Three-Dimensional Electrical Control of the Excitonic Fine Structure for a Quantum Dot in a Cavity.

The excitonic fine structure plays a key role for the quantum light generated by semiconductor quantum dots, both for entangled photon pairs and single photons. Controlling the excitonic fine structure has been demonstrated using electric, magnetic, or strain fields, but not for quantum dots in optical cavities, a key requirement to obtain high source efficiency and near-unity photon indistinguishability. Here, we demonstrate the control of the fine structure splitting for quantum dots embedded in micropillar cavities. We propose and implement a scheme based on remote electrical contacts connected to the pillar cavity through narrow ridges. Numerical simulations show that such a geometry allows for a three-dimensional control of the electrical field. We experimentally demonstrate tuning and reproducible canceling of the fine structure, a crucial step for the reproducibility of quantum light source technology.

Fiber-based angular filtering for high-resolution Brillouin spectroscopy in the 20-300†GHz frequency range.

Brillouin spectroscopy emerges as a promising non-invasive tool for nanoscale imaging and sensing. One-dimensional semiconductor superlattice structures are eminently used for selectively enhancing the generation or detection of phonons at few GHz. While commercially available Brillouin spectrometers provide high-resolution spectra, they consist of complex experimental techniques and are not suitable for semiconductor cavities operating at a wide range of optical wavelengths. We develop a pragmatic experimental approach for conventional Brillouin spectroscopy that can integrate a widely tunable excitation-source. Our setup combines a fibered-based angular filtering and a spectral filtering based on a rotating single etalon and a double grating spectrometer for sequential reconstruction of Brillouin spectra. This configuration allows probing confined acoustic phonon modes in the 20-300 GHz frequency range with excellent laser rejection and high spectral resolution. Remarkably, our scheme based on the excitation and collection of the enhanced Brillouin scattering signals through the optical cavity allows for better angular filtering with decreasing phonon frequency. It can be implemented for the study of cavity optomechanics and stimulated Brillouin scattering over broadband optical and acoustic frequency ranges.

Optomechanical properties of GaAs/AlAs micropillar resonators operating in the 18 GHz range.

Recent experiments demonstrated that GaAs/AlAs based micropillar cavities are promising systems for quantum optomechanics, allowing the simultaneous three-dimensional confinement of near-infrared photons and acoustic phonons in the 18-100 GHz range. Here, we investigate through numerical simulations the optomechanical properties of this new platform. We evidence how the Poisson's ratio and semiconductor/vacuum boundary conditions lead to very distinct features in the mechanical and optical three-dimensional confinement. We find a strong dependence of the mechanical quality factor and strain distribution on the micropillar radius, in great contrast to what is predicted and observed in the optical domain. The derived optomechanical coupling constants g0 reach ultra-large values in the 106 rad/s range.

Probing gigahertz coherent acoustic phonons in TiO2 mesoporous thin films.

Ultrahigh-frequency acoustic-phonon resonators usually require atomically flat interfaces to avoid phonon scattering and dephasing, leading to expensive fabrication processes, such as molecular beam epitaxy. Mesoporous thin films are based on inexpensive wet chemical fabrication techniques that lead to relatively flat interfaces regardless the presence of nanopores. Here, we report mesoporous titanium dioxide-based acoustic resonators with resonances up to 90 GHz, and quality factors from 3 to 7. Numerical simulations show a good agreement with the picosecond ultrasonics experiments. We also numerically study the effect of changes in the speed of sound on the performance of the resonator. This change could be induced by liquid infiltration into the mesopores. Our findings constitute the first step towards the engineering of building blocks based on mesoporous thin films for reconfigurable optoacoustic sensors.

Sequential generation of linear cluster states from a single photon emitter.

Light states composed of multiple entangled photons-such as cluster states-are essential for developing and scaling-up quantum computing networks. Photonic cluster states can be obtained from single-photon sources and entangling gates, but so far this has only been done with probabilistic sources constrained to intrinsically low efficiencies, and an increasing hardware overhead. Here, we report the resource-efficient generation of polarization-encoded, individually-addressable photons in linear cluster states occupying a single spatial mode. We employ a single entangling-gate in a fiber loop configuration to sequentially entangle an ever-growing stream of photons originating from the currently most efficient single-photon source technology-a semiconductor quantum dot. With this apparatus, we demonstrate the generation of linear cluster states up to four photons in a single-mode fiber. The reported architecture can be programmed for linear-cluster states of any number of photons, that are required for photonic one-way quantum computing schemes.

Conformational heterogeneity and spin-labeled -SH groups: pulsed EPR of Na,K-ATPase.

Membranous Na,K-ATPase from shark salt gland and from pig kidney was spin-labeled on class I -SH groups in the presence of glycerol, or on class II -SH groups in the absence of glycerol. The class I-labeled preparations retain full enzymatic activity, whereas the class II-labeled preparations are at least partially inactivated. This provides an excellent testbed on which to demonstrate how advanced electron paramagnetic resonance (EPR) can provide novel information on specific residues in unique environments in a complex, membrane-bound transport system. The polarity of the environment, and the librational dynamics and conformational exchange, of the spin-labeled groups were studied with pulsed EPR by using electron spin echo envelope modulation (ESEEM) spectroscopy and spin-echo detected (ED) EPR spectroscopy, respectively. 2H-ESEEM spectra of membranes dispersed in D2O reveal that class I groups of the shark enzyme are more exposed to water than are those of the pig enzyme or class II groups of either species, consistent with the more superficial membrane location in the former case. Spin-echo decay curves indicate conformational heterogeneity at low temperatures (<150 K), but a more homogeneous conformational state at higher temperatures that is characterized by a single phase-memory T2M relaxation time. Conventional EPR lineshapes also demonstrate conformational microheterogeneity at low temperatures: the inhomogeneously broadened lines narrow progressively with increasing temperature reaching an almost pure Lorentzian line shape at temperatures of ca. 220 K and above. The inhomogeneous broadening at low temperature is well described by a Gaussian distribution of Lorentzian lines. ED spectra as a function of echo-delay time demonstrate the onset of rapid librational motions of appreciable amplitude, and slower conformational exchange, at temperatures above 220 K. These motions could drive transitions between the different conformational substates, which are frozen in at lower temperatures but contribute to the pathways between the principal enzymatic intermediates at higher temperatures.

Precipitation of solubilized Na+/K+-ATPase by divalent cations.

A method for preparation of membranous fragments of pure and highly active shark rectal gland Na+/K+-ATPase by Mn2+ precipitation of C12E8-solubilized enzyme is described. The method is rapid and inexpensive, and yields enzyme with a specific Na+/K+-ATPase activity of up to 1800 mumol/mg per h at 37 degrees C. The influence of the detergent/protein and lipid/protein ratios on the yield of membrane bound enzyme is described.

Sulphydryl groups of (Na+ + K+)-ATPase from rectal glands of Squalus acanthias. Detection of ligand-induced conformational changes.

1. Modification of the Class II sulphydryl groups on the (Na+ + K+)-ATPase from rectal glands of Squalus acanthias with N-ethylmaleimide has been used to detect conformational changes in the protein. The rates of inactivation of the enzyme and the incorporation of N-ethylmaleimide depend on the ligands present in the incubation medium. With 150 mM K+ the rate of inactivation is largest (k1 = 1.73 mM-1 . min-1) and four SH groups per alpha-subunit are modified. The rate of inactivation in the presence of 150 mM Na+ is smaller (k1 = 1.08 mM-1 . min-1) but the incorporation of N-ethylmaleimide is the same as with K+. 2. ATP in micromolar concentrations protects the Class II groups in the presence of Na+ (k1 = 0.08 mM-1 . min-1 at saturating ATP) and the incorporation is drastically reduced. ATP in millimolar concentrations protects the Class II groups partially in the presence of K+ (k1 = 1.08 mM-1 . min-1) and three SH groups are labelled per alpha subunit. 3. The K+-dependent phosphatase is inhibited in parallel to the (Na+ + K+)-ATPase under all conditions, and the ligand-dependent incorporation of N-ethylmaleimide was on the alpha-subunit only. 4. It is shown that the difference between the Na+ and K+ conformations sensed with N-ethylmaleimide depends on the pH of the incubation medium. At pH 6 there is a very small difference between the rates of inactivation in the presence of Na+ and K+, but at higher pH the difference increases. It is also shown that the rate of inactivation has a minimum at pH 6.9, which suggests that the conformation of the enzyme changes with pH. 5. Modification of the Class III groups with N-ethylmaleimide--whereby the enzyme activity is reduced from about 16% to zero--shows that these groups are also sensitive to conformational changes. As with the Class II groups, ATP in micromolar concentrations protects in the presence of Na+ relative to Na+ or K+ alone. ATP in millimolar concentrations with K+ present increases the rate of inactivation relative to K+ alone, in contrast to the effect on the Class II groups. 6. Modification of the Class II groups with a maleimide spin label shows a difference between Class II groups labelled in the presence of Na+ (or K+) and Class II groups labelled in the presence of K + ATP, in agreement with the difference in incorporation of N-ethylmaleimide. The spectra suggest that the SH group protected by ATP in the presence of K+ is buried in the protein. 7. The results suggest that at least four different conformations of the (Na+ + K+)-ATPase can be sensed with N-ethylmaleimide: (i) a Na+ form of the enzyme with ATP bound to a high-affinity site (E1-Na-ATP); (ii) a Na+ form without ATP bound (E1-Na); (iii) a K+ form without ATP bound (E2-K); and (iv) an enzyme form with ATP bound to a low-affinity site in the presence of K+, probably and E1-K-ATP form.

Sulphydryl groups of (Na+ + K+)-ATPase from rectal glands of Squalus acanthias. Titrations and classification.

1. (Na+ + K+)-ATPase from rectal glands of Squalus acanthias contains 34 SH groups per mol (Mr 265000). 15 are located on the alpha subunit (Mr 106000) and two on the beta subunit (Mr 40000). The beta subunit also contains one disulphide bridge. 2. The reaction of (Na+ + K+)-ATPase with N-ethylmaleimide shows the existence of at least three classes of SH groups. Class I contains two SH groups on each alpha subunit and one on each beta subunit. Reaction of these groups with N-ethylmaleimide in the presence of 40% glycerol or sucrose does not alter the enzyme activity. Class II contains four SH groups on each alpha subunit, and the reaction of these groups with 0.1 mM N-ethylmaleimide in the presence of 150 mM K+ leads to an enzyme species with about 16% activity. The remaining enzyme activity can be completely abolished by reaction with 5-10 mM N-ethylmaleimide, indicating a third class of SH groups (Class III). This pattern of inactivation is different from that of the kidney enzyme, where only one class of SH groups essential to activity is observed. 3. It is also shown that N-ethylmaleimide and DTNB inactivate by reacting with the same Class II SH groups. 4. Spin-labelling of the (Na+ + K+)-ATPase with a maleimide derivative shows that Class II groups are mostly buried in the membrane, whereas Class I groups are more exposed. It is also shown that spin label bound to the Class I groups can monitor the difference between the Na+- and K+-forms of the enzyme.

Occlusion of Rb+ by detergent-solubilized (Na+ + K+)-ATPase from shark salt glands.

Occlusion of Rb+ by C12E8-solubilized (Na+ + K+)-ATPase from shark salt glands has been measured. The rate of de-occlusion at room temperature is about 1 s-1, which is the same as for the membrane-bound enzyme. The amount of Rb+ occluded is 3 moles Rb+ per mole membrane-bound shark enzyme, whereas only about 2 moles Rb+ are occluded by the C12E8-solubilized enzyme.