摘要：
The phenomenon of quantum revivals resulting from the self-interference of wave packets has been observed in several quantum systems and utilized widely in spectroscopic applications. Here, we present a combined analytical and numerical study on the generation of orientational quantum revivals (OQRs) exclusively using a single-cycle terahertz pulse. As a proof of principle, we examine the scheme in the linear polar molecule HCN with experimentally accessible pulse parameters and obtain strong field-free OQR without requiring the condition of the sudden-impact limit. To visualize the involved quantum mechanism, we derive a three-state model using the Magnus expansion of the time-evolution operator. Interestingly, the terahertz pulse interaction with the electric-dipole moment can activate direct multiphoton processes, leading to OQR enhancements beyond that induced by a rotational ladder-climbing mechanism from the rotational ground state. We further show that the theoretical maximum degree of orientation (0.774) for three populated rotational states can be obtained with available terahertz pulses.
摘要：
We present a numerical method for solving the separability problem of Gaussian quantum states in continuous-variable quantum systems. We show that the separability problem can be cast as an equivalent problem of determining the feasibility of a set of linear matrix inequalities. Thus, it can be efficiently solved using existent numerical solvers. We apply this method to the identification of bound entangled Gaussian states. We show that the proposed method can be used to identify bound entangled Gaussian states that could be simple enough to be producible in quantum optics.
作者机构：
[Guo, Yu; Shu, Chuan-Cun] Cent S Univ, Sch Phys & Elect, Hunan Key Lab Super Microstruct & Ultrafast Proc, Changsha 410083, Hunan, Peoples R China.;[Guo, Yu] Changsha Univ Sci & Technol, Sch Phys & Elect Sci, Hunan Prov Key Lab Flexible Elect Mat Genome Engn, Changsha 410114, Hunan, Peoples R China.;[Luo, Xiaobing] Jinggangshan Univ, Dept Phys, Inst Atom Mol Phys & Funct Mat, Jian 343009, Jiangxi, Peoples R China.;[Ma, Shan] Cent S Univ, Sch Automat, Changsha 410083, Hunan, Peoples R China.
通讯机构：
[Shu, Chuan-Cun] C;Cent S Univ, Sch Phys & Elect, Hunan Key Lab Super Microstruct & Ultrafast Proc, Changsha 410083, Hunan, Peoples R China.
摘要：
We present a quantum optimal control theory study combined with theoretical analysis to determine a pulsed laser field, capable of generating a maximally entangled state in two trapped two-level atoms. By expanding the time-dependent unitary operator to the first leading term of Magnus expansion, we reexamine the pulse area theorem for the trapped atoms driven by an arbitrarily temporary field. Due to the dipole-dipole interaction blockade, we find that the two trapped atoms described by a three-level ladder system can be reduced into an equivalent two-level model by using narrow-bandwidth pulses, leading to an analytical solution for generating the maximally entangled state. We also solve a highly constrained optimization problem to search for optimal laser pulses with broad bandwidths. A zero pulse-area constraint is employed to remove the dc offset of the optimized laser pulses, and a fixed fluence limitation combined with a constant pulse-area constraint at the resonant frequency of the equivalent two-level system are utilized to restrict the unitary evolution of quantum systems by the first leading term of Magnus expansion. This work provides a potentially useful approach to find all-optical control schemes for generating the maximally entangled state by using ultrafast laser pulses while satisfying multiple strict limitations.
摘要：
The possibility to manipulate quantum coherence and interference, apart from its fundamental interest in quantum mechanics, is essential for controlling nonlinear optical processes such as high harmonic generation, multiphoton absorption, and stimulated Raman scattering. We show, analytically and numerically, how a nonlinear optical process via resonance Raman scattering (RRS) can be manipulated in a four-level double- Λ system by using pulsed laser fields. We find that two simultaneously excited RRS paths involved in the system can generate an ultimately destructive interference in the broad-bandwidth-limit regime. This, in turn, reduces the four-level system to an equivalent three-level system in a V configuration capable of naturally vanishing RRS effects. We further show that this counterintuitive phenomenon, i.e., the RRS vanishing, can be prevented by transferring a modulated phase of the laser pulse to the system at resonance frequencies. This work demonstrates a clear signature of both quantum destructive and constructive interference by actively controlling resonant multiphoton processes in multilevel quantum systems, and it therefore has potential applications in nonlinear optics, quantum control, and quantum information science.
期刊：
PHYSICAL CHEMISTRY CHEMICAL PHYSICS,2018年20(14):9498-9506 ISSN：1463-9076
通讯作者：
Shu, Chuan-Cun
作者机构：
[Guo, Yu] Changsha Univ Sci & Technol, Sch Phys & Elect Sci, Changsha 410114, Hunan, Peoples R China.;[Guo, Yu; Dong, Daoyi; Shu, Chuan-Cun] Univ New South Wales, Sch Engn & Informat Technol, Canberra, ACT 2600, Australia.;[Guo, Yu] Hunan Normal Univ, Minist Educ, Key Lab Low Dimens Quantum Struct & Quantum Contr, Changsha 410081, Hunan, Peoples R China.;[Shu, Chuan-Cun] Cent S Univ, Sch Phys & Elect, Inst Super Microstruct & Ultrafast Proc Adv Mat, Changsha 410083, Hunan, Peoples R China.
通讯机构：
[Shu, Chuan-Cun] U;[Shu, Chuan-Cun] C;Univ New South Wales, Sch Engn & Informat Technol, Canberra, ACT 2600, Australia.;Cent S Univ, Sch Phys & Elect, Inst Super Microstruct & Ultrafast Proc Adv Mat, Changsha 410083, Hunan, Peoples R China.
摘要：
Achieving fast and efficient quantum state transfer is a fundamental task in physics, chemistry and quantum information science. However, the successful implementation of the perfect quantum state transfer also requires robustness under practically inevitable perturbative defects. Here, we demonstrate how an optimal and robust quantum state transfer can be achieved by shaping the spectral phase of an ultrafast laser pulse in the framework of frequency domain quantum optimal control theory. Our numerical simulations of the single dibenzoterrylene molecule as well as in atomic rubidium show that optimal and robust quantum state transfer via spectral phase modulated laser pulses can be achieved by incorporating a filtering function of the frequency into the optimization algorithm, which in turn has potential applications for ultrafast robust control of photochemical reactions.
关键词：
Quantum optimal control theory;Population transfer;Ultrafast laser pulses
摘要：
High fidelity population transfer is of fundamental importance in quantum information processing. This paper employs a multiple-constraint optimal control theory method to find optimal control fields for a quantum system of two two-level particles to implement fast population transfer with very high fidelity. Numerical simulations show that high fidelity and fast population transfer can be achieved by shaping ultrafast laser pulses in the framework of time domain. The dependence of fidelity on the time duration and the field amplitude of the laser pulses is also discussed.
关键词：
Classical correlations;Quantum correlations;Quantum discord;Spin-boson model
摘要：
Based on an open exactly solvable system coupled to an environment with nontrivial spectral density, we connect the features of quantum and classical correlations with some features of the environment, initial states of the system, and the presence of initial system–environment correlations. Some interesting features not revealed before are observed by changing the structure of environment, the initial states of system, and the presence of initial system–environment correlations. The main results are as follows. (1) Quantum correlations exhibit temporary freezing and permanent freezing even at high temperature of the environment, for which the necessary and sufficient conditions are given by three propositions. (2) Quantum correlations display a transition from temporary freezing to permanent freezing by changing the structure of environment. (3) Quantum correlations can be enhanced all the time, for which the condition is put forward. (4) The one-to-one dependency relationship between all kinds of dynamic behaviors of quantum correlations and the initial states of the system as well as environment structure is established. (5) In the presence of initial system–environment correlations, quantum correlations under local environment exhibit temporary multi-freezing phenomenon. While under global environment they oscillate, revive, and damp, an explanation for which is given.
关键词：
Cold atoms;Coherent control;Tunneling dynamics;Coherent destruction of tunneling
摘要：
We study the tunneling dynamics of bosons with periodically modulated interaction held in a triple-well potential. In high-frequency approximation, we derive a set of reduced coupled equations and the corresponding Floquet solutions are obtained. Based on the analytical results and their numerical correspondence, the directed selective-tunneling effect of a single atom is demonstrated when all bosons are prepared in middle well initially. A scheme for separating a single atom from N bosons is presented, in which the atom can be trapped in right or left well by adjusting the modulation strength. (C) 2015 Elsevier B.V. All rights reserved.
摘要：
We investigate the reduced dynamics of a central spin coupled to a spin environment with non-uniform coupling. Through using the method of time-dependent density-matrix renormalization group (t-DMRG), we nonperturbatively show the dissipative dynamics of the central spin beyond the case of uniform coupling between the central spin and the environment spins. It is shown that only when the system-environment coupling is weak enough, the central spin system shows Markovian effect and will finally reach the steady state; otherwise, the reduced dynamics is non-Markovian and exhibits a quasi-periodic oscillation. The frequency spectrum and the correlation between the central spin system and the environment are also studied to elucidate the dissipative dynamics of the central spin system for different coupling strengths.
摘要：
We investigate the entanglement dynamics of a two-spin-qubit system coupled to a spin environment with nonuniform coupling through using the time-dependent density-matrix renormalization group method. We show that the entanglement generation and decay depend on the number of environment spins, the coupling strength between the central spin system and the environment, and the initial state of the central spin system.
期刊：
EUROPEAN PHYSICAL JOURNAL D,2013年67(11):218-null ISSN：1434-6060
通讯作者：
Wang, Z. H.
作者机构：
[Zhou, D. L.; Wang, Z. H.] Chinese Acad Sci, Inst Phys, Beijing Natl Lab Condensed Matter Phys, Beijing 100190, Peoples R China.;[Guo, Y.] Changsha Univ Sci & Technol, Sch Phys & Elect Sci, Changsha 410114, Hunan, Peoples R China.
通讯机构：
[Wang, Z. H.] C;Chinese Acad Sci, Inst Phys, Beijing Natl Lab Condensed Matter Phys, Beijing 100190, Peoples R China.
关键词：
Quantum Optics
摘要：
In most cases, a small system weakly interacting with a thermal bath will eventually reach a thermal state with the temperature of the bath. We show that this intuitive picture is not always true, by using a spin star model where the non-Markov effect dominates the dynamical process. The spin star system consists of a central spin homogeneously interacting with an ensemble of identical noninteracting spins. We find that the correlation time of the bath is infinite, which implies that the bath has a perfect memory, and that the dynamical evolution of the central spin must be non-Markovian. A direct consequence of this is that the final state of the central spin is not the thermal state in equilibrium with the bath, but a steady state which depends on the initial state of the spin.
摘要：
A fundamental task in quantum information science is to transfer an unknown state from particle <i>A<i/> to particle <i>B<i/> (often in remote space locations) by using a bipartite quantum operation . We suggest the power of for quantum state transfer (QST) to be the maximal average probability of QST over the initial states of particle <i>B<i/> and the identifications of the state vectors between <i>A<i/> and <i>B<i/>. We find the QST power of a bipartite quantum operations satisfies four desired properties between two <i>d<i/>-dimensional Hilbert spaces. When <i>A<i/> and <i>B<i/> are qubits, the analytical expressions of the QST power is given. The numerical result on a QST scheme via a quantum wire shows the necessity to optimize the average fidelity. In particular, we obtain the exact results of the QST power for a general two-qubit unitary transformation, and we find a necessary and sufficient condition for the two-qubit unitary gates with perfect QST.
