Voltage-tunable quantum traps confining individual spatially indirect and long-living excitons are realized by providing a coupled double quantum well with nanoscale gates. This enables us to study the transition from confined multiexcitons down to a single, electrostatically trapped indirect exciton. In the few exciton regime, we observe discrete emission lines identified as resulting from a single dipolar exciton, a biexciton, and a triexciton, respectively. Their energetic splitting is well described by Wigner-like molecular structures reflecting the interplay of dipolar interexcitonic repulsion and spatial quantization.

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We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.In 1962 a Bose-Einstein-condensation of excitons was predicted by Blatt 1 and Moskalenko 2 . Dipolar excitons, where the composing electrons and holes are located in adjacent quantum wells of a semiconductor heterostructure 3,4 , are particularly suited to probe such a condensate. Due to the reduced overlap of the electron and hole wave functions, the lifetime of these quasi-particles is strongly increased. This enables them to cool down to the lattice temperature and to relax to their quantum mechanical ground state. To reach a Bose-Einstein condensate, a high particle density and low temperatures are a prerequisite, as demonstrated for related quasi-particles, namely polaritons 5 . For dipolar excitons, a condensation has been recently reported 6-8 . In particular, shiftinterferometry was used to study the spatial coherence of the excitonic emission which is considered as an indication of a condensed excitonic phase 6,[8][9][10] . Yet, there has been a controversial discussion about the impact of a limited spatial resolution on the ability to resolve the spatial coherence of the excitonic emission 11,12 . Here, we introduce a fiber-based confocal interferometer which operates down to cryogenic temperatures. We demonstrate that the approach allows measuring the coherence variation of the photoluminescence (PL) of trapped, dipolar excitons with a high spatial resolution. We extract a coherence length on the micrometer scale which is consistent with earlier reports 8 . However, we also observe that this length does not exceed the point spread function of our confocal set-up with two objectives even at the lowest temperature of 250 mK. In turn, we interpret the data such that a temporal coherence dominates the excitonic emission in our samples. We begin with the interferometer. It comprises two confocal objectives which can be positioned individually by piezo-positioners [Fig.

We explore the photoluminescence of spatially indirect, dipolar Mahan excitons in a gated double quantum well diode containing a mesoscopic electrostatic trap for neutral dipolar excitons at low temperatures down to 250 mK and in quantizing magnetic fields. Mahan excitons in the surrounding of the trap, consisting of individual holes interacting with a degenerate two-dimensional electron system confined in one of the quantum wells, exhibit strong quantum Hall signatures at integer filling factors and related anomalies around filling factor ν = 2 3 , 3 5 , and 1 2 , reflecting the formation of composite fermions. Interactions across the trap perimeter are found to influence the energy of the confined neutral dipolar excitons by the presence of the quantum Hall effects in the two-dimensional electron system surrounding the trap.Interband optical excitations in solids containing degenerate electron systems provide a probe for many-body interactions as reflected, for example, in the so-called Mahan exciton observed in degenerate semiconductors. 1 There, many-body interactions between an exciton and remaining free electrons in the Fermi sea cause a so-called Fermi edge singularity of the optical absorption or emission, similar to singularities observed in the x-ray photoemission spectra of metals. 2 Mahan excitons have been studied extensively in doped quantum wells containing a two-dimensional electron system 3 (2DES) and were found to also exhibit signatures in quantizing magnetic fields related to both the integer 4 and fractional 5 quantum Hall effects (QHE). 6-11 Coupled double quantum wells (CDQW), exhibiting long-living spatially indirect excitons, are of particular interest for the study of many-body interactions between dipolar excitons, in part to explore potential Bose-Einstein condensation of excitons 12-14 predicted several decades ago, 15 but also to study the role of the strong dipolar interactions between excitons. 16-19 A related CDQW system, composed of two adjacent 2DES, is the bilayer quantum Hall system in which excitonic quasiparticles are formed in the ground state around half-filling of Landau levels (LLs) and investigated via magnetotransport experiments. 20,21 Here, we report on luminescence studies of excitons in a CDQW, consisting of an individual hole in one QW and a voltage-controlled 2DES in the other QW. Such types of dipolar quasiparticles we introduce as Mahan-type indirect excitons (MIX). In the presence of a quantizing magnetic field, we find such MIX to exhibit characteristic signatures at both integer and fractional Landau-level filling factors. Furthermore, at the perimeter of an electrostatically created circular excitonic trap, we find the QHE of the 2DES located outside the trap perimeter to influence the energies of neutral dipolar excitons confined within the trap, thus reflecting nonlocal interactions across the trap perimeter. Our studies enable new insights into complex many-body interactions in systems with restricted dimensionalities. Dipolar excitons are spatially indi...

The coherent state preparation and control of single quantum systems is an important prerequisite for the implementation of functional quantum devices. Prominent examples for such systems are semiconductor quantum dots, which exhibit a fine structure split single exciton state and a V-type three level structure, given by a common ground state and two distinguishable and separately excitable transitions. In this work we introduce a novel concept for the preparation of a robust inversion by the sequential excitation in a V-type system via distinguishable paths.