an entangled state is created with certainty, and this entanglement can be used to implement gates on qubits which are stored in different internal degrees of freedom of the atoms. This method, based on measurement induced dynamics, has a higher fidelity than schemes making use of controlled unitary dynamics.     

Two-pulse correlations in noisy quantum channels

The modification of initially entangled light pulses passing through noisy quantum channels, modeled by dispersive and absorbing four-port devices, is studied with the help of recently obtained results on quantum state transformations. The fidelity and indices of quantum correlations based on the von Neumann entropy are calculated. Their dependence on both the pulse shape and the four-port device parameters is studied. It is shown that due to dispersion and absorption, the quantum correlations are reduced, and they decrease with increasing initial entanglement.     

Time-resolved homodyne characterization of individual quadrature-entangled pulses

We describe a simple and efficient set-up to generate and characterize femtosecond quadrature-entangled pulses. Quantum correlations equivalent to about 2.5 dB squeezing are efficiently and easily reached using the non-degenerate parametric amplification of femtosecond pulses through a single-pass in a thin (100 m) potassium niobate crystal. The entangled pulses are then individually sampled to characterize the non-separability and the entropy of formation of the states. The complete experiment is analysed in the time-domain, from the pulsed source of quadrature entanglement to the time-resolved homodyne detection. This particularity allows for applications in quantum communication protocols using continuous-variable entanglement.     

Nonlinear optics and quantum entanglement of ultraslow single photons

Two light pulses propagating with slow group velocities in a coherently prepared atomic gas exhibit dissipation-free nonlinear coupling of an unprecedented strength. This enables a single-photon pulse to coherently control or manipulate the quantum state of the other. Processes of this kind result in generation of entangled states of radiation field and open up new prospectives for quantum information processing.     

Entangled double-helix phase

We present a novel phase with an entangled double-helix structure. Beams with this phase have the same transverse patterns as those of interference between two doughnut beams. This proposed method allows a complete set of the superpositions of the doughnut modes or the orbital angular momentum states with different topological orders to be obtained. Furthermore, it introduces a simple continuous and controlled rotation of the transverse patterns by use of a spatial light modulator. It can be used to form a three-dimensional structure by three-dimensional trapping in an optical tweezers setup or to study the quantum characteristics of an optical vortex.     

Soliton squeezing in microstructure fiber

We demonstrate, for the first time to our knowledge, the generation of squeezed light by means of soliton self-phase modulation in microstructure fiber. We observe and characterize the formation of solitons in the microstructure fiber at 1550 nm. A maximum squeezing of 2.7 dB is observed, corresponding to 4.0 dB after correcting for detection losses. The dependence of this quantum-noise reduction on various system parameters is studied in detail. Features of the microstructure fiber can be exploited for generation of low-energy continuous-variable entangled pulses for use in all-fiber teleportation experiments.     

Manipulating a Bose-Einstein condensate with a single photon

We propose a method to create macroscopic superpositions, so-called Schr?dinger cat states, of different motional states of an ideal Bose-Einstein condensate. The scheme is based on the scattering of a freely expanding condensate by the light field of a high-finesse optical cavity in a quantum superposition state of different photon numbers. The atom-photon interaction creates an entangled state of the motional state of the condensate and the photon number, which can be converted into a pure atomic Schr?dinger cat state by operations only acting on the cavity field. We discuss in detail the fully quantised theory and propose an experimental procedure to implement the scheme using short coherent light pulses. Received 26 June 2000 and Received in final form 2nd October 2000     

Stark效应对量子纠缠信息交换传递的影响

用多个“二能级原子与单模光场”组成的联合物理模型,考虑Stark效应后,推导了该系统的态矢演化式,得到光场纠缠度的数值计算结果.讨论了Stark效应和初始场强对量子纠缠信息交换传递的影响.结果发现,在一定条件下,原子纠缠态与光场纠缠态之间可以转化,实现了量子纠缠信息的交换传递,且Stark效应和初始场强对此过程有着显著的影响:光场的纠缠程度随着初始场强的增强而增大;在强场条件下,光场纠缠度出现崩塌-回复现象.Stark移位参量越大,光场纠缠度振荡越剧烈,说明Stark效应破坏了光场纠缠程度的稳定性.     

Hyperentangled mixed phased Dicke states: optical design and detection

A recently introduced family of multipartite entangled states, the 4-qubit phased Dicke states, has been created by 2-photon hyperentanglement. Our experimental method allows high state fidelity and generation rate. By introducing quantum noise in the multipartite system in a controlled way, we have tested the robustness of these states. To this purpose the entanglement of the resulting multipartite entangled mixed states has been verified by using a new kind of structural witness.