Continuous-Variable Entanglement Generation from a Four-State Atom under Raman Excitation

We investigate the generation and the evolution of continuous-variable (CV) entanglement from a laser-driven four-state atom inside a doubly resonant cavity under Raman excitation. Two transitions in the four-state atom independently interact with the two cavity modes, while two other transitions are driven by coupling laser fields. By including the atomic relaxation as well as cavity losses, we show that the CV entanglement with large mean number of
photons can be generated in our scheme. We also show that the intensity of the coupling laser fields can influence effectively the entanglement period of the cavity field. Different from the conventional resonant excitation scheme where zero one-photon detuning are required, it is found that the intensity and period of entanglement between the two cavity modes as well as the total mean photon number of the cavity field can be adjusted by properly
modulating the frequency detuning.     

Robust scheme for implemention of quantum phase gates for two atoms

We propose a scheme for implementing conditional quantum phase gates for two four-state atoms trapped in a cavity. The two ground states of the atoms are coupled through two Raman processes induced by the cavity mode and two classical fields. Under certain conditions nonresonant Raman processes lead to two-atom coupling and can be used to produce conditional phase gates. The scheme is insensitive to cavity decay, thermal photons, and atomic spontaneous emission. The scheme does not require individual addressing of the atoms.     

离散调制连续变量量子密钥分发的安全边界

对一种结合离散调制和反向协调,适用于长距离传输的连续变量量子密钥分发四态协议的安全性进行了严格证明.这种协议中Alice发送的态与高斯调制协议中的有一定差异,这种差异可以等价成信道衰减和额外噪声.另外,由于Alice不可能做到精确调制,这会导致其发送的相干态中含有噪声.把这种调制引起的噪声看作光源的噪声,并推导出了在光源噪声不能被窃听者所利用的条件下的安全码率的下界.为了避免实验上快速、随机的控制本地振荡光的相位,还将无开关协议和四态协议相结合,分析了其安全性.     

Quantum key distribution with two-qubit quantum codes

We propose a prepare-and-measure scheme for quantum key distribution with two-qubit quantum codes. The protocol is unconditionally secure under all types of intercept-and-resend attack. Given the symmetric and independent errors to the transmitted qubits, our scheme can tolerate a bit of an error rate up to 26% in four-state protocol and 30% in six-state protocol, respectively. These values are higher than all currently known threshold values for the prepare-and-measure protocols. Moreover, we give a practically implementable linear optics realization for our scheme.     

Four-State Continuous-Variable Quantum Key Distribution with Photon Subtraction

Four-state continuous-variable quantum key distribution (CVQKD) is one of the discretely modulated CVQKD which generates four nonorthogonal coherent states and exploits the sign of the measured quadrature of each state to encode information rather than uses the quadrature \(\hat {x}\) or \(\hat {p}\) itself. It has been proven that four-state CVQKD is more suitable than Gaussian modulated CVQKD in terms of transmission distance. In this paper, we propose an improved four-state CVQKD using an non-Gaussian operation, photon subtraction. A suitable photon-subtraction operation can be exploited to improve the maximal transmission of CVQKD in point-to-point quantum communication since it provides a method to enhance the performance of entanglement-based (EB) CVQKD. Photon subtraction not only can lengthen the maximal transmission distance by increasing the signal-to-noise rate but also can be easily implemented with existing technologies. Security analysis shows that the proposed scheme can lengthen the maximum transmission distance. Furthermore, by taking finite-size effect into account we obtain a tighter bound of the secure distance, which is more practical than that obtained in the asymptotic limit.     

Efficient multiwave mixing in the ultraslow propagation regime and the role of multiphoton quantum destructive interference

We analyze a lifetime-broadened four-state four-wave-mixing (FWM) scheme in the ultraslow propagation regime and show that the generated FWM field can acquire the same group velocity and pulse shape as those of an ultraslow pump field. We show that a new type of induced transparency resulted from multiphoton destructive interference that significantly reduced the pump field loss. Such induced transparency based on multphoton destructive interference may have important applications in other nonlinear optical processes.     

Balancing four-state continuous-variable quantum key distribution with linear optics cloning machine

We show that the secret key generation rate can be balanced with the maximum secure distance of four-state continuous-variable quantum key distribution(CV-QKD) by using the linear optics cloning machine(LOCM). Benefiting from the LOCM operation, the LOCM-tuned noise can be employed by the reference partner of reconciliation to achieve higher secret key generation rates over a long distance. Simulation results show that the LOCM operation can flexibly regulate the secret key generation rate and the maximum secure distance and improve the performance of four-state CV-QKD protocol by dynamically tuning parameters in an appropriate range.     

Time-dependent theoretical approach to the influence of laser fields on the resonance enhanced multi-photon ionization of SH radical

This paper reports that the (2+1) resonance enhanced multi-photon ionization spectra of SH radical in external fields are simulated using the split-operator scheme of time-dependent wave-packet method. Two ionic states, i.e.~a¹Δ and b¹∑⁺, are involved in the simulation. It gives the simulated photoelectron spectra, the population in each electronic state, as well as the projection of the wave-packet in each electronic state on different vibrational states. These results show that the so-called four-state model can represent the experimental results well.     

基于量子催化的离散调制连续变量量子密钥分发

相比于离散变量量子密钥分发,连续变量量子密钥分发虽然具备更高的安全码率等优势,但是在安全传输距离上却略有不足.尽管量子催化的运用对高斯调制连续变量量子密钥分发协议的性能,尤其在安全传输距离方面有着显著的提升,然而能否用来改善离散调制协议的性能却仍然未知.鉴于上述分析,本文提出了一种基于量子催化的离散调制协议的方案,试图在安全密钥率、安全传输距离和最大可容忍过噪声方面进一步提升协议性能.研究结果表明,在相同参数下,当优化量子催化引入的透射率T,相比于原始四态调制协议,所提方案能够有效地提升量子密钥分发的性能.特别是,对于可容忍过噪声为0.002,量子催化可将安全通信距离突破300 km,密钥率为10^-8bits/pulse,而过大的可容忍噪声会抑制量子催化对协议性能的改善效果.此外,为了彰显量子催化的优势,本文给出了点对点量子通信的最终极限Pirandola-Laurenza-Ottaviani-Banchi边界,仿真结果表明,虽然原始方案与所提方案都未能突破这种边界,但是相比于前者,后者能够在远距离通信上逼近于这种边界,这为实现全球量子安全通信的最终目标提供理论依据.     

A Hierarchical Modulation Coherent Communication Scheme for Simultaneous Four-State Continuous-Variable Quantum Key Distribution and Classical Communication

We present a hierarchical modulation coherent communication protocol, which simultaneously achieves classical optical communication and continuous-variable quantum key distribution. Our hierarchical modulation scheme consists of a quadrature phase-shifting keying modulation for classical communication and a four-state discrete modulation for continuous-variable quantum key distribution. The simulation results based on practical parameters show that it is feasible to transmit both quantum information and classical information on a single carrier. We obtained a secure key rate of \(10^{-3}\) bits/pulse to \(10^{-1}\) bits/pulse within 40 kilometers, and in the meantime the maximum bit error rate for classical information is about \(10^{-7}\). Because continuous-variable quantum key distribution protocol is compatible with standard telecommunication technology, we think our hierarchical modulation scheme can be used to upgrade the digital communication systems to extend system function in the future.     

Four-State Quantum Chain as a Model of Sequence Evolution

A variety of selection-mutation models for DNA (or RNA) sequences, well known in molecular evolution, can be translated into a model of coupled Ising quantum chains. This correspondence is used to investigate the genetic variability and error threshold behaviour in dependence of possible fitness landscapes. In contrast to the two-state models treated hitherto, the model explicitly takes the four-state nature of the nucleotide alphabet into account and allows for the distinction of mutation rates for the different base substitutions, as given by standard mutation schemes of molecular phylogeny. As a consequence of this refined treatment, new phase diagrams for the error threshold behaviour are obtained, with appearance of a novel phase in which the nucleotide ordering of the wildtype sequence is only partially conserved. Explicit analytical and numerical results are presented for evolution dynamics and equilibrium behaviour in a number of accessible situations, such as quadratic fitness landscapes and the Kimura 2 parameter mutation scheme.     

Stochastic Four-State Mechanochemical Model of F1-ATPase

F1-ATPase, a part of ATP synthase, can synthesize and hydrolyze ATP moleculars in which the central γ-subunit rotates inside the α3β3 cylinder. A stochastic four-state mechanochemical coupling model of F1-ATPase is studied with the aid of the master equation. In this model, the ATP hydrolysis and synthesis are dependent on ATP, ADP, and Pi concentrations. The effects of ATP concentration, ADP concentration, and the external torque on the occupation probability of binding-state, the rotation rate and the diffusion coefficient of F1-ATPase are investigated. Moreover, the results from this model are compared with experiments. The mechanochemical mechanism F1-ATPase is qualitatively explained by the model.     

Equal-power 4m-th power sum squeezing of generalized magnetic-field component in vacuum state immiting four-state superposition multimode entangled state light field

By utilizing multimode squeezed states theory, we study the generalized nonlinear equal-power higherpower sum squeezing properties of the generalized magnetic-field component in four-state superposition multimode entangled state light field ｜ψ1^（4）〉q. The state is composed of multimode vacuum state, multimode coherent state and its contrary state, multimode imaginary coherent state. It is found that the state ｜ψ1^（4）〉q is a type of four-state superposition multimode nonclassical light field, and under certain fixed conditions, the generalized magnetic-field component in the state ｜ψ1^（4）〉q can display generalized nonlinear equal-power 4m-th power sum squeezing effects which change periodically.     

Upper bound on key generation rate of quantum key distribution with two-way or two-step quantum channels

We present an asymptotic security proof of deterministic quantum key distribution (DQKD) with a two-way quantum channel. The security proof of DQKD with a two-way quantum channel is different from that of BB84, because Eve can attack the travel qubits twice, both in line Bob to Alice and in line Alice to Bob. With the no-signaling principle and the property of mutual information, we obtain an upper bound of the final key generation of entanglement-based DQKD and hence single-photon four-state DQKD. Our results can be applied to the protocol of QKD with two-step quantum channels.     

Ultraslow optical solitons in a cold four-state medium

We show the formation of ultraslow optical solitons in a lifetime broadened four-state atomic medium under Raman excitation. With appropriate conditions we demonstrate, both analytically and numerically, that both bright and dark ultraslow optical solitons can occur in such a highly resonant medium with remarkable propagation characteristics. This work may open other research opportunities in condensed matter and may result in a substantial impact on technology.     

激光场强度对NO电子态粒子数布居影响的理论研究

用含时波包方法研究了激光场强度对NO各电子态上粒子数布居的影响.在计算中,采用了四态模型.利用分裂算符傅里叶变换方法求解Schrdinger方程,得到各个电子态的波函数,从而求得各个电子态上粒子数的布居.计算结果表明,不同强度的抽运光对各个电子态上粒子数布居会产生不同的影响.通过改变激光强度可以实现对电离产率的控制,这种思想对实现原子分子过程的激光操控具有重要意义.     

A reformulation of the Feynman chessboard model

The chessboard model is reviewed and reformulated as a four-state process. In this formulation both the Dirac propagator of the chessboard model and the partition function of the associated Ising chain are observed to be projections of a single matrix of partition functions onto two orthogonal eigenspaces. This helps clarify the role played by the phase associated with Feynman paths in this model.     

Pump-induced nonlinear refractive-index change in erbium- and ytterbium-doped fibers: theory and experiment

A 980-nm pump-induced nonlinear refractive-index (n2) change in erbium-doped (20-m) and ytterbium-doped (20-m) fibers has been measured at 1064 nm by time-delayed photorefractive beam coupling in a co-propagating and counterpropagating geometry. It was found that n2 decreases at the wavelength of the probe beam when the pump beam is present. We present a semiclassical theory based on a four-state system that accounts for the pump-induced change of n2. Both theoretical and experimental results show that a significant index change, of the order of 10%, can be obtained for cw pump powers as small as 10 mW.