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Reflections are omnipresent tools in quantum algorithms. We consider the task of reflecting through the eigenspace of an implementable unitary. Such reflections are generally designed using phase estimation or linear combination of unitaries. These methods have size and depth that scale favorably with the desired precision and the spectral gap of the unitary. However, they require a number of ancilla qubits that grows with both parameters. Here, we present a simple algorithm with the same size and depth scaling but requiring only 3 ancilla qubits for all problem instances. As such, this algorithm is expected to become the reference method to reflect through eigenspaces of unitaries., Comment: 5 pages

We report a new scanning nanomechanical noise microscopy platform enabling to both heat and acquire the fluctuations of mechanical nanostructures with nanometric resolution. We use this platform to image the thermally activated nanomechanical dynamics of a model system consisting of a $40\,\mathrm{nm}$ diameter single-defect nanowire, while scanning a localized heat source across its surface. We develop a thermal backaction model, which we use to demonstrate a close connection between the structure of the nanowire, its thermal response, its dissipation and its fluctuations. We notably show that the defect behaves as a single fluctuation hub, whose e-beam excitation yields a far off-equilibrium vibrational state, largely dominated by the quantum fluctuations of the heating source. Our platform is of interest for future quantitative investigation of fundamental nanoscale dynamical phenomena, and appears as a new playground for investigating quantum thermodynamics in the strongly dissipative regime and at room temperature., Comment: 10 pages, 4 figures

We show that a compact K\"ahler manifold bimeromorphic to a weakly K\"ahler hyperbolic manifold is weakly K\"ahler hyperbolic, providing an answer to a problem raised by J. Koll\'ar in his 1995 book "Shafarevic maps and automorphic forms", Comment: 16 pages, no figures, comments are very welcome! v2: fixed some typos (including in the title), Corollary 1.3 and Theorem 5.1 are improved, and Corollary 1.4 is new

In this short note we prove two projectivity criteria for fibrations between mildly singular compact K\"ahler spaces. They are the relative versions of the celebrated criteria of Kodaira and Moishezon. As an application we obtain that the MRC fibration always has a model that is a projective morphism., Comment: 11 pages

Controlled operations are fundamental building blocks of quantum algorithms. Decomposing $n$-control-NOT gates ($C^n(X)$) into arbitrary single-qubit and CNOT gates, is a crucial but non-trivial task. This study introduces $C^n(X)$ circuits outperforming previous methods in the asymptotic and non-asymptotic regimes. Three distinct decompositions are presented: an exact one using one borrowed ancilla with a circuit depth $\Theta\left(\log(n)^{3}\right)$, an approximating one without ancilla qubits with a circuit depth $\mathcal O \left(\log(n)^{3}\log(1/\epsilon)\right)$ and an exact one with an adjustable-depth circuit which decreases with the number $m\leq n$ of ancilla qubits available as $O(log(2n/m)^3+log(m/2))$. The resulting exponential speedup is likely to have a substantial impact on fault-tolerant quantum computing by improving the complexities of countless quantum algorithms with applications ranging from quantum chemistry to physics, finance and quantum machine learning.

Quantum Computing allows, in principle, the encoding of the exponentially scaling many-electron wave function onto a linearly scaling qubit register, offering a promising solution to overcome the limitations of traditional quantum chemistry methods. An essential requirement for ground state quantum algorithms to be practical is the initialisation of the qubits to a high-quality approximation of the sought-after ground state. Quantum State Preparation (QSP) allows the preparation of approximate eigenstates obtained from classical calculations, but it is frequently treated as an oracle in quantum information. In this study, we conduct QSP on the ground state of prototypical strongly correlated systems, up to 28 qubits, using the Hyperion GPU-accelerated state-vector emulator. Various variational and non-variational methods are compared in terms of their circuit depth and classical complexity. Our results indicate that the recently developed Overlap-ADAPT-VQE algorithm offers the most advantageous performance for near-term applications.

In this paper, we demonstrate that a few-period circular Bragg reflector around an infinite nanowire can increase the $\beta$ factor of the fundamental mode up to 0.999 due to further suppression of the emission into radiation modes caused by a photonic band gap effect. We then apply this strategy in the practically relevant case of the finite-sized SPS based on tapered nanowires and demonstrate that the collection efficiency can be further increased. Additionally, we also show the beneficial effects of adding optimized high-index rings around the micropillar SPS.

Hybrid quantum-classical adaptive Variational Quantum Eigensolvers (VQE) already hold the potential to outperform classical computing for simulating quantum many-body systems. However, their practical implementation on current quantum processing units (QPUs) is very challenging due to the noisy evaluation of a polynomially scaling number of observables, undertaken for operator selection and optimisation of a high-dimensional cost function. To overcome this, we propose new techniques to execute adaptive algorithms on a 25-qubit error-mitigated QPU coupled to a GPU-accelerated HPC simulator. Targeting physics applications, we compute the ground state of a 25-body Ising model using the newly introduced Greedy Gradient-free Adaptive VQE (CGA-VQE) requiring only five circuit measurements per iteration, regardless of the number of qubits and size of the operator pool. Towards chemistry, we combine the GGA-VQE and Overlap-ADAPT-VQE algorithms to approximate a molecular system ground state. We show that the QPU successfully executes the algorithms and yields the correct choice of parametrised unitary operators. While the QPU evaluation of the resulting ansatz wave-function is polluted by hardware noise, a single final evaluation of the sought-after observables on a classical GPU-accelerated/noiseless simulator allows the recovery of the correct approximation of the ground state, thus highlighting the need for hybrid quantum-classical observable measurement.

Let $(X, \Delta)$ be a compact K\"ahler klt pair, where $K_X + \Delta$ is ample or numerically trivial, and $\Delta$ has standard coefficients. We show that if equality holds in the orbifold Miyaoka-Yau inequality for $(X, \Delta)$, then its orbifold universal cover is either the unit ball (ample case) or the affine space (numerically trivial case).

We investigate the structure of transversely K\"ahler foliations with quasi-negative tranverse Ricci curvature. In particular, we prove a de Rham type theorem decomposition on the leaf space where we characterize each factor., Comment: 48 pages, comments are more than welcome!

We present a detailed analysis of the physics governing the collection efficiency and the Purcell enhancement of the nanopost single-photon source. We show that a standard single-mode Fabry-P\'erot model is insufficient to describe the device performance, which benefits significantly from scattering from the fundamental mode to radiation modes. We show how the scattering mechanism decouples the collection efficiency from the Purcell enhancement, such that maximum collection efficiency is obtained off-resonance. Finally, we discuss how this scattering mechanism can be beneficial for future single-photon source designs.

An on-going challenge within scalable optical quantum information processing is to increase the collection efficiency $\varepsilon$ and the photon indistinguishability $\eta$ of the single-photon source towards unity. Within quantum dot-based sources, the prospect of increasing the product $\varepsilon \eta$ arbitrarily close to unity was recently questioned. In this work, we discuss the influence of the trade-off between efficiency and indistinguishability in the presence of phonon-induced decoherence, and we show that the photonic "hourglass" design allows for improving $\varepsilon \eta$ beyond the predicted maximum for the standard micropillar design subject to this trade-off. This circumvention of the trade-off is possible thanks to control of the spontaneous emission into background radiation modes, and our work highlights the importance of engineering of the background emission in future pursuits of near-unity performance of quantum dot single-photon sources., Comment: Added some small sections discussing possible reduction in indistinguishability due to charge noise, the possibility of electrical pumping, and some additional excitation schemes. Added small clarifications regarding the indistinguishability of QDs in bulk, the usage of dielectric screening in photonic crystals, and on the strong coupling regime

This article gives a characterization of quotients of complex tori by finite groups acting freely in codimension two in terms of a numerical vanishing condition on the first and second Chern class. This generalizes results previously obtained by Greb--Kebekus--Peternell in the projective setting, and by Kirschner and the second author in dimension three. As a key ingredient to the proof, we obtain a version of the Bogomolov--Gieseker inequality for stable sheaves on singular spaces, including a discussion of the case of equality., Comment: 26 pages, v2: exposition improved thanks to the referee's suggestions, added Proposition 3.5 in the semistable case, to appear in Comment. Math. Helv

Hybrid quantum optomechanical systems offer an interface between a single two-level system and a macroscopical mechanical degree of freedom. In this work, we build a hybrid system made of a vibrating microwire coupled to a single semiconductor quantum dot (QD) via material strain. It was shown a few years ago, that the QD excitonic transition energy can thus be modulated by the microwire motion. We demonstrate here the reverse effect, whereby the wire is set in motion by the resonant drive of a single QD exciton with a laser modulated at the mechanical frequency. The resulting driving force is found to be almost 3 orders of magnitude larger than radiation pressure. From a fundamental aspect, this state dependent force offers a convenient strategy to map the QD quantum state onto a mechanical degree of freedom., Comment: 16 pages, 12 figures

We conducted a pilot study to evaluate the potential and feasibility of back-support exoskeletons to help the caregivers in the Intensive Care Unit (ICU) of the University Hospital of Nancy (France) executing Prone Positioning (PP) maneuvers on patients suffering from severe COVID-19-related Acute Respiratory Distress Syndrome. After comparing four commercial exoskeletons, the Laevo passive exoskeleton was selected and used in the ICU in April 2020. The first volunteers using the Laevo reported very positive feedback and reduction of effort, confirmed by EMG and ECG analysis. Laevo has been since used to physically assist during PP in the ICU of the Hospital of Nancy, following the recrudescence of COVID-19, with an overall positive feedback.

Let $X$ be a compact K\"ahler space with klt singularities and vanishing first Chern class. We prove the Bochner principle for holomorphic tensors on the smooth locus of $X$: any such tensor is parallel with respect to the singular Ricci-flat metrics. As a consequence, after a finite quasi-\'etale cover $X$ splits off a complex torus of the maximum possible dimension. We then proceed to decompose the tangent sheaf of $X$ according to its holonomy representation. In particular, we classify those $X$ which have strongly stable tangent sheaf: up to quasi-\'etale covers, these are either irreducible Calabi--Yau or irreducible holomorphic symplectic. As an application of these results, we show that if $X$ has dimension four, then it satisfies Campana's Abelianity Conjecture., Comment: 28 pages; v2: exposition improved following the referee's suggestions, new title, section 7 shortened and section 8 removed; to appear in Crelle

Controlling the strain level in nanowire heterostructures is critical for obtaining coherent interfaces of high crystalline quality and for the setting of functional properties such as photon emission, carrier mobility or piezoelectricity. In a nanowire axial heterostructure featuring a sharp interface, strain is set by the materials lattice mismatch and the nanowire radius. Here, we show that introducing a graded interface in nanowire heterostructures offers an additional parameter to control strain. For a given interface length and lattice mismatch, we first derive theoretically the maximum nanowire radius below which coherent growth is possible. We validate these findings by growing and characterizing various In(Ga)As/GaAs nanowire heterostructures with graded interfaces. Furthermore, we perform a complete chemical and structural characterization of the interface by combining energy-dispersive X-ray spectroscopy and high resolution transmission electron microscopy. In the case of coherent growth, we directly observe that the mismatch strain relaxes elastically on the side walls of the nanowire around the interface area, while the core of the nanowire remains partially strained. Moreover, our experimental data show good agreement with finite element calculations. This analysis confirms in particular that mechanical strain is largely reduced by interface grading. Overall, our work extends the parameter space for the design of nanowire heterostructures, thus opening new opportunities for nanowire optoelectronics., Comment: 8 pages, 6 figures

We report on the generation of few-ps long spontaneous emission pulses by quantum dots (QDs) in a switched optical microcavity. We use a pulsed optical injection of free charge carriers to induce a large frequency shift of the fundamental mode of a GaAs/AlAs micropillar. We track in real time by time-resolved photoluminescence its fundamental mode during its relaxation, using the emission of the QD ensemble as a broadband internal light source. Sub-ensembles of QDs emitting at a given frequency, interact transiently with the mode and emit an ultrashort spontaneous emission pulse into it. By playing with switching parameters and with the emission frequency of the QDs, selected by spectral filtering, pulse durations ranging from 300 ps down to 6 ps have been obtained. These pulses display a very small coherence length, which opens potential applications in the field of ultrafast imaging. The control of QD-mode coupling on ps-time scales establishes also cavity switching as a key resource for quantum photonics., Comment: 11 pages, 8 figures; includes supplemental material

Detecting nanomechanical motion has become an important challenge in Science and Technology. Recently, electromechanical coupling to focused electron beams has emerged as a promising method adapted to ultra-low scale systems. However the fundamental measurement processes associated with such complex interaction remain to be explored. Here we report highly sensitive detection of the Brownian motion of um-long semiconducting nanowires (InAs). The measurement imprecision is found to be set by the shot noise of the secondary electrons generated along the electromechanical interaction. By carefully analysing the nano-electromechanical dynamics, we demonstrate the existence of a radial backaction process which we identify as originating from the momentum exchange between the electron beam and the nanomechanical device, which is also known as radiation pressure.

We use strain to statically tune the semiconductor band gap of individual InAs quantum dots (QDs) embedded in a GaAs photonic wire featuring very efficient single photon collection efficiency. Thanks to the geometry of the structure, we are able to shift the QD excitonic transition by more than 20 meV by using nano-manipulators to apply the stress. Moreover, owing to the strong transverse strain gradient generated in the structure, we can relatively tune two QDs located in the wire waveguide and bring them in resonance, opening the way to the observation of collective effects such as superradiance.

Optical non-linearities usually appear for large intensities, but discrete transitions allow for giant non-linearities operating at the single photon level. This has been demonstrated in the last decade for a single optical mode with cold atomic gases, or single two-level systems coupled to light via a tailored photonic environment. Here we demonstrate a two-modes giant non-linearity by using a three-level structure in a single semiconductor quantum dot (QD) embedded in a photonic wire antenna. The large coupling efficiency and the broad operation bandwidth of the photonic wire enable us to have two different laser beams interacting with the QD in order to control the reflectivity of a laser beam with the other one using as few as 10 photons per QD lifetime. We discuss the possibilities offered by this easily integrable system for ultra-low power logical gates and optical quantum gates.

We demonstrate the application of a fiber-coupled quantum-dot-in-a-tip as a probe for scanning electric field microscopy. We map the out-of-plane component of the electric field induced by a pair of electrodes by measurement of the quantum-confined Stark effect induced on a quantum dot spectral line. Our results are in agreement with finite element simulations of the experiment. Furthermore, we present results from analytic calculations and simulations which are relevant to any electric field sensor embedded in a dielectric tip. In particular, we highlight the impact of the tip geometry on both the resolution and sensitivity., Comment: 10 pages, 4 figures

We report on the growth of axial InAs-on-GaAs nanowire heterostructures on silicon by molecular beam epitaxy using 20 nm diameter Au catalysts. First, the growth parameters of the GaAs nanowire segment were optimized to achieve a pure wurtzite crystal structure. Then, we developed a two-step growth procedure to enhance the yield of vertical InAs-on-GaAs nanowires. We achieved 90% of straight InAs-on-GaAs nanowires by further optimizing the growth parameters. We investigated the composition change at the interface by energy dispersive X-ray spectroscopy and the nanowire crystal structure by transmission electron microscopy. The nominal composition of the InAs segment is found to be In$_{x}$Ga$_{1-x}$As with $x$=0.85 and corresponds to 6% of lattice mismatch with GaAs. Strain mapping performed by the geometrical phase analysis of high-resolution images revealed a dislocation-free GaAs/InAs interface. In conclusion, we successfully fabricated highly mismatched heterostructures, confirming the prediction that axial GaAs/InAs interfaces are pseudomorphic in nanowires below 40 nm diameter.

We prove that a representation of the fundamental group of a quasi-projective manifold into the group of formal diffeomorphisms of one variable either is virtually abelian or, after taking the quotient by its center, factors through an orbicurve., Comment: 8 pages; comments are welcome. v2: precisions added

Let $X$ be a compact K{\"a}hler manifold of dimension three. We prove that there exists a projective manifold $Y$ such that $\pi\_1(X)\simeq \pi\_1(Y)$. We also prove the bimeromorphic existence of algebraic approximations for compact K{\"a}hler manifolds of algebraic dimension $\dim(X)-1$. Together with the work of Graf and the third author, this settles in particular the bimeromorphic Kodaira problem for compact K{\"a}hler threefolds., Comment: 27 pages. Main result improved: the algebraic approximation holds now for elliptic compact K{\"a}hler manifolds over a projective base in any dimension

The proof of the celebrated Viehweg's hyperbolicity conjecture is a consequence of two remarkable results: Viehweg and Zuo's existence results for global pluri-differential forms induced by variation in a family of canonically po-larised manifolds and Campana and P\v{a}un's vast generalisation of Miyaoka's generic semipositivity result for non-uniruled varieties to the context of pairs. The aim of this chapter is an exposition of Campana-P\v{a}un's generic semipositivity theorem ., Comment: 30 pages. To appear as a chapter of a book about complex hyperbolicity

Coupling a microscopic mechanical resonator to a nano-scale quantum system enables control of the mechanical resonator via the quantum system, and vice versa. The coupling is usually achieved through functionalization of the mechanical resonator but this results in additional mass and dissipation channels. An alternative is an intrinsic coupling based on strain. We employ here a monolithic semiconductor system. The nano-scale quantum system is a quantum dot; the mechanical resonator a microscopic trumpet which simultaneously optimizes the mechanical and photonic properties. The quantum dot transition is driven resonantly. Via the resonance fluorescence, we observe mechanical Brownian motion even at 4K, and demonstrate a coupling to mechanical modes of different types. We identify a mechanical mode with a cooperativity larger than one. We show analytically that the Heisenberg limit on displacement measurement can be reached with an embedded two-level system in the case of a transform-limited optical emitter with perfect photon detection. We argue that operation close to the Heisenberg limit is achievable with state-of-the-art quantum dot devices.

In the context of fast developing quantum technologies, locating single quantum objects embedded in solid or fluid environment while keeping their properties unchanged is a crucial requirement as well as a challenge. Such "quantum microscopes" have been demonstrated already for NV-centers embedded in diamond [1], and for single atoms within an ultracold gas [2]. In this work, we demonstrate a new method to determine non-destructively the position of randomly distributed semiconductor quantum dots (QDs) deeply embedded in a solid photonic waveguide. By setting the wire in an oscillating motion, we generate large stress gradients across the QDs plane. We then exploit the fact that the QDs emission frequency is highly sensitive to the local material stress [3-5] to infer their positions with an accuracy ranging from +/- 35 nm down to +/-1 nm for close-to-axis QDs.

That short note, meant as an addendum to [CCE14], enhances the results contained in loc. cit. In particular it is proven here that a linear K{\"a}hler group is already the fundamental group of a smooth complex projective variety. This is achieved by studying the relative deformation of the total space of a smooth family of tori in an equivariant context., Comment: A flaw in the previous version. The main theorem is now stated for smooth families of tori with compact base. It does not affect the application to linear K{\"a}hler groups. To appear in Annales de la Facult{\'e} des Sciences de Toulouse

We perform coherent non-linear spectroscopy of individual excitons strongly confined in single InAs quantum dots (QDs). The retrieval of their intrinsically weak four-wave mixing (FWM) response is enabled by a one-dimensional dielectric waveguide antenna. Compared to a similar QD embedded in bulk media, the FWM detection sensitivity is enhanced by up to four orders of magnitude, over a broad operation bandwidth. Three-beam FWM is employed to investigate coherence and population dynamics within individual QD transitions. We retrieve their homogenous dephasing in a presence of spectral wandering. Two-dimensional FWM reveals off-resonant F\"orster coupling between a pair of distinct QDs embedded in the antenna. We also detect a higher order QD non-linearity (six-wave mixing) and use it to coherently control the FWM transient. Waveguide antennas enable to conceive multi-color coherent manipulation schemes of individual emitters., Comment: 7 pages, 8 Figures

Proven by A. Parshin and S. Arakelov in the early 70's, Shafaverich hyperbolicity conjecture states that a family of curves of genus $g\ge2$ parametrized by a non hyperbolic curve (\emph{i.e.} isomorphic to $\mathbb{P}^1$, $\mathbb{C}$, $\mathbb{C}^*$ or an elliptic curve) has to be isotrivial : the moduli of smooth fibres are constant. In higher dimensions, Viehweg's works on moduli of canonically polarized manifolds led him to generalize this statement in the following way: if a family of canonically polarized manifolds (parametrized by a quasi-projective base) has maximal variation, the base is then of log-general type. It can be thought as an algebraic hyperbolicity property which is expected to hold for the moduli space.Adapting results due to Y. Miyoaka on generic semi-positivity of cotangent bundle to the framework of pairs, F. Campana and M. P\u{a}un recently obtained a positive answer to Viehweg's conjecture. We will also take the opportunity of this talk to present the classification of orbifolds as developed in Campana's works. This setting is moreover the right one to state the optimal version of Viehweg's conjecture proven by B. Taji, Comment: S\'eminaire Bourbaki du 7 novembre 2015, in French

We introduce a method which enables to directly compare the impact of elastic strain on the optical properties of distinct quantum dots (QDs). Specifically, the QDs are integrated in a cross-section of a semiconductor core wire which is surrounded by an amorphous straining shell. Detailed numerical simulations show that, thanks to the mechanical isotropy of the shell, the strain field in a core section is homogeneous. Furthermore, we use the core material as an in situ strain gauge, yielding reliable values for the emitter energy tuning slope. This calibration technique is applied to self-assembled InAs QDs submitted to incremental tensile strain along their growth axis. In contrast to recent studies conducted on similar QDs stressed perpendicularly to their growth axis, optical spectroscopy reveals 5-10 times larger tuning slopes, with a moderate dispersion. These results highlight the importance of the stress direction to optimise QD response to applied strain, with implications both in static and dynamic regimes. As such, they are in particular relevant for the development of wavelength-tunable single photon sources or hybrid QD opto-mechanical systems.

This article studies codimension one foliations on projective man-ifolds having a compact leaf (free of singularities). It explores the interplay between Ueda theory (order of flatness of the normal bundle) and the holo-nomy representation (dynamics of the foliation in the transverse direction). We address in particular the following problems: existence of foliation having as a leaf a given hypersurface with topologically torsion normal bundle, global structure of foliations having a compact leaf whose holonomy is abelian (resp. solvable), and factorization results.

We have switched GaAs/AlAs and AlGaAs/AlAs planar microcavities that operate in the "Original" (O) telecom band by exploiting the instantaneous electronic Kerr effect. We observe that the resonance frequency reversibly shifts within one picosecond. We investigate experimentally and theoretically the role of several main parameters: the material backbone and its electronic bandgap, the pump power, the quality factor, and the duration of the switch pulse. The magnitude of the shift is reduced when the backbone of the central $\lambda-$layer has a greater electronic bandgap; pumping with photon energies near the bandgap resonantly enhances the switched magnitude. Our model shows that the magnitude of the resonance frequency shift depends on the pump pulse duration and is maximized when the duration matches the cavity storage time that is set by the quality factor. We provide the settings for the essential parameters so that the frequency shift of the cavity resonance can be increased to one linewidth.

We present the experimental realization of a quantum fiber-pigtail. The device consists of a semiconductor quantum-dot embedded into a conical photonic wire that is directly connected to the core of a fiber-pigtail. We demonstrate a photon collection efficiency at the output of the fiber of 5.8% and suggest realistic improvements for the implementation of a useful device in the context of quantum information. We finally discuss potential applications in scanning probe microscopy.

We show that the general fibres of the Albanese morphism of a projective special manifold are special as well (a question raised by the first-named author). The main ingredient of the proof is a version (established by Birkar and Chen) with boundary of the famous conjecture of Iitaka on the subadditivity of the Kodaira dimension in an algebraic fibre space. Some consequences of our main result are discussed., Comment: 19 pages, in French. Comments are welcome

Ever since the advent of nonlinear optics, the generation of light by frequency-conversion is drawing continued attention, and leading to emerging applications such as supercontinuum sources for ultra stable clocks and advanced microscopy. A modern approach to frequency-conversion is to switch light confined in micro- and nanocavity resonances to enable on-chip operation. Supposedly, nonlinear frequency conversion in such confined media differs from traditional non-linear optics in three key features regarding output spectrum, frequency shift, and critical time scale. Therefore, we switch GaAs-AlAs microcavities by the electronic Kerr effect, and study a range of quality factors to bridge the confined and the traditional non-linear regimes. We uncover the key role of the density of vacuum fluctuations, i.e., the local density of optical states (LDOS) for newly generated frequencies, a concept inspired by cavity quantum electrodynamics (cQED). As a result, we succeed to establish a framework, which not only describes nonlinear optics both in traditional bulk and in confined media but also opens a new control dimension in changing the color of light., Comment: 7 double column pages, 5 figures

In this note we establish the following result (announced in a previous work): if a linear group is the image of a representation of a K\"ahler group, then it has a finite index subgroup which is the image of a representation of the fundamental group of a smooth projective variety. In particular, a K\"ahler group which is linear is virtually projective., Comment: 12 pages, no figure, in French. Comments are welcome

We perform frequency- and time-resolved all-optical switching of a GaAs-AlAs micropillar cavity using an ultrafast pump-probe setup. The switching is achieved by two-photon excitation of free carriers. We track the cavity resonances in time with a high frequency resolution. The pillar modes exhibit simultaneous frequency shifts, albeit with markedly different maximum switching amplitudes and relaxation dynamics. These differences stem from the non-uniformity of the free carrier density in the micropillar, and are well understood by taking into account the spatial distribution of injected free carriers, their spatial diffusion and surface recombination at micropillar sidewalls., Comment: 4 pages, 3 figures

Recent progress in nanotechnology has allowed to fabricate new hybrid systems where a single two-level system is coupled to a mechanical nanoresonator. In such systems the quantum nature of a macroscopic degree of freedom can be revealed and manipulated. This opens up appealing perspectives for quantum information technologies, and for the exploration of quantum-classical boundary. Here we present the experimental realization of a monolithic solid-state hybrid system governed by material strain: a quantum dot is embedded within a nanowire featuring discrete mechanical resonances corresponding to flexural vibration modes. Mechanical vibrations result in a time-varying strain field that modulates the quantum dot transition energy. This approach simultaneously offers a large light extraction efficiency and a large exciton-phonon coupling strength $g_0$. By means of optical and mechanical spectroscopy, we find that $g_0/2\pi$ is nearly as large as the mechanical frequency, a criterion which defines the ultra-strong coupling regime., Comment: 6 pages, 3 figures