Coherent Quantum Dynamics Research Articles

Dynamics of quantum coherence and entanglement in an intrinsic noise model of a V-type qutrit system interacting with a coherent field

An analytical solution of the dynamical evolution of an intrinsic noise model of a V-type three-level atom confined in a cavity having one-mode coherent field via multi-photon transitions is obtained. We investigate the effect of the physical parameters of the qutrit-cavity unitary interaction, the intrinsic noise and the photon multiplicity, on important quantum effects such as: the atomic inversion, the quantum coherence, the entanglement and the total correlation. These physical parameters control the phenomenon of the collapses and revivals, and the regularity of the oscillatory behavior of the atomic-inversion. When the intrinsic noise is considered, the dynamics of the entropy, the negativity and the mutual information are different, while the entropy grows, the entanglement negativity and the mutual information degrades to their stationary values. It is found that the quantum coherence, the entanglement, the total correlation and their stationary values are very sensitive to any change in the physical parameters.

Dynamics of quantum coherence and quantum Fisher information after a sudden quench

The dynamics of relative entropy and $l_{1}$-norm of coherence, as well as, the Wigner-Yanase-skew and quantum Fisher information are studied for a time-dependent coupled XY spin chain in presence of a time-dependent transverse magnetic field. Independent of the initial state of the system and while the relative entropy of coherence, $l_{1}$-norm of coherence, and quantum Fisher information are incapable, surprisingly, the dynamic Wigner-Yanase-skew information can truly spotlight the equilibrium critical point. We also observe that when the system is quenched to the critical point, these quantities show suppressions and revivals. Moreover, the first suppression (revival) time scales linearly with the system size and its scaling ratio is unique for all quenches independent to the initial phase. This is the promised universality of the first suppression (revival) time.

Simulation of quantum coherence dynamics in macroscopic domain using a three-waveguide directional coupler

We consider a three-waveguide (WG) directional coupler with an attempt to explore an optical analogue of the quantum coherence dynamics, which is found to be achievable in the present model at the macroscopic domain. In order to simulate the characteristics of such opto-quantum analogy, a field-coupled three-level atomic system has been undertaken in our study. Mutual correspondence between the field-induced coherence properties in the atomic system and the light transfer related optical properties in the evanescently coupled WGs has been discussed in the dissipative cases at different parameter conditions. The signature of optical impedance is highlighted in this context and the impact of these terms in visualizing the coherence dynamics inherent to a nano-photonic structure is explicitly reported.

Non-classicality dynamics of Schrödinger cat states in a non-Markovian environment

By virtue of the superoperator technique, we solve analytically the non-Markovian master equation that describes single-mode continuous variable system interacting with a structure reservoir. Our analytical solution allows us to investigate the non-Markovian effects on the dynamics of quantum coherence and non-classicality of Schrödinger cat states embedded in various types of structured reservoirs. We show that the dynamical behaviors of the fidelity, entanglement potentials, and higher-order antibunching of such states can be divided into two phases: they decay rapidly within a short time, but would either vanish completely or tend to a nonzero steady-state value in the long-time limit depending on the initial Schrödinger cat states and system parameters. We also show that by examining the Wigner function and higher-order sub-Poissonian statistics, the super-Ohmic environment can better help to protect the non-classicality, whereas the sub-Ohmic environment causes the disappearance of the non-classicality. Our approach is concise and can generalize to other open quantum system coupled to non-Markovian environment at finite-temperature or to linear non-Markovian Gaussian system.

Quantum coherence and correlation dynamics of two-qubit system in spin bath environment**Project supported by the National Natural Science Foundation of China (Grant Nos. 61605225, 11704238, and 61772295), the Educational Science and Technology Program of Shandong Province, China (Grant No. J18KZ012), and the Natural Science Foundation of Shanghai (Grant No. 16ZR1448400).

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.

Universal Dynamics of a Degenerate Bose Gas Quenched to Unitarity.

Motivated by an unexpected experimental observation from the Cambridge group, [Eigen etal., Nature 563, 221 (2018)], we study the evolution of the momentum distribution of a degenerate Bose gas quenched from the weakly interacting regime to the unitary regime. For the two-body problem, we establish a relation that connects the momentum distribution at a long time to a subleading term in the initial wave function. For the many-body problem, we employ the time-dependent Bogoliubov variational wave function and find that, in certain momentum regimes, the momentum distribution at long times displays the same exponential behavior found by the experiment. Moreover, we find that this behavior is universal and is independent of the short-range details of the interaction potential. Consistent with the relation found in the two-body problem, we also numerically show that this exponential form is hidden in the same subleading term of the Bogoliubov wave function in the initial stages. Conceptually, our results show that, for quench to the universal regime and coherent quantum dynamics afterward, the universal longtime behavior is hidden in the initial state.

Multipartite quantum coherence under electromagnetic vacuum fluctuation with a boundary

We investigate the dynamics of multipartite quantum coherence (QC) for accelerated detector qubit coupled with fluctuating electromagnetic field by presence of a perfectly reflecting boundary. We firstly derive the master equation that the detector evolution obeys. We find that without boundary, QC declines under the impact of Unruh thermal effect and vacuum fluctuation. However, with a boundary, the degradation, floating and protection of QC are closely related to boundary effect, detector polarization and acceleration. The presence of boundary can effectively protect QC under the influence of vacuum fluctuation and Unruh thermal effect in some certain conditions and give us more freedom of adjusting the QC behaviors. Besides, it is shown that the QC of W state manifests better robustness than that of GHZ state under the influence of the vacuum fluctuation and acceleration.

Dynamics of quantum coherence in a spin-star system: Bipartite initial state and coherence distribution

We investigate the transient dynamics of quantum coherence for a system of two central spins in a spin-star environment by employing a numerical procedure based on a Laguerre polynomial expansion scheme. The dynamics of the total, local, and global coherence are calculated for different values of the anisotropy parameter, the system-bath interaction strengths, and temperature for different initial bipartite states. Significant dynamical features of quantum coherence are found as follows: (i) an $X$ state can only have global coherence; (ii) a state with only initial local coherence gains global coherence during the course of evolution by the induced correlations between the two-qubit system and the common bath; (iii) an incoherent state gains coherence by interacting with an external bath. We find there are two primary ways to gain coherence for an incoherent state: one is by interacting with the external quantum bath and the other is through interconversion of other quantum properties such as purity into coherence. Finally, we demonstrate that our results for the system in an infinite bath also hold qualitatively when the system is in contact with a finite bath.

Revealing many-body effects on interband coherence through adiabatic charge pumping

The adiabatic charge pumping of a non-equilibrium state of spinless fermions in a one-dimensional lattice is investigated, with an emphasis placed on its usefulness in revealing many-body interaction effects on interband coherence. For a non-interacting system, the pumped charge per adiabatic cycle depends not only on the topology of the occupied bands but also on the interband coherence in the initial state. This insight leads to an interesting opportunity for quantitatively observing how quantum coherence is affected by many-body interaction that is switched on for a varying duration prior to adiabatic pumping. In particular, interband coherence effects can be clearly observed by adjusting the switch-on rates with different adiabatic pumping protocols and by scanning the duration of many-body interaction prior to adiabatic pumping. The time dependence of single-particle interband coherence in the presence of many-body interaction can then be examined in detail. As a side but interesting result, for relatively weak interaction strength, it is found that the difference in the pumped charges between different pumping protocols vanishes if a coherence measure defined from the single-particle density matrix in the sublattice representation reaches its local minima. Our results hence provide an interesting means to quantitatively probe the dynamics of quantum coherence in the presence of many-body interaction (e.g., in a thermalization process).

Features of the Simultaneous Generation of Low-Q and High-Q Modes in Heterolasers Based on Quantum Dots with a Long Incoherent Relaxation Time of Optical Dipole Oscillations

The features of the multimode steady-state generation of superradiant heterolasers, in which quantum dots with a long incoherent relaxation time serve as an active medium, while low-Q combined Fabry–Perot cavities with a distributed feedback of counter-propagating waves serve as cavities, are studied. It is shown that due to the quantum-coherent dynamics of optical dipole oscillations and population inversion of the working levels in a quantum-dot ensemble with strong inhomogeneous broadening of the spectral line, the simultaneous lasing of modes with various degrees of phasing and/or correlation and with qualitatively different dynamic behavior, notably, quasi-stationary, metastable, self-modulation, pulse-periodic, and quasi-chaotic is possible.

Photoexcitation Induced Quantum Dynamics of Charge Density Wave and Emergence of a Collective Mode in 1T-TaS2.

Photoexcitation is a powerful means in distinguishing different interactions and manipulating the states of matter, especially in charge density wave (CDW) materials. The CDW state of 1T-TaS2 has been widely studied experimentally mainly because of its intriguing laser-induced ultrafast responses of electronic and lattice subsystems. However, the microscopic atomic dynamics and underlying electronic mechanism upon photoexcitation remain unclear. Here, we demonstrate photoexcitation induced ultrafast dynamics of CDW in 1T-TaS2 using time-dependent density functional theory molecular dynamics. We discover a novel collective oscillation mode between the CDW state and a transient state induced by photodoping, which is significantly different from thermally induced phonon mode and attributed to the modification of the potential energy surface from laser excitation. In addition, our finding validates nonthermal melting of CDW induced at low light intensities, supporting that conventional hot electron model is inadequate to explain photoinduced dynamics. Our results provide a deep insight into the coherent electron and lattice quantum dynamics during the formation and excitation of CDW in 1T-TaS2.

Coherent quantum dynamics of systems with coupling-induced creation pathways

Many technologies emerging from quantum information science heavily rely upon the generation and manipulation of entangled quantum states. Here, we propose and demonstrate a new class of quantum interference phenomena that arise when states are created in and coherently converted between the propagating modes of an optical microcavity. The modal coupling introduces several new creation pathways to a nonlinear optical process within the device, which quantum mechanically interfere to drive the system between states in the time domain. The coherent conversion entangles the generated biphotons between propagation pathways, leading to cyclically evolving path-entanglement and the manifestation of coherent oscillations in second-order temporal correlations. Furthermore, the rich device physics is harnessed to tune properties of the quantum states. In particular, we show that the strength of interference between pathways can be coherently controlled, allowing for manipulation of the degree of entanglement, which can even be entirely quenched. The states can likewise be made to flip-flop between exhibiting initially correlated or uncorrelated behavior. The phenomena presented here open a route to creating higher dimensional entanglement and exotic multi-photon states.

Quantum coherence of the spin-1 system under an XY spin chain with three-spin interaction

We investigate the performance of quantum coherence and nonlocal advantage of quantum coherence (NAQC) for the central spin-1 system which couple to an XY spin chain with three-spin interaction in a transverse magnetic field. For the case that the central spin system couple weakly to the spin chain, both quantum coherence and NAQC endure sudden change around the critical point of quantum phase transitions when the three-spin interaction is switched off. While there is three-spin interaction, the performances of quantum coherence and NAQC indicate that the critical point has been shifted away from its ordinary position. Therefore, quantum coherence and NAQC can be used to detect the critical point of quantum phase transitions of the XY spin chain for the weak-coupling case. However, three-spin interaction has no obvious effect on the dynamics of quantum coherence and NAQC for the strong-coupling case.

Dynamics of Quantum Coherence in Bell-Diagonal States under Markovian Channels**Supported by the National Natural Science Foundation of China under Grant No. 11675113 and Natural Science Foundation of Beijing under Grant No. KZ201810028042

We study the curves of coherence for the Bell-diagonal states including l1-norm of coherence and relative entropy of coherence under the Markovian channels in the first subsystem once. For a special Bell-diagonal state under bit-phase ip channel, we find frozen coherence under l1 norm occurs, but relative entropy of coherence decrease. It illustrates that the occurrence of frozen coherence depends on the type of the measure of coherence. Also, we study the coherence evolution of Bell-diagonal states under Markovian channels in the first subsystem n times and find that coherence under depolarizing channel decreases initially then increases for small n and tends to zero for large n. The dynamics of coherence of the Bell-diagonal state under two independent same type local Markovian channels is discussed.

Time dynamics of quantum coherence and monogamy in a non-Markovian environment

The time evolution of the distribution and shareability of quantum coherence of a tripartite system in a non-Markovian environment is examined. The total coherence can be decomposed into various contributions, ranging from local, global bipartite and global tripartite, which characterize the type of state. We identify coherence revivals for non-Markovian systems for all the contributions of coherence. The local coherence is found to be much more robust under the environmental coupling due to an effective smaller coupling to the reservoir. This allows us to devise a characterization of a quantum state in terms of a coherence tuple on a multipartite state simply by examining various combinations of reservoir couplings. The effect of the environment on the shareability of quantum coherence, as defined using the monogamy of coherence, is investigated and found that the sign of the monogamy is a preserved quantity under the decoherence. We conjecture that the monogamy of coherence is a conserved property under local incoherent processes.

Revealing the Presence of Potential Crossings in Diatomics Induced by Quantum Cavity Radiation.

We propose an experiment to find evidence of the formation of light-induced crossings provoked by cavity quantum radiation on simple molecules by using state-of-the-art optical cavities, molecular beams, pump-probe laser schemes, and velocity mapping detectors for fragmentation. The procedure is based on prompt excitation and subsequent dissociation in a three-state scheme of a polar diatomic molecule, with two ^{1}Σ states (ground and first excited) coupled first by the UV pump laser and then by the cavity radiation, and a third fully dissociative state ^{1}Π coupled through the delayed UV/V probe laser. The observed enhancement of photodissociation yields in the ^{1}Π channel at given time delays between the pump and probe lasers unambiguously indicates the formation of a light-induced crossing between the two ^{1}Σ field-dressed potential energy curves of the molecule. Also, the production of cavity photons out of the vacuum field state via nonadiabatic effects represents a showcase of a molecular dynamical Casimir effect. To simulate the experiment outcome, we perform abinitio coherent quantum dynamics of the molecule LiF subject to external lasers and quantum cavity interactions in the strong coupling regime, using a product grid representation of the total polaritonic wave function for both vibrational and photon degrees of freedom.

Quantum Coherence Behaviors for a Uniformly Accelerated Atom Immersed in Fluctuating Vacuum Electromagnetic Field with a Boundary

We investigate the dynamics of quantum coherence (QC) for a uniformly accelerated atom interacting with fluctuating electromagnetic field subject to a conductor boundary. We firstly derive the master equation that the atom evolution obeys. We find that without boundary, QC declines under the effect of Unruh thermal bath and vacuum fluctuation. However, with a boundary, the degradation, fluctuation, and preservation of QC are closely related to boundary effect, atomic polarization, and acceleration. Furthermore, in the presence of a boundary, QC can effectively be protected under the influence of the vacuum fluctuation and Unruh thermal effect when the atom is transversely polarizable and near this boundary, and the presence of boundary gives us more freedom of controlling the QC behaviors.

Особенности одновременной генерации низко- и высокодобротных мод в гетеролазерах на квантовых точках с большим временем некогерентной релаксации оптических дипольных колебаний

We investigate the features of multimode steady-state generation of the superradiant heterolasers which have an active medium formed by the quantum dots with long incoherent relaxation time and a cavity of a combined Fabry-Perot type with a distributed feedback of the counter-propagating waves. We show that a quantum-coherent dynamics of the optical dipole oscillations and a population inversion of the working levels in an ensemble of the quantum dots with a strong inhomogeneous broadening of a spectral line may lead to a simultaneous lasing of modes with a various degree of phasing and/or correlation and with qualitatively different dynamical behavior, including quasi-stationary, metastable, self-modulated, pulsed-periodic, quasi-chaotic ones.

Nonlocal advantage of quantum coherence under relativistic frame

In this paper, we investigate the influence of the Unruh effect on the achievement of the nonlocal advantage of quantum coherence for a two-qubit system under a relativistic frame. The results show that with the increase of acceleration, it is difficult to realize the nonlocal advantage of quantum coherence and when the acceleration exceeds a certain value, nonlocal advantage of quantum coherence cannot be realized. In addition, we explore the dynamics of Bell nonlocality, steering, quantum coherence, entanglement and quantum discord (QD) under Unruh thermal noise. It is shown that nonlocal advantage of quantum coherence, Bell nonlocality, steering and entanglement experience “sudden death” for a finite acceleration, while quantum coherence and QD vanish only in the limit of an infinite acceleration. We also find that not all nonlocal states can achieve the nonlocal advantage of quantum coherence. It is also demonstrated that the robustness of Bell nonlocality is better than nonlocal advantage of quantum coherence under the influence of the Unruh noise.

Coherent dynamics of a qubit–oscillator system in a noisy environment

We investigate the non-Markovian dynamics of a qubit–oscillator system embedded in a noisy environment by employing the hierarchical equations of motion approach. It is shown that the decoherence rate of the whole qubit–oscillator–bath system can be significantly suppressed by enhancing the coupling strength between the qubit and the harmonic oscillator. Moreover, we find that the non-Markovian memory character of the bath is able to facilitate a robust quantum coherent dynamics in this qubit–oscillator–bath system. Our findings may be used to engineer some tunable coherent manipulations in mesoscopic quantum circuits.