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We propose, design and fabricate an on-chip AlGaAs waveguide capable of generating a controlled phase delay of π/2 between the guided transverse electric and magnetic modes. These modes possess significantly strong longitudinal field components as a direct consequence of their strong lateral confinement in the waveguide. We demonstrate that the effect of the device on a linearly polarised input beam is the generation of a field, which is circularly polarised in its transverse components and carries a phase vortex in its longitudinal component. We believe that the discussed integrated platform enables the generation of light beams with tailored phase and polarisation distributions.

In view of real world applications of quantum information technologies, the combination of miniature quantum resources with existing fibre networks is a crucial issue. Among such resources, on-chip entangled photon sources play a central role for applications spanning quantum communications, computing and metrology. Here, we use a semiconductor source of entangled photons operating at room temperature in conjunction with standard telecom components to demonstrate multi-user quantum key distribution, a core protocol for securing communications in quantum networks. The source consists of an AlGaAs chip emitting polarization entangled photon pairs over a large bandwidth in the main telecom band around 1550 nm without the use of any off-chip compensation or interferometric scheme; the photon pairs are directly launched into a dense wavelength division multiplexer (DWDM) and secret keys are distributed between several pairs of users communicating through different channels. We achieve a visibility measured after the DWDM of 87% and show long-distance key distribution using a 50-km standard telecom fibre link between two network users. These results illustrate a promising route to practical, resource-efficient implementations adapted to quantum network infrastructures., Comment: 13 pages, 5 figures, 2 tables

There are reported results of numerical calculations of energy structure and carrier wavefunctions in a quantum mechanically coupled complex system consisting of an InGaAsN injector quantum well (QW) (with various N contents), separated by a thin GaAs barrier from an InAs/GaAs quantum dot (QD) layer covered with an InGaAs strain reducing layer. The investigated system consists in a tunnel injection structure, whose operation principle is to transfer the carriers captured by the injector QW to QDs by means of tunnelling. The QDs emit light at the 1.3 µm telecom window at room temperature, so in order to match the energy levels of the confined states in the injector QW to those in the QDs, nitrogen has been added to the former. The obtained calculation results are compared with measured photoreflectance spectra and used to explain their features. A detailed discussion on the spatial distribution of carrier wavefunctions over both parts of the tunnelling system versus nitrogen concentration in the injector QW is presented. The importance of the control of wavefunctions for the performance of tunnel-injection-based devices is discussed.

Conference on Quantum Dots 2010 (QD2010), Nottingham, ENGLAND, APR 26-30, 2010; International audience; We report on coherent control of the resonantly driven exciton state in a single quantum dot in the strong coupling regime. With picosecond laser pulses, the two-level system undergoes Rabi oscillations and a given coherent superposition of states can be addressed. Using a pair of phase-locked pulses, optical manipulation is achieved during the coherence time of the qu-bit. The dephasing processes are also studied and we show that in typical quantum dots, the coherence time T(2) can be of the same order of magnitude as the effective lifetime of the exciton T(1) (a few hundreds of ps) although not reaching the upper theoretical limit of 2T(1). We conclude that energy relaxation and pure dephasing processes due to virtual scattering with phonons contribute on an equal footing to the loss of coherence.

We present several examples of the interesting phenomenology shown by a moving polariton condensate in semiconductor microcavities. The superfluid behavior is probed by colliding the polariton condensate against physical obstacles in the form of natural defects of the sample, demonstrating a clear suppression of scattering when the speed of the flow lies below the critical velocity. At higher velocities Cerenkov-like shock waves around the defect and disruption of the condensate are also observed.

15th International Conference on Photoacoustic and Photothermal Phenomena, Catholic Univ Leuven, Leuven, Belgique, du 19 au 23 juillet 2009; International audience; Experimental results about light-to-sound transduction with semiconductor superlattices are presented. Picosecond ultrasonics with pump and probe incident on opposite sides of the substrate allows a full decoupling between generation and detection processes. Associating a metallic transducer, we show that superlattices generate quasi-monochromatic wave packets which can propagate over macroscopic distances in the underlying substrate but are also very selective and sensitive detectors. At the end, using two superlattices simultaneously as phonons generator and detector we evidence that these new transducers allow to perform acoustic experiments at the challenging frequency of 1THz.

Exciton-polaritons, two-dimensional composite bosons arising from the quantum mixture of excitons and photons, can manifest many-body quantum effects at liquid He temperatures (4 K). Interestingly, polaritons are predicted to behave as particular quantum fluids due to their out of equilibrium character, arising from their reduced lifetime (shorter than their thermalization time). Here we report the observation of superfluid motion of polaritons in semiconductor microcavities both under cw and pulsed excitation. Among other signatures, superfluidity is manifested via the absence of scattering of the polariton condensates when encountering a localized defect in their flow path.

Observation of quantized vortices in non-equilibrium polariton condensates, suggesting parallelisms with conventional superfluids, have been reported either by spontaneous formation and pinning in the presence of disorder [1,2] or by imprinting it onto the signal or idler of an optical parametric oscillator (OPO) [2]. Here we report the first observation of a polariton condensate receiving a quantized angular momentum by means of a short optical pulse and maintaining its rotation for a time much longer than the pulse duration, the polariton and coherence lifetimes. This observation shows a peculiar character of polariton condensates and reveals analogies with supercurrents in superconductors or persistent flow in condensates [3].

We report on the electronic properties of GaAs-substrate-based structures designed as a tunnel-injection system composed of self-assembled InAs quantum dots and an In0.3Ga0.7As quantum well separated by a GaAs barrier. We have performed photoluminescence and photoreflectance measurements which have allowed the determination of the optical transitions in the QW–QD tunnel structure and its respective references with just quantum dots or a quantum well. The effective mass calculations of the band structure dependence on the tunnelling barrier thickness have shown that in spite of an expected significant tunnelling between both parts of the system, its strong asymmetry and the strain distribution cause that the quantum-mechanical-coupling-induced energy shift of the optical transitions is almost negligible for the lowest energy states and weakly sensitive to the width of the barrier, which finds confirmation in the existing experimental data.

We describe multilayer acoustic nanowave devices based on aperiodic stacks of GaAs and AlAs layers and achievable with standard Molecular Beam Epitaxy (MBE) technology. These nanostructures were designed to display optimized acoustic reflectivity curves in the terahertz range. We address the design and optimization of these devices using a downhill simplex algorithm. We analyze different strategies of optimization, and we study the different engineering parameters relevant for the conception of aperiodic filters.

International audience; Coherent acoustic phonons confined in a nanocavity are generated in an optical microcavity. The confinement of the femtosecond light pulse in the optical resonator amplifies both processes, generation and detection of the acoustic phonons. In addition, due to the standing wave character of the photon field, phonons of wavevector q = 0 and q = 2k (k is the light wavenumber) contribute to both the generation and detection in time resolved reflectivity measurements, further optimizing the pump and probe experiments. Time resolved differential reflectivity experiments are reported as a function of laser energy. The optical cavity resonance is apparent in the amplitude and spectral features of the Fourier transformed signals.