Ka-Di Zhu

Voltage-controlled slow light in asymmetry double quantum dots

The authors demonstrate theoretically that there exists electromagnetically induced transparency in an asymmetric double quantum dot system using tunneling instead of pump laser. The group velocity slowdown factor is theoretically analyzed as a function of electron tunneling at different broadened linewidths. With feasible parameters for applications to a 100Gbits∕s optical network, numerical calculation infers group velocity as low as 300m∕s. The scheme is expected to be useful in constructing a variable semiconductor optical buffer based on electromagnetically induced transparency in an asymmetric double quantum dot controlled by voltage.

Non-Markovian reduced dynamics and entanglement evolution of two coupled spins in a quantum spin environment

The exact quantum dynamics of the reduced density matrix of two coupled spin qubits in a quantum Heisenberg XY spin star environment in the thermodynamic limit at arbitrarily finite temperatures is obtained using a novel operator technique. In this approach, the transformed Hamiltonian becomes effectively Jaynes-Cumming like and thus the analysis is also relevant to cavity quantum electrodynamics. This special operator technique is mathematically simple and physically clear, and allows us to treat systems and environments that could all be strongly coupled mutually and internally. To study their entanglement evolution, the concurrence of the reduced density matrix of the two coupled central spins is also obtained exactly. It is shown that the dynamics of the entanglement depends on the initial state of the system and the coupling strength between the two coupled central spins, the thermal temperature of the spin environment and the interaction between the constituents of the spin environment. We also investigate the effect of detuning which in our model can be controlled by the strength of a locally applied external magnetic field. It is found that the detuning has a significant effect on the entanglement generation between the two spin qubits.

Slow light in an artificial hybrid nanocrystal complex

A slow light effect in a hybrid nanocrystal complex composed of a semiconductor quantum dot (SQD) and a metal nanoparticle (MNP) is investigated theoretically. It is shown that a hole induced by coherent population oscillation appears at the absorption spectrum of the probe when an exciton and plasmon interact. Then a slow light effect may become possible. The numerical results further indicate that the slow light effect is greatly modified by the distance between the SQD and the MNP due to the coupling of the exciton and plasmon. For experimental conditions, the group velocity indexes with Gauss distribution and Lorenz distribution of the distance between the SQD and the MNP are evaluated. It is found that for small dispersion, almost the same values of the group velocity index can be obtained in these different distributions.

Enhancing Kerr nonlinearity of a strongly coupled exciton-plasmon in hybrid nanocrystal molecules

The optical nonlinearity of hybrid nanocrystal molecules composed of semiconductor and metal nanoparticles with a weak probe in a strong control field is investigated theoretically. Excitons in such a hybrid molecule demonstrate novel optical properties due to the coupling between excitons and plasmons. It is shown that Kerr nonlinear coefficients as functions of the detuning of the control field from the exciton frequency and the centre-to-centre distance between the two nanoparticles are greatly enhanced due to exciton–plasmon couplings.

Coupling-rate determination based on radiation-pressure-induced normal mode splitting in cavity optomechanical systems.

We theoretically propose a precise way to measure the coupling rate in an optomechanical system based on radiation-pressure-induced normal mode splitting. This all-optical method is effective in both weak and strong coupling regions. Simultaneously the vibrational frequency of the mechanical mode can also be detected easily in the reflected probe spectrum. The results are much useful for the extensive applications in optomechanical systems.

Charge qubit dynamics in a double quantum dot coupled to phonons

Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, China(Dated: February 2, 2008)The dynamics of charge qubit in a double quantum dot coupled to phonons is investigated the-oretically in terms of a perturbation treatment based on a unitary transformation. The dynamicaltunneling current is obtained explicitly. The result is compared with the standard perturbation the-ory at Born-Markov approximation. The decoherence induced by acoustic phonons is analyzed atlength. It is shown that the contribution from deformation potential coupling is comparable to thatfrom piezoelectric coupling in small dot size and large tunneling rate case. A possible decouplingmechanism is predicted.

Strong coupling among semiconductor quantum dots induced by a metal nanoparticle

Based on cavity quantum electrodynamics (QED), we investigate the light-matterinteraction between surface plasmon polaritons (SPP) in a metal nanoparticle (MNP)and the excitons in semiconductor quantum dots (SQDs) in an SQD-MNP coupled system.We propose a quantum transformation method to strongly reveal the exciton energyshift and the modified decay rate of SQD as well as the coupling among SQDs. Toobtain these parameters, a simple system composed of an SQD, an MNP, and a weaksignal light is designed. Furthermore, we consider a model to demonstrate thecoupling of two SQDs mediated by SPP field under two cases. It is shown that two SQDscan be entangled in the presence of MNP. A high concurrence can be achieved, which isthe best evidence that the coupling among SQDs induced by SPP field in MNP. Thisscheme may have the potential applications in all-optical plasmon-enhanced nanoscaledevices.

Slow light control with electric fields in vertically coupled InGaAs/GaAs quantum dots

Tunneling-induced transparency in vertically coupled InGaAs/GaAs quantum dots using tunneling instead of pump laser, analogous to electromagnetically induced transparency in atomic systems, is studied. The interdot quantum coupling strength is tuned by static electric fields. The group velocity slow-down factor is theoretically analyzed as a function of electron tunneling at different broadened linewidths. For parameters appropriate to a 100 Gbits/s optical network, group velocities as low as 850 m/s are calculated. The scheme is expected to be useful to construct a variable semiconductor optical buffer based on electromagnetically induced transparency in vertically coupled InGaAs/GaAs quantum dots controlled by electric fields.

A scheme for measuring vibrational frequency and coupling strength in a coupled nanomechanical resonator-quantum dot system

The authors theoretically propose a precise way to measure the vibrational frequency and coupling strength in a coupled nanomechanical resonator-quantum dot system in terms of mechanically induced coherent population oscillation. The pump-probe spectroscopy that exhibits new features such as mechanically induced three-photon resonance and ac Stark effect is obtained. Simultaneously, the coupling strength between nanomechanical resonator and quantum dot can also be detected from Rabi-splitting-like peak in the probe spectrum. Our results could be applied to other systems such as coupled DNA-quantum dot systems.

Optical determination of vacuum Rabi splitting in a semiconductor quantum dot induced by a metal nanoparticle

We propose a theoretical scheme to determine the vacuum Rabi splitting in a single semiconductor quantum dot (SQD) induced by a metal nanoparticle (MNP). Based on cavity quantum electrodynamics, the exciton-plasmon interaction between the SQD and the MNP is considered while a strong pump laser and a weak probe laser are simultaneously presented. By decreasing the distance between them, we can increase the coupling strength. At resonance, thanks to the strong coupling, a vacuum Rabi splitting can be observed clearly in the probe absorption spectrum. The coupling strength can be obtained by measuring the vacuum Rabi splitting. This strong coupling is significant for the investigation of surface-plasmon-based quantum information processing.