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Room-temperature optically detected magnetic resonance experiments on spin-$\frac{3}{2}$ silicon vacancies in $4H$-SiC are reported. The ${m}_{s}=+\frac{1}{2}\ensuremath{\leftrightarrow}\ensuremath{-}\frac{1}{2}$ transition is accessed using a two-microwave-frequency excitation protocol. The ratio of the Rabi frequencies of the $+\frac{3}{2}\ensuremath{\leftrightarrow}+\frac{1}{2}$ and $+\frac{1}{2}\ensuremath{\leftrightarrow}\ensuremath{-}\frac{1}{2}$ transitions is measured to be $(0.901\ifmmode\pm\else\textpm\fi{}0.013)$. The deviation from $\sqrt{3}/2$ is attributed to small difference in $g$ factor for different magnetic dipole transitions. Whereas a spin-$\frac{1}{2}$ system is characterized by a single-spin lifetime ${T}_{1}$, we experimentally demonstrate that the spin-$\frac{3}{2}$ system has three distinct relaxation modes that can be preferentially excited and detected. The measured relaxation times are $(0.41\ifmmode\pm\else\textpm\fi{}0.02){T}_{\mathrm{slow}}={T}_{d}=(3.3\ifmmode\pm\else\textpm\fi{}0.5){T}_{\mathrm{fast}}$. This differs from the values of ${T}_{p}/3={T}_{d}=2{T}_{f}$ expected for pure dipole (${T}_{p}$), quadrupole (${T}_{d}$), and octupole (${T}_{f}$) relaxation modes, respectively, and implies admixing of the slow dipole and fast octupole relaxation modes.

Microwave-to-optical conversion via ferromagnetic magnons has so far been limited by the optical coupling rates achieved in millimeter-scale whispering-gallery-mode resonators. Toward overcoming this limitation, we propose and demonstrate an open magneto-optical cavity containing a thin film of yttrium iron garnet (YIG). We achieve a 0.1 pL ($100\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{3}$) optical mode volume, approximately $50$ times smaller than previous devices. From this, we estimate that the magnon single-photon coupling rate is $G\ensuremath{\approx}50$ Hz. This open-cavity design offers the prospect of wavelength-scale mode volumes, small polarization splittings, and good magneto-optical mode overlap. With achievable further improvements and optimization, efficient microwave-optical conversion and magnon cooling devices become a realistic possibility.

We experimentally identify the magnetostatic modes active for Brillouin light scattering in the optical whispering gallery modes of a yttrium iron garnet sphere. Each mode is identified by magnetic field dispersion of ferromagnetic-resonance spectroscopy and coupling strength to the known field distribution of the microwave drive antenna. Our optical measurements confirm recent predictions that higher-order magnetostatic modes can also generate optical scattering, according to the selection rules derived from the axial symmetry. From this we summarize the selection rules for Brillouin light scattering. We give experimental evidence that the optomagnonic coupling to non-uniform magnons can be higher than that of the uniform Kittel mode., 5 pages, 5 figures

We demonstrate that multiply coupled spinor polariton condensates can be optically tuned through a sequence of spin-ordered phases by changing the coupling strength between nearest neighbors. For closed four-condensate chains these phases span from ferromagnetic (FM) to antiferromagnetic (AFM), separated by an unexpected crossover phase. This crossover phase is composed of alternating FM-AFM bonds. For larger eight-condensate chains, we show the critical role of spatial inhomogeneities and demonstrate a scheme to overcome them and prepare any desired spin state. Our observations thus demonstrate a fully controllable nonequilibrium spin lattice., We acknowledge Grants No. EPSRC EP/L027151/1, No. EU INDEX 289968, No. ERC “POLAFLOW” Starting Grant, ERC LINASS 320503, Spanish MEC (MAT2008- 01555), Mexican CONACYT 251808, Leverhulme Trust Grant No. VP1-2013-011 and Fundación La Caixa. H. S. and I. S. acknowledge support by the Research Fund of the University of Iceland, The Icelandic Research Fund, Grant No. 163082-051. T. L. was supported by the MOE AcRF Tier 1 Grant No. 2016-T1-001-084. P. S. acknowledges financial support from the Stavros Niarchos Foundation, “ARCHERS” project.

Domain wall motion driven by ultra-short laser pulses is a pre-requisite for envisaged low-power spintronics combining storage of information in magnetoelectronic devices with high speed and long distance transmission of information encoded in circularly polarized light. Here we demonstrate the conversion of the circular polarization of incident femtosecond laser pulses into inertial displacement of a domain wall in a ferromagnetic semiconductor. In our study, we combine electrical measurements and magneto-optical imaging of the domain wall displacement with micromagnetic simulations. The optical spin-transfer torque acts over a picosecond recombination time of the spin-polarized photo-carriers that only leads to a deformation of the initial domain wall structure. We show that subsequent depinning and micrometre-distance displacement without an applied magnetic field or any other external stimuli can only occur due to the inertia of the domain wall., Domain wall motion driven by ultra-short laser pulses has potential for storage of information in magnetoelectronic devices. Here the authors demonstrate the conversion of a circularly polarized femtosecond laser light into inertial displacement of a domain wall in a ferromagnetic semiconductor.

We report spin and intensity coupling of an exciton-polariton condensate to the mechanical vibrations of a circular membrane microcavity. We optically drive the microcavity resonator at the lowest mechanical resonance frequency while creating an optically-trapped spin-polarized polariton condensate in different locations on the microcavity, and observe spin and intensity oscillations of the condensate at the vibration frequency of the resonator. Spin oscillations are induced by vibrational strain driving, whilst the modulation of the optical trap due to the displacement of the membrane causes intensity oscillations in the condensate emission. Our results demonstrate spin-phonon coupling in a macroscopically coherent condensate., 5 pages

An enhancement in Brillouin light scattering of optical photons with magnons is demonstrated in magneto-optical whispering gallery mode resonators tuned to a triple-resonance point. This occurs when both the input and output optical modes are resonant with those of the whispering gallery resonator, with a separation given by the ferromagnetic resonance frequency. The identification and excitation of specific optical modes allows us to gain a clear understanding of the mode-matching conditions. A selection rule due to wave vector matching leads to an intrinsic single-sideband excitation. Strong suppression of one sideband is essential for one-to-one frequency mapping in coherent optical-to-microwave conversion., Engineering and Physical Sciences Research Council (Grant ID: EP/ M50693X/1), European Research Council (Grant ID: 648613), Hitachi (Research Fellowship), Royal Society (University Research Fellowship), Winton Programme for the Physics of Sustainability, This is the author accepted manuscript. The final version is available from the American Physical Society via https://doi.org/10.1103/PhysRevLett.117.133602

We demonstrate that yttrium iron garnet microspheres support optical whispering gallery modes similar to those in non-magnetic dielectric materials. The direction of the ferromagnetic moment tunes both the resonant frequency via the Voigt effect as well as the degree of polarization rotation via the Faraday effect. An understanding of the magneto-optical coupling in whispering gallery modes, where the propagation direction rotates with respect to the magnetization, is fundamental to the emerging field of cavity optomagnonics., Engineering and Physical Sciences Research Council (Grant IDs: EP/M50693X/1, EP/L027151/1), European Research Council (Grant ID: 648613), Hitachi (research fellowship), Royal Society (University Research Fellowship), Winton programme for the Physics of Sustainability, This is the author accepted manuscript. The final version is available from the American Physical Society via http://dx.doi.org/10.1103/PhysRevA.92.063845

We study optically driven Rabi rotations of a quantum dot exciton transition between 5 and 50 K, and for pulse-areas of up to $14\pi$. In a high driving field regime, the decay of the Rabi rotations is nonmonotonic, and the period decreases with pulse-area and increases with temperature. By comparing the experiments to a weak-coupling model of the exciton-phonon interaction, we demonstrate that the observed renormalization of the Rabi frequency is induced by fluctuations in the bath of longitudinal acoustic phonons, an effect that is a phonon analogy of the Lamb-shift., Comment: 4-pages, 2 figs, final version published in PRL

The extrinsic dephasing of a hole spin confined to a self-assembled quantum dot is dominated by charge noise acting on an electric-field dependent g-factor. Here we propose the use of the optical Stark effect to reduce the sensitivity of the effective hole Zeeman energy to fluctuations in the local electric-field. Calculations using measured quantum dot parameters are presented, and demonstrate a factor of 10-100 reduction in the extrinsic dephasing. Compared to active stabilization methods, this technique should benefit from reduced experimental complexity., Comment: 7 pages, 4 figs, final version

A two-photon absorption resonance is observed when the sum-frequency of two laser pulses equals the creation energy of a biexciton in a single InAs/GaAs semiconductor quantum dot. Resonant excitation of the two-color two-photon transition results in a coherent oscillation that is remarkably similar to a single-color two-photon Rabi oscillation of the crystal-ground to biexciton optical transition. A model explaining the coherent two-photon absorption is presented.

A hole spin is a potential solid-state q-bit, that may be more robust against nuclear spin induced dephasing than an electron spin. Here we propose and demonstrate the sequential preparation, control and detection of a single hole spin trapped on a self-assembled InGaAs/GaAs quantum dot. The dot is embedded in a photodiode structure under an applied electric field. Fast, triggered, initialization of a hole spin is achieved by creating a spin-polarized electron–hole pair with a picosecond laser pulse, and in an applied electric field, waiting for the electron to tunnel leaving a spin-polarized hole. Detection of the hole spin with picoseconds time resolution is achieved using a second picosecond laser pulse to probe the positive trion transition, where a trion is created conditional on the hole spin being detected as a change in photocurrent. Finally, using this setup we observe a Rabi rotation of the hole–trion transition that is conditional on the hole spin, which for a pulse area of 2 π can be used to impart a phase shift of π between the hole spin states, a non-general manipulation of the hole spin.

We study the coherent properties of a two-level system based on the ground state neutral exciton of a single InGaAs self-assembled quantum dot (SAQD) embedded in an n-i-Schottky diode. By using a photocurrent technique, we observe Rabi oscillations of the exciton population for pulse-areas of up to 6π. We perform two pulse quantum interference experiments to deduce the decoherence rate.

We demonstrate a new method to produce a population inversion in an InGaAs quantum dot by quasi-resonant, incoherent excitation within the LA phonon sideband. A maximum exciton population of 0.67 is measured; applications include single photon sources and single QD lasers.

We demonstrate optical manipulation of the position of a domain wall in a dilute magnetic semiconductor, GaMnAsP. Two main contributions are identified. Firstly, photocarrier spin exerts a spin transfer torque on the magnetization via the exchange interaction. The direction of the domain wall motion can be controlled using the helicity of the laser. Secondly, the domain wall is attracted to the hot-spot generated by the focused laser. Unlike magnetic field driven domain wall depinning, these mechanisms directly drive domain wall motion, providing an optical tweezer like ability to position and locally probe domain walls., 5 pages, 4 figs, final version improved by referee feedback

We demonstrate a new method to realize the population inversion of a single InGaAs/GaAs quantum dot excited by a laser pulse tuned within the neutral exciton phonon sideband. In contrast to the conventional method of inverting a two-level system by performing coherent Rabi oscillation, the inversion is achieved by rapid thermalization of the optically dressed states via incoherent phonon-assisted relaxation. A maximum exciton population of 0.67±0.06 is measured for a laser tuned 0.83 meV to higher energy. Furthermore, the phonon sideband is mapped using a two-color pump-probe technique, with its spectral form and magnitude in very good agreement with the result of path-integral calculations.

GaAs provides a versatile platform for developing on-chip integrated quantum-optical circuits. In this paper we review progress in developing spin-photon interfaces, on-chip beam-splitters and coherent single photon sources based on GaAs photonics.

The electron–phonon coupling in self-assembled InGaAs quantum dots is relatively weak at low light intensities, which means that the zero-phonon line in emission is strong compared to the phonon sideband. However, the coupling to acoustic phonons can be dynamically enhanced in the presence of an intense optical pulse tuned within the phonon sideband. Recent experiments have shown that this dynamic vibronic coupling can enable population inversion to be achieved when pumping with a blueshifted laser and for rapid de-excitation of an inverted state with red detuning. In this paper we confirm the incoherent nature of the phonon-assisted pumping process and explore the temperature dependence of the mechanism. We also show that a combination of blueshifted and redshifted pulses can create and destroy an exciton within a timescale of ∼20 ps as determined by the pulse duration and ultimately limited by the phonon thermalization time.

An in-plane spin-photon interface is essential for the integration of quantum dot spins with optical circuits. The optical dipole of a quantum dot lies in the plane and the spin is optically accessed via circularly polarized selection rules. Hence, a single waveguide, which can transport only one in-plane linear polarization component, cannot communicate the spin state between two points on a chip. To overcome this issue, we introduce a spin-photon interface based on two orthogonal waveguides, where the polarization emitted by a quantum dot is mapped to a path-encoded photon. We demonstrate operation by deducing the spin using the interference of in-plane photons. A second device directly maps right and left circular polarizations to antiparallel waveguides, surprising for a nonchiral structure but consistent with an off-center dot.

The preparation of a coherent heavy-hole spin via ionization of a spin-polarized electron-hole pair in an InAs/GaAs quantum dot in a Voigt geometry magnetic field is investigated. For a dot with a 17 ueV bright-exciton fine-structure splitting, the fidelity of the spin preparation is limited to 0.75, with optimum preparation occurring when the effective fine-structure of the bright-exciton matches the in-plane hole Zeeman energy. In principle, higher fidelities can be achieved by minimizing the bright-exciton fine-structure splitting., 8 pages, 10 figs, published PRB 85 155310 (2012)

We propose a device architecture for an in-plane network of optically connected quantum dots. At each node of the network, the dot resides at the intersection of two orthogonal waveguides which transmit the full polarization of an emitted photon to another node. A prototype device is presented.

We report coherent optical control of a single hole spin confined to an InAs/GaAs quantum dot. The hole spin was initialized by fast dissociation of a spin-polarized electron-hole pair, and controlled by combining coherent rotations about two perpendicular axes: Larmor precession about an external Voigt-geometry magnetic field, and optical rotation caused by the geometric phase shift induced by a picosecond laser pulse.

This thesis describes experiments on the initialisation, coherent control, and readout of a single hole spin trapped in a self assembled InGaAs semiconductor quantum dot. High fidelity initialisation of a hole spin state is achieved by the fast ionisation of a spin polarised neutral exciton under an applied electric field and in a Faraday geometry magnetic field. The preparation of a coherent superposition state is demonstrated by observing the precession of the hole spin about a Voigt geometry magnetic field. The hole spin dephasing time is deduced from the decay of the spin contrast. Coherent optical rotation of the hole spin state about the z-axis is demonstrated using the geometric phase shift induced by a picosecond laser pulse. By combining the precession of the spin about the x-axis, and optical rotations about the z-axis, full quantum control of a hole spin is demonstrated over the surface of the Bloch sphere. This is an important prerequisite for the use of a hole spin as a qubit for quantum information processing applications.

Highly polarized nuclear spins within a semiconductor quantum dot (QD) induce effective magnetic (Overhauser) fields of up to several Tesla acting on the electron spin or up to a few hundred mT for the hole spin. Recently this has been recognized as a resource for intrinsic control of QD-based spin quantum bits. However, only static long-lived Overhauser fields could be used. Here we demonstrate fast redirection on the microsecond time-scale of Overhauser fields of the order of 0.5 T experienced by a single electron spin in an optically pumped GaAs quantum dot. This has been achieved using full coherent control of an ensemble of 10^3-10^4 optically polarized nuclear spins by sequences of short radio-frequency (rf) pulses. These results open the way to a new class of experiments using rf techniques to achieve highly-correlated nuclear spins in quantum dots, such as adiabatic demagnetization in the rotating frame leading to sub-micro K nuclear spin temperatures, rapid adiabatic passage, and spin squeezing.

We demonstrate fast initialization of a single hole spin trapped in an InGaAs quantum dot with a fidelity F>99% and a 1/e time of ∼30 ps, by applying a magnetic field parallel to the growth direction. We show that the fidelity of the hole spin, prepared by ionization of a photo-generated electron-hole pair, is limited by the precession of the exciton spin due to the anisotropic exchange interaction.

We report experimental evidence identifying acoustic phonons as the principal source of the excitation-induced-dephasing (EID) responsible for the intensity damping of quantum dot excitonic Rabi rotations. The rate of EID is extracted from temperature dependent Rabi rotation measurements of the ground-state excitonic transition, and is found to be in close quantitative agreement with an acoustic-phonon model., Comment: 4 pages, 2 figs, Accepted Physical review letters changes to referee comments + supplement

We report picosecond control of excitonic dressed states in a single semiconductor quantum dot. A strong laser pulse couples the exciton and biexciton states, to form an Autler-Townes doublet of the neutral exciton transition. The Rabi-splitting, and hence the admixture of the dressed states follows the envelope of the picosecond control laser. We create a superposition of dressed states, and observe the resulting beat: a direct measurement of a Rabi oscillation in time delay rather than the usual power domain.

The four-level exciton/biexciton system of a single semiconductor quantum dot acts as a two qubit register. We experimentally demonstrate an exciton-biexciton Rabi rotation conditional on the initial exciton spin in a single InGaAs/GaAs dot. This forms the basis of an optically gated two-qubit controlled-rotation (CROT) quantum logic operation where an arbitrary exciton spin is selected as the target qubit using the polarization of the control laser., 4- pages, 4 figures, to be submitted to PRL

We demonstrate the fast optical control of a single hole spin in an InGaAs quantum dot by using picosecond pulse excitation and photocurrent readout techniques. The optical control is implemented with 20ps gate time.

We demonstrate conditional Rabi rotations on the exciton-biexciton transition in a single self-assembled InGaAs/GaAs quantum dot using picosecond optical excitation and photocurrent readout. This is an implementation of the two-qubit C-ROT quantum logic gate.

We propose and demonstrate the sequential initialization, optical control, and read-out of a single spin trapped in a semiconductor quantum dot. Hole spin preparation is achieved through ionization of a resonantly excited electron-hole pair. Optical control is observed as a coherent Rabi rotation between the hole and charged exciton states, which is conditional on the initial hole spin state. The spin-selective creation of the charged exciton provides a photocurrent read-out of the hole spin state., Comment: 5 pages, 3 figures, resubmitted to PRL, changes due to referee comments

We investigate the coherence decay of a single-exciton-based qubit with electrical readout. Pairs of laser pulses initialize the qubit state and then map the target basis onto the measurement basis, probing the decay of either the population inversion or electronic polarization components of the state vector. A comparison of coherence and population decay confirms that the exciton coherence is lifetime limited by fast voltage-tunable electron tunneling. Optimum coherence times are limited by a heavy-hole tunneling rate slow compared with the repetition rate of the laser. © 2007 The American Physical Society.

We observe strong nonlinear absorption in the excitation profile of a single InGaAs quantum dot exciton driven by a rectangular spectrum optical pulse, in a region of detuning where the linear response is weak. Here the exciton is resonant with the temporal side-lobes of the pulse, which drive a damped Rabi oscillation. Good agreement is found between experiment and a two-level atom model. In the nonlinear regime the excitation profile exhibits features from both the rectangular spectrum and the two peaks present in the spectrum of the autocorrelation of the pulse.

We experimentally demonstrate coherent control of a laser. A resonant 100-fs optical pulse is injected into a vertical cavity surface emitting laser to introduce a field component with well-defined phase and thereby excite beating oscillations between the transverse lasing modes. By changing the relative phase between two injected pulses, we can enhance or destroy the beating oscillations and select which lasing modes are excited. We discuss resonant pulse injection into lasers and show how mode competition improves controllability by suppressing the phase-sensitive effects of the carriers.

Summary form only given. For room-temperature coherent control, semiconductor lasers are promising candidates because they offer a long optical coherence time and can provide gain for the optical signal. We present coherent control of optical modes in a VCSEL with resonant 100 fs optical pulse injection (center wavelength = 855 nm). It shows 40-210 GHz oscillations in the VCSEL after single pulse injection. Similar fast oscillations have been shown to originate from the beating between different transverse optical modes. This interpretation was confirmed by the optical spectra. Oscillations in the response indicate that the injected pulse establishes a fixed phase relationship between different transverse optical modes.

Using the helicity of a non-resonant excitation laser, control over the emission direction of an InAs/GaAs quantum dot is demonstrated. The quantum dot is located off-center in a crossed-waveguide structure, such that photons of opposite circular polarization are emitted into opposite waveguide directions. By preferentially exciting spin-polarized excitons, the direction of emission can therefore be controlled. The directional control is quantified by using the ratio of the intensity of the light coupled into the two waveguides, which reaches a maximum of ±35%.

We observe the resonantly excited emission of a single self assembled quantum dot (QD) positioned in a planar μ-cavity, free of any background scattering from defects in the surrounding lattice matrix. Making use of this technique, we are able to obtain the second-order correlation function g2(τ) and, therefore, observe its oscillations under strong continuous-wave excitation. By determining that its correlation function is near zero for g2(τ = 0), we are able to verify its status as a single quantum emitter. By altering the input scheme geometry, we are also able to demonstrate further direct control of a single QD's properties by gaining access to and observing emission from both of its polarization states πx and πy by merely tuning the excitation wavelength.

We propose and demonstrate a hybrid quantum well/quantum dot structure to enhance the gain and spontaneous emission bandwidth of a quantum dot active region. A single quantum well is introduced into a multi-layer stack of quantum dots, spectrally positioned to cancel the losses due to the second excited state of the dots. As a result, the modal gain at room temperature is extended to 300 nm, covering the wavelength range of 1100-1400 nm, and spontaneous emission has a 250 nm, 3 dB linewidth from 1080-1335 nm, of interest to short-haul telecom and optical coherence tomography applications.

Recently, longitudinal acoustic phonons have been identified as the main source of the intensity damping observed in Rabi rotation measurements of the ground-state exciton of a single InAs/GaAs quantum dot. Here we report experiments of intensity damped Rabi rotations in the case of detuned laser pulses, the results have implications for the coherent optical control of both excitons and spins using detuned laser pulses., Comment: 6 pages, 3 figs, invited conference paper ICPS 2010

The spin of a carrier trapped in a self-assembled quantum dot has the potential to be a robust optically active qubit that is compatible with existing III?V semiconductor device technology. A key requirement for building a quantum processor is the ability to dynamically prepare, control and detect single quantum states. Here, experimental progress in the coherent optical control of single semiconductor quantum dots over the past decade is reviewed, alongside an introductory discussion of the basic principles of coherent control.

We present a method for time-resolving the coherent precession of the spin of a single neutral exciton confined to an InAs/GaAs quantum dot. A pair of circularly polarized π pulses are used to pump and probe the exciton spin, rather than a pair of interferometrically stable π/2-pulses in a quantum interference type experiment, faster, technically simpler, measurements of the small 15 μeV exciton fine-structure can be made in the photocurrent regime. A small change in the fine-structure splitting with vertically applied electric-field is observed.

We demonstrate picosecond control of dressed states in a single quantum dot. The Rabi oscillations of the dressed states are directly resolved in the time domain through beating of the Autler-Townes doublet.

The source of intensity damping of Rabi-oscillations in quantum dots has long been in question. Here we identify acoustic phonons as the principal source of dephasing and demonstrate the re-normalization of the Rabi-frequency.

We demonstrate technique to time-resolve the population inversion of a quantum dot exciton, and measure T1. Combined with measurements of the coherence time T2. T2 is limited by electron tunnelling from the dot.

Quantum dot (QD) systems containing electron spins may hold a role in a future photonic quantum circuit as a means of storing a quantum state in a spin superposition. In general, the spin superposition state maps directly to a photon emitted out of the plane (kz) with the photon in a polarization superposition. In waveguides, however, this is more difficult: one requires a waveguide mode that is able to transmit both TEx and TEy modes in coherent superposition. We have already demonstrated a cross-waveguide design, as shown in Fig 1(a), that converts spin superposition states to a path encoding when the QD is located close to the waveguide centre.

The integration of InAs quantum dots as on-chip single-photon sources in GaAs photonic circuits is reviewed. A Hanbury Brown-Twiss effect is demonstrated using a monolithic directional coupler, together with coherent emission under resonant excitation.