We study Rabi oscillations detected in the coherent optical response from various exciton complexes in a 20 nm-thick CdTe/(Cd,Mg)Te quantum well using time-resolved photon echoes. In order to evaluate the role of exciton localization and inhomogeneous broadening we use selective excitation with spectrally narrow ps-pulses. We demonstrate that the transient profile of the photon echo from the localized trion (X − ) and the donor-bound exciton (D 0 X) transitions strongly depends on the strength of the first pulse. It acquires a non-Gaussian shape and experiences significant advancement for pulse areas larger than π due to non-negligible inhomogeneity-induced dephasing of the oscillators during the optical excitation. Next, we observe that an increase of the area of either the first (excitation) or the second (rephasing) pulse leads to a significant damping of the photon echo signal, which is strongest for the neutral excitons and less pronounced for the donor-bound exciton complex (D 0 X). The measurements are analyzed using a theoretical model based on the optical Bloch equations which accounts for the inhomogeneity of optical transitions in order to reproduce the complex shape of the photon echo transients. In addition, the spreading of Rabi frequencies within the ensemble due to the spatial variation of the intensity of the focused Gaussian beams and excitation-induced dephasing are required to explain the fading and damping of Rabi oscillations. By analyzing the results of the simulation for the X − and the D 0 X complexes we are able to establish a correlation between the degree of localization and the transition dipole moments determined as µ(X − )=73 D and µ(D 0 X)=58 D. PACS numbers:Introduction. Coherent control of excitonic states in semiconductor nanostructures under resonant excitation with intense optical pulses attracts a lot of attention in relation with possible applications in quantum information [1]. These ideas exploit coherent rotations of the Bloch vector in the photoexcited two-level system (TLS), which depends on the area of the exciting pulse via Rabi oscillations [2][3][4]. Since stronger localization of excitons is in favor of longer decoherence times, most of the studies of coherent control have concentrated on quantum dots (QD) [3,5,6]. However, the strong localization in QDs is accompanied by large variations in QD size, shape, and composition which consequently leads to the large inhomogeneous broadening of the optical transitions when an ensemble of emitters is used. Therefore, most Rabi oscillation studies were performed on single QDs [7-10].In semiconductor quantum well (QW) structures the inhomogeneous broadening of the optical transitions is significantly smaller as compared to QD systems, i.e. it is possible to selectively address different exciton complexes, such as free and localized excitons, localized charged excitons (trions, X − ), and donor-bound excitons (D 0 X). Therefore, QW structures can be considered as a model system for the investigation of Rabi oscillations and t...

The optically induced electron dynamics at a Si(001) surface is studied using a five-wave-mixing setup which measures the diffracted second-harmonic intensity induced by three ultrashort (13 fs) laser pulses. Depending on the time ordering of the pulses, this technique is capable of monitoring the temporal evolution of photoexcited one- or two-photon coherences, or populations. For a particular pulse sequence, the experiments show a delayed rise and a decay of the diffracted signal intensity on time scales of 50 and 250 fs, respectively. This response can be described by optical Bloch equations by including rapid scattering of the photoexcited carriers in the D(down) band of Si(001).

We present and discuss perspectives of current developments on advanced quantum optical circuits monolithically integrated in the lithium niobate platform. A set of basic components comprising photon pair sources based on parametric down conversion (PDC), passive routing elements and active electro-optically controllable switches and polarisation converters are building blocks of a toolbox which is the basis for a broad range of diverse quantum circuits. We review the state-of-the-art of these components and provide models that properly describe their performance in quantum circuits. As an example for applications of these models we discuss design issues for a circuit providing on-chip twophoton interference. The circuit comprises a PDC section for photon pair generation followed by an actively controllable modified mach-Zehnder structure for observing Hong-Ou-Mandel interference. The performance of such a chip is simulated theoretically by taking even imperfections of the properties of the individual components into account.

Parametric down-conversion (PDC) forms one of the basic building blocks for quantum optical experiments. However, the intrinsic multimode spectral-temporal structure of pulsed PDC often poses a severe hindrance for the direct implementation of the heralding of pure single-photon states or, for example, continuous-variable entanglement distillation experiments. To get rid of multimode effects narrowband frequency filtering is frequently applied to achieve a single-mode behavior. A rigorous theoretical description to accurately describe the effects of filtering on PDC, however, is still missing. To date, the theoretical models of filtered PDC are rooted in the discrete-variable domain and only account for filtering in the low-gain regime, where only a few photon pairs are emitted at any single point in time. In this paper we extend these theoretical descriptions and put forward a simple model, which is able to accurately describe the effects of filtering on PDC in the continuous-variable domain. This developed straightforward theoretical framework enables us to accurately quantify the tradeoff between suppression of higher-order modes, reduced purity, and lowered Einstein-Podolsky-Rosen entanglement, when narrowband filters are applied to multimode type-II PDC.

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