Strong Coupling Conditions Research Articles

Strong coupling in a single quantum dot–semiconductor microcavity system

Cavity quantum electrodynamics, a central research field in optics and solid-state physics, addresses properties of atom-like emitters in cavities and can be divided into a weak and a strong coupling regime. For weak coupling, the spontaneous emission can be enhanced or reduced compared with its vacuum level by tuning discrete cavity modes in and out of resonance with the emitter. However, the most striking change of emission properties occurs when the conditions for strong coupling are fulfilled. In this case there is a change from the usual irreversible spontaneous emission to a reversible exchange of energy between the emitter and the cavity mode. This coherent coupling may provide a basis for future applications in quantum information processing or schemes for coherent control. Until now, strong coupling of individual two-level systems has been observed only for atoms in large cavities. Here we report the observation of strong coupling of a single two-level solid-state system with a photon, as realized by a single quantum dot in a semiconductor microcavity. The strong coupling is manifest in photoluminescence data that display anti-crossings between the quantum dot exciton and cavity-mode dispersion relations, characterized by a vacuum Rabi splitting of about 140 microeV.

Polyelectrolyte adsorption and multilayering on charged colloidal particles

AbstractIn this article we review the complexation behavior of charged polymers adsorbed onto charged substrates. In the first part we summarize the results obtained on the adsorption properties of a single polyelectrolyte chain with one or several oppositely charged spherical colloids. The results of various simulational studies are compared with theoretical predictions. After that, we show that under strong electrostatic coupling conditions one can also obtain complexations, where chain and colloid carry the same charge (like‐charge complexation). This effect is due to counterion correlation effects. Finally, we investigate the case of polyelectrolyte multilayers structures, which are composed of alternating polycations and polyanions on various substrate geometries. This structure, normally obtained by a so‐called layer‐by‐lay deposition, is very stable experimentally, but only poorly understood in terms of theoretical approaches. Our simulation results suggest that short range attractive interactions (e.g., van der Waals) are important for obtaining a stable sequence of multilayers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3557–3570, 2004

Focus section on Cavity QED

Focus section on Cavity QED

Resonant nonstationary amplification of polychromatic laser pulses and conical emission in an optically dense ensemble of neon metastable atoms

Experimental and numerical investigation of single-beam and pump-probe interaction with a resonantly absorbing dense extended medium under strong and weak field-matter coupling is presented. Significant probe beam amplification and conical emission were observed. Under relatively weak pumping and high medium density, when the condition of strong coupling between field and resonant matter is fulfilled, the probe amplification spectrum has a form of spectral doublet. Stronger pumping leads to the appearance of a single peak of the probe beam amplification at the transition frequency. The greater probe intensity results in an asymmetrical transmission spectrum with amplification at the blue wing of the absorption line and attenuation at the red one. Under high medium density, a broadband of amplification appears. The theoretical model is based on the solution of the Maxwell-Bloch equations for a two-level system. Different types of probe transmission spectra obtained are attributed to complex dynamics of a coherent medium response to broadband polychromatic radiation of a multimode dye laser.

Electromagnetically induced tunnelling suppression in a flux qubit

Motivated by recent experiments wherein Josephson devices are irradiated by microwaves fields or are coupled to LC resonators, we theoretically investigate the dynamics of a flux qubit coupled to a monochromatic bosonic mode. We define strong coupling conditions under which the qubit tunnelling frequency between the localized flux states can be controlled and even suppressed. The practical realization of such a regime leading to this hindered dynamics is discussed.

Dephasing due to background charge fluctuations

In quantum computation, quantum coherence must be maintained during gate operation. However, in physical implementations, various couplings with the environment are unavoidable and can lead to a dephasing of a quantum bit(qubit). The background charge fluctuations are an important dephasing process, especially in a charge qubit system. We examined the dephasing rate of a qubit due to random telegraph noise. Solving stochastic differential equations, we obtained the dephasing rate of a qubit constructed of a coupled-dot system; we applied our results to the charge Josephson qubit system. We examined the dephasing rates due to two types of couplings between the coupled-dot system and the background charge, namely, fluctuation in the tunnel coupling constant and fluctuation in the asymmetric bias. For a strong coupling condition, the dephasing rate was inversely proportional to the time constant of the telegraph noise. When there is fluctuation in the tunnel coupling constant, Gaussian decay occurs in the initial regime. We also examined the rate of dephasing due to many impurity sites. For a weak coupling condition with fluctuation in the asymmetric bias, the obtained dephasing rate coincided with that obtained by the perturbation method using the spectral weight of a boson thermal bath, which is proportional to the inverse of the frequency.

Current Sheet Formation in Weakly Ionized Plasmas

Using the two fluid model, the problem of two-dimensional steady-state magneticreconnection in incompressible resistive weakly ionized plasmas isanalysed. Under the strong-coupling condition such as in the Sun'sphotosphere, it allows us to express the current sheet analytically. Inthis case the current sheet scales as η1/2. Then this model isused to understand magnetic flux cancellation which is observed in theSun's magnetograms. To account for a cancelling flux of 1×1018 Max in a time interval of 77 min, the width of the current sheetis about ten kilometres.

Quantum-dot microlaser operating on the whispering gallery mode—A source of squeezed (sub-Poissonian) light

The reduced density matrix method is used to calculate the quantum-statistical properties of the radiation of a quantum-dot laser operating on the whispering gallery mode of a dielectric microsphere. It is shown that under the conditions of strong coupling between the quantum dot and an electromagnetic field the radiation of such a laser can be in a nonclassical (sub-Poissonian) state. The laser scheme considered is characterized by an extremely low lasing threshold and a small number of saturation photons, as result of which lasing is possible with close to zero population inversion of the working levels, if g ≫ P ≫ γ ≫ Γ, where g is the field-matter interaction constant, P is the pumping rate, γ is the loss rate of the resonator, and Γ is the spontaneous emission rate. The largest squeezing inside the resonator-microsphere (the Fano factor F = 0.75) obtains for g ≫ P ≫ γ ≫ Γ, and the greatest squeezing in the fluctuation spectrum outside the resonator [V(ω = 0) ≈ 0.25] occurs for g ∼ P ∼ γ ≫ Γ, and in this case a substantial deviation of the photon number statistics of the radiation leaving the resonator from the Poissonian statistics is observed.

Electrostatic Attraction and/or Repulsion Between Charged Colloids: A (NVT) Monte-Carlo Study

We have performed canonical Monte-Carlo simulation of the distribution of counter-ions between two uniformly charged colloids of different geometries (infinite slabs, discoids and spheres). We have calculated the net force (or the pressure) between the colloids as a function of the interparticle separation in order to determine their stability. Simulations were performed within the primitive model which describes short-ranged excluded volume effects and long-ranged electrostatic interactions. Long-ranged behavior of the Coulomb potential has been handle by different numerical procedures: Ewald summations, hypersphere's method or external self-consistent field's approximation. In all cases, the net force between a pair of colloids results from the balance between the electrostatic attraction and the contact repulsion exerted by the condensed counter-ions. In the case of two infinite slabs, both contributions are (in absolute value) of the same order of magnitude; the resulting net force depends on the so-called electrostatic coupling which is the ratio of the counter-ion/surface electrostatic term at contact divided by the thermal energy. At high coupling (high surface density, polyvalent ions and/or low dielectric constant), we have demonstrated the existence of an attractive domain responsible for the cohesion of various lamellar materials (calcic-clay, cement, organic dispersion…). At low coupling (monovalent counterions in water), we have only detected a monotonous swelling behavior (repulsion) of the charged interfaces. We discuss these results on the basis of ionic correlations within the double-layers of condensed counter-ions. In addition to the infinite-slab case, we present results for a pair of discoid and spherical colloids in order to evaluate finite-size effects. By contrast to infinite interfaces, the electrostatic attraction is found to be negligible for a pair of parallel discoids neutralized by monovalent counter-ions (weak coupling conditions); the net force is then repulsive and driven by the contact force. A net attraction is also found in the case of divalent counter-ions (strong coupling conditions). No attractive regime is found in the case of interacting spheres neutralized by monovalent counter-ions.

Quantum theory of microcavity-modified fluorescence decay rate under a strong coupling condition

In this article, we propose a simple approach to deal with the fluorescence of a quasi-three-level system in $\ensuremath{\Lambda}$ configuration in which one of the ``levels'' is actually a band with possibly a large width. By solving this quasi-three-level molecule model, we obtain an analytical expression for the microcavity-modified fluorescence decay rate suitable for arbitrary cavities with any geometrical shape under a strong coupling condition. Applying the general result to a microsphere, we express explicitly the fluorescence decay rate's dependence on the cavity radius, emission linewidth, and the quantities characterizing the involved cavity modes, and find it in remarkable agreement with the recent fluorescence experiment by Barnes et al. [Phys. Rev. Lett. 76, 3931 (1996)]. In addition we briefly discuss the suitability of the approach for other microcavity-modified optical processes such as lasing and Raman scattering.

Longitudinal collective modes of strongly coupled dusty plasmas at finite frequencies and wavevectors

Dusty plasmas offer a unique method for testing dynamical wave theories in the strong Coulomb coupling regime. Recently, there have been many theoretical models, based on either a generalized hydrodynamic or on kinetic descriptions, developed to describe dust acoustic waves under conditions of strong coupling. These theories attempt to extend the usual acoustic wave dispersion relation to the strong coupling regime and, in some cases, to finite frequencies and wave vectors beyond the hydrodynamic limit. Here a multicomponent kinetic theory is used to obtain the dust acoustic wave dispersion relation in terms of an approximate dynamic local field correction, and comparison is made with the viscoelastic Navier–Stokes description.

Boundary conditions, solitons, and spin configuration in interchain coupled organic ferromagnetic polymers

With an extended Su–Schrieffer–Heeger Hamiltonian including Hubbard energy and intersite Coulomb repulsion, we study the effects of two kinds of boundary conditions on the spin configuration and lattice distortion in organic ferromagnetic polymers with interchain coupling. It is shown that if the length L of the lattice is even, there is dimerization along the main chain and uniform spin density along side radicals. If L is odd, there exist domain wall solitons describing the displacement of the lattice along the main chain, and spin soliton with envelope shape describing the localization of spin at side radicals. Under the condition of strong interchain coupling, a great fluctuation of spin and distortion appear along the main chain and side radicals, and the system transfers to two-dimensional one. The e–e correlation reduces the amplitude of spin soliton and domain wall while electron–phonon coupling has the reverse effect.

Mathematical modeling of complex oscillatory phenomena during CO oxidation over Pd zeolite catalysts

A mathematical model, which simulates the complicated dynamic behavior experimentally observed during CO oxidation over Pd zeolite catalysts is presented. It describes the coupling of reaction rate oscillations, generated by various parts of the inhomogeneous catalytic layer through the gas phase. It can be shown, that the resulting dynamic behavior depends upon the difference between natural frequencies of local oscillators and the strength of coupling, which is defined mostly by the degree of conversion. Chaotic behavior could be identified under the condition of weak coupling for local oscillators with widely different natural frequencies. In the range of strong coupling the phenomenon of phase death has been obtained. A special type of intermittency chaos (“on–off” chaos) was observed in a small region of parameters under the conditions of strong coupling.

Effect of geometrical confinement on the interaction between charged colloidal suspensions.

The effective interaction between charged colloidal particles confined between two planar like-charged walls is investigated using computer simulations of the primitive model describing asymmetric electrolytes. In detail, we calculate the effective force acting onto a single macroion and onto a macroion pair in the presence of slitlike confinement. For moderate Coulomb coupling, we find that this force is repulsive. Under strong-coupling conditions, however, the sign of the force depends on the distance to the plates and on the interparticle distance. In particular, the particle-plate interaction becomes strongly attractive for small distances which may explain the occurrence of colloidal crystalline layers near the plates observed in recent experiments.

Single-electron tunnelling in two vertically coupled quantum dots

Electron states in two vertically coupled quantum dots are analysed using an exact-diagonalization method. In the two-electron system, the ground state at zero field is a spin-singlet state, and this state is eventually replaced by spin-triplet states by applying a vertical magnetic field. One of the triplet states is stable under the conditions of very weak coupling and larger asymmetry between the two dots. In the transition from the three- to the four-electron state, we find a parameter region where the tunnelling current is strongly suppressed because of the orthogonality of the quantum states. An isospin selection rule is effective under strong-coupling conditions.

Effect of exciton dephasing in a semiconductor microcavity

We study the interaction between delocalized excitons in a semiconductor quantum well and a longitudinal mode of the radiation field in a semiconductor microcavity with Bragg mirrors. The drastic enhancement of the spontaneous emission rate, that occurs under strong coupling conditions, is found to be surprisingly robust with respect to incoherent processes leading to dephasing of the exciton mode.

Lasing threshold for whispering-gallery-mode microsphere lasers

The analytical expression for the lasing threshold for whispering-gallery-mode microsphere lasers under the strong-coupling condition is obtained by solving a quasi-three-level quantized model. It explicitly displays the dependence of the lasing threshold on the cavity radius, the homogeneous linewidth, the quality factors and linewidths of the relevant cavity modes.

Ntwo-level atoms in a driven optical cavity: Quantum dynamics of forward photon scattering for weak incident fields

We investigate the photon statistics of the light transmitted from a driven optical cavity containing N two-level atoms, with emphasis on the weak driving field limit. This limit is of most interest from the point of view of quantum fluctuations. We find that various types of nonclassical behavior are possible, even with large numbers of atoms in the cavity, under conditions of strong atom-field coupling, which we refer to as the cavity quantum electrodynamics limit. We describe the system with a pure-state formalism valid for weak fields, and also use a Fokker-Planck equation obtained by a small fluctuation linearization. We examine the conditions under which the linearized theory is appropriate, and explore the sensitivity of nonclassical effects in the system to atom number, transit-time broadening, and detunings.

Coupling Characteristics of Semiconductor Coupled Quantum Dots in Coulomb Blockade Regime

Coupled quantum dots in series have been realized in two dimensional electron gas (2DEG) in GaAs/AlGaAs heterostructures using the surface gate, and transport measurements have been performed at the dilution refrigerator temperature. The electrical transport was found to be determined by the Coulomb blockade and the resonant tunneling. The energy width of the resonant peak can be smaller than the thermal energy when two dots are sufficiently isolated. The charge stability diagram, by changing two gates attached to each dot, was measured for weak and strong coupling conditions. The width of the current resonant peak changed significantly as the coupling was increased, while the capacitive coupling did not change so much. The experimental result was analyzed using a simple capacitance model.

Drastic Increase of the Nuclear Deformation in Bromine betweenA=79and 77

The electric quadrupole interaction frequencies eQVzzyh of 77Br (I ­ 3y2; T1y2 ­ 57 h) and 82Br (I ­ 5; T1y2 ­ 35 h) in hcp Co were measured with modulated adiabatic fast passage on oriented nuclei. The electric quadrupole moment of 77Br is deduced to be Q ­ 10.530s22d b, which, on the condition of strong coupling, implies a nuclear deformation of b2 ­ 10.354s15d. This manifests a drastic increase of the nuclear deformation between 79Br (b2 , 10.22) and 77Br. [S0031-9007(98)06361-3]