External Field Control Research Articles

Controllable double tunneling induced transparency and solitons formation in a quantum dot molecule

We consider the coupling effect between interdot tunneling coupling and external optical control field to study the linear optical property and the formation of temporal optical solitons in a quantum dot molecules system, analytically. The results show that the double tunneling induced transparency (TIT) windows are appeared in the absorption curve of probe field because of the formation of dynamic Stark splitting and quantum destructive interference effect from the two upper levels. Interestingly, the width of the TIT window becomes wider with the increasing intensity of the optical control field. We also find that the Kerr nonlinear effect of the probe field can be modulated effectively through coherent control both the control field and the interdot tunneling coupling in this system. Meanwhile, we demonstrate that the formation of dark or bright solitons can be practical regulated by varying the intensity of the optical control field.

External field control of spin-dependent rotational decoherence of ultracold polar molecules

We determine trapping conditions for ultracold polar molecules, where pairs of internal states experience identical trapping potentials. Such conditions could ensure that detrimental effects of inevitable inhomogeneities across an ultracold sample are significantly reduced. In particular, we investigate the internal rovibronic and hyperfine quantum states of ultracold fermionic ground-state 40K87Rb polar molecules, when static magnetic, static electric and trapping laser fields are simultaneously applied. Understanding the effect of changing the relative orientation or polarisation of these three fields is of crucial importance for the creation of decoherence-free subspaces built from two or more rovibronic states. Moreover, we evaluate the induced dipole moment of the molecule in the presence of these fields, which will allow control of interactions between molecules in different sites of an optical lattice, and study the influence of the interaction anisotropy on the ability to entangle polar molecules.

Theory and simulation of combined mechanisms limiting the safe operating area of power semiconductor microelectronic switches

A generalized analytical model for the turn-off process of bipolar switches with microgates is developed taking into account the role of technological and design imperfections of real structures, which limit their safe operating area. Using a thyristor microchip with external field control, operating in a voltage inverter circuit, as an example, the boundary of the safe operating area with respect to the “turned-off current” is quantitatively determined. It is found that this boundary for a non-ideal structure in the low-voltage region is controlled by the effect of regenerative unlocking [triggering] of the cathode emitter; in the high-voltage region, this boundary is controlled by the onset of current localization in “perturbed” cells, involving dynamic breakdown. The possible applications of the developed model to indicate the directions of device-structure optimization to increase their maximum switching current are discussed. The adequacy of the results of model applications is validated using numerical simulation.

Frequency‐sensitive switching control effect induced by two‐photon resonance in an EIT‐based layered medium

The optical response of an atomic vapor can be coherently manipulated by tunable quantum interference occurring in atomic transition processes. A periodic layered medium whose unit cells consist of a dielectric and an EIT (electromagnetically induced transparency) atomic vapor is designed for light propagation manipulation. Such an EIT‐based periodic layered medium exhibits a flexible frequency‐sensitive optical response, where a very small change in probe frequency can lead to a drastic variation of reflectance and transmittance. As the destructive quantum interference relevant to two‐photon resonance arises in EIT atoms interacting with both control and probe fields, the controllable optical processes that depend sensitively on the external control field will take place in this EIT‐based periodic layered medium. Such a frequency‐sensitive and field‐controlled optical behavior of reflection and transmission in the EIT photonic crystal can be applicable to designs of new devices such as photonic switches, photonic logic gates and photonic transistors, where one laser field can be controlled by the other one, and would have potential applications in the areas of integrated optical circuits and other related techniques (e.g., all‐optical instrumentations).

Electric field control in ultralong-range triatomic polar Rydberg molecules

We theoretically explore the external electric field control of a species of ultralong-range molecules that emerge from the interaction of a ground-state polar molecule with a Rydberg atom. The external field mixes the Rydberg electronic states and, therefore, strongly alters the electric field seen by the polar diatomic molecule due to the Rydberg electron. As a consequence, the adiabatic potential energy curves responsible for the molecular binding can be tuned in such a way that an intersection with neighboring curves occurs. The latter leads to admixture of $s$-wave character in the Rydberg wave function and will substantially facilitate the experimental preparation and realization of this particular class of Rydberg molecule species.

Effects of current localization in high-power microgate bipolar switches with nonideal interconnection between controlled cells

The influence of nonideal interconnection between controlled cells in bipolar microgate switches on current localization at the turn-off stage is analyzed. To estimate the resistance of the distributed electrodes, the entire device is represented as two parallel subsystems of controlled cells interconnected via the effective resistance of the gate circuit. Different scenarios of the turn-off process at nonideal couplings between the cells are numerically simulated for three turn-off circuit regimes by the example of an integrated thyristor with external field control. The turn-off scenarios are studied versus the effective resistance of the gate-off circuit and the ratio between the working surface areas of the cellular subsystems. Limitations on the ultimate switched current are compared. For small-scale inhomogeneities in the series resistance of the gate circuit, the maximum turn-off current in the cascode mode and using a negative voltage source remains an order of magnitude higher than that under the emitter short-circuit conditions.

Probe frequency- and field intensity-sensitive coherent control effects in an EIT-based periodic layered medium

A periodic layered medium, with unit cells consisting of a dielectric and an electromagnetically-induced transparency (EIT)-based atomic vapor, is designed for light propagation manipulation. Considering that a destructive quantum interference relevant to a two-photon resonance emerges in EIT-based atoms interacting with both control and probe fields, an EIT-based periodic layered medium exhibits a flexible frequency-sensitive optical response, where a very small variation in the probe frequency can lead to a drastic variation in reflectance and transmittance. The present EIT-based periodic layered structure can result in controllable optical processes that depend sensitively on the external control field. The tunable and sensitive optical response induced by the quantum interference of a multi-level atomic system can be applied in the fabrication of new photonic and quantum optical devices. This material will also open a good perspective for the application of such designs in several new fields, including photonic microcircuits or integrated optical circuits.

Coherent control of the Goos-Hnchen shift in four-level atomic medium

The manipulation of the Goos-Hnchen shifts in the reflected and transmitted light beam through a cavity containing four-level atomic medium is investigated by modifying the dispersion-absorption properties of the intracavity medium. Via two external coherent control fields, the Goos-Hnchen shifts of the reflected and transmitted beam can be easily controlled. It is found that around the transparent region of the medium, the Goos-Hnchen shifts are very sensitive and can be enhanced as compared with strong absorption and amplification.

Experimental Verification of Circuit Analog of Three- and Four-Level Electromagnetically Induced Transparency

Since a two-level resonant atomic system can be simulated by a simple circuit, three- and four-level electromagnetically induced transparency (EIT) that occur due to light-atom interaction can find its classical counterpart in circuit analog. As the optical response of an EIT atomic medium (including atomic vapors and semiconductor-quantum-dot dielectrics) can be controlled via tunable quantum interference induced by applied external control fields, in the scheme of circuit analog, such a controllable manipulation is achieved via capacitor coupling, where two loops are coupled by a capacitor that can represent the applied control fields in atomic EIT. Both numerical simulation and experimental demonstration of three- and four-level EIT were performed based on such a scenario of circuit analog. The classical “coherence” relevant to quantum interference among transitions pathways driven by both probe and control fields in EIT atomic systems has been manifested in the present circuit analog of EIT.

All possible coupling schemes inXYspin chains for perfect state transfer

We investigate quantum state transfer in XY spin chains and propose a recursive procedure to construct the nonuniform couplings of these chains with arbitrary length to achieve perfect state transfer(PST). We show that this method is capable of finding all possible coupling schemes for PST. These schemes, without external control fields, only involve preengineered couplings but not dynamical control of them, so they can be simply realized experimentally. The analytical solutions provide all information for coupling design.

Dimension reduction by balanced truncation: Application to light-induced control of open quantum systems

In linear control, balanced truncation is known as a powerful technique to reduce the state-space dimension of a system. Its basic principle is to identify a subspace of jointly easily controllable and observable states and then to restrict the dynamics to this subspace without changing the overall response of the system. This work deals with a first application of balanced truncation to the control of open quantum systems which are modeled by the Liouville-von Neumann equation within the Lindblad formalism. Generalization of the linear theory has been proposed to cope with the bilinear terms arising from the coupling between the control field and the quantum system. As an example we choose the dissipative quantum dynamics of a particle in an asymmetric double well potential driven by an external control field, monitoring population transfer between the potential wells as a control target. The accuracy of dimension reduction is investigated by comparing the populations obtained for the truncated system versus those for the original system. The dimension of the model system can be reduced very efficiently where the degree of reduction depends on temperature and relaxation rate.

Photonic Logic Gate Design Based on Frequency-Sensitive Optical Response of an EIT Photonic Crystal

The frequency-sensitive optical response due to two-photon resonance of electromagnetically induced transparency (EIT) in a tunable band structure of an EIT-based layered medium is considered. The unit cells of this periodic layered structure are composed of dielectric (e.g., GaAs) and EIT atomic vapor. The frequency-sensitive behavior of controllable reflectance and transmittance depending on the external control field can be applicable to new device design (e.g., it can serve as the fundamental working mechanism for photonic switches and photonic logic gates). Some two-input logic gates (e.g., OR and NAND gates) are designed based on the present effect of sensitive switching control that results from the two-photon resonance.

External field control of charge transmission through single molecules: Switching effects and transient currents

Net electron transfer through a single molecule attached to nano electrodes can undergo a pronounced change by resonant optical excitation. Caused by the population of excited electronic states of the molecule new transmission channels are opened which alter the current–voltage characteristics of the junction. This recent suggestion is further investigated here by studying the transient behavior of the current if the external laser pulse excitation is switched-on and -off within a certain time-interval. The computations concentrate on the case of weak and intermediate molecule–lead coupling. They are carried out in using rate equations which govern the populations of the different electron-vibrational states of the molecule being in its neutral or charged state. The interrelation of the switching-time and the time of charging and discharge of the molecule as well as of the time of vibrational relaxation is demonstrated by working in a switching-time range of some hundreds fs up to about 50ps.

Vibrational effect on external field control of charge transmission in molecular nano-junction

With the weak-coupling approach in molecular vibrational normal coordinate, the charge transmission under external optical field through molecular nano-junction is studied. From the charging energy point of view, the sequential charge transmission is analyzed and the current-voltage characteristics under external optical field are studied. It is found that the vibrational effect of molecule is the key factor of the removal of Franck-Condon blockage under external optical field. Furthermore, the effects of charging energy and intra-molecular vibrational energy redistribution (IVR) in charge transmission under external optical field are studied in detail.

Light trapping in high-density ultracold atomic gases for quantum memory applications

High-density and ultracold atomic gases have emerged as promising media for storage of individual photons for quantum memory applications. In this paper we provide an overview of our theoretical and experimental efforts in this direction, with particular attention paid to manipulation of light storage (a) through complex recurrent optical scattering processes in very high density gases (b) by an external control field in a characteristic electromagnetically induced transparency configuration.

Frequency-Sensitive Optical Response via Tunable Band Structure in an EIT-Based Layered Medium

The optical response of an atomic vapor can be controlled by using tunable quantum interference induced by external control field. A periodic layered medium whose unit cells consist of dielectric (e.g., GaAs) and EIT (electromagnetically induced transparency) atomic vapor is suggested. It is demonstrated that such an EIT-based periodic layered medium shows more flexible optical response (sensitive to frequency) than a conventional photonic crystal does. The controllable band structure that depends on the external control field can be applicable to designs of new devices such as photonic switches and photonic logic gates, where one laser field can be controlled by the other one, and would have potential applications in the field of integrated optical circuits and other related areas, e.g., the all-optical technique.

External electric field control in the charge qubits including three Josephson junctions

Based on the standard canonical quantization principle, this paper gives the quantization scheme for the charge qubits mesoscopic circuit including three Josephson junctions coupled capacitively. By virtue of the Heisenberg equation, the time evolution of the phase difference operators across the polar plates and the number operators of the Cooper-pairs on the island are investigated and the modification of the Josephson equation is discussed. The time evolution of the phase difference operators is analysed when the Josephson junctions are irradiated by the external electrical field, which is referred to as also the obtainable controlling parameter.

External field control of collective spin excitations in an optical lattice of 2Σ molecules

We show that an ensemble of 2Σ molecules in the ro-vibrational ground state trapped on an optical lattice exhibits collective spin excitations that can be controlled by applying superimposed electric and magnetic fields. In particular, we show that the lowest energy excitation of the molecular ensemble at certain combinations of electric and magnetic fields leads to the formation of a magnetic Frenkel exciton. The exciton bandwidth can be tuned by varying the electric or magnetic fields. We show that the exciton states can be localized by creating vacancies in the optical lattice. The localization patterns of the magnetic exciton states are sensitive to the number and distribution of vacancies, which can be exploited for engineering many-body entangled spin states. We consider the dynamics of magnetic exciton wavepackets and show that the spin excitation transfer between molecules in an optical lattice can be accelerated or slowed down by tuning an external magnetic or electric field.

Characteristics of MHD Equilibrium and Related Issues on LHD

In the vacuum of the Large Helical Device (LHD), we can change the plasma volume, the aspect ratio, the ellipticity, the rotational transform, and the height of the magnetic hill through the control of the vertical and the qudrupole components of the magnetic field and the helical coil pitch parameter. The two effects of the finite beta on the magnetohydrodynamic (MHD) configuration, the magnetic surface torus outward shift and the invasion of the stochastic region into the plasma core, are discussed. The former is qualitatively the same as that by the external vertical field control. According to the comparison between a theoretical prediction in the finite beta and the vacuum field calculation in the vertical field control, the latter effect is strongly affected by the nonaxisymmetric component of the equilibrium current. A theoretical prediction suggests that an MHD equilibrium beta limit different from the conventional one exists due to the lack of the equilibrium force balance in the stochastic region. The key parameters to improve the accuracy of the identification of the MHD equilibrium configuration are shown to be the identification of the toroidal current profile, the anisotropic pressure effect, and the identification of the plasma boundary shape.

Multi-normal mode-splitting for an optical cavity with electromagnetically induced transparency medium

We theoretically study the cavity transmission spectra with three-level atoms coupled by a coherent external control field in the superstrong coupling regime (atoms-cavity coupling strength g [square root] N is near or larger than the cavity free-spectral range DeltaFSR). When satisfying the superstrong coupling condition by increasing the number of the interaction atoms, more than one FSR cavity modes interact with atoms and each mode will split three peaks, which can be well explained by the linear dispersion enhancement of electromagnetically induced transparency medium due to the largely increased atomic density in the cavity.