Coherent Quantum Dynamics Research Articles

Quantum cascade driving: Dissipatively mediated coherences

Quantum cascaded systems offer the possibility to manipulate a target system with the quantum state of a source system. Here, we study in detail the differences between a direct quantum cascade and coherent/incoherent driving for the case of two coupled cavity-QED systems. We discuss qualitative differences between these excitations scenarios, which are particular strong for higher-order photon-photon correlations: $g^{(n)}(0)$ with $ n>2 $. Quantum cascaded systems show a behavior differing from the idealized cases of individual coherent/incoherent driving and allow to produce qualitatively different quantum statistics. Furthermore, the quantum cascaded driving exhibits an interesting mixture of quantum coherent and incoherent excitation dynamics. We develop a measure, where the two regimes intermix and quantify these differences via experimentally accessible higher-order photon correlations.

Non-Markovian dynamics of quantum coherence of two-level system driven by classical field

In this paper, we study the quantum coherence dynamics of two-level atom system embedded in non-Markovian reservoir in the presence of classical driving field. We analyze the influence of memory effects, classical driving, and detuning on the quantum coherence. It is found that the quantum coherence has different behaviors in resonant case and non-resonant case. In the resonant case, in stark contrast with previous results, the strength of classical driving plays a negative effect on quantum coherence, while detuning parameter has the opposite effect. However, in non-resonant case through a long time, classical driving and detuning parameter have a different influence on quantum coherence compared with resonant case. Due to the memory effect of environment, in comparison with Markovian regime, quantum coherence presents vibrational variations in non-Markovian regime. In the resonant case, all quantum coherence converges to a fixed maximum value; in the non-resonant case, quantum coherence evolves to different stable values. For zero-coherence initial states, quantum coherence can be generated with evolution time. Our discussions and results should be helpful in manipulating and preserving the quantum coherence in dissipative environment with classical driving field.

Dynamics of quantum correlation and coherence in de Sitter universe

In this article, we investigate the dynamics of quantum correlation and coherence for two atoms interacting with massless scalar field in the background de Sitter spacetime. We firstly analyze the solving process of master equation that describes the system evolution with initial Werner state. Then, we discuss the degradation, generation, revival and enhancement of quantum correlation and coherence for three cases of different initial states: zero correlation state, nonzero correlation separable state and maximally entangled state.

Dynamics of quantum coherence in two-dimensional quantum walk on finite lattices

We study the dynamics of the l1 norm coherence in a two-dimensional quantum walk on finite lattices with four-dimensional (4D) and two-dimensional (2D) coins. It is observed that the boundaries suppress the growth of coherence of both the whole system and the position subsystem. The coherence of the quantum walk with a 2D coin is larger than that of the quantum walk with a 4D coin when it stabilizes after a number of steps. We also analyze the influence of two kinds of noise, i.e., broken links and lattice congestion, on the coherence of a bounded quantum walk. Experimental results show that both the broken links and the lattice congestion with low probability slightly increase the coherence of the whole system and the position subsystem. However, a high noise level significantly suppresses the growth of coherence, especially for static noise. The coherence of the coin subsystem is also analyzed and we find that the boundaries result in a large fluctuation of coherence of the coin subsystem.

A coherent quantum annealer with Rydberg atoms

There is a significant ongoing effort in realizing quantum annealing with different physical platforms. The challenge is to achieve a fully programmable quantum device featuring coherent adiabatic quantum dynamics. Here we show that combining the well-developed quantum simulation toolbox for Rydberg atoms with the recently proposed Lechner–Hauke–Zoller (LHZ) architecture allows one to build a prototype for a coherent adiabatic quantum computer with all-to-all Ising interactions and, therefore, a platform for quantum annealing. In LHZ an infinite-range spin-glass is mapped onto the low energy subspace of a spin-1/2 lattice gauge model with quasi-local four-body parity constraints. This spin model can be emulated in a natural way with Rubidium and Caesium atoms in a bipartite optical lattice involving laser-dressed Rydberg–Rydberg interactions, which are several orders of magnitude larger than the relevant decoherence rates. This makes the exploration of coherent quantum enhanced optimization protocols accessible with state-of-the-art atomic physics experiments.

Evolution of multiple quantum coherences with scaled dipolar Hamiltonian

In this article, we introduce a pulse sequence which allows the monitoring of multiple quantum coherences distribution of correlated spin states developed with scaled dipolar Hamiltonian. The pulse sequence is a modification of our previous Proportionally Refocused Loschmidt echo (PRL echo) with phase increment, in order to verify the accuracy of the weighted coherent quantum dynamics. The experiments were carried out with different scaling factors to analyze the evolution of the total magnetization, the time dependence of the multiple quantum coherence orders, and the development of correlated spins clusters. In all cases, a strong dependence between the evolution rate and the weighting factor is observed. Remarkably, all the curves appeared overlapped in a single trend when plotted against the self-time, a new time scale that includes the scaling factor into the evolution time. In other words, the spin system displayed always the same quantum evolution, slowed down as the scaling factor decreases, confirming the high performance of the new pulse sequence.

Attosecond Dynamics of Molecular Electronic Ring Currents.

Ultrafast charge migration is of fundamental importance to photoinduced chemical reactions. However, exploring such a quantum dynamical process requires demanding spatial and temporal resolutions. We show how electronic coherence dynamics induced in molecules by a circularly polarized UV pulse can be tracked by using a time-delayed circularly polarized attosecond X-ray pulse. The X-ray probe spectra retrieve an image at different time delays, encoding instantaneous pump-induced circular charge migration information on an attosecond time scale. A time-dependent ultrafast electronic coherence associated with the periodical circular ring currents shows a strong dependence on the helicity of the UV pulse, which may provide a direct approach to access and control the electronic quantum coherence dynamics in photophysical and photochemical reactions in real time.

Payoffs and coherence of a quantum two-player game under noisy environment

In this work, we mainly analyse the dynamics of quantum coherence, entanglement, geometric quantum discord and payoffs of a two-player quantum game under noisy environment. Our results indicate that unitary strategies do not change resource measures but change players' payoffs, and entangled operator changes both. Different measures have different variations with noise parameter and entanglement parameter. The behaviors of all quantifiers and payoffs in Markovian regime are different from the case of the non-Markovian regime. The case when only one of the two players is affected by noise gives different results from the case when the noise acts on both players. We find that the game symmetry is broken under noisy environment.

Strong Analog Classical Simulation of Coherent Quantum Dynamics**Funding support from NSERC of Canada and a research fellowship at Department of Physics and Astronomy, University of British Columbia are acknowledged

A strong analog classical simulation of general quantum evolution is proposed, which serves as a novel scheme in quantum computation and simulation. The scheme employs the approach of geometric quantum mechanics and quantum informational technique of quantum tomography, which applies broadly to cases of mixed states, nonunitary evolution, and infinite dimensional systems. The simulation provides an intriguing classical picture to probe quantum phenomena, namely, a coherent quantum dynamics can be viewed as a globally constrained classical Hamiltonian dynamics of a collection of coupled particles or strings. Efficiency analysis reveals a fundamental difference between the locality in real space and locality in Hilbert space, the latter enables efficient strong analog classical simulations. Examples are also studied to highlight the differences and gaps among various simulation methods.

Dynamics of quantum correlation and coherence for two atoms coupled with a bath of fluctuating massless scalar field

In this article, the dynamics of quantum correlation and coherence for two atoms interacting with a bath of fluctuating massless scalar field in the Minkowski vacuum is investigated. We firstly derive the master equation that describes the system evolution with initial Bell-diagonal state. Then we discuss the system evolution for three cases of different initial states: non-zero correlation separable state, maximally entangled state and zero correlation state. For non-zero correlation initial separable state, quantum correlation and coherence can be protected from vacuum fluctuations during long time evolution when the separation between the two atoms is relatively small. For maximally entangled initial state, quantum correlation and coherence overall decrease with evolution time. However, for the zero correlation initial state, quantum correlation and coherence are firstly generated and then drop with evolution time; when separation is sufficiently small, they can survive from vacuum fluctuations. For three cases, quantum correlation and coherence first undergo decline and then fluctuate to relatively stable values with the increasing distance between the two atoms. Specially, for the case of zero correlation initial state, quantum correlation and coherence occur periodically revival at fixed zero points and revival amplitude declines gradually with increasing separation of two atoms.

Time-correlated blip dynamics of open quantum systems

The non-Markovian dynamics of open quantum systems is still a challenging task, particularly in the non-perturbative regime at low temperatures. While the Stochastic Liouville-von Neumann equation (SLN) provides a formally exact tool to tackle this problem for both discrete and continuous degrees of freedom, its performance deteriorates for long times due to an inherently non-unitary propagator. Here we present a scheme which combines the SLN with projector operator techniques based on finite dephasing times, gaining substantial improvements in terms of memory storage and statistics. The approach allows for systematic convergence and is applicable in regions of parameter space where perturbative methods fail, up to the long time domain. Findings are applied to the coherent and incoherent quantum dynamics of two- and three-level systems. In the long time domain sequential and super-exchange transfer rates are extracted and compared to perturbative predictions.

Origin of long-lived quantum coherence and excitation dynamics in pigment-protein complexes.

We explore the mechanism for the long-lived quantum coherence by considering the discrete phonon modes: these vibrational modes effectively weaken the exciton-environment interaction, due to the new composite (polaron) formed by excitons and vibrons. This subsequently demonstrates the role of vibrational coherence which greatly contributes to long-lived feature of the excitonic coherence that has been observed in femtosecond experiments. The estimation of the timescale of coherence elongated by vibrational modes is given in an analytical manner. To test the validity of our theory, we study the pigment-protein complex in detail by exploring the energy transfer and coherence dynamics. The ground-state vibrational coherence generated by incoherent radiations is shown to be long-survived and is demonstrated to be significant in promoting the excitation energy transfer. This is attributed to the nonequilibriumness of the system caused by the detailed-balance-breaking, which funnels the downhill migration of excitons.

Optimal Protection of Quantum Coherence in Noisy Environment

In this paper, we analyse the quantum coherence dynamics of a single qubit locally interacting with a zero-temperature reservoir. We compare the behaviors of quantum coherence in Markovian and non-Markovian regime. We find that the system coherence is transferred to the reservoir and decreases with time. In non-Markovian regime, quantum coherence exists instantaneous disappearance at some discrete time points. Furthermore, we propose an optimal scheme to protect quantum coherence by executing prior weak measurement and post-measurement reversal. It is worth noticing that the scheme can get better quantum coherence with the larger weak measurement strength, while at the cost of reducing success probability.

Impact of Vibrational Coherence on the Quantum Yield at a Conical Intersection.

We study the vibrationally coherent quantum dynamics of an electronic wave packet in the vicinity of a conical intersection within a three-state two-mode model. By transforming the coherent tuning and coupling modes into the bath, the underdamped dynamics of the resulting effective three-state model is solved efficiently by the numerically exact hierarchy equation of motion approach. The transient excited-state absorption and two-dimensional spectra reveal the impact of vibrational coherence on the relaxation pathways of the wave packet. We find that both the quantum yield and the isomerization rate are crucially influenced by the vibrational coherence of the wave packet. A less coherent wave packet can traverse the conical intersection more rapidly, while the resulting quantum yield is smaller. Finally, we show that repeated passages of the wave packet through the conical intersection can lead to measurable interference effects in the form of Stueckelberg oscillations.

Inhibiting decoherence of two-level atom in thermal bath by presence of boundaries

We study, in the paradigm of open quantum systems, the dynamics of quantum coherence of a static polarizable two-level atom which is coupled with a thermal bath of fluctuating electromagnetic field in the absence and presence of boundaries. The purpose is to find the conditions under which the decoherence can be inhibited effectively. We find that without boundaries, quantum coherence of the two-level atom inevitably decreases due to the effect of thermal bath. However, the quantum decoherence, in the presence of a boundary, could be effectively inhibited when the atom is transversely polarizable and near this boundary. In particular, we find that in the case of two parallel reflecting boundaries, the atom with a parallel dipole polarization at arbitrary location between these two boundaries will be never subjected to decoherence provided we take some special distances for the two boundaries.

Irreversible degradation of quantum coherence under relativistic motion

We study the dynamics of quantum coherence under Unruh thermal noise and seek under which condition the coherence can be frozen in a relativistic setting. We find that the frozen condition is either (i) the initial state is prepared as a incoherence state, or (ii) the detectors have no interaction with the external field. That is to say, the decoherence of detectors' quantum state is irreversible under the influence of thermal noise induced by Unruh radiation. It is shown that quantum coherence approaches zero only in the limit of an infinite acceleration, while quantum entanglement could reduce to zero for a finite acceleration. It is also demonstrated that the robustness of quantum coherence is better than entanglement under the influence of the atom-field interaction for an extremely large acceleration. Therefore, quantum coherence is more robust than entanglement in an accelerating system and the coherence type quantum resources are more accessible for relativistic quantum information processing tasks.

Quantum coherence and quantum correlation of two qubits mediated by a one-dimensional plasmonic waveguide.

We investigate the dynamics of quantum coherence and quantum correlation of two qubits mediated by a one-dimensional plasmonic waveguide. The analytical expression of the dissipative dynamics of the two qubits is obtained for the initial X state. The dynamical behaviors of the quantum coherence and quantum correlation are shown to be largely dependent on the parameters of the initial state. Starting from a product state, quantum coherence and quantum correlation can be induced by the plasmonic waveguide. Under continuous drivings, steady quantum correlation can be obtained at specific distance larger than the operating wavelength and large values of steady quantum coherence are attainable at arbitrary distance. The detuning effect on the dissipation-driven generation of steady quantum coherence and quantum correlation is also explored.

Molecular Packing Determines Charge Separation in a Liquid Crystalline Bisthiophene-Perylene Diimide Donor-Acceptor Material.

Combined electronic structure and quantum dynamical calculations are employed to investigate charge separation in a novel class of covalently bound bisthiophene-perylene diimide type donor-acceptor (DA) co-oligomer aggregates. In an earlier spectroscopic study of this DA system in a smectic liquid crystalline (LC) film, efficient and ultrafast (subpicosecond) initial charge separation was found to be followed by rapid recombination. By comparison, the same DA system in solution exhibits ultrafast resonant energy transfer followed by slower (picosecond scale) charge separation. The present first-principles study explains these contrasting observations, highlighting the role of an efficient intermolecular charge-transfer pathway that results from the molecular packing in the LC phase. Despite the efficiency of this primary charge-transfer step, long-range charge separation is impeded by a comparatively high Coulomb barrier in conjunction with small electron- and hole-transfer integrals. Quantum dynamical calculations are carried out for a fragment-based model Hamiltonian, parametrized by ab initio second-order Algebraic Diagrammatic Construction (ADC(2)) and Time-Dependent Density Functional Theory (TDDFT) electronic structure calculations. Simulations of coherent vibronic quantum dynamics for up to 156 electronic states and 48 modes are performed using the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method. Excellent agreement with experimentally determined charge separation time scales is obtained, and the spatially coherent nature of the dynamics is analyzed.

Doppler effect induced spin relaxation boom.

We study an electron spin qubit confined in a moving quantum dot (QD), with our attention on both spin relaxation, and the product of spin relaxation, the emitted phonons. We find that Doppler effect leads to several interesting phenomena. In particular, spin relaxation rate peaks when the QD motion is in the transonic regime, which we term a spin relaxation boom in analogy to the classical sonic boom. This peak indicates that a moving spin qubit may have even lower relaxation rate than a static qubit, pointing at the possibility of coherence-preserving transport for a spin qubit. We also find that the emitted phonons become strongly directional and narrow in their frequency range as the qubit reaches the supersonic regime, similar to Cherenkov radiation. In other words, fast moving excited spin qubits can act as a source of non-classical phonons. Compared to classical Cherenkov radiation, we show that quantum dot confinement produces a small but important correction on the Cherenkov angle. Taking together, these results have important implications to both spin-based quantum information processing and coherent phonon dynamics in semiconductor nanostructures.

Lindblad dynamics of a quantum spherical spin

The coherent quantum dynamics of a single bosonic spin variable, subject to a constraint derived from the quantum spherical model of a ferromagnet, and coupled to an external heat bath, is studied through the Lindblad equation for the reduced density matrix. Closed systems of equations of motion for several quantum observables are derived and solved exactly. The relationship to the single-mode Dicke model from quantum optics is discussed. The analysis of the interplay of the quantum fluctuation and the dissipation and their influence on the relaxation of the time-dependent magnetisation leads to the distinction of qualitatively different regimes of weak and strong quantum couplings. Considering the model’s behaviour in an external field as a simple mean-field approximation of the dynamics of a quantum spherical ferromagnet, the magnetic phase diagram appears to be re-entrant and presents a quantum analogue of well-established classical examples of fluctuation-induced order.