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Wei-Min Zhang

Publications and source records attributed to Wei-Min Zhang.

At least 73 records · Page 4Linked to original sources
arXiv2010

Dynamics of open quantum systems with initial system-reservoir correlations

In this paper, the exact dynamics of open quantum systems in the presence of initial system-reservoir correlations is investigated for a photonic cavity system coupled to a general non-Markovian reservoir. The exact time-convolutionless master equation incorporating with initial system-reservoir correlations is obtained. The non-Markovian dynamics of the reservoir and the effects of the initial correlations are embedded into the time-dependent coefficients in the master equation. We show that the effects induced by the initial correlations play an important role in the non-Markovian dynamics of the cavity but they are washed out in the steady-state limit in the Markovian regime. Moreover, the initial two-photon correlation between the cavity and the reservoir can induce nontrivial squeezing dynamics to the cavity field.

quant-ph↗
arXiv2010

High frequency limit for single-electron pumping operations

In this Letter, we study the transient electron transfer phenomena of single-electron devices with alternating external gate voltages. We obtain a high frequency limit for pumping electrons one at a time in single-electron devices. Also, we find that in general the electrical current is not proportional to the frequency of the external signals in the single-electron devices, due to the strong quantum coherence tunneling effect.

cond-mat.mes-hall↗
arXiv2010

Intrinsic coherence dynamics and phase localization in Aharonov-Bohm Interferometers

The nonequilibrium real-time dynamics of electron coherence is explored in the quantum transport through the double-dot Aharonov-Bohm interferometers. We solve the exact master equation to find the exact quantum state of the device, from which the changes of the electron coherence through the magnetic flux in the nonequilibrium transport processes is obtained explicitly. We find that the relative phase between the two charge states of the double dot localizes to $\fracπ{2}$ or $-\fracπ{2}$ for all different magnetic flux. This nontrivial phase localization process can be manifested in the measurable occupation numbers.

cond-mat.mes-hall↗
arXiv2010

Non-Markovian dynamics of a microcavity coupled to a waveguide in photonic crystals

In this paper, the non-Markovian dynamics of a microcavity coupled to a waveguide in photonic crystals is studied based on Fano-type tight binding model. Using the exact master equation, we solve analytically and numerically the temporal evolution of the cavity coherent state and the associated physical observables. A critical transition is revealed when the coupling increase between the cavity and the waveguide. In particular, the cavity field becomes dissipationless when the coupling strength goes beyond a critical value, as a manifestation of strong non-Markovian memory effect. The result also indicates that the cavity can maintain in a coherent state with arbitrary small number of photons when it strongly couples to the waveguide at very low temperature. These properties can be measured experimentally through the photon current flowing over the waveguide in photonic crystals.

quant-ph↗
arXiv2010

Exact non-Markovian cavity dynamics strongly coupled to a reservoir

The exact non-Markovian dynamics of a microcavity strongly coupled to a general reservoir at arbitrary temperature is studied. With the exact master equation for the reduced density operator of the cavity system, we analytically solve the time evolution of the cavity state and the associated physical observables. We show that the non-Markovian dynamics is completely determined by the propagating (retarded) and correlation Green functions. Compare the non-Markovian behavior at finite temperature with those at zero-temperature limit or Born-Markov limit, we find that the non-Markovian memory effect can dramatically change the coherent and thermal dynamics of the cavity. We also numerically study the dissipation dynamics of the cavity through the mean mode amplitude decay and the average photon number decay in the microwave regime. It is shown that the strong coupling between the cavity and the reservoir results in a long-time dissipationless evolution to the cavity field amplitude, and its noise dynamics undergoes a critical transition from the weak to strong coupling due to the non-Markovian memory effect.

quant-ph↗
arXiv2009

Decoherence of coherent electronic excited state in the reaction center of the photosynthetic purple bacterium Rhodobacter sphaeroides

In this paper, we present a theoretical description to the quantum coherence and decoherence phenomena of energy transfer in photosynthesis observed in a recent experiment [see Science 316, 1462 (2007)]. As a successive two-color laser pulses with selected frequencies cast on a sample of the photosynthetic purple bacterium Rb. sphaeroides two resonant excitations of electrons in chromophores can be generated. However, this effective two-level subsystem will interact with its protein environment and decoherence is inevitable. We describe this subsystem coupled with its environment as a dynamical spin-boson model. The non-Markovian decoherence dynamics is described using a quasi-adiabatic propagator path integral (QUAPI) approach. With the photon-induced effective time-dependent level splitting energy and level flip coupling coefficient between the two excited states and the environment-induced non-Markovian decoherence dynamics, our theoretical result is in good agreement with the experimental data.

quant-ph↗
arXiv2009

A nonequilibrium theory for transient transport dynamics in nanostructures via the Feynman-Vernon influence functional approach

In this paper, we develop a nonequilibrium theory for transient electron transport dynamics in nanostructures based on the Feynman-Vernon influence functional approach. We extend our previous work on the exact master equation describing the non-Markovian electron dynamics in the double dot [Phys. Rev. B78, 235311 (2008)] to the nanostructures in which the energy levels of the central region, the couplings to the leads and the external biases applied to leads are all time-dependent. We then derive nonperturbatively the exact transient current in terms of the reduced density matrix within the same framework. This provides an exact non-linear response theory for quantum transport processes with back-reaction effect from the contacts, including the non-Markovian quantum relaxation and dephasing, being fully taken into account. The nonequilibrium steady-state transport theory based on the Schwinger-Keldysh nonequilibrium Green function technique can be recovered as a long time limit. For a simple application, we present the analytical and numerical results of transient dynamics for the resonance tunneling nanoscale device with a Lorentzian-type spectral density and ac bias voltages, where the non-Markovian memory structure and non-linear response to the bias voltages in transport processes are demonstrated.

cond-mat.mes-hall↗
arXiv2009

Magnetic field switching in parallel quantum dots

We show that the Coulomb blockade in parallel dots pierced by magnetic flux $Φ$ completely blocks the resonant current for any value of $Φ$ except for integer multiples of the flux quantum $Φ_0$. This non-analytic (switching) dependence of the current on $Φ$ arises only when the dot states that carry the current are of the same energy. The time needed to reach the steady state, however, diverges when $Φ\to nΦ_0$.

cond-mat.other↗
arXiv2009

Exact Master Equation and Non-Markovian Decoherence for Quantum Dot Quantum Computing

In this article, we report the recent progress on decoherence dynamics of electrons in quantum dot quantum computing systems using the exact master equation we derived recently based on the Feynman-Vernon influence functional approach. The exact master equation is valid for general nanostructure systems coupled to multi-reservoirs with arbitrary spectral densities, temperatures and biases. We take the double quantum dot charge qubit system as a specific example, and discuss in details the decoherence dynamics of the charge qubit under coherence controls. The decoherence dynamics risen from the entanglement between the system and the environment is mainly non-Markovian. We further discuss the decoherence of the double-dot charge qubit induced by quantum point contact (QPC) measurement where the master equation is re-derived using the Keldysh non-equilibrium Green function technique due to the non-linear coupling between the charge qubit and the QPC. The non-Markovian decoherence dynamics in the measurement processes is extensively discussed as well.

cond-mat.mes-hall↗
arXiv2009

Non-Markovian decoherence dynamics of entangled coherent states

We microscopically model the decoherence dynamics of entangled coherent states under the influence of vacuum fluctuation. We derive an exact master equation with time-dependent coefficients reflecting the memory effect of the environment, by using the Feynman-Vernon influence functional theory in the coherent-state representation. Under the Markovian approximation, our master equation recovers the widely used Lindblad equation in quantum optics. We then investigate the non-Markovian entanglement dynamics of the quantum channel in terms of the entangled coherent states under noise. Compared with the results in Markovian limit, it shows that the non-Markovian effect enhances the disentanglement to the initially entangled coherent state. Our analysis also shows that the decoherence behaviors of the entangled coherent states depend sensitively on the symmetrical properties of the entangled coherent states as well as the interactions between the system and the environment.

quant-ph↗
arXiv2008

Entanglement oscillation and survival induced by non-Markovian decoherence dynamics of entangled squeezed-state

We study the exact decoherence dynamics of the entangled squeezed state of two single-mode optical fields interacting with two independent and uncorrelated environments. We analyze in detail the non-Markovian effects on the entanglement evolution of the initially entangled squeezed state for different environmental correlation time scales. We find that the environments have dual actions on the system: backaction and dissipation. In mparticular, when the environmental correlation time scale is comparable to the time scale for significant change in the system, the backaction would counteract the dissipative effect. Interestingly, this results in the survival of some residual entanglement in the final steady state.

quant-ph↗
arXiv2008

Exact wave-packet decoherence dynamics in a discrete spectrum environment

We find an exact analytical solution of the reduced density matrix from the Feynman-Vernon influence functional theory for a wave packet influenced by an environment containing a few discrete modes. We obtain two intrinsic energy scales relating to the time scales of the system and the environment. Different relationship between these two scales alters the overall form of the solution of the system. We also introduce a decoherence measure for a single wave packet which is defined as the ratio of Schrödinger uncertainty over the delocalization extension of the wave packet and characterizes the time-evolution behavior of the off-diagonal reduced density matrix element. We utilize the exact solution and the docherence measure to study the wave packet decoherence dynamics. We further demonstrate how the dynamical diffusion of the wave packet leads to non-Markovian decoherence in such a microscopic environment.

quant-ph↗
arXiv2008

A non-Markovian decoherence theory for double dot charge qubit

In this paper, we develop a non-perturbation theory for describing decoherence dynamics of electron charges in a double quantum dot gated by electrodes. We extend the Feynman-Vernon influence functional theory to fermionic environments and derive an exact master equation for the reduced density matrix of electrons in the double dot for a general spectral density at arbitrary temperature and bias. We then investigate the decoherence dynamics of the double dot charge qubit with back-action of the reservoirs being fully taken into account. Time-dependent fluctuations and leakage effects induced from the dot-reservoir coupling are explicitly explored. The charge qubit dynamics from the Markovian to non-Markovian regime is systematically studied under various manipulating conditions. The decay behavior of charge qubit coherence and the corresponding relaxation time $T_1$ and dephasing time $T_2$ are analyzed in details.

cond-mat.mes-hall↗
arXiv2008

Non-equilibrium theory of charge qubit decoherence in the quantum point contact measurement

A non-equilibrium theory describing the charge qubit dynamics measured by a quantum point contact is developed based on Schwinger-Keldysh's approach. Using the real-time diagram technique, we derive the master equation to all orders in perturbation expansions. The non-Markovian processes in the qubit dynamics is naturally taken into account. The qubit decoherence, in particular, the influence of the tunneling-electron fluctuation in the quantum point contact with a longer time correlation, is studied in the framework. We consider the Lorentzian-type spectral density to characterize the channel mixture of the electron tunneling processes induced by the measurement and determine the correlation time scale of the tunneling-electron fluctuation. The result shows that as the quantum point contact is casted with a narrower profile of the spectral density, tunneling electrons can propagate with a longer time correlation and lead to the non-Markovian processes of the qubit dynamics. The qubit electron in the charge qubit will be driven coherently. The quantum point contact measurement with the minimum deviation of the electron tunneling processes prevents the qubit state from the decoherence.

cond-mat.mes-hall↗
arXiv2007

Non-Markovian entanglement dynamics of noisy continuous variable quantum channels

We investigate the entanglement dynamics of continuous-variable quantum channels in terms of an entangled squeezed state of two cavity fields in a general non-Markovian environment. Using the Feynman-Vernon influence functional theory in the coherent-state representation, we derive an exact master equation with time-dependent coefficients reflecting the non-Markovian influence of the environment. The influence of environments with different spectral densities, e.g., Ohmic, sub-Ohmic, and super-Ohmic, is numerically studied. The non-Markovian process shows its remarkable influences on the entanglement dynamics due to the sensitive time-dependence of the dissipation and noise functions within the typical time scale of the environment. The Ohmic environment shows a weak dissipation-noise effect on the entanglement dynamics, while the sub-Ohmic and super-Ohmic environments induce much more severe noise. In particular, the memory of the system interacting with the environment contributes a strong decoherence effect to the entanglement dynamics in the super-Ohmic case.

quant-ph↗
arXiv2007

Charge-to-spin conversion of electron entanglement states and spin-interaction-free solid-state quantum computation

Without resorting to spin-spin coupling, we propose a scalable spin quantum computing scheme assisted with a semiconductor multiple-quantum-dot structure. The techniques of single electron transitions and the nanostructure of quantum-dot cellular automata (QCA) are used to generate charge entangled states of two electrons, which are then converted into spin entanglement states using single-spin rotations only. Deterministic two-qubit quantum gates are also manipulated using only single-spin rotations with the help of QCA. A single-shot readout of spin states can be carried out by coupling the multiple dot structure to a quantum point contact. As a result, deterministic spin-interaction-free quantum computing can be implemented in semiconductor nanostructure.

cond-mat.mes-hall↗
arXiv2007

Implementing controlled-NOT gate based on free spin qubits with semiconductor quantum-dot array

Based on electron spins in semiconductor quantum dots as qubits, a new quantum controlled-NOT(CNOT) gate is constructed in solid nanostructure without resorting to spin-spin interactions. Single electron tunneling technology and coherent quantum-dot cellular automata architecture are used to generate an ancillary charge entangled state. Using the ancillary charge entangled state as an intermediate state, we obtain a spin entangled state and design a CNOT gate by using only single spin rotations.

quant-ph↗