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A source of bright and pure single photons is an essential tool for photonic quantum technologies, predicted to enable secure communication and networking. Self-assembled quantum dots are among the leading candidates to realise such a source. The currently highest-performing quantum dots emit at wavelengths unsuitable for fibre transmission, with telecom quantum dots lagging in performance. An intermediate step is to use quantum frequency conversion. Here we report to our knowledge the brightest quantum dot based source of telecom photons by frequency converting a near-infrared quantum dot embedded in a micropillar cavity to the telecom C-band. In a single-photon BB84 protocol, this source is capable of producing asymptotic keys rates of 1 kbps at over 150 km of optical fibre.

On-demand indistinguishable single photon sources are essential for quantum networking and communication. Semiconductor quantum dots are among the most promising candidates, but their typical emission wavelength renders them unsuitable for use in fibre networks. Here, we present quantum frequency conversion of near-infrared photons from a bright quantum dot to the telecommunication C-band, allowing integration with existing fibre architectures. We use a custom-built, tunable 2400 nm seed laser to convert single photons from 942 nm to 1550 nm in a difference frequency generation process. We achieve an end-to-end conversion efficiency of $\sim$35%, demonstrate count rates approaching 1 MHz at 1550 nm with $g^{\left(2\right)}\left(0\right) = 0.04$, and achieve Hong-Ou-Mandel visibilities of 60%. We expect this scheme to be preferable to quantum dot sources directly emitting at telecom wavelengths for fibre based quantum networking., Comment: 7 pages, 4 figures

Stimulated Raman adiabatic passage (STIRAP) describes adiabatic population transfer between two states coherently coupled via a mediating state that remains unoccupied. This renders STIRAP robust against loss in the mediating state, leading to profound applications in atomic- and molecular-beam research, trapped-ion physics, superconducting circuits, other solid-state systems, optics, in entanglement generation and qubit operations. STIRAP in optomechanics has been considered for optical frequency conversion where a mechanical mode provides the mediating state. Given the advances of optomechanical devices with exceptionally high mechanical-quality factors, STIRAP between mechanical modes has the prospect of generating macroscopic quantum superposition and of supporting quantum information protocols. An optical cavity mode can mediate the coupling between mechanical modes, without detrimental effects of optical losses. We demonstrate STIRAP between two mechanical modes of a phononic membrane-in-the-middle system with an efficiency of 86% and immune against photon loss through the mediating optical cavity., 10 pages

Coherent optical states consist of a quantum superposition of different photon number (Fock) states, but because they do not form an orthogonal basis, no photon number states can be obtained from it by linear optics. Here we demonstrate the reverse, by manipulating a random continuous single-photon stream using quantum interference in an optical Sagnac loop, we create engineered quantum states of light with tunable photon statistics, including approximate weak coherent states. We demonstrate this experimentally using a true single-photon stream produced by a semiconductor quantum dot in an optical microcavity, and show that we can obtain light with $g^{(2)}(0)\rightarrow1$ in agreement with our theory, which can only be explained by quantum interference of at least 3 photons. The produced artificial light states are, however, much more complex than coherent states, containing quantum entanglement of photons, making them a resource for multi-photon entanglement., 6 pages + supplemental material

We present a simple extension of the semi-classical model for a two-level system in a cavity, in order to incorporate multiple polarized transitions, such as those appearing in neutral and charged quantum dots (QDs), and two nondegenerate linearly polarized cavity modes. We verify the model by exact quantum master equation calculations, and experimentally using a neutral QD in a polarization non-degenerate micro-cavity, in both cases we observe excellent agreement. Finally, the usefulness of this approach is demonstrated by optimizing a single-photon source based on polarization postselection, where we find an increase in the brightness for optimal polarization conditions as predicted by the model., Comment: 8 pages, for simple code see https://doi.org/10.5281/zenodo.3476660

Maxwell equations in vacuum allow for solutions with a non-trivial topology in the electric and magnetic field line configurations at any given moment in time. One example is a space filling congruence of electric and magnetic field lines forming circles lying on the surfaces of nested tori. In this example the electric, magnetic and Poynting vector fields are orthogonal everywhere. As time evolves the electric and magnetic fields expand and deform without changing the topology and energy, while the Poynting vector structure remains unchanged while propagating with the speed of light. The topology is characterized by the concept of helicity of the field configuration. Helicity is an important fundamental concept and for massless fields it is a conserved quantity under conformal transformations. We will review several methods by which linked and knotted electromagnetic (spin-1) fields can be derived. A first method, introduced by A. Ranada, uses the formulation of the Maxwell equations in terms of differential forms combined with the Hopf map from the three-sphere S 3 to the two-sphere S 2 . A second method is based on spinor and twistor theory developed by R. Penrose in which elementary twistor functions correspond to the family of electromagnetic torus knots. A third method uses the Bateman construction of generating null solutions from complex Euler potentials. And a fourth method uses special conformal transformations, in particular conformal inversion, to generate new linked and knotted field configurations from existing ones. This fourth method is often accompanied by shifting singularities in the field to complex space–time points. Of course the various methods must be closely related to one another although they have been developed largely independently and they suggest different directions in which to expand the study of topologically non-trivial field configurations. It will be shown how the twistor formulation allows for a direct extension to massless fields of other spin values, such as spin-2 fields satisfying the linearized Einstein vacuum equation, and how the formulation by A. Ranada can be extended to fields for which the electric and magnetic fields are not orthogonal everywhere. Underlying the various methods is the fact that electric and magnetic field lines can be described as the level curves of complex functions. Compactification of R 3 naturally leads to finite energy solutions because the fields at infinity in all directions should all converge towards zero. An intriguing question that is raised by the finite energy is whether there is a connection to the quantization of the classical electromagnetic field. We will review some issues related to this question. Another interesting question is why the general formulation of topologically non-trivial solutions uses the electric and magnetic fields instead of the electromagnetic vector potentials. This leads to a discussion of the Clebsch representation of the electromagnetic field strength 2-form. Finally, a topic of great interest is the possibility of experimentally generating and investigating linked and knotted field configurations. Since the non-trivial topological field solutions exploit the special conformal symmetry of the underlying vacuum wave-equations it will only be possible to approximate the solutions in an experiment, which necessarily introduces material objects that will break the special conformal symmetry. We will review the research on plasma configurations in which the magnetic field-line configuration approximates plasma torus knots leading to the prediction of topological solitons in plasma.

We present the design and implementation of a mechanical low-pass filter vibration isolation used to reduce the vibrational noise in a cryogen-free dilution refrigerator operated at 10 mK, intended for scanning probe techniques. We discuss the design guidelines necessary to meet the competing requirements of having a low mechanical stiffness in combination with a high thermal conductance. We demonstrate the effectiveness of our approach by measuring the vibrational noise levels of an ultrasoft mechanical resonator positioned above a superconducting quantum interference device. Starting from a cryostat base temperature of 8 mK, the vibration isolation can be cooled to 10.5 mK, with a cooling power of 113 µW at 100 mK. We use the low vibrations and low temperature to demonstrate an effective cantilever temperature of less than 20 mK. This results in a force sensitivity of less than 500 zN/Hz and an integrated frequency noise as low as 0.4 mHz in a 1 Hz measurement bandwidth.

We study the resistive evolution of a localized self-organizing magnetohydrodynamic equilibrium. In this configuration the magnetic forces are balanced by a pressure force caused by a toroidal depression in the pressure. Equilibrium is attained when this low pressure region prevents further expansion into the higher-pressure external plasma. We find that, for the parameters investigated, the resistive evolution of the structures follows a universal pattern when rescaled to resistive time. The finite resistivity causes both a decrease in the magnetic field strength and a finite slip of the plasma fluid against the static equilibrium. This slip is caused by a Pfirsch-Schl\"uter type diffusion, similar to what is seen in tokamak equilibria. The net effect is that the configuration remains in Magnetostatic equilibrium whilst it slowly grows in size. The rotational transform of the structure becomes nearly constant throughout the entire structure, and decreases according to a power law. In simulations this equilibrium is observed when highly tangled field lines relax in a high-pressure (relative to the magnetic field strength) environment, a situation that occurs when the twisted field of a coronal loop is ejected into the interplanetary solar wind. In this paper we relate this localized MHD equilibrium to magnetic clouds in the solar wind., Comment: accepted for publication in Journal of Plasma Physics

Quantum teleportation -- the transmission and reconstruction over arbitrary distances of the state of a quantum system -- is demonstrated experimentally. During teleportation, an initial photon which carries the polarization that is to be transferred and one of a pair of entangled photons are subjected to a measurement such that the second photon of the entangled pair acquires the polarization of the initial photon. This latter photon can be arbitrarily far away from the initial one. Quantum teleportation will be a critical ingredient for quantum computation networks., Comment: 7 pages, 5 figures

We calculate the optomechanical coupling strength for a multi-membrane in a cavity system. The optimal configuration for an array of $N$ membranes placed near the center of a cavity is identified. This results in a coupling strength much greater than previously proposed multi-membrane configurations. We find that the coupling strength scales exponentially with the number of membranes until saturating due to complete localization of the field within the array. Furthermore we explore two sources of loss, those due to light absorption within the membrane(s) and leakage through the cavity's end-mirrors, and evaluate how they affect the possibility of achieving strong coupling., Comment: 12 Pages, 8 Figures

Surface acoustic waves (SAWs) have proven a useful tool for dynamically manipulating the optical properties of semiconductor nanostructures. Recently we have shown that the mechanical deformation induced by a SAW on a two-dimensional photonic crystal membrane (PCM) spectrally tunes the resonance frequency of nanocavities within the PCM employing phase locked stroboscopic excitation.

We present an oxide aperture microcavity with embedded quantum dots that utilizes a three contact design to independently tune the quantum dot wavelength and birefringence of the cavity modes. A polarization splitting tuning of $\sim$5 GHz is observed. For typical microcavity polarization splittings, the method can be used to achieve perfect polarization degeneracy that is required for many polarization-based implementations of photonic quantum gates. The embedded quantum dot wavelength can be tuned into resonance with the cavity, independent of the polarization tuning.

We observe the unconventional photon blockade effect in quantum dot cavity QED, which, in contrast to the conventional photon blockade, operates in the weak coupling regime. A single quantum dot transition is simultaneously coupled to two orthogonally polarized optical cavity modes, and by careful tuning of the input and output state of polarization, the unconventional photon blockade effect is observed. We find a minimum second-order correlation g((2))(0) approximate to 0.37, which corresponds to g((2))(0) approximate to 0.005 when corrected for detector jitter, and observe the expected polarization dependency and photon bunching and antibunching; close by in parameter space, which indicates the abrupt change from phase to amplitude squeezing.

Experimental observation of the decoherence of macroscopic objects is of fundamental importance to the study of quantum collapse models and the quantum to classical transition. Optomechanics is a promising field for the study of such models because of its fine control and readout of mechanical motion. Nevertheless, it is challenging to monitor a mechanical superposition state for long enough to investigate this transition. We present a scheme for entangling two mechanical resonators in spatial superposition states such that all quantum information is stored in the mechanical resonators. The scheme is general and applies to any optomechanical system with multiple mechanical modes. By analytic and numeric modeling, we show that the scheme is resilient to experimental imperfections such as incomplete pre-cooling, faulty postselection and inefficient optomechanical coupling. This proposed procedure overcomes limitations of previously proposed schemes that have so far hindered the study of macroscopic quantum dynamics.

We demonstrate 8.5 dB thermal squeezing of a membrane oscillator using the dynamical backaction effect and electrostatic feedback in an optomechanical membrane-in-the-middle setup. We show that strong squeezing can be obtained even in the far detuning regime of a sideband-resolved system. By using the dielectrophoretic force of a metallic needle kept in close proximity to the membrane, we implement the one-quadrature active feedback scheme to prevent the divergence of the amplified quadrature and surpass the 3 dB limit of mechanical squeezing. We also discuss different regions of the sideband spectrum where strong squeezing can be obtained. Although the demonstration here is classical, this technique is equally applicable to prepare the mechanical oscillator in a quantum squeezed state., 5 pages, 4 figures

High quality sources of single and identical (indistinguishable) photons are essential for photonic quantum information processing including boson sampling, cluster-state quantum computing, and optical quantum repeaters. To enable the use of several single photon sources in parallel and to allow the implementation of complex photonic networks, it is essential that the single photon source can be integrated easily and efficiently with optical fiber technology. Here we show a first all-solid-state InGaAs quantum dot based, integrated cavity-QED enhanced, single photon source. The source is operated by resonant optical pumping through an excitation fiber, and the single photons are collected in the fiber attached to the transmission channel of the cavity.

Author(s): Markesevic, N; Oemrawsingh, SSR; Schultz, D; Gwinn, EG; Bouwmeester, D | Abstract: Polarization-resolved excitation and emission measurements on individual 10-15 atom silver clusters formed on DNA are presented. The emission is highly linearly polarized, typically around 90% for all emitters, whereas the polarization dependence of the excitation strongly varies from emitter to emitter. These observations support the hypothesis that the luminescence arises from collective electron oscillations along rod-shaped silver clusters, whereas the excitation process does not necessarily have a strong preferential polarization direction and thus may involve energy and/or charge transfer between the cluster and DNA. In addition to stabilizing the clusters, the DNA also appears to strongly influence the available paths for excitation. © 2014 WILEY-VCH Verlag GmbH a Co. KGaA, Weinheim.

We develop approaches to hold fluorescent silver clusters composed of only 10-20 atoms in nanoscale proximity, while retaining the individual structure of each cluster. This is accomplished using DNA clamp assemblies that incorporate a 10 atom silver cluster and a 15 or 16 atom silver cluster. Thermally modulated fluorescence resonance energy transfer (FRET) verifies assembly formation. Comparison to Förster theory, using measured spectral overlaps, indicates that the DNA clamps hold clusters within roughly 5 to 6 nm separations, in the range of the finest resolutions achievable on DNA scaffolds. The absence of spectral shifts in dual-cluster FRET pairs, relative to the individual clusters, shows that select few-atom silver clusters of different sizes are sufficiently stable to retain structural integrity within a single nanoscale DNA construct. The spectral stability of the cluster persists in a FRET pair with an organic dye molecule, in contrast to the blue-shifted emission of the dye.

Torus-knot solitons have recently been formulated as solutions to the ideal incompressible magnetohydrodynamics (MHD) equations. We investigate numerically how these fields evolve in resistive, compressible, and viscous MHD. We find that certain decaying plasma torus knots exhibit magnetic surfaces that are topologically distinct from a torus. The evolution is predominantly determined by a persistent zero line in the field present when the poloidal winding number . Dependence on the toroidal winding number n t is less pronounced as the zero line induced is contractible and disappears. The persistent zero line intersects the new magnetic surfaces such that, through the Hopf–Poincare index theorem, the sum of zeroes on the new surfaces equals their (in general non-zero) Euler characteristic. Furthermore we observe the formation of magnetic islands between the surfaces. These novel persistent magnetic structures are of interest for plasma confinement, soliton dynamics and the study of dynamical systems in general.

We demonstrate a simple method to measure optomechanically induced transparency (OMIT) in a Fabry-Perot based system using a trampoline resonator. In OMIT, the transmitted intensity of a weak probe beam in the presence of a strong control beam is modified via the optomechanical interaction, leading to an ultra-narrow optical resonance. To retrieve both the magnitude and the phase of the probe beam, a homodyne detection technique is typically used. We have greatly simplified this method by using a single acousto-optical modulator to create a control and two probe beams. The beat signal between the transmitted control and probe beams shows directly the typical OMIT characteristics. This method therefore demonstrates an elegant solution when a homodyne field is needed but experimentally not accessible.

A high-fidelity source of identical single photons is essential for numerous quantum technologies such as quantum repeaters and optical quantum information processing. Hallmarks thereof are a near-unity single-photon purity, near-unity indistinguishability of consecutively emitted photons, and high brightness through a near-unity number of photons per time bin. In order to embed such sources in quantum networks, optical fiber integration is essential but complicated by cryogenic compatability and noise. Here we demonstrate a resonantly pumped, quantum dot (QD) based, transmission operated, single-mode fiber coupled single photon source with a purity of 97%, indistinguishability of 90%, and a brightness of 17%. This is achieved by deploying a unique micropillar cavity design in a closed-cycle cryostat, which is operated using a through-fiber cross-polarization technique to remove the pump laser light from the resonantly scattered single photons. These results pave the way for fully fiber integrated photonic quantum networks, as our technology is equally applicable for cavity-QED based photonic quantum gates., Comment: 10 pages, 10 figures

Scientific Reports 6: Article number: 37897; published online: 30 November 2016; updated: 12 April 2017 In the ‘Introduction’ and ‘Results and Discussion’ sections of this manuscript, the authors used 2 sequences that were previously published and cited already as references 35 and 44. To better reflect the use of these sequences within the Introduction

Experiments involving micro- and nanomechanical resonators need to be carefully designed to reduce mechanical environmental noise. A small scale on-chip approach is to add an additional resonator to the system as a mechanical low-pass filter. Unfortunately, the inherent low frequency of the low-pass filter causes the system to be easily excited mechanically. Fixating the additional resonator ensures that the resonator itself can not be excited by the environment. This, however, negates the purpose of the low-pass filter. We solve this apparent paradox by applying active feedback to the resonator, thereby minimizing the motion with respect the front mirror of an optomechanical cavity. Not only does this method actively stabilize the cavity length, but it also retains the on-chip vibration isolation., Comment: Minor adjustments made

Experiments in the field of optomechanics do not yet fully exploit the photon polarization degree of freedom. Here experimental results for an optomechanical interaction in a polarization nondegenerate system are presented and schemes are proposed for how to use this interaction to perform accurate side-band thermometry and to create novel forms of photon-phonon entanglement. The experimental system utilizes the compressive force in the mirror attached to a mechanical resonator to create a micro-mirror with two radii of curvature which leads, when combined with a second mirror, to a significant polarization splitting of the cavity modes., 5 pages, 4 figures

DNA-stabilized silver clusters (Ag-DNA) show excellent promise as a multi-functional nanoagent for molecular investigations in living cells. The unique properties of these fluorescent nanomaterials allow for intracellular optical sensors with tunable cytotoxicity based on simple modifications of the DNA sequences. Three Ag-DNA nanoagent designs are investigated, exhibiting optical responses to the intracellular environments and sensing-capability of ions, functional inside living cells. Their sequence-dependent fluorescence responses inside living cells include (1) a strong splitting of the fluorescence peak for a DNA hairpin construct, (2) an excitation and emission shift of up to 120 nm for a single-stranded DNA construct, and (3) a sequence robust in fluorescence properties. Additionally, the cytotoxicity of these Ag-DNA constructs is tunable, ranging from highly cytotoxic to biocompatible Ag-DNA, independent of their optical sensing capability. Thus, Ag-DNA represents a versatile live-cell nanoagent addressable towards anti-cancer, patient-specific and anti-bacterial applications.

A silicon nitride ring resonator with implanted ytterbium ions offers a means for greatly enhanced ion–light interactions in an integrated optics platform. Rare-earth ions in solids are of particular interest for quantum information storage and processing because of the long coherence times of the 4f states1. In the past few years, substantial progress has been made by using ensembles of ions2,3,4,5,6 and single ions7,8,9,10. However, the weak optical transitions within the 4f manifold pose a great challenge to the optical interaction with a single rare-earth ion on a single-photon level. Here, we demonstrate a ninefold enhanced ion–light interaction (Purcell effect11) in an integrated-optics-based, fibre-coupled silicon nitride (Si3N4) ring resonator with implanted ytterbium ions (Yb3+). We unveil the one-, two- and three-dimensional contributions to the Purcell factor as well as the temperature-dependent decoherence and depolarization of the ions. The results indicate that this cavity quantum electrodynamics (QED) system has the potential of interfacing single rare-earth ions with single photons on a chip.

We report on the first single emitter fluorescence spectra of DNA-stabilized silver clusters (Ag:DNAs) at ambient and cryogenic temperatures. While Ag:DNAs have received much attention recently due to their sequence-tunable emission wavelengths, the nature of the optical transitions (molecule-like or collective, cluster-like) is an open question. By removing the ensemble broadening present in previous spectroscopic studies, we probe the line widths Γ and brightness of individual Ag:DNA emitters. A roughly 5-fold increase in brightness from 295 to 1.7 K is accompanied by a factor of 2 decrease in Γ. The symmetric emission line shape, its insensitivity to embedding medium, and the independence of emission wavelength on excitation energy together indicate that the measured Γ represents the homogeneous line width, while the large, ∼100 meV values of Γ suggest rapid dephasing of a collective excited state of the silver cluster.

Photonic crystal membranes (PCM) provide a versatile planar platform for on-chip implementations of photonic quantum circuits. One prominent quantum element is a coupled system consisting of a nanocavity and a single quantum dot (QD) which forms a fundamental building block for elaborate quantum information networks and a cavity quantum electrodynamic (cQED) system controlled by single photons. So far no fast tuning mechanism is available to achieve control within the system coherence time. Here we demonstrate dynamic tuning by monochromatic coherent acoustic phonons formed by a surface acoustic wave (SAW) with frequencies exceeding 1.7 gigahertz, one order of magnitude faster than alternative approaches. We resolve a periodic modulation of the optical mode exceeding eight times its linewidth, preserving both the spatial mode profile and a high quality factor. Since PCMs confine photonic and phononic excitations, coupling optical to acoustic frequencies, our technique opens ways towards coherent acoustic control of optomechanical crystals., Comment: 11 pages 4 figures

Maxwell’s equations describing electric and magnetic fields limit the shapes field lines can take. But exotic solutions exist where the field lines are linked and knotted. A proposal now shows how such solutions could be realized experimentally. Maxwell’s equations allow for curious solutions characterized by the property that all electric and magnetic field lines are closed loops with any two electric (or magnetic) field lines linked. These little-known solutions, constructed by Ranada1, are based on the Hopf fibration. Here we analyse their physical properties to investigate how they can be experimentally realized. We study their time evolution and uncover, through a decomposition into a spectrum of spherical harmonics, a remarkably simple representation. Using this representation, first, a connection is established to the Chandrasekhar–Kendall curl eigenstates2, which are of broad importance in plasma physics and fluid dynamics. Second, we show how a new class of knotted beams of light can be derived, and third, we show that approximate knots of light may be generated using tightly focused circularly polarized laser beams. We predict theoretical extensions and potential applications, in fields ranging from fluid dynamics, topological optical solitons and particle trapping to cold atomic gases and plasma confinement.

Two major challenges in the development of optomechanical devices are achieving a low mechanical and optical loss rate and vibration isolation from the environment. We address both issues by fabricating trampoline resonators made from low pressure chemical vapor deposition (LPCVD) Si$_3$N$_4$ with a distributed bragg reflector (DBR) mirror. We design a nested double resonator structure with 80 dB of mechanical isolation from the mounting surface at the inner resonator frequency, and we demonstrate up to 45 dB of isolation at lower frequencies in agreement with the design. We reliably fabricate devices with mechanical quality factors of around 400,000 at room temperature. In addition these devices were used to form optical cavities with finesse up to 181,000 $\pm$ 1,000. These promising parameters will enable experiments in the quantum regime with macroscopic mechanical resonators.

We construct a family of exact solutions to Maxwell's equations in which the points of zero intensity form knotted lines topologically equivalent to a given but arbitrary algebraic link. These lines of zero intensity, more commonly referred to as optical vortices, and their topology are preserved as time evolves and the fields have finite energy. To derive explicit expressions for these new electromagnetic fields that satisfy the nullness property, we make use of the Bateman variables for the Hopf field as well as complex polynomials in two variables whose zero sets give rise to algebraic links. The class of algebraic links includes not only all torus knots and links thereof, but also more intricate cable knots. While the unknot has been considered before, the solutions presented here show that more general knotted structures can also arise as optical vortices in exact solutions to Maxwell's equations., Comment: 5 pages, 3 figures; revised abstract, introduction, and conclusion; results unchanged

Single photon nonlinearities based on a semiconductor quantum dot in an optical microcavity are a promising candidate for integrated optical quantum information processing nodes. In practice, however, the finite quantum dot lifetime and cavity-quantum dot coupling lead to reduced fidelity. Here we show that, with a nearly polarization degenerate microcavity in the weak coupling regime, polarization pre- and postselection can be used to restore high fidelity. The two orthogonally polarized transmission amplitudes interfere at the output polarizer; for special polarization angles, which depend only on the device cooperativity, this enables cancellation of light that did not interact with the quantum dot. With this, we can transform incident coherent light into a stream of strongly correlated photons with a second-order correlation value up to 40, larger than previous experimental results, even in the strong-coupling regime. This purification technique might also be useful to improve the fidelity of quantum dot based logic gates., Single-photon optical nonlinearity is possible using an optical cavity to create strong coupling between a cavity mode and a two-level quantum system. Here, the authors demonstrate it is also possible in the weak-coupling regime by using quantum interference in a polarization-degenerate cavity.

Optoelectronic devices that provide non-classical light states on demand have a broad range of applications in quantum information science1, including quantum‐key‐distribution systems2, quantum lithography3 and quantum computing4. Single-photon sources5,6 in particular have been demonstrated to outperform key distribution based on attenuated classical laser pulses7. Implementations based on individual molecules8, nitrogen vacancy centres9 or dopant atoms10 are rather inefficient owing to low emission rates, rapid saturation and the lack of mature cavity technology. Promising single-photon-source designs combine high-quality microcavities11 with quantum dots as active emitters12. So far, the highest measured single-photon rates are ∼ 200 kHz using etched micropillars13,14. Here, we demonstrate a quantum-dot-based single-photon source with a measured single-photon emission rate of 4.0 MHz (31 MHz into the first lens, with an extraction efficiency of 38%) due to the suppression of exciton dark states. Furthermore, our microcavity design provides mechanical stability, and voltage-controlled tuning of the emitter/mode resonance and of the polarization state.

We demonstrate experimental exploration of the attractor diagram of an optomechanical system where the optical forces compensate for the mechanical losses. In this case stable self-induced oscillations occur but only for specific mirror amplitudes and laser detunings. We demonstrate that we can amplify the mechanical mode to an amplitude 500 times larger than the thermal amplitude at 300K. The lack of unstable or chaotic motion allows us to manipulate our system into a non-trivial steady state and explore the dynamics of self-induced oscillations in great detail., 6 pages, 4 figures

Micromechanical resonators, when cooled down to near their ground state, can be used to explore quantum effects such as superposition and entanglement at a macroscopic scale. Previously, it has been proposed to use electronic feedback to cool a high frequency (10 MHz) resonator to near its ground state. In other work, a low frequency resonator was cooled from room temperature to 18 K by passive optical feedback. Additionally, active optical feedback of atomic force microscope cantilevers has been used to modify their response characteristics, and cooling to approximately 2 K has been measured. Here we demonstrate active optical feedback cooling to 135 +/- 15 mK of a micromechanical resonator integrated with a high-quality optical resonator. Additionally, we show that the scheme should be applicable at cryogenic base temperatures, allowing cooling to near the ground state that is required for quantum experiments--near 100 nK for a kHz oscillator.

In a Fabry–Perot cavity optomechanical system the mechanical mode can be excited by placing the laser at the blue sideband of the cavity resonance. As soon as the optical driving force overcomes the mechanical losses, self-induced oscillations start to occur. When trying to achieve large parametric amplification in most experiments these self-induced oscillations cross over into chaotic motion due to secondary effects such as heating of the mirror by optical absorption. Here we demonstrate an optomechanical system in which the amplitude of mechanical mode can be greatly amplified due to the use of a low-absorption Bragg mirror on the mechanical oscillator. By scanning the laser across the optical cavity resonance and analyzing the optical output, we demonstrate that we can amplify the mechanical mode to an amplitude 450 times larger than the thermal amplitude at 300 K, without any appearance of unstable or chaotic motion.

We extend the definition of hopfions to include a class of spin-$h$ fields and use this to introduce the electromagnetic and gravitational hopfions of different algebraic types. The fields are constructed through the Penrose contour integral transform, thus the singularities of the generating functions are directly related to the geometry of the resulting physical fields. We discuss this relationship and how the topological structure of the fields is related to the Robinson congruence. Since the topology appears in the lines of force for both electromagnetism and gravity, the gravito-electromagnetic formalism is used to analyze the gravitational hopfions and describe the time evolution of their tendex and vortex lines. The correspondence between fields of different spin results in analogous configurations based on the same topological structure. The null and type N fields propagate at the speed of light, while the non-null and type D fields radiate energy outward from the center. Finally we discuss the type III gravitational hopfion, which has no direct electromagnetic analog, but find that it still exhibits some of the characteristic features common to the other hopfion fields., 18 pages, 5 figures

In an oxide apertured quantum dot (QD) micropillar cavity-QED system, we found strong QD hysteresis effects and lineshape modifications even at very low intensities corresponding to less than 0.001 intracavity photons. We attribute this to the excitation of charges by the intracavity field; charges that get trapped at the oxide aperture, where they screen the internal electric field and blueshift the QD transition. This in turn strongly modulates light absorption by cavity QED effects, eventually leading to the observed hysteresis and lineshape modifications. The cavity also enables us to observe the QD dynamics in real time, and all experimental data agrees well with a power-law charging model. This effect can serve as a novel tuning mechanism for quantum dots., 7 pages, 6 figures

We perform full-magnetohydrodynamics simulations on various initially helical configurations and show that they reconfigure into a state where the magnetic field lines span nested toroidal surfaces. This relaxed configuration is not a Taylor state, as is often assumed for relaxing plasma, but a state where the Lorentz force is balanced by the hydrostatic pressure, which is lowest on the central ring of the nested tori. Furthermore, the structure is characterized by a spatially slowly varying rotational transform, which leads to the formation of a few magnetic islands at rational surfaces. We then obtain analytic expressions that approximate the global structure of the quasistable linked and knotted plasma configurations that emerge, using maps from S^{3} to S^{2} of which the Hopf fibration is a special case. The knotted plasma configurations have a highly localized magnetic energy density and retain their structure on time scales much longer than the Alfvenic time scale.

For experimental investigations of macroscopic quantum superpositions and the possible role of gravitational effects on the reduction of the corresponding quantum wave function it is beneficial to consider large mass, low frequency optomechanical systems. We report optical side-band cooling from room temperature for a 1.5×10⁻¹⁰ kg (mode mass), low frequency side-band resolved optomechanical system based on a 5 cm long Fabry-Perot cavity. By using high-quality Bragg mirrors for both the stationary and the micromechanical mirror we are able to construct an optomechanical cavity with an optical linewidth of 23 kHz. This, together with a resonator frequency of 315 kHz, makes the system operate firmly in the side-band resolved regime. With the presented optomechanical system parameters cooling close to the ground state is possible. This brings us one step closer to creating and verifying macroscopic quantum superpositions.

Ideal magnetohydrodynamics relaxation is the topology-conserving reconfiguration of a magnetic field into a lower energy state where the net force is zero. This is achieved by modeling the plasma as perfectly conducting viscous fluid. It is an important tool for investigating plasma equilibria and is often used to study the magnetic configurations in fusion devices and astrophysical plasmas. We study the equilibrium reached by a localized magnetic field through the topology conserving relaxation of a magnetic field based on the Hopf fibration in which magnetic field lines are closed circles that are all linked with one another. Magnetic fields with this topology have recently been shown to occur in non-ideal numerical simulations. Our results show that any localized field can only attain equilibrium if there is a finite external pressure, and that for such a field a Taylor state is unattainable. We find an equilibrium plasma configuration that is characterized by a lowered pressure in a toroidal region, with field lines lying on surfaces of constant pressure. Therefore, the field is in a Grad-Shafranov equilibrium. Localized helical magnetic fields are found when plasma is ejected from astrophysical bodies and subsequently relaxes against the background plasma, as well as on earth in plasmoids generated by, e.g., a Marshall gun. This work shows under which conditions an equilibrium can be reached and identifies a toroidal depression as the characteristic feature of such a configuration.