arXiv subjects

Wei-Min Zhang

Publications and source records attributed to Wei-Min Zhang.

At least 55 records · Page 3Linked to original sources

arXiv2013

Breakdown of Bose-Einstein distribution in photonic crystals

In the last two decades, considerable advances have been made in the investigation of nano-photonics in photonic crystals. Previous theoretical investigations of photon dynamics were carried out at zero temperature. Here, we investigate micro/nano cavity photonics in photonic crystals at finite temperature. Due to photonic-band-gap-induced non-Markovian dynamics, we discover that cavity photons in photonic crystals do not obey the standard Bose-Einstein statistical distribution. Within the photonic band gap and in the vicinity of the band edge, cavity photons combine nontrivial quantum dissipation with thermal fluctuations to form photon states that can memorize the initial cavity state information. As a result, Bose-Einstein distribution is completely broken down in these regimes, even if the thermal energy is larger than the photonic band gap.

quant-ph↗

arXiv2013

Non-Markovian Dynamics Impact on the Foundations of Statistical Mechanics

The foundations of statistical mechanics, namely how equilibrium hypothesis emerges microscopically from quantum theory, is explored through investigating the environment-induced quantum decoherence processes. Based on the recent results on non-Markovian dynamics [Phys. Rev. Lett. 109, 170402 (2012)], we find that decoherence of quantum states manifests unexpected complexities. Indeed, an arbitrary given initial quantum state, under the influence of different reservoirs, can evolve into four different steady states: thermal, thermal-like, quantum memory and oscillating quantum memory states. The first two steady states \textit{de facto} provided a rigorous proof how the system relaxes to thermal equilibrium with its environment. The latter two steady states, with strong non-Markovian effects, will maintain the initial state information and not reach thermal equilibrium, which is beyond the conventional wisdom of statistical mechanics.

quant-ph↗

arXiv2013

Transient Current-Current Correlations and Noise Spectra

In this paper, we present an exact formalism for transient current-current correlations and transient noise spectra. The exact solution of transient current correlations in both the time domain and the frequency domain are obtained. Without taking the wide band limit, we investigate transient current-current correlations with different bias voltages and different finite temperatures. Transient noise spectra over the whole frequency range are calculated and that in the steady-state limit are also reproduced. From transient current-current correlations and noise spectra, we analyze the frequency-dependence of electron transport for the system evolving far away from equilibrium to the steady state. Various time scales associated with the energy structure of the nanosystem are also obtained from the transient current-current correlations and transient noise spectra.

cond-mat.mes-hall↗

arXiv2013

Precision control of charge coherence in parallel double dot systems through spin-orbit interaction

In terms of the exact quantum master equation solution for open electronic systems, the coherent dynamics of two charge states described by two parallel quantum dots with one fully polarized electron on either dot is investigated in the presence of spin-orbit interaction. We demonstrate that the double dot system can stay in a dynamically decoherence free space. The coherence between two double dot charge states can be precisely manipulated through a spin-orbit coupling. The effects of the temperature, the finite bandwidth of lead, and the energy deviations during the coherence manipulation are also explored.

cond-mat.mes-hall↗

arXiv2012

Exact Non-Markovian Master Equation and Dispersive Probing of Non-Markovian Process

For a bosonic (fermionic) open system in a bath with many bosons (fermions) modes, we derive the exact non-Markovian master equation in which the memory effect of the bath is reflected in the time dependent decay rates. In this approach, the reduced density operator is constructed from the formal solution of the corresponding Heisenberg equations. As an application of the exact master equation, we study the active probing of non-Markovianity of the quantum dissipation of a single boson mode of electromagnetic (EM) field in a cavity QED system. The non-Markovianity of the bath of the cavity is explicitly reflected by the atomic decoherence factor.

quant-ph↗

arXiv2012

Quantum-criticality-induced strong Kerr nonlinearities in optomechanical systems

We investigate a hybrid electro-optomechanical system that allows us to obtain controllable strong Kerr nonlinearities in the weak-coupling regime. We show that when the controllable electromechanical subsystem is close to its quantum critical point, strong photon-photon interactions can be generated by adjusting the intensity (or frequency) of the microwave driving field. Nonlinear optical phenomena, such as the appearance of the photon blockade and the generation of nonclassical states (e.g., Schrödinger cat states), are predicted in the weak-coupling regime, which is feasible for most current optomechanical experiments.

quant-ph↗

arXiv2012

Dynamically stabilized decoherence-free states in non-Markovian open fermionic systems

Decoherence-free subspaces (DFSs) provide a strategy for protecting the dynamics of an open system from decoherence induced by the system-environment interaction. So far, DFSs have been primarily studied in the framework of Markovian master equations. In this work, we study decoherence-free (DF) states in the general setting of a non-Markovian fermionic environment. We identify the DF states by diagonalizing the non-unitary evolution operator for a two-level fermionic system attached to an electron reservoir. By solving the exact master equation, we show that DF states can be stabilized dynamically.

quant-ph↗

arXiv2012

Real-time dynamics of single-electron pumping

In this Letter, we study the real-ime dynamics of single-electron pumpings. We modulate the left tunneling barrier along with the dot level but slightly changing the right barrier to operate the single-electron pumping device at zero bias. We show the adiabatic to the non-adiabatic transition in pumping operations. We also find that the relation $I=eω/2π$ is only valid in the low operating frequency adiabatic regime.

cond-mat.mes-hall↗

arXiv2012

General non-Markovian dynamics of open quantum systems

We present a general theory of non-Markovian dynamics for open quantum systems. We explore the non-Markovian dynamics by connecting the exact master equations with the non-equilibirum Green functions. Environmental back-actions are fully taken into account. The non-Markovian dynamics consists of non-exponential decays and dissipationless oscillations. Non-exponential decays are induced by the discontinuity in the imaginary part of the self-energy corrections. Dissipationless oscillations arise from band gaps or the finite band structure of spectral densities. The exact analytic solutions for various non-Markovian environments show that the non-Markovian dynamics can be largely understood from the environmental-modified spectra of the open systems.

quant-ph↗

arXiv2012

Noise spectrum of quantum transport through quantum dots: a combined effect of non-Markovian and cotunneling processes

Based on our recently developed quantum transport theory in term of an exact master equation, the corresponding particle-number resolved ($n$-resolved) master equation and the related shot noise spectrum formalism covering the full frequency range are constructed. We demonstrate that the noise spectra of transport current through single quantum dot and double quantum dots show characteristic steps and/or peak-dips in different tunneling frequency regimes through tuning the applied bias voltage and/or gate voltage at low temperatures. The peak-dips crossing the tunneling resonant frequencies is a combination effect of non-Markovian and cotunneling processes. These voltage-dependent tunneling resonance characteristics can be utilized to effectively modulate the internal Rabi resonance signature in the noise spectrum.

cond-mat.mes-hall↗

arXiv2012

Transient quantum transport in double-dot Aharonov-Bohm interferometers

Real-time nonequilibrium quantum dynamics of electrons in double-dot Aharonov-Bohm (AB) interferometers is studied using an exact solution of the master equation. The building of the coherence between the two electronic paths shows up via the time-dependent amplitude of the AB oscillations in the transient transport current, and can be enhanced by varying the applied bias on the leads, the on-site energy difference between the dots and the asymmetry of the coupling of the dots to the leads. The transient oscillations of the transport current do not obey phase rigidity. The circulating current has an anti-symmetric AB oscillation in the flux. The non-degeneracy of the on-site energies and the finite bias cause the occupation in each dot to have an arbitrary flux dependence as the coupling asymmetry is varied.

cond-mat.mes-hall↗

arXiv2012

Coherent control of artificial molecules using an Aharonov-Bohm magnetic flux

Bonding and anti-bonding states of artificial molecules have been realized in experiments by directly coupling two quantum dots. Without a direct coupling between two nearby quantum dots, here we show that a continuous crossover, from symmetric to anti-symmetric molecular state, can be achieved by changing the flux through a double quantum dot Aharonov-Bohm (AB) interferometer. We explicitly present the flux-dependent real-time processes of molecular-state formation. In contrast to the transport current, which has a $2π$ period, the quantum state of the DQD molecule has a $4π$ period in the AB flux.

quant-ph↗

arXiv2011

Decoherence suppression of open quantum systems through a strong coupling to non-Markovian reservoirs

In this paper, we provide a mechanism of decoherence suppression for open quantum systems in general, and that for "Schrodinger cat-like" state in particular, through the strong couplings to non-Markovian reservoirs. Different from the usual strategies of suppressing decoherence by decoupling the system from the environment in the literatures, here the decoherence suppression employs the strong back-reaction from non-Markovian reservoirs. The mechanism relies on the existence of the singularities (bound states) of the nonequilibrium retarded Green function which completely determines the dissipation and decoherence dynamics of open systems. As an application, we examine the decoherence dynamics of a photonic crystal nanocavity that is coupled to a waveguide. The strong non-Markovian suppression of decoherence for the optical cat state is attained.

quant-ph↗

arXiv2011

Single-electron turnstile pumping with high frequencies

In this Letter, we present a theoretical analysis to single-electron pumping operation in a large range of driving frequencies through the time-dependent tunneling barriers controlled by external gate voltages. We show that the single-electron turnstile works at the frequency lower than the characteristic frequency which is determined by the mean average electron tunneling rate. When the driving frequency is greater than the characteristic frequency of electron tunnelings, fractional electron pumping occurs as an effect of quantum coherence tunneling.

cond-mat.mes-hall↗

arXiv2011

Quantum Transport Theory for Photonic Networks

In this paper, we develop a quantum transport theory to describe photonic transport in photonic networks. The photonic networks concerned in the paper consist of all-optical circuits incorporating photonic bandgap waveguides and driven resonators. The photonic transport flowing through waveguides are entirely determined from the exact master equation of the driven resonators. The master equation of the driven resonators is obtained by explicitly eliminating all the waveguide degrees of freedom while the back-reactions between resonators and waveguides are fully taken into account. The relations between the driven photonic dynamics and photocurrents are obtained. The non-Markovian memory structure and quantum coherence and decoherence effects in photonic transport are also fully included. This quantum transport theory unifies two fundamental nonequilibrium approaches, the Keldysh's nonequilibrium Green function technique and the Feynman-Vernon influence functional approach, together to make the investigation of the transient quantum photonic transport become more powerful. As an illustration, the theory is applied to the transport phenomena of a driven nanocavity coupled to two waveguides in photonic crystals. The controllability of photonic transport through the driven resonator is demonstrated.

quant-ph↗

arXiv2011

Entangling two distant nanocavities via a waveguide

In this paper, we investigate the generation of continuous variable entanglement between two spatially-separate nanocavities mediated by a coupled resonator optical waveguide in photonic crystals. By solving the exact dynamics of the cavity system coupled to the waveguide, the entanglement and purity of the two-mode cavity state are discussed in detail for the initially separated squeezing inputs. It is found that the stable and pure entangled state of the two distant nanocavities can be achieved with the requirement of only a weak cavity-waveguide coupling when the cavities are resonant with the band center of the waveguide. The strong couplings between the cavities and the waveguide lead to the entanglement sudden death and sudden birth. When the frequencies of the cavities lie outside the band of the waveguide, the waveguide-induced cross frequency shift between the cavities can optimize the achievable entanglement. It is also shown that the entanglement can be easily manipulated through the changes of the cavity frequencies within the waveguide band.

quant-ph↗

arXiv2011

Suppressing non-Markovian noises by coupling the qubit to a chaotic device

To suppress decoherence of solid-state qubits which are coupled to the non-Markovian noises, we propose a strategy to couple the qubit with a chaotic device, of which the broad power distribution in the high-frequency domain can be used to freeze the noises just like the dynamical decoupling control (DDC) method. Compared with the DDC, high-frequency components can be generated by the chaotic device even driven by a low-frequency field and we do not need to optimize the control fields to generate complex control pulses. As an application to superconducting circuits, we find that various noises in a wide frequency domain, including low-frequency $1/f$, high-frequency Ohmic, sub-Ohmic, and super-Ohmic noises, can be efficiently suppressed by coupling the qubit to a Duffing oscillator, and the decoherence rate of the qubit is efficiently decreased for about 100 times in magnitude.

quant-ph↗

arXiv2011

Chaos can act as a decoherence suppressor

We propose a strategy to suppress decoherence of a solid-state qubit coupled to non-Markovian noises by attaching the qubit to a chaotic setup with the broad power distribution in particular in the high-frequency domain. Different from the existing decoherence control methods such as the usual dynamics decoupling control, high-frequency components of our control are generated by the chaotic setup driven by a low-frequency field, and the generation of complex optimized control pulses is not necessary. We apply the scheme to superconducting quantum circuits and find that various noises in a wide frequency domain, including low-frequency $1/f$, high-frequency Ohmic, sub-Ohmic, and super-Ohmic noises, can be efficiently suppressed by coupling the qubits to a Duffing oscillator as the chaotic setup. Significantly, the decoherence time of the qubit is prolonged approximately $100$ times in magnitude.

quant-ph↗