Spatial quantum coherent modulation with perfect hybrid vector vortex beam based on atomic medium

The perfect hybrid vector vortex beam(PHVVB)with helical phase wavefront structure has aroused significant concern in recent years,as its beam waist does not expand with the topological charge(TC).In this work,we investigate the spatial quantum coherent modulation effect with PHVVB based on the atomic medium,and we observe the absorption characteristic of the PHVVB with different TCs under variant magnetic fields.We find that the transmission spectrum linewidth of PHVVB can be effectively maintained regardless of the TC.Still,the width of transmission peaks increases slightly as the beam size expands in hot atomic vapor.This distinctive quantum coherence phenomenon,demonstrated by the interaction of an atomic medium with a hybrid vector-structured beam,might be anticipated to open up new opportunities for quantum coherence modulation and accurate magnetic field measurement.

Dynamic manipulation of probe pulse and coherent generation of beating signals based on tunneling-induced inference in triangular quantum dot molecules

We investigate the dynamic propagation of a probe field via the tunneling-induced interference effect in a triple model of quantum dot molecules. By theoretical analysis and numerical simulation, we find that the number of transparency window relate to the energy splitting and the group velocity of probe field can be effectively controlled by the tunneling coupling intensity. In addition, in the process of light storage and retrieval, when the excited states have no energy splitting in the storage stage but opposite values of the energy splitting in the retrieval stage, the beating signals can be generated.

Continuous-variable quantum teleportation of even and odd coherent states through varied gain channels

This paper has investigated quantum teleportation of even and odd coherent states in terms of the EPR entanglement states for continuous variables. It discusses the relationship between the fidelity and the entanglement of EPR states, which is characterized by the degree of squeezing and the gain of classical channels. It shows that the quality of teleporting quantum states also depends on the characteristics of the states themselves. The properties of teleporting even and odd coherent states at different intensities are investigated. The difference of teleporting two such kinds of quantum states are analysed based on the quantum distance function.

Passive decoy-state quantum key distribution for the weak coherent photon source with finite-length key

Passive decoy-state quantum key distribution systems, proven to be more desirable than active ones in some scenarios, also have the problem of device imperfections like finite-length keys. In this paper, based on the WCP source which can be used for the passive decoy-state method, we obtain the expressions of single-photon error rates, single-photon counts, and phase error rates. According to the information of smooth min-entropy, we calculate the key generation rate under the condition of finite-length key. Key generation rates with different numbers of pulses are compared by numerical simulations. From the results, it can be seen that the passive decoy-state method can have good results if the total number of pulses reaches 1010. We also simulate the passive decoy-state method with different probabilities of choosing a pulse for parameter estimation when the number of pulses is fixed.

Managing Quantum Heat Transfer in a Nonequilibrium Qubit-Phonon Hybrid System with Coherent Phonon States

We investigate quantum heat transfer in a nonequilibrium qubit-phonon hybrid open system,dissipated by external bosonic thermal reservoirs.By applying coherent phonon states embedded in the dressed quantum master equation,we are capable of dealing with arbitrary qubit-phonon coupling strength.It is counterintuitively found that the effect of negative differential thermal conductance is absent at strong qubit-phonon hybridization,but becomes profound at weak qubit-phonon coupling regime.The underlying mechanism of decreasing heat flux by increasing the temperature bias relies on the unidirectional transitions from the up-spin displaced coherent phonon states to the down-spin counterparts,which seriously freezes the qubit and prevents the system from completing a thermodynamic cycle.Finally,the effects of perfect thermal rectification and giant heat amplification are unraveled,thanks to the effect of negative differential thermal conductance.These results of the nonequilibrium qubit-phonon open system would have potential implications in smart energy control and functional design of phononic hybrid quantum devices.

Quantum Properties of Two-Mode Squeezed Even and Odd Coherent States

Two new types of quantum states are constructed by applying the operator s（ξ） = exp（ξ＊ ab - ξa＋b＋） on the two-mode even and odd coherent states. The mathematical and quantum statistical properties of such states are investigated. Various nonclassical features of these states, such as squeezing properties, the inter-mode photon bunching, and the violation of Cauchy-Schwarz inequality, are discussed. The Wigner function in these states are studied in detail.

Incoherent control of two classes of finite-level quantum systems

The incoherent control of finite-level quantum systems is investigated. Following a brief introduction to coherent control paradigms in quantum control, a control problem that can not be accomplished using only coherent control is presented. For such a control problem, it is proved that it can be accomplished using incoherent control based on projective measurement and coherent control for two classes of finite-level quantum systems, i.e., eigenstate controllable quantum systems and wavefunction controllable quantum systems.

Statistical properties of coherent photon-subtracted two-mode squeezed vacuum and its application in quantum teleportation

We introduce a kind of non-Gaussian entangled state, which can be obtained by operating a non-local coherent photon-subtraction operation on a two-mode squeezed vacuum. It is found that its normalization factor is only related to the Legendre polynomials, which is a compact expression. Its statistical properties are discussed by the negative region Wigner function with the analytical expression. As an application, the quantum teleportation for coherent states is considered by using the non-Gaussian state as an entangled channel. It is found that the teleportation fidelity can be enhanced by this non-Gaussian operation.

Passive decoy-state quantum key distribution using weak coherent pulses with modulator attenuation

Passive decoy-state quantum key distribution is more desirable than the active one in some scenarios. It is also affected by the imperfections of the devices. In this paper, the influence of modulator attenuation on the passive decoy-state method is considered. We introduce and analyze the unbalanced Mach-Zehnder interferometer, briefly, and combining with the virtual source and imaginary unitary transformation, we characterize the passive decoy-state method using a weak coherent photon source with modulator attenuation. According to the attenuation parameter 6, the pass efficiencies are given. Then, the key generation rate can be acquired. From numerical simulations, it can be seen that modulator attenuation has a non- negligible influence on the performance of passive-state QKD protocol. Based on the research, the analysis method of virtual source and imaginary unitary transformation are preferred in analyzing passive decoy state protocol, and the passive decoy-state method is better than the active one and is close to the active vacuum ＋ weak decoy state under the condition of having the same modulator attenuation.

Phase-controlled coherent population trapping in superconducting quantum circuits

We investigate the influences of the-applied-field phases and amplitudes on the coherent population trapping behavior in superconducting quantum circuits. Based on the interactions of the microwave fields with a single A-type three-level fluxonium qubit, the coherent population trapping could be obtainable and it is very sensitive to the relative phase and amplitudes of the applied fields. When the relative phase is tuned to 0 or π, the maximal atomic coherence is present and coherent population trapping occurs. While for the choice of π/2, the atomic coherence becomes weak. Meanwhile, for the fixed relative phase π/2, the value of coherence would decrease with the increase of Rabi frequency of the external field coupled with two lower levels. The responsible physical mechanism is quantum interference induced by the control fields, which is indicated in the dressed-state representation. The microwave coherent phenomenon is present in our scheme, which will have potential applications in optical communication and nonlinear optics in solid-state devices.

Properties of field quantum entropy evolution in the Jaynes-Cummings model with initial squeezed coherent states field

The properties of the field quantum entropy evolution in a system of a single-mode squeezed coherent state field interacting with a two-level atom is studied by utilizing the complete quantum theory, and we focus our attention on the discussion of the influences of field squeezing parameter γ, atomic distribution angle θ and coupling strength g between the field and the atom on the properties of the evolution of field quantum entropy. The results obtained from numerical calculation indicate that the amplitude of oscillation of field quantum entropy evolution decreases with the increasing of squeezing parameter γ, and that both atomic distribution angle θ and coupling strength g between the field and the atom can influence the periodicity of field quantum entropy evolution.

Application of a Discrete Phase-Randomized Coherent State Source in Round-Robin Differential Phase-Shift Quantum Key Distribution

Recently, a novel kind of quantum key distribution called the round-robin differential phase-shift （RRDPS） protocol was proposed, which bounds the amount of leakage without monitoring signal disturbance. The protocol can be implemented by a weak coherent source. The security of this protocol with a simply characterized source has been proved. The application of a common phase shift can improve the secret key rate of the protocol. In practice, the randomized phase is discrete and the secret key rate is deviated from the continuous case. In this study, we analyze security of the RRDPS protocol with discrete-phase-randomized coherent state source and bound the secret key rate. We fix the length of each packet at 32 and 64, then simulate the secret key rates of the RRDPS protocol with discrete-phase randomization and continuous-phase randomization. Our simulation results show that the performance of the discrete-phase randomization case is close to the continuous counterpart with only a small number of discrete phases. The research is practically valuable for experimental implementation.

Entanglement diversion and quantum teleportation of entangled coherent states

The proposals on entanglement diversion and quantum teleportation of entangled coherent states are presented. In these proposals, the entanglement between two coherent states, ｜α〉 and ｜-α〉, with the same amplitude but a phase difference of π is utilized as a quantum channel. The processes of the entanglement diversion and the teleportation are achieved by using the 50/50 symmctric beam splitters, the phase shifters and the photodetectors with the help of classical information.

Quantum block coherence with respect to projective measurements

Quantum coherence serves as a defining characteristic of quantum mechanics,finding extensive applications in quantum computing and quantum communication processing.This study explores quantum block coherence in the context of projective measurements,focusing on the quantification of such coherence.Firstly,we define the correlation function between the two general projective measurements P and Q,and analyze the connection between sets of block incoherent states related to two compatible projective measurements P and Q.Secondly,we discuss the measure of quantum block coherence with respect to projective measurements.Based on a given measure of quantum block coherence,we characterize the existence of maximal block coherent states through projective measurements.This research integrates the compatibility of projective measurements with the framework of quantum block coherence,contributing to the advancement of block coherence measurement theory.

Controllable preparation of two-mode entangled coherent states in circuit QED

Although the multi-level structure of superconducting qubits may result in calculation errors, it can be rationally used to effectively improve the speed of gate operations. Utilizing a current-biased Josephson junction （A-type rf-SQUID） as a tunable coupler for superconducting transmission line resonators （TLRs）, under the large detuning condition, we demonstrate the controllable generation of entangled coherent states in circuit quantum electrodynamics （circuit QED）. The coupling between the TLRs and the qubit can be effectively regulated by an external bias current or coupling capacitor. Further investigations indicate that the maximum entangled state can be obtained through measuring the excited state of the superconducting qubits. Then, the influence of the TLR [tecay on the prepared entangled states is analyzed.

Coherent coupling between vibrational modes of C-H bonds at different positions studied by femtosecond time-resolved coherent anti-Stokes Raman scattering

We performed femtosecond time-resolved coherent anti-Stokes Raman scattering （fs-CARS） measurements on liquid toluene and PVK film. For both samples, we selectively excited the CH stretching vibrational modes and observed the expected quantum beat signals. The frequency of the well-defined beats is in good agreement with the energy difference between the two simultaneously excited modes, which demonstrates that a coherent coupling between the vibrational modes of the C H chemical bonds exists at the different positions of the molecules. The dephasing times of the excited modes are obtained simultaneously.

Quantum coherence and correlation dynamics of two-qubit system in spin bath environment

The quantum entanglement,discord,and coherence dynamics of two spins in the model of a spin coupled to a spin bath through an intermediate spin are studied.The effects of the important physical parameters including the coupling strength of two spins,the interaction strength between the intermediate spin and the spin bath,the number of bath spins and the temperature of the system on quantum coherence and correlation dynamics are discussed in different cases.The frozen quantum discord can be observed whereas coherence does not when the initial state is the Bell-diagonal state.At finite temperature,we find that coherence is more robust than quantum discord,which is better than entanglement,in terms of resisting the influence of environment.Therefore,quantum coherence is more tenacious than quantum correlation as an important resource.

Coherent Assembly of Ultracold Polyatomic Molecules: Two-Channel Interference

The evidences of three-body and four-body bound states have been reported in a series of very recent experiments with ultracold atoms.Here we study coherent creation of polyatomic molecules via a generalized atom-molecule dark-state technique.By keeping the intermediate trimer or tetramer state essentially unpopulated,the constructive quantum two-channel interference is shown to play an important role in,e.g.coherent atom-pentamer conversion at ultracold temperature.

Coherent H^(∞)Control for Linear Quantum Systems With Uncertainties in the Interaction Hamiltonian

This work conducts robust H^(∞)analysis for a class of quantum systems subject to perturbations in the interaction Hamiltonian.A necessary and sufficient condition for the robustly strict bounded real property of this type of uncertain quantum system is proposed.This paper focuses on the study of coherent robust H^(∞)controller design for quantum systems with uncertainties in the interaction Hamiltonian.The desired controller is connected with the uncertain quantum system through direct and indirect couplings.A necessary and sufficient condition is provided to build a connection between the robust H^(∞)control problem and the scaled H^(∞)control problem.A numerical procedure is provided to obtain coefficients of a coherent controller.An example is presented to illustrate the controller design method.